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The use of the glass electrode as a reference electrode

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THE USE OP THE GLASS ELECTRODE AS A REFERENCE ELECTRODE
PAUL Si BROOKS '
T hesis submitted t o th e F a c u lty of th e Graduate School
of t h e U n iv e r s ity o f Maryland in p a r t i a l
f u l f i l l m e n t of th e requirem ents f o r
th e degree of Doctor of Philosophy
1940
UMI Number: DP70081
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Ac knowle dgement
The w r i t e r ta k e s t h i s o p p o rtu n ity t o express h i s a p p r e c ia tio n
to Dr. M. M* Haring f o r h is sug g estio n of th e problem and h i s c o n tin u a l
a id d u rin g i t s completion.
TABLE OP CONTENTS
I.
INTRODUCTION..........................
II.
THEORETICAL DISCUSSION
III.
Page
1
. .........................................................................
2
Theory o f th e Glass E l e c t r o d e .....................................
2
Standard E le c tro d e P o t e n t i a l
5
..............
A p p lic a tio n of th e Glass E l e c t r o d e ............... . . . . . . . . ...............
10
EXPERIMENTAL.....................................................................................................................
14
Apparatus
>. *
.
14
Reagents ...................................................................................................................
17
P r e p a r a tio n o f E l e c t r o d e s .............................
18
Calomel E le c t r o d e s .................................................. . ................................... 18
Mercury-Mercurous S u l f a t e - S u l f a t e - i o n E le c t r o d e s
18
Quinhydrone E le c tr o d e s .
.......
18
S i l v e r - S i l v e r C h lo rid e -C h lo rid e -io n E l e c t r o d e s . .............
19
Measurement of th e E. M. p. o f C ells
IV.
................................................. 21
RESULTS.....................................................................................................
23
C a l c u l a t i o n ..................................
23
D iscussion of R e su lts
30
......................
The S04 = |HgaS04 | Hg E l e c t r o d e ..........................................................
30
The 01“ |AgClJAg E le c tro d e
......
32
..........
33
.............................. . < - ...........
33
The Cr04“ |pt>Cr04 |pb E l e c t r o d e ............................. . . . . . . . . . . .
35
The Glass E le c tro d e ..........................................................................
35
The Quinhydrone E l e c t r o d e ....................
The Calomel E le c tro d e
V.
VI.
DISCUSSION OF ERRORS .........................................................................
CONCLUSION................................................................................
-
37
- . . . . ...........
41
V II.
SUMMARY.................................................................................................................................
42
V III.
BIBLIOGRAPHY.....................................................................................................................
43
I.
INTRODUCTION
A p r e lim in a r y r e s e a rc h ( l ) was undertaken t o determine th e p o s s i ­
b i l i t y of applying th e g la s s e le c tr o d e to th e measurement of sta n d a rd
e le c t r o d e p o t e n t i a l s .
That i t could be used f o r such purposes seemed
l o g i c a l sin ce i t i s known t o behave l i k e a hydrogen e l e c tr o d e .
I t has
t h e g r e a t advantage of e l im i n a t in g th e n e c e s s i t y of c a l c u l a t i n g a
j u n c t i o n p o t e n t i a l since in th e manner in which i t was employed, th e
j u n c t io n p o t e n t i a l i s absent from th e thermodynamically derived equation
f o r th e E° v a lu e .
The r e s u l t s of t h i s work were encouraging so a second re s e a rc h
was c a r r i e d out to i n v e s t i g a t e more f u l l y th e p o s s i b i l i t i e s and th e
p r e c i s i o n t o be expected.
In t h i s work much th e same technique was
employed, but th e measurements covered a wider range o f re fe re n c e e le c t r o d e s
and g r e a t l y improved apparatus was employed.
II.
THEORETICAL DISCUSSION
Theory o f th e Glass E le c tr o d e .
(2 )
The use of th e g la s s e le c t r o d e f o r r e s e a rc h and r o u tin e work
d a te s from about 1929 when Maclnnes and Dole s tu d ie d d i f f e r e n t ty pes of
g l a s s f o r making th e e le c t r o d e s .
A th e o r y of the g la s s e l e c tr o d e must account f o r i t s behavior in
alk alin e so lu tio n s,
a c id ity .
in very ac id s o l u t i o n s , and in s o lu tio n s o f in te rm e d ia te
In s o lu tio n s in which th e g la s s e le c tro d e a c ts as a hydrogen
e l e c t r o d e , thermodynamics gives a c l e a r fo rm u la tio n .
Imagine two h y d ro c h lo ric acid s o lu tio n s o f d i f f e r e n t c o n c e n tra ­
t i o n s in which two connected platinum e le c tr o d e s are immersed.
c i r c u i t i s completed through re f e re n c e calomel e l e c t r o d e s .
of e l e c t r i c i t y flows through th e c e l l ,
The
As one Faraday
one e q u iv a le n t of hydrogen ions i s
t r a n s f e r r e d from th e c o n c e n tra te d to th e d i l u t e s o l u t i o n , and th e f r e e
energy of t r a n s f e r , measured by th e E. M. F . , i s equal to th e d if f e r e n c e
i n f r e e energy of th e hydrogen ion in th e two s o l u t i o n s .
I f we c o n sid e r an analogous g la ss e le c tro d e system in which th e
two platinum e l e c t r o d e s are re p la ce d by a sin g le g la s s membrane, and
again allow one Faraday of c u r r e n t to p a ss, one e q u iv a le n t of hydrogen
ions i s r e v e r s i b l y t r a n s f e r r e d from th e c o n c e n tra te d to th e d i l u t e s o l u ­
tio n .
In t h i s c a s e , although th e mechanism o f the process i s e n t i r e l 3r
d i f f e r e n t from t h a t of th e hydrogen e l e c tr o d e , th e n e t r e s u l t i s e x a c tly
th e same, th e E. M. F. o f th e g la s s e le c tro d e i s i d e n t i c a l w ith t h a t of
th e platinum e le c t r o d e system, and th e g la ss e le c tro d e behaves as a t r u e
hydrogen e le c t r o d e .
I t i s i n t e r e s t i n g t o compare th o mechanism involved in th e t r a n s ­
f e r o f hydrogen ion in th e two c a se s.
In th e case of th e platinum e le c tr o d e
d ip p in g i n t o t h e c o n c e n tra te d s o lu t i o n , hydrogen ions are d e p o site d on
th e e l e c t r o d e , gain one e l e c t r o n each, and form hydrogen gas which escapes
from th e s o l u tio n .
At th e e le c tr o d e dipping in to th e d i l u t e s o l u t i o n ,
hydrogen gas d is s o lv e s to form hydrogen io n s, g iv in g one e l e c t r o n p e r ion
to th e e l e c tr o d e .
Any o th e r r e a c t i o n which may ta k e p la c e on the e le c tro d e
and in v o lv in g e l e c t r o n s , o x i d a tio n - r e d u c tio n f o r example, w i l l i n t e r f e r e
w ith th e process of t r a n s f e r r i n g hydrogen ions and so cause an e r r o r in
th e E. M. F. re a d in g .
For t h i s reason th e hydrogen e le c tr o d e cannot he
employed t o measure pH in th e presence of o x id iz in g or reducing substances#
The mechanism in th e case o f th e g la s s e le c tro d e i s e n t i r e l y
d iffe re n t.
No e l e c tr o n s are involved, so th e p o t e n t i a l i s e n t i r e l y un­
a f f e c t e d by o x id iz in g and reducing su b sta n c e s.
Hydrogen ions are n e i t h e r
d ischarged as hydrogen gas on th e g la s s nor does gaseous hydrogen form
hydrogen io n s ; i n s t e a d , th e hydrogen ions pass through th e g la ss as such
w ithout e l e c t r o n in te rc h a n g e .
For s o l u t io n s of high pH, v/here hydrogen
ion c o n c e n tra tio n i s very low, th e flow of e l e c t r i c i t y may r e s u l t in th e
flow of o th e r io n s.
Hence, th e E. M. F. no longer measures s o l e l y th e
f r e e energy of t r a n s f e r o f hydrogen ions and th e g la s s e le c tro d e no longer
fu n c tio n s w ithout e r r o r i
Another e x p la n a tio n of th e mechanism ^ ) »
(4) hop^g t h a t th e
conduction of e l e c t r i c i t y from one sid e of th e g la s s t o th e o th e r ta k e s
p la c e almost e x c lu s iv e ly through the sodium ions of th e g la s s .
The
mechanism involves an exchange r e a c ti o n between th e hydrogen ions and
th e sodium and calcium ions o f th e g l a s s .
The s u i t a b i l i t y of a g la s s as
an e le c tr o d e depends upon i t s a b i l i t y to exchange th e s e ions.
I t is k n o w n t h a t
a l l g l a s s e l e c t r o d e s show a small r e s i d u a l
E* M. P. a cro ss th e g la s s membrane when i d e n t i c a l s o lu tio n s are brought
in c o n ta c t w ith th e in n e r and o u te r s u r f a c e s .
The ex act cause of t h i s
so c a l l e d
One hy po th esis i s t h a t
“asymmetry p o t e n t i a l 11 i s n o t known.
th e p o t e n t i a l i s due to s t r a i n s in th e g la s s sin c e i t seems to be sm a lle r
w ith t h i n th a n w ith t h i c k membranes.
Another t h e o r y ^ ) holds t h a t t h i s
p o t e n t i a l i s due t o a d if f e r e n c e in c u r v a tu r e ; t h i s would suggest t h a t a
d i f f e r e n c e in atomic arrangement in th e two s u rfa c e s i s r e s p o n s ib le f o r
th e asymmetry p o t e n t i a l .
The r e l a t i o n s h i p between asymmetry p o t e n t i a l
and membrane th ic k n e s s would i n d i c a t e t h a t th e atomic arrangement of th e
e le c t r o d e membrane i s l i k e l y d i f f e r e n t both lengthw ise and c ro ssw ise .
At any r a t e , each g la s s membrane has i t s own asymmetry p o t e n t i a l
which i s n o t c o n s ta n t even f o r th e same membrane.
Hence, i n a c tu a l u se,
th e asymmetry p o t e n t i a l must be measured f r e q u e n tl y .
The e f f e c t of th e
asymmetry p o t e n t i a l w i l l be e x h ib ite d in the n e t E. M. P. of th e c e l l .
Standard E le c tro d e P o t e n t i a l
Since in t h i s work th e g la s s e le c t r o d e i s employed as an instrum ent
f o r in terco m p arin g s e v e r a l e le c tr o d e s through t h e i r sta n d a rd p o t e n t i a l s ,
a d is c u s s io n of th e sta n d a rd p o t e n t i a l i s in o r d e r ,
A u s e f u l concept f o r th e mechanism involved in a s in g le e le c tr o d e
was proposed in 1880 by N e r n s t ^ ^ .
According to t h i s th e o ry t h e r e i s a
tendency f o r th e atoms composing an e le c tro d e to go i n t o s o l u t i o n ,
and a
corresponding tendency f o r th e ions of th e s o l u t i o n to d e p o sit on the
e le c t r o d e as atoms.
The tendency of th e atoms to g o . in to s o l u t i o n as ions
i s expressed by th e e l e c t r o l y t i c
s o lu tio n p r e s s u r e , P.
The re v e rs e
tendency, which i s determined by th e io n ic c o n c e n tra tio n i s p r o p o r tio n a l
to th e osmotic p r e s s u r e , p , . o f th e s o lu tio n .
I f t h i s be t r u e ,
l)
it
i s ev ident t h a t t h r e e c o n d itio n s are p o s s i b l e :
I f P i s g r e a t e r th an p„ th e metal w i l l send ions in to
th e s o l u t io n u n t i l f u r t h e r a c ti o n i s stopped by th e e l e c t r o s t a t i c
attra c tio n .
.2 )
I f P i s l e s s than p, ions w i l l d e p o s it on th e e le c tro d e
from th e s o l u t i o n u n t i l th e charges accumulated oppose f u r t h e r
a c tio n .
3)
I f P i s equal t o p, n e i t h e r a c tio n w i l l occur and no
p o t e n t i a l w i l l be developed*
A fte r th e e s ta b lis h m e n t of e q u ilib riu m th e e le c tro d e w i l l be
surrounded by a l a y e r of e l e c t r i c a l charges, known as a Helmholtz e l e c t r i c a l
double l a y e r , which w i l l be p o s i t i v e when F i s g r e a t e r th an p, o r n eg a tiv e
when P i s l e s s th a n p.
In o rd e r to get th e E value f o r a s in g le e le c t r o d e , Nernst developed
a thermodynamic cy cle in which ions are t r a n s f e r r e d from a s o l u t i o n of
osmotic p r e s s u r e , p, to a s o l u ti o n equal in osmotic p re s s u re to th e
e l e c t r o l y t i c s o lu t io n p r e s s u r e , P.
T ra n s fe r may be e f f e c t e d e i t h e r by
passage o f an e l e c t r i c c u r r e n t , or by osmosis of th e s o lv e n t, each pro cess
being c a r r i e d out r e v e r s i b l y .
The work in th e f i r s t case i s nPE, where
E i s t h e d if f e r e n c e in p o t e n t i a l between"the two s o l u t i o n s ,
The work
done i n th e second case i s
P
RTdp or RTln £
P
F
( 1)
These two e x pression s are equal because th e work done in each case i s
maximum work, and,
E = RT
nF
I f th e f i r s t
In £
P
2)
s o l u t i o n has osmotic p r e s s u r e , p, equal t o th e
e l e c t r o l y t i c s o l u t i o n p r e s s u r e , P, in th e second s o l u t i o n , th e above formula
gives th e p o t e n t i a l of an e le c tro d e in terms of i t s e l e c t r o l y t i c s o l u ti o n
p r e s s u r e and th e osmotic p re s s u re of th e s o lu ti o n of i t s ions in which i t
has been placed .
At 25° C
E = RT In £ = *059 log
nF
P
n
£
P
3)
This eq u atio n prov ides only a q u a l i t a t i v e i n t e r p r e t a t i o n of
e le c tr o d e p o t e n t i a l s since E
l a t i o n s employing some
and
P cannot beev a lu a ted except from
a r b i t r a r i l y chosen s ta n d a rd ,
c a lc u ­
th e choice of which
w i l l determine th e value found.
Since th e value of a s in g le e le c tro d e p o t e n t i a l i s dependent on
th e tendency o f th e metal to go in to s o lu tio n as i t s
io n s, and on th e
osmotic p r e s s u r e , th e r e w i l l be a v a r i a t i o n w ith th e c o n c e n tra tio n of
th e ions in th e s o l u t io n .
For t h i s reason th e d e f i n i t i o n of th e standard
p o t e n t i a l must s p e c if y a p a r t i c u l a r io n ic c o n c e n tr a tio n and e le c tro d e
s ta te *
Keeping t h i s
(a S
d e f i n e d ' * as
in mind, th e sta n d a rd e l e c t r o e p o t e n t i a l has te e n
"the p o t e n t i a l of a m etal in i t s
in a s o l u t i o n of i t s ions a t u n i t a c t i v i t y ,
of 1 gram ion per 1000 grams o f s o lv e n t. "
sta n d a rd s t a t e v/hen immersed
i.e .
e f f e c t i v e c o n c e n tra tio n
F u r th e r ,
sin ce no method f o r
e v a lu a tio n of th e a b so lu te p o t e n t i a l o f an e l e c tr o d e e x i s t s a t p r e s e n t ,
a s ta n d a rd re fe re n c e p o t e n t i a l must he chosen.
The custom i s to r e f e r
a l l e le c tr o d e s t o th e hydrogen e le c t r o d e a t one atmosphere p re s s u re i n a
s o l u t i o n o f hydrogen ions a t u n i t a c t i v i t y ^ \
The value o f E° = 0 a t
a l l tem p eratu res i s assigned to t h i s e l e c tr o d e .
I n o rd e r
t o determine th e sign of
convention has te e n
e stab lish ed .
th e p o t e n t i a l
th e fo llo w in g
"The normal hydrogen e l e c tr o d e
i s placed
a t th e l e f t f o r re fe re n c e and combined w ith th e e le c tro d e i n q u e stio n a t
th e r i g h t .
The sig n of an e le c tro d e gives th e sign o f th e charge of th e
e le c tro d e a g a in s t th e s o l u t i o n when connected t o a normal hydrogen
e le c tro d e "
This convention gives a n e g a tiv e sign t o e le c tr o d e s of
m etals above hydrogen and a p o s i t i v e sign to th o se below hydrogen in the
u su a l a c t i v i t y s e r i e s .
The sta n d a rd p o t e n t i a l or E° value f o r an e le c tro d e may be expressed
(ll}
i n terms of u n i t a c t i v i t y as shown below'
From thermodynamics,
AF = - nFE
4)
and in a chemical r e a c t io n as
5)
aA + bB = gG + hH
- AF' = RTlnK - RTln
as-ai
6,
a i - a i
I f th e r e a c t i n g m a te r i a l s are a l l a t u n i t a c t i v i t y ( i . e . th e e f ­
f e c t i v e c o n c e n tra tio n i s one mol p e r 1000 grams o f s o lv e n t) ,AF i s w r i t t e n
as / \ F ° , and th e l a s t term of e q u a tio n 6 ) becomes z ero .
4 F ° = - RTlnK
S u b s t i t u t i n g e q u a tio n 4)
Thus,
( 7)
eq u a tio n 6 ),
E = RT InK - RE’ In d i d \ i
8)
“ at-ai
Here again in th e s p e c i a l case t h a t r e a c t a n t s and products a.re
a l l a t u n i t a c t i v i t y e q u a tio n 8 ) becomes
E° = RT InK
nF
S u b stitu tin g
9) i n
9)
8)
E = E° - RT In L7 G
nF
/? a
10)
m-a
/7 b
B
The sta n d a rd e le c tr o d e p o t e n t i a l has a lre a d y been defined as th e
p o t e n t i a l of a m etal in i t s
ions a t -unit a c t i v i t y .
stan d ard s t a t e immersed in a s o l u t i o n of i t s
T herefore, equ ation
10) can be employed to ex­
p re ss th e r e l a t i o n between th e standard e le c tr o d e p o t e n t i a l , E ° , and th e
e le c tro d e p o t e n t i a l , E, f o r any a c t i v i t y of th e io n s.
4-
For an e le c tr o d e of a m etal, LI, i n a s o lu tio n of i t s io n s, M , o
a c t i v i t y , ( X y f » Q^d. connected to th e r e fe re n c e hydrogen e l e c tr o d e , the
c e ll is:
Ha (p = 1 a tm .), H+ (a = l )
The r e a c t io n
M*, M
11)
fo r th is c e ll is w ritte n :
1/2 Ha. + H+ = Il+ + M
Applying eq u atio n
10)
&X+ *^M
E = E° - RT In
"
But in eq u atio n
12 )
13
13)
i
and
(X
M are f ix e d a t u n i t y .
So
th e e q u a tio n "becomes
E = E° - RT In _1__ .
14)
*
or i t s e q u iv a le n t,
E = E ° + RT I n
/I .
T h e re fo re , knov/ing th e value of E and th e value o f
, th e E° value
M+
can "be c a lc u late d *
S im ila r c o n s id e r a tio n s w i l l be app lie d to th e use of th e g la s s
e le c tr o d e i n th e fo llo w in g section*
A p p lic a tio n of th e Glass E le c tro d e
The fo re g o in g c o n s id e r a ti o n s may be a p p lie d t o th e measurement
of sta n d a rd p o t e n t i a l s w ith th e g la s s e l e c t r o d e in t h e fo llo w in g manner*
For th e e l e c t r o d e , S04 “ |Hg2 S04 Hg^a c e l l u s in g a calomel re f e re n c e
e l e c t r o d e was assembled as shown*
Hg|HgaCla | HC1 |G lass| H2 S04 j Hga S04 1Hg
%
V
r 0~ ^ .......—
--v '■
^
a
b
c
d
For such a c e l l , th e o v e r a l l E« M* F* should be th e sum of th e
v a rio u s ju n c t io n p o t e n t i a l s , o r ,
■ c e l l = Ea + Eh + E0 + Bd
16)
E v a lu a tin g th e s e ju n c t io n p o t e n t i a l s and s u b s t i t u t i n g in equ atio n
1 6 ) r e s u l t s in th e fo llo w in g eq u atio n f o r EC6-q a t 25° C.
e ° - .0 5 9 1 5 .log
1
Ec e l l
Eg - .05915 l o g
a-ci-j
/
+ Eg - .05915 lo g ^ H+
ES ~ »°.5915 lo g ( X s o - -
1
17)
But, sin ce th e g la s s e le c tr o d e i s s e n s i t i v e only t o H »
Eg = - Eg
18 )
and th e se two terms cancel from equ a tio n 17)*
By t h i s c a n c e l l a t i o n and
combination where p o s s i b l e i n e q u a t i o n .1? ) , equ a tio n 19) r e s u l t s :
E
c e ll =
E^ - .02958 lo g
E° - .05915 lo g _ 1
&H.C1
From which
Eo e l l =
Ea - *1183 lo g _ 1 —
+
E° - .02958 lo g
20)
Ym-HC1
Solving eq uation 20) f o r Eg, th e sta n d a rd p o t e n t i a l ,
Ed = Eo e l l ‘ Ea + - 1183 lo g
„ 1
ifmHC1
+ -02958 lo g /)( O n ^ g ^ ) 3
2 1)
The e q u a tio n in t h i s form was employed f o r th e c a l c u l a t i o n of the Ec
value f o r th e e le c tro d e S04
|Hg2 S04 |Hg*
For c a l c u l a t i o n of E° f o r th e e le c t r o d e C l“ |AgCl Ag u sin g a
quinhydrone re f e re n c e h a l f c e l l , th e c e l l employed was
P t | (HgQt O |H C l| Glass|HCl|AgCl Ag
a
b
e
d
The e q u a tio n f o r th e E° value of th e h a l f c e l l , Cl” |AgCl|Ag, th e n r e s u l t s
from a s i m i l a r d e r i v a t i o n i
Ec e l l = Ea + Eb + Ec + Ed
22)
E v a lu a tin g th e v a rio u s p o t e n t i a l s as b e f o r e ,
E
c e ll
E° - .05915 l o g f la *
Eg - .05915 log __1_
& H t,
Eg - .05915 log d H+
■if E | - .05915 lo g ( i u -
23)
Combining and c a n c e ll in g where p o s s i b l e gives
E o e ll = Et * Ed -
l o 6 d HC1
2k)
or th e e q u iv a le n t e x p re s sio n ,
Ec e l l = E° + Efl - *l l 8 3 l o S X'mHC1 ,
25)
Ed = Eo e l l - El + • 1183 log h a Hclo
26)
from which
For purposes/of in terco m p ariso n , each c e l l stu d ie d was measured
a g a in s t th e o th e r s as r e f e r e n c e s .
The d e r i v a t io n f o r each c e l l w i l l not
be given, but i n s te a d th e c e l l s s tu d ie d w i l l be l i s t e d and w ith each the
f i n a l form of th e e q u a tio n used f o r c a l c u l a t i o n of th e E° value.
In each
case the h a l f - c e l l used as r e f e re n c e i s placed on the l e f t hand sid e
w hile th e h a l f - c e l l on th e r i g h t i s th e one whose E° value is t o be
found.
The fo llo w in g l i s t i s complete, in c lu d in g the two f o r which the
E° value has been derived above.
I
Hg| HggClg HC1 Glass H2 S04 Hg2S04 |Hg
V.
a
E& " Ec e l l ** Ea + *1183 log ___ 1 __ + .0 2 9 5 8 log
YmHc i
^
SG ^
4
27)
12.
II
Ag |AgCl |hC1 Glass |h2 S0 4 Hg2 S04 Hg
a
b
c
d
Ed “ Ec e l l “ Ea +
III
lo £
-1
- + *02958 log 4 ( Kmg s 0
*mHCl
X2 4
28)
F t |(H2Q,f Q) | hC1 | G la s s | hs S04 |Hga S04 |Hg/
a
" b o
d
Ed = Eo e l l - Ea + -°2958 log 4< VmH2S04 )3
29)
XV Hg| HggClg | HClj Glass] HC11 AgCl |Ag_
a
Ed = E c e l l
V
- Ea +
- 1183
____
lo 6
K TCI.
+ - 1183
\
fc|
f V
■
/ V.
I
1» /
E° = Em 1 1 - E° + .02958 lo g ___ 1
*■ .1183 lo g
F t |(H2 Q,Q) HCl IGlass I HCl AgCl Ag
a
E°
VH
=
Eo e l l
S' m-HCl
- Ea + • 1133 l o g
" b e
d
Ed + Ec e l l - E° + .1183 log
1___
ft mHCl
T
Hg|lig2 S04 | H2 S04 IGlass] HCl | (H2 Q,Q)]Pt
---- y,----------'------v----M---V
----''--------- V------------'
'-----1
a
E d = Eo e l l
IX
32)
Hg Hga Cla IHCl 1Glassj IlCl |(Ha Q>Q ) | p t /
a
V III
30)
Hg|Hg2 S04 |HgS04 |Glass| HCI AgCllAg
*
VI
log If mHCl„
b
e
33)
d
" E a - *0 2 5 5 8 log 4(>'mH3S04)3
Ag| AgCl [ HCl | Glass |HC1 |(H2 Q,,Q)|pt
E3 + Eo e l l " Ea + *1183
lQg rj- A TCi
35)
3k)
mHCl
81 ^
13-
X
Agf AgCl 1HClj Glassj HCl [HggCl2 1Hg ^
a
b
^d = ^ e e l l “
XI
e
d
+ *
1°S
1
+ .1183 lo g
ymHClb
^^01
36)
°
Hg|HgaS04 |H2S04 |Glass| HCl| Hg3CIa |Hg _
a
b
e
d
Ea = Eo e l l “ Ea + *°29 58 lo g
+ .1 1 3 3 log If mHC1
. .1________
^
“ HsSO* ) 3
37)
In a l l of th e s e eq u a tio n s th e symbols used have th e foH ew ing
sig n ifican ce.
E^
E°
The stan dard p o t e n t i a l of th e e le c t r o d e measured as unknown.
The standard p o t e n t i a l of th e r e f e re n c e e le c tro d e .
Ecei i
The measured E. M. F* of th e unknown c e l l c o r r e c te d f o r
asymmetry p o t e n t i a l .
%
A c t i v i t y c o e f f i c i e n t of acid used,
m
M o la lity of a c id .
By measuring th e s e c e l l combinations a t v a rio u s c o n c e n tra tio n s
i t was p o s s ib le to determine th e E° v alu es f o r th e h a l f - c e l I s and to
intercom pare th e v a rio u s r e f e re n c e s through th e v alu es obtained.
E°
f o r t h e r e fe re n c e was, in every c a se , taken as th e b e s t l i t e r a t u r e value.
14-
III.
EXPERIMENTAL
Apparatus
Glass E le c tr o d e
The e l e c t r o d e used was a commercial type designed by E l l i s and
Kiehl and marketed by th e E e l l ig e Company.
The e le c t r o d e i s shown
s c h e m a tic a lly i n f i g u r e I in which th e l e t t e r e d p a r t s are as f o llo w s :
A.
D etachable e le c tr o d e
B.
Permanent e le c t r o d e
C.
Small cup f o r h o ld ing th e e l e c t r o l y t e
D*
Stopcock
E.
Rubber bulb f o r f i l l i n g th e e le c tr o d e
F.
C y l i n d r ic a l g la s s e le c tr o d e membrane
G.
C a p i l l a r y tube
H.
Metal c o n ta c t
I#
E x ten sio n on g la s s membrane
The tech n iq u e employed w i l l be d iscu ssed in a l a t e r s e c tio n .
Galvanometer
The galvanometer used was the Leeds and horthrup dual type galvano­
m eter, 2480-C.
I t has th e fo llo w in g c h a r a c t e r i s t i c s :
S en sitiv ity ,
0.0005
/mm.
P e rio d , 3 seconds
-Damping r e s i s t a n c e ,
Coil r e s i s t a n c e ,
15,000 ohms
1000 ohms.
Thermionic Am plifier
In any high r e s i s t a n c e system, such as th e g la ss e l e c tr o d e system,
s p e c i a l equipment i s n e c e s sa ry because of th e v e r y small c u r r e n ts to be
measured.
F u r th e r , since th e c u r r e n ts are so small th e e f f e c t of e l e c t r o -
--31
C
FIGURE
I
GLASS ELECTRODE
15 s t a t i c d is tu r b a n c e s and s t r a y c u r r e n t sjof va rio u s kinds become so p ro nounced t h a t th e system must be th o ro u gh ly s h ie ld e d in o rd e r to make the
use o f s e n s i t i v e in stru m e n ts p o s s i b l e .
For measurements of t h i s kind
ap p a ratu s employing e l e c t r o n tu b e s have been found most a p p lic a b le because
of t h e i r s e n s i t i v i t y and th e small amount of c u r r e n t drawn.
In th e p r e s e n t re s e a rc h th e instrum ent used was a Leeds and
Northrup Thermionic a m p li f ie r , No. 7673*
t h i s a m p l i f i e r i s shown i n f i g u r e I I .
The w irin g diagram ^ 2 ) por
The e l e c t r o n tu be in t h i s a m p lif ie r
i s one manufactured by th e Westinghouse Company and l i s t e d as e le c tro m e te r
tub e RH-507*
This tu b e i s known as an “in v e rte d t r i o d e " t u b e , and has
th e normal fu n c tio n s of g r i d and p l a t e re v e rsed .
The anode c u r r e n t was su p p lie d by a small C b a t t e r y .
was 6 V. r a t h e r th a n 4*5 V* as shown in th e diagram.
allov/ed t o flow c o n tin u o u sly to keep i t c o n s ta n t.
The c u r r e n t
The c u r r e n t was
The fila m e n t c u r r e n t
was, i n th e e a r l y p a r t of th e work, supplied by a lead s to ra g e b a t t e r y ,
and l a t e r by a group of Eveready a i r c e l l s .
more s a t i s f a c t o r y .
The l a t t e r were found to be
While th e a m p li f ie r was not i n u se, th e c u r r e n t was
allowed to flow through a r e s i s t a n c e equal to t h a t o f th e a m p li f ie r in
o r d e r to keep i t c o n s ta n t.
In o p e r a tin g th e a m p li f ie r , f and j are used to a d j u s t th e
e l e c t r i c a l zero o f th e galvemometer.
The unknown E. M. F. i s measured by
connecting a p o te n tio m e te r to te rm in a ls marked POT and th e unknown c e l l
t o te r m in a ls marked E. Iff. F.
By r e l e a s i n g swri t c h X to th e upper p o s i ti o n
th e c o n t r o l e le c tr o d e i s charged to a n eg a tiv e p o t e n t i a l equal to the
p o t e n t i a l of th e housing p lu s th e unknown E. hi. F. » i f th e potentiom eter
i s z ero .
By a d j u s t i n g th e p o te n tio m e te r u n t i l the galvanometer r e tu r n s
t o i t s o r i g i n a l p o s i t i o n , th e unknown E. M. F. can be determined and i s
equal to th e p o te n tio m e te r reading a t b alan ce.
kA
EM F
CONTROL
POT
CATHODE
17
-ANODE
GA.
(MA
FIG URE
AMPLIFIER
I I
CIRCUIT
The c i r c u i t employed f o r E. M. P. measurement i s shown diagramm a t i c a l l y in Figure I I I .
The E. M. P. le a d s p a s sin g in to th e metal "box
o f th e a i r b a th were i n s u la t e d by 2 1 /2 inch quartz tu b e s s e t in la r g e
rubber s to p p e r s .
These le a d s between th e a m p lif ie r and th e a i r ba th
were s h ie ld e d by a U shaped aluminum s h e e t.
P o te n t iomete r
A ll E. M. F. measurements were made w ith a Leeds and Northrup
Type K p o te n tio m e te r.
Constant Temperature A ir Bath
Constant te m p e ratu re f o r c e l l measurements v/as m aintained i n an
a i r b a th r e g u l a te d by a mercury th e rm o re g u lato r and s u p e r s e n s i t i v e r e l a y
which kept th e tem p eratu re c o n sta n t a t 25° i
0 .1 ° C.
H eating was
accomplished by means of a small 12 w a tt carbon f ila m e n t l i g h t bulb
mounted in th e top of th e box.
Cooling was e f f e c t e d by a copper c o i l
through which cold w ater was c i r c u l a t e d .
S t i r r i n g was e f f e c t e d by a
r e l a t i v e l y la r g e aluminum p r o p e ll o r d riv e n e x t e r n a l l y .
The s t e e l box, 8 " x 1 2 11 x 16 ” in s i z e , was covered w ith p r e s t wood which a c te d as i n s u l a t i o n to p rev en t ex c e ssiv e h e a t lo s s e s through
th e m etal w a l ls .
The m etal "walls served as a s h i e l d f o r th e unknown
c e l l and prevented e r r o r s due to s t r a y c u r r e n t s .
Weights
The weights used were c a l i b r a t e d a g a in s t a 10 gram weight
c a l i b r a t e d by th e Bureau o f Standards.
6
100
A
DPDT
J V
DPDT
/V
STD
AIR-BATH
25
^
-f6 V .—
t
GA
EMF
—
+—
POT
______I
FIGURE I I I
EM F.
CIRCUIT
BA STD
EMF
Reagents
Water
C o n d u c tiv ity w ater of 1*5 micromhos was used in th e p r e p a r a t io n
of a l l s o l u t i o n s .
Sodium Carbonate
The sodium carbo n ate used in th e s ta n d a r d i z a t i o n of th e a c id
s o lu t i o n s was of re a g e n t q u a l i t y .
I t was prepared f o r use by h e a tin g
to a tem p eratu re of 270° to 280° C . , a t which tem p erature i t was m aintained
f o r 1 l / 2 h ours.
as t h e i n d i c a t o r .
T i t r a t i o n s were run in t r i p l i c a t e u sin g methyl orange
The d e s ire d c o n c e n tra tio n s were prepared from th e
s ta n d a rd acid s o l u t io n s by d i l u t i o n on a weight b a s i s .
A ll c o n c e n tra tio n s
a re expressed on a molal b a s i s .
Mercurous S u l f a te
The mercurous s u l f a t e used in th e study of th e electro de^
S04~J HggSO4.| Hg^was prepared by th e e l e c t r o l y t i c method of Clark and
Weston
Mercury
The mercury used was cleaned and t r e a t e d according to th e method
o f H u le tt and m i n c h i n ^ ^ .
Other Reagents
A ll o th e r r e a g e n ts employed were of reagent q u a l it y .
P r e p a r a t io n of E le c tr o d e s
Calomel E le c tr o d e s
The calomel e l e c t r o d e s used were prepared i n th e follow ing manner.
The e l e c t r o d e compartment was th o rou g h ly cleaned and r in s e d w ith acid of
th e c o n c e n tr a tio n to he used in th e h a l f - c e l l .
The bottom o f th e compart­
ment was d r ie d in th e v i c i n i t y of the platinum c o n ta c t and mercury added
to a depth of about 0*5 bo 0 .7 c e n tim e te r .
Over th e mercury was placed
about th e same depth of p a s te prepared from calomel, mercury, and acid
of th e c o n c e n t r a ti o n t o be used.
The c e l l was th e n f i l l e d w ith hydro­
c h l o r i c a c id o f th e proper c o n c e n tra tio n .
The e le c tr o d e used as a
r e f e re n c e was prepared i n compartment B of Figure I .
H a l f - c e l l s to be
measured were p repared i n u n i t s corresponding to compartment A o f Figure
I.
For measuring th e asymmetry p o t e n t i a l , th e e le c tr o d e s in A and B
were of th e same composition.
Mercury-Mercurous S u l f a t e - S u l f a t e - i o n E le c tro d e s
These e le c tr o d e s were p re p a red in a manner s im ila r to th e above,
f i r s t c le a n in g th e u n i t s ,
r i n s i n g w ith acid of th e proper c o n c e n tra tio n ,
th e n d ryin g th e p a r t of th e e l e c tr o d e v/hich was t o hold th e mercury.
In
t h i s c a se , the l a y e r of mercury was covered w ith a l a y e r of mercurous
s u l f a t e p r e v io u s ly r in s e d s i x tim es w ith s u l f u r i c acid of th e c o n c e n tra tio n
used i n th e c e l l .
The r i n s i n g was done j u s t before use of th e mercurous
su lfa te .
Quinhydrone E le c tro d e s
H ydrochloric acid of va rio u s c o n c e n tra tio n s was fre e d from d isso lv ed
a i r by h e a tin g to b o i l i n g in a long necked f l a s k .
A fte r cooling to room
te m p e ratu re th e acid was s a t u r a te d w ith quinhydrone in a g la s s stoppered
fla sk .
C e lls were made up by p la c in g t h i s s a tu r a te d s o lu ti o n in the
e l e c t r o d e v e s s e l s and i n s e r t i n g a platinum s p ir s .l e le c tro d e p re v io u s ly
c lean ed i n dichromate c le a n in g s o l u t io n .
To in su re s a t u r a t i o n some s o li d
quinhydrone was added to each h a l f - c e l l p re p a red .
This s o l u t i o n was
f r e s h l y prep ared f o r each measurement employing quinhydrone e i t h e r as
a r e f e re n c e or as an unknown c e l l .
S i l v e r - S i l v e r C h lo rid e -C h lo rid e -io n E le c tro d e s
These e le c tr o d e s were p repared on a r o l l of platinum gauze welded
t o a platinum w ire se a le d in s o f t g l a s s .
The t o t a l a re a of fo u r of th e se
e le c t r o d e s was c a l c u l a t e d t o he 27 square c e n tim e te rs .
The gauze a f t e r
thorough c le a n in g was s i l v e r p la t e d in a h a th c o n ta in in g 3k grams of
sodium cyanide and 24 grams of s i l v e r
of w a te r.
(from s i l v e r n i t r a t e ) in a l i t e r
This h a th i s o f the composition recommended hy Blum and
Hogahoom ( l^ ) .
Four e le c tr o d e s were p l a t e d sim u ltaneo u sly u sin g a s i l v e r
anode.
The e le c tr o d e s were connected in p a r a l l e l ,
sym metrically arranged
ahout t h e anode, and p l a t e d f o r s ix hours at a c u r r e n t d e n s i ty of fo ur
m illiam p eres p e r square c e n tim e te r .
The p l a t i n g was done a t room tem perature
w ith o c c a s io n a l s t i r r i n g .
A fte r p l a t i n g th e e le c t r o d e s were washed over a p e rio d of fo u r
days u s in g 12 changes o f d i s t i l l e d w a te r.
Each change of w ater was heated
to c o i l i n g w ith th e e le c tr o d e s immersed in an attem pt t o in c r e a s e th e
r a t e of washing.
A fte r t h i s thorough c le a n in g th e e le c t r o d e s were o h lo r id iz e d in
a 0 .1 molal HC1 s o l u t i o n u sin g th e same c u r r e n t d e n s ity as b e f o r e .
The
e le c t r o d e s were connected in p a r a l l e l ,
The
and t r e a t e d sim ultan eou sly .
cathode was platinum.
The p r e p a r a ti o n was completed hy washing in d i s t i l l e d w a te r f o r
two days, th e n p u t t i n g th e e l e c tr o d e s to soak in acid of th e c o n c e n tra tio n
20.
t o be used i n t h e i r measurement*
The time of soaking of each e le c tro d e
b e fo re measuring a g a in s t th e v a rio u s re f e re n c e s i s given in th e fo llow ing
ta b le.
Cone, of HC1
on E le c tro d e
Time o f soaking (in days) b efo re measurement
A gainst
Against
Against ,
Hg[ HgQCl0 1Cl" Hg iHg^SO^ [S0A"~ Pt |HqQ, Q, Ih
0 . 20 m
7
10
33
0 . 10m
6
1
34
0 . 05m
12
14
17
0 . 01 m
12
14
17
Table I - A
For measurement th e s e e le c tr o d e s were placed i n small e le c tro d e
v e s s e l s w ith side-arm dipping in to cup C, o f Figure I .
In a l l c a se s th e e le c tr o d e used as a re fe re n c e was placed in
th e permanent e le c t r o d e compartment, r e p re se n te d by B of Figure I .
The
e l e c t r o d e s measured were run in d u p lic a te in u n i t s corresponding to A
of Figure I .
In th e cases of th e quinhydrone e l e c tr o d e , and th e s i l v e r -
s i l v e r c h lo r id e e l e c tr o d e c o n ta c t was not made through the metal con tact
a t t h e bottom o f th e e le c t r o d e v e s s e l , but through a w ir e dipping in to
mercury in th e g l a s s tube h o ld in g th e platinum.
The re f e re n c e e le c tr o d e compartment, B of Figure I , was always
r in s e d a t l e a s t s i x tim es w ith th e a p p ro p ria te s o lu ti o n before assembling
th e e l e c t r o d e .
M e a s u r e m e n t o f th e E .
M. P. of C ells
The complete c e l l f o r measurement was assembled as fo llo w s.
R e f e r r in g t o Figure X, th e stopcock D vv'as f i r s t opened and the lower end
o f th e tu b e , I ,
p laced in cup C ,
to be used in a p a r t i c u l a r c e l l .
c o n ta in in g a c id o f th e c o n c e n tra tio n
By p r e s s in g bulb E ,a c id was drawn up
i n s i d e th e g la s s e l e c tr o d e membrane.
In t h i s manner th e in n er su rface
of th e e l e c t r o d e was r in s e d w ith s e v e ra l p o r tio n s of th e a p p ro p ria te
ac id b e fo re measuring a c e l l .
F i n a l l y a p o r ti o n of acid was drawn up
in s i d e th e membrane t o th e h e ig h t of th e surrounding ja c k e t and th e
stopcock c lo s e d .
E le c tro d e s in u n i t s corresponding to A of Figure I
were placed i n p o s i t i o n w ith th e side-arms dipping in to cup C.
The acid
in th e s e e le c tr o d e s was o f th e same c o n c e n tra tio n as t h a t in th e cup.
E le c tro d e s were measured in d u p li c a te .
To o b ta in th e o v e r a ll E. M. F. of any c e l l t h r e e measurements
were n e c e s s a ry .
Using, f o r example, measurement o f the E° f o r th e
S04“ | Hg2 S04 | Hg e le c t r o d e employing 0.1 molal E^SO^. a g a in s t a calomel
r e f e r e n c e , th e t h r e e measurements are *
l)
Measurement o f th e asymmetry p o t e n t i a l o f the g la ss e le c tro d e
u sin g th e same h a l f - c e l l on each sid e o f the membran e t
Hg| Hg2 Cl2| HC1 (0. lm)| Glass | HC1 (. lm)|Hg2 Cls | Hg
Any E .
M. F. observed h ere would be due to a d if f e r e n c e in behavior of
th e g la s s membrane on th e two s i d e s .
This c e l l was allowed about one
hour to come t o tem p eratu re e q u ilib riu m .
2)
Measurement of th e unknown E. M. F. of th e c e l ls
H 6 | h 6 2C 12 | H C 1 (.ln i)|G la s s |H 3 S04 (. l m )| HgaS04 | H g
The unknown c e l l s were allowed an hour to reach tem perature e q u ilib riu m ,
th e n readin g s ta k e n u n t i l co n sta n t w ith in 0.1 m i l l i v o l t .
This u s u a l l y
re q u ire d
a p e rio d
e q u ilib riu m
reach
th e
m ost
d e sire d
3)
as
in
th e
o f
from
3 to
ra p id ly .
The
7 h o u rs.
calo m el
c e lls
q u in h y d ro n e
c e lls
reached
re q u ired
le a s t
a day to
a t
c o n stan cy .
R em easurem ent
f i r s t
The
m easu rem en t.
o f th e
asy m m etry p o te n ti a l
u sin g th e
sam e
c e ll
IV.
RESULTS
C a lc u la t io n
For c a l c u l a t i o n of th e E° values measured^ equations 27) to 37)
were used to g e t h e r w ith th e d a ta of th e t a b l e s which follow .
I-B and I-C th e
From t a b l e s
m values were o b tain ed ; from I-D, th e E° values o f
th e r e f e re n c e e le c t r o d e s employed; and from t a b l e s I t o XI, th e c e l l
E. M .
F. v a lu e s .
Tables I to XI correspond t o eq uations I to XI (these
e q u a tio n s a re 2 7 ) t o 37) t u t were given Roman numerals t o correspond to
th e t a b l e s assembled by t h e i r u s e . )
Taking f o r example th e c a l c u l a t i o n of E° f o r th e h a l f - c e l l
S04 " | H 6 aS04 |Hs u sin g 0 . 1 molal HgS04 acid measured a g a in s t a calomel
r e fe re n c e the fo llo w in g c a l c u l a t i o n r e s u l t s .
E quation 27) (or I ) i s used:
E° = Ec e l l - E° + .1183 l o g
1
+ .0 2 9 5 8 l o g 4 U r n - s0 )3
a mHCl
4
S u b s t i t u t i n g th e values
Ec e i i = 0.3390 (Table I )
E° = - 0-2676 (Table I-D)
a
.1183 log
1
= 0.1300 (Table I-B)
*mHCl
• 02958 lo g 4 ( ^ ^ H 2 S04 ^
= 0.3390 -
= “ 0 .1 2 2 1
(-0.2676) +■ 0.1300 + (-0.1221)
Eg = 0.6145
The E° valu es f o r a l l e le c tr o d e s measured are c a l c u l a te d in th e
same manner u s in g d a ta from t h e a p p ro p ria te t a b l e s .
These r e s u l t s are
l i s t e d in th e l a s t column of each t a b l e from t a b l e I to t a b l e XI.
t a b l e s include a lso th e c e l l measured, th e sig n of th e E. M. F.
experim entally)* and th e •asymmetry p o t e n t i a l s .
These
(found
The average asymmetry
p o t e n t i a l was used in c a l c u l a t i n g a l l E° valu es l i s t e d .
A c t i v i t y C o e f f i c ie n t s f o r Hydrochloric Acid
M o la lity
of HCl
A c tiv ity ( l 6 )
C o e f f ic i e n t
Value of
• I I 83 lo g V'rojjQi
0
CVj
.
0
molal
0 .7 6 6
- 0 .0 9 6 4
0 .1 0
ti
0 .7 9 6
- 0.1 30 0
0 .0 5
ti
0 .8 2 9
-O .I636
0 .0 1
n
0 .9 0 4
-0 .2 4 1 8
Table I-B
A c t i v i t y C o e f f i c ie n t s f o r S u lf u r ic Acid
M o la lity of
HgSO^
A ctiv ity
(17)
C o e f f ic ie n t
0 .3 0 molal
0 .1 8 0
- 0 .0 9 4 7
.0295$ log 4 ( * mHoS04 ^
0 .2 0
11
0 .2 0 9
- 0.1045
0 .1 0
it
0 .2 6 5
-0 .1 2 2 1
0 .0 5
11
0 .3 4 0
-0 .1 3 9 2
0 .0 2
it
0 .4 5 3
- 0.1635
0 .0 1
11
0 .5 4 4
- 0 .1 8 3 1
Table I-C
E° Values f o r Reference E le c tro d e s
Reference
E le c tro d e
E° Value
HE|H6s Cla | c i -
- 0 .2 6 7 6
(1 8 )
H g|HgaS0jS04 ~
-0.6152
(19)
Ag |A g C l |d -
-0.2223
(2 0 )
P t | (HgQ.Q)! H+
-0.6990
(2 1 )
Table I-D
Asy.
P o t.:
C e ll :
R ef. +
H g | H g 2C l 2 | H C l ( . l m ) | G l a s s [ H C 1 ( . l m ) | H g 2C l 2 | H g
—
+
H g | H g a C l 2 | H G l ( . l m ) | G l a s s | H q S O ^ ( x m ) j H g 2 S 04 Ji£g
O v e ra ll E is
Cone, of
H^SO*
Asy. P o t.
Before
sum
o f
tw o
E
is -
E
is +
p o te n tia ls.
Asy. P ot.
A fte r
Asy. Pot.
Average
Ec e l l
Measured
O verall
E of Cell
E° of Ce
Eq. 27
O.3 O m
-0.0088
-0.0091
-0.0090
0.3022
0.3112
0.6141
0.20
-0.0091
-0.0094
-0.0093
0.3119
0.3212
0.6143
0 .10
-0 .0 0 9 4
-0.0093
-0.0094
0.3296
0.3390
0 .6 1 4 5
0 .0 5
- 0 . 00.90
-0.0091
-0.0091
0.3475
0.3566
0 .6 1 5 0
0 .0 2
-0.0091
-0 .0 0 9 3
-0.0092
0.3715
O.38O7
0 .6 1 4 8
0 .0 1
-0.C091
-0.0092
-0.0092
0.3906
0.3998
0 .6 1 4 3
Table I
Asy. P o t:
Ref. +
Ag| AgCl IfiCl (. lm) (Glass |HC1 (.. lm) |AgCl| Ag
E is -
C e ll :
Ag|AgCl|HCl ( . lm) | G la s s |h 3 S04 (xm)| Iig2S04 |Hg
E is +
O v e ra ll E is sum of two p o te n tia ls *
Cone. of
Asy. P ot.
Before
Asy. Pot.
A fte r
Asy. Pot.
Average
0 .3 0 m
- 0 .0079
-0 . 0 0 7 1
- 0 .0 0 7 5
0 .2 0
-0.0073
- 0 .0079
0 . 10
- 0 .0 0 7 9
0 .0 5
E of C e ll
O verall
E° of C ell
Eq. 23
0.3491
0 .3 5 6 6
0.6142
-0.0079
0.3596
0 . 3675
0 .6 1 5 3
-0.0069
-0.0074
0.3764
O.3 8 3 8
0 .6 1 4 0
-0.0073
-0 .0078
-0 .0 0 7 6
0.3941
0,4017
0 .6 1 4 8
0 .0 2
- 0 .0 0 7 8
- 0 .0 0 5 2
-0 .0 0 6 5
O.Z|171
0.4236
0.6124
0 .0 1
-0.0032
- 0 .0 0 5 6
-0 .0 0 4 4
0.4358
O. 4 4 0 2
0.6094
T ab le
II
E of C e ll
Measured
+
F t | (HgQ#Q) | h C 1 ( • l m ) [ G l a s s | Ha S04 (xm;jHg2 S04 |Hg
C e ll:
O v e ra ll E
2one.
of
is
sum
o f tw o
A sy. P o t.
B efore
A sy. P o t.
A fte r
A sy. P o t.
A verage
0*30 m
- 0 .0 0 6 4
- 0 .0 0 6 0
- 0 .0 0 6 2
0 .2 0
- 0 .0 0 6 5
-0 .0 0 6 4 .
0 .1 0
- 0 .0 0 6 5
0 .0 5
E
is
+
p o te n tia ls.
E o f C ell
M easured
E of C ell
O v erall
E °of C e ll
E q . 29
0 .0 0 3 8
0 .0 1 0 0
0 .6 1 4 4
- 0 .0 0 6 5
0 .0 1 4 4
0 .0 2 0 9
0 .6 1 5 4
- 0 .0 0 6 5
- 0 .0 0 6 5
0 .0 3 1 5
0 .0 3 8 0
0 .6 1 4 9
- 0 .0 0 6 0
- 0 .0 0 6 7
- 0 .0 0 6 4
0 .0 4 8 8
0 .0 5 5 2
0 .6 1 5 0
0 .0 2
-0 .C 0 6 8
- 0 .0 0 5 8
- 0 .0 0 5 8
0 .0 7 3 0
0 .0 7 8 8
0 .6 1 4 3
0 .0 1
- o . 0055
-0 .0 0 4 8
- 0 .0 0 5 2
0 .0 9 2 8
0 .0 9 8 0
0 .6 1 3 9
__
Table I I I
Asy. P o t:
C e ll :
Ref, +
Hg| Hg2 Cl2 | HC1 (. 1*) |G lass| HC1. (. lm) |HggCla |Hg
4*
—
Hg| Hg2 Cl2| EC1 (. lm) |G lass| HC1 (xm) | AgCl |Ag
E is ■
E is -
O v erall E i s d if f e r e n c e of two p o te n tia ls ^
Cone• of
HC1
Asy. P ot.
Before
Asy. P o t .
A fte r
Asy. P ot.
Average
0 .2 0 m
- 0.0087
-0 . 0087
-0.0087
-0.0893
-0 .0 8 0 6
0 .2 2 0 6
0 .1 0
-0.0087
-0-0089
-0-0088
-0.0557
-0.0469
0 .2 2 0 7
0 .0 5
-0.0059
- 0 .0 0 6 1
-0 .0 0 6 0
-0.0192
-0.C132
0 .2 2 0 8
0 .0 1
-0 .0 0 6 1
-0 .0 0 6 1
-O-CO6 I
+0 . 0 5 7 8
+0.0639
0.2197
Table IV
E of C e ll
Measured
E of C ell
O verall
E° of C ell
E1 -..SQ
27.
Asy. P o t . :
Ref • +
—
Hgl HggSO^I HgSO^ ( . lm) I Glass| H2 S0l ( . Ira) | Hg2S04 1Hg
3
1s -
E
is -
+
C e ll:
Hg[ Hg2 S0/J H2SO^ ( . lm) | 01ass| HC1 (xm) | AgOl |.Ag
O verall S is d i f f e r e n c e of two p o t e n t i a l s .
Gone, of
HOI______
Asy. P o t.
Before
0 .2 0 m
- 0 .0 0 7 7
-0.0079
-0.0078
- o .4 2 6 6
- 0 .4 1 8 8
0. 2210
0 .1 0
- 0 .0 0 8 4
-0.0073
- 0 .0 0 8 1
-0.3944
-O.3 8 6 3
0 .2221
0 .0 5
- 0 .0 0 6 5
-0.0069
-0.0067
-0.3591
-O.3 5 2 4
0 .2 2 1 3
I—1
0•
0
Asy. P o t. Asy. P o t .
.After
Average
3 of c e l l
Meas ured
5 of Cell
Overall
E° of Cell
E g . 31
- 0 .0 0 6 9
-0.0069
-0.0069
- 0 .2 8 0 5
-O.2 7 3 6
0.2219
Table V
Asy. P o t . :
Ref. +
PV|(H2 q , q ) | h c i
( . lm)| 01 ass |HC1 (-lm) | (H2 Q ,Q )|pt
E
Cell:
F t| (H2 q , q ) | h c i
( . lm)| Class | HOI ( xm) I AgO1 |as
E is -
is -
O verall E is d if f e r e n c e of two p o te n t 1 al s .
Asy. P o t.
Before
Asy. p o t.
A fter
Asy* P o t.
Ave ra£ e
E of Cell
Measured
E of C ell
Overall
E ° of C ell
Eq- 32
0 .2 0 m
- 0 .0 0 5 9
- 0 .0 0 0 7
- 0 .0 0 3 3
-O.3 8 4 6
-O .3 8 I 3
0 .2 2 2 5
0
1—
1
•
0
-0.0069
- 0 .0 0 4 2
- 0 .0 0 5 6
- 0 .3 5 2 1
-0.3465
0 .2 2 2 5
0 .0 5
-O.OO83
-0.0071
-0.0077
-0.3123
0 .2 2 2 6
0 .0 1
- 0 .0 0 7 6
- 0 .0 0 8 2
-0.0079
- 0 .2 3 5 6
0 .2 2 1 6
Cone, of
HOI
Table VI
- 0 .3 2 0 5
-0.2435
23.
Asy. Pot:
Ref * +
—
Hg| Hg2Cl2 | HCl ( . lm)| G lass|h c1 ( - lm)| Hg2d l 2 |Hg
E is -
+
C ell:
Hg| Hg2 Cl2 | HCl
(• lm)| Glass |HCl (xm) |(H2 ^ » 'l) |p t
E is +
O verall E is sum of two p o t e n t i a l s .
Cone, of
HCl______
Asy. P o t.
Before
Asy. p o t .
A fter
Asy. P o t.
Average
E of d e ll
Measured
E of d e l l
Overall
E° of Cell
Eq. 33*
0 .2 0 m
- 0 .0 0 6 0
- 0 .0 0 6 0
- 0 .0 0 6 0
0.2918
0.2973
0.6954
0 .1 0
- 0 .0 0 6 0
- 0 .0 0 6 0
- 0 .0 0 6 0
0.2919
0.2979
0.6955
0 .0 5
- 0 .0 0 6 0
- 0 .0 0 6 0
- 0 . 0 0 60
0 .2 9 1 8
0.2978
0 .6 9 5 4
0 .0 1
- 0 .0 0 6 0
- 0 .0 0 5 2
- 0 .0 0 5 6
0.2923
0.2979
0.6955
Table VII
Asy. Pot:
C ell;
Ref. +
Hg| Hg2 S0^| H2S0^ ( .lm) | G la s s |h 2 SC^ ( • lm)| Hg2SO^ |Hg
4**
Hg|Hg2 S0^| H2 SO^ ( . lm) | C-lass[HCl ( xm) |(H2 Q,Q)| F t
E is E is -
Cone, of
HCl
Asy. P o t.
Before
Asy. p o t .
..After
Asy. P o t.
Average
E of d e l l
Measured
E of d e l l
Overall
E° of d e ll
Eq* 34
0 .2 0 m
-0.0069
- 0 .0 0 7 0
- 0 .0 0 7 0
-0.0467
-0.0397
0 .6 9 7 6
0 .1 0
- 0 .0 0 7 0
-0.0069
-i0.0070
-0.0469
-0.0399
0 .6 9 7 4
0 .0 5
-0.0069
-0.0067
- 0 .0 0 6 8
- o .0 4 6 6
-O.O398
0.6975
0
•
0
i—
1
O v erall E is d if f e r e n c e of two p o t e n t i a l s .
-0.0067
- 0 .0 0 6 8
- 0 .0 0 6 8
- 0 .0 4 6 8
- 0 .0 4 0 0
0.6973
Table V I I I
29 .
Asy. P o t . :
C e ll:
R ef. +
Ag| AgCl| HC1 ( . o5m) | 01 as s| HCl ( .05nj)|AgCl IAs
.
+
'
As|AsCl|HCl (.05m)|Class) HCl( xm) | (H2 Q,Q) | p t
E is E is +
O verall E is sum of two p o t e n t i a l s
Cono.
of
HCl________
Asy. P o t. Asy. p o t .
Bef ore
A f t er
Asy. P o t.
Ave r aye
E of Cell
M e as ured
E of Cell
O verall
E ° of Cell
Eg. 35_____
0 .2 0 m
-0.0077
- 0 .0 0 6 8
-0.0073
0 .3 0 4 5
O.3 I I 8
0.6977
0 .1 0
- 0 .0 0 6 8
- 0 .0 0 7 5
- 0 .0 0 7 2
0.3025
0.3097
0.6956
0 .0 5
-0 .0 0 7 5
-0.0079
-0.0077
0.3029
O.3 IO6
0 .6 9 6 5
0 .0 1
- 0 .0 0 7 9
-O.OO83
- 0 .0 0 8 1
0 .3 0 2 5
O.3 IO8
0.6965
T ab le
A sy.
IX
R ef. +
A s | A s C l l HC1 ( .05m)J G l a s s | H C l
P o t:
C ell:
Ag|ASCI|H C l
( .05m)| C l a s s | H C l
O v erall
E is
sum
of
( -0 5 m)| A s C l 1As
+
(xm) | Ke2 C12 Hg
E is
-
E is
+
tw o p o t e n t i a l s
E°
of
C ell
..
A sy. P o t.
B efo re
A sy. P o t.
A fte r
Asy. P o t .
A verage
E of C ell
M easured
E of C ell
O v erall
0 .1 0 m
- 0 .0 0 9 2
- 0 .0 0 8 2
-O.OO87
0 .0 0 2 6
0 .0 1 1 3
0 .2 6 7 2
0 .05 m
-0.0078
-0.0076
-0.0077
O.O367
0.0444
0•2667
Cone*
HCl
of
T ab le
A sy.
C ell:
P o t:
Ref • +
H-|Hs2S0^|H2 S0^ ( . l m ) | C l a s s ) H 2 S < f y ( . l m ) | HS230^ Hg
h s | h S2so 4 | h 2 so 4
O v erall
Cone,
HCl
0 . 10
X
E
is
( . lm) | C l a s s ) H C l
d iffe re n c e
of
( x m ) | K s 2 c i 2 |h s
E
is
-
E
is
-
tw o p o t e n t i a l s
of
A sy. P o t.
B efore
A sy. P o t.
A fter
A sy. P o t.
A verage
E of C ell
M easured
E of C e l l
O v erall
S° of C ell
Eg. 37
m
-0.0070
-0.0069
- 0.0 0 7 0
-0-3473
-O.3 4 0 3
0.2 6 7 0
-0 .0 0 7 4
-0 .0 0 7 3
-0.0074
- O .3 I 42
- 0.3 0 6 8
0.2669
0 .0 5 m
Table XI
30 .
D isc u ssio n of R e su lts
For convenience a l l
of the E^ values determined have been ^rouned
to g e t h e r in t a b l e s XII t o XV.
In t a b l e s XII and X III have been placed
a l s o th e r e s u l t s obtained f o r the same e le c tro d e s in th e p r e l j.r. inary
r e s e a rc h .
The SC^= |Hg2S0Zj_|Hg E le c tro d e
R e fe rrin g t o t a b l e XII in which are given th e values found f o r E°
of t h e S04= |Hg2 S0idHg e l e c t r o d e ,
i t is seen t h a t in each case th e average
v a lu e f o r th e e le c t r o d e is n e a rly the same.
The values obtained using
th e calomel re f e re n c e are th e most c o n s is te n t showing an o v e ra ll v a r i a t i o n
of 0.9 m i l l i v o l t .
With the s i l v e r - s i l v e r c h lo r id e reference (d isre g a rd in g
th e l a s t two v a lu e s) th e o v e r a ll v a r i a t i o n is 1 . 3 m i l l i v o l t s , and with the
quinhydrone r e f e r e n c e , 1 . 5 m i l l i v o l t s .
In a l l Gases, the average value
is w i t h i n 0 .7 m i l l i v o l t of th e accepted value of O.6 1 5 2 VI-D)
This is an e r r o r of 0.11 p e r c e n t.
(See t a b l e
The r e s u l t s found previ ously
gave an average value of .6134 V, n e a rly 2 m i l l i v o l t s from the accepted
v a lu e .
The improved r e s u l t s a re b elieved due t o b e t t e r equipment f o r
E. M. F. measurement since the c e l l s were prepared in the same manner.
The S0^— |Hg2S02j.|Hg e le c tr o d e was found t o reach eq uilib riu m q u ite
r a p id ly ;
c o n s ta n t re a d in g s were obtained w ith in th r e e hours.
Reproducible
r e s u l t s were obtained using f r e s h l y prepared e l e c t r o l y t i c mercurous s u l f a t e
and another sample which had been prepared f o r over a y ear.
The measurements were not c a r r ie d t o low acid c o n c e n tra tio n s since
i t has been shown (22) t h a t the values f a l l r a p i d l y , due to h y d ro ly s is
of th e mercurous s u l f a t e .
The l a s t two values f o r t h i s e le c tro d e measured a g a in s t the s i l v e r s i l v e r chlori.de re fe re n c e are d isre g a rd ed in ta k in g the average since they
31 .
S O ^ = | h s 2 s c 4 |h 2 E l e c t r o d e
E ° F o r
done. of
H2 so 4
C alo m el
R eference
1933
C alo m el
R eference
1939
S ilv e r
C hi or? d e
R eference
Q uinhydrone
R eference
0 .3 0 m
0 .6 1 2 8
0 .6 1 4 1
0 .6 1 4 2
0 .6 1 4 4
0 .2 0
0 .6 1 4 0
0 .6 1 4 3
0.6153
0.6154
0 .1 0
0,6139
0.6145
0 .6 1 4 0
0 .6 1 4 9
0 .0 5
0 .6 1 4 1
0 .6 1 5 0
0 .6 1 4 8
0.6150
0 .0 2
0 .6 1 3 0
0 .6 1 4 8
0.6124
0.6143
0 .0 1
0.6127
0 .6 1 3 4
A verage
0.6143
0.6145
A verage
0 . 60 94
O.6 1 4 6
A verage
0.6139
0.6147
Ave ra g e
T ab le
E° F o r
0o n e •
HCl
of
C alom el
R eference
1933
0 .2 0 m
X II
C l" | A gC l | Ag E le c tr o d e
C alo m el
R eference
1939
M ercurous
S u lfa te
R eference
Q uinh ydrone
R eference
0 .2 2 0 6
0 .2 2 1 0
0 .2 2 2 5
0 .1 0
0.2197
0 .2 2 0 7
0 .2 2 2 1
0.2225
0 .0 5
0.2191
0 .2 2 0 8
0 .2 2 1 3
0 .2 2 2 6
0 .0 1
0 .2 1 9 6
0.2195
Ave rag e
0.2197
0 .2 2 0 5
A verage
0.2219
0 .2 2 1 6
A verage
0 .2 2 1 6
0 .2 2 2 3
A verage
T ab le
E° F o r
X III
Q u .in h y d .ro n e E l e c t r o d e
S ilv e r
Chi o ride
R eference
C alo m el
R eference
M ercurous
S u lfa te
R eference
0 .2 0 m
0 .6 9 5 4
0 .6 9 7 6
0.6977 '
0 .1 0
0 .6 9 5 5
0 . 6974
0 . 695^
0 .0 5
0 .6 9 5 4
0 .6 9 7 5
0 • 69 65
0 .0 1
0 .6 9 6 5
0 .6 9 5 5
0 .6 9 7 3
0.6955
0.6975
0.6966
A v e r a g e ______A v e r a g e_______ A v e r a g e
Cone*
HCl
of
32 .
E° For Cl*| Hs2 C12 |h £ E le c tro d e
Cone, of
HCl
__________
S ilv e r
Chloride
Reference
Mercurous
S u lfate
Reference
0 .1 0 m
0 .2 6 7 2
0 .2 6 7 0
0.0.5 m
0.2667
0 .2 6 7 0
Average
0.2669
0.2670
Average
Table XV
are obviously c o n sid e ra b ly in e r r o r .
The Cl” | AgCl |As E le c tro d e
In t a b l e X III th e r e s u l t s f o r the Cl*|AsCl|AS e le c tro d e are given.
Here again th e more re c e n t values are a m i l l i v o l t nearer th e accepted
v alue of 0.2223 V (se e t a b l e I-D) than the previous v a lu e s .
The agreement
f o r d i f f e r e n t re fe re n c e s i s not good, the o v e ra ll v a r i a t i o n being 1 .8
m i l l i v o l t s f o r th e averages under each re fe re n c e .
This may be du e (in
p a r t t o th e f a c t t h a t the e le c tr o d e s were o lder (see ta b le I-A) when
measured w ith th e quinhydrone re fe re n c e th an when measured with the
calomel r e f e r e n c e .
The d if f e r e n c e in the asymmetry p o t e n t i a l seen in
t a b l e s IV, V, and VI i s not a sudden change;f o r the e le c tro d e s using 0.2
m and 0 .1 m HCl were measured se v e ra l weeks before those using 0.05 m and
0.01 m.
Hence the lowering of t h e asymmetry p o t e n t i a l occurred over a
c o n s id e ra b le p e rio d of tim e.
When a quick change of asymmetry p o t e n ti a l
o c c u rs, e s p e c i a l l y before and a f t e r a sin g le c e l l , the r e s u l t s have been
found t o be u n r e l i a b l e .
The g r e a t e s t d e v i a ti o n from the t r u e E° value was found when using
th e calomel r e f e re n c e .
Here th e d e v ia t io n was 1.8 m i l l i v o l t s from the
t r u e v a lu e of 0.2223 V, or a d i f f e r e n c e of 0 . 8 lf£.
The p e rio d re q u ire d f o r measurement of th e s e e le c tro d e s was not long
s in c e they had heen standing in th e pro per s o lu ti o n f o r s e v e ra l days
b efo re measurement.
The Quinhydrone E lectro d e
Table XIV shows th e E° values found f o r th e quinhydrone e le c tro d e
measured a g a i n s t d i f f e r e n t r e f e re n c e s .
Agreement f o r d i f f e r e n t concentra­
t i o n s of acid is good f o r any one re f e re n c e , but values found f o r E° of
th e quinhydrone e l e c tr o d e with d i f f e r e n t re fe re n c e s are in poor agreement
(see ta b le ) .
The v alue c l o s e s t the c o r r e c t value of 0.6990 V ( t a b l e j-D)
was obtained using th e Hg| Hg^SO^jSO^- e le c tro d e as a re fe re n c e .
value of 0.6975 was o btained.
0.21 p e r c e n t .
An average
This is in e r r o r by 1 . 5 m i l l i v o l t s , or
The p o o r e s t value i s in e r r o r by 3 . 5 m i l l i v o l t s , or 0 .5
p e r c e n t.
D e riv a tio n of the equation f o r E° f o r th e quinhydrone e le c tro d e shows
t h a t th e measured E» M. P.
should be the same f o r c e l l s using d i f f e r e n t
c o n c e n tra tio n s of acid with the quinhydrone.
case in a c tu a l measurements.
TbHs was found to be the
The values were e r r a t i c in the case of the
Ag | AgCl | Cfl" r e f e r e n c e , but the c a lc u la te d S° values were e r r a t i c to the
same e x t e n t .
The quinhydrone e le c tr o d e s were observed t o reach equilibrium r a p id ly ,
most of th e time being used t o allow' the e le c tro d e s t o reach the c o rre c t
tem p eratu re.
The Oa-lomel E le c tro d e
Table XV g iv e s the r e s u l t s f o r th e measurements made to determine
th e E° value f o r th e calomel e l e c t r o d e .
This e le c tro d e comes to equilibrium
much more slow ly, a t' l e a s t a f u l l day being re q u ire d .
ments were made.
Hence fewer measure­
Two co n c e n tra tio n s were Mieasured a g a in s t each of two
referen ces.
The r e s u l t s agree w ell w jth each o th e r , the average values
being th e same f o r th e two r e f e re n c e s .
This value of 0 . 2 670 is 0 .6
m i l l i v o l t below th e c o r r e c t v alue of O.2 6 7 6 y (see t a b le I-D ).
This is
an e r r o r of 0 . 2 2 %.
The calomel e le c tr o d e gave some d i f f i c u l t y
in measurement when used
as an unknown sin c e i t r e q u ir e s more time to become c o nstan t.
However,
when used as a re f e re n c e e l e c t r o d e i t was kept assembled f o r longer
p e r io d s and became very c o n s ta n t and re p ro d u c ib le .
Reference to t a b l e s
Xll» X I I I , and XIV, shows t h a t th e most c o n s i s t e n t T° values were obtained
by measurements a g a in s t t h i s
e le c tro d e .
In order t o determine the r e l a t i v e s u i t a b i l i t y of the various ele c tro d e s
as r e f e re n c e s they were compared in the follow ing manner.
The d e v ia tio n s
from the average 5° valu e of each e le c tro d e measured ag ain st a given reference
were added (w ithout regard to s ign) and th e t o t a l divided by the number
of measurements.
The r e s u l t s were as fo llo w s.
Hg |Hg2 Gl2 | CJl” e le c tro d e v a rie d by
0 .2 m illiv o lts
^g2 ’^ 4. | ' ^ 4 ~ el e ° tro d e varied by _+ 0 . 2 m i l l i v o l t s
A g | A gCfl | c l ”
e le c tro d e
v a rie d
by
4- 0 - 5 m i l l i v o l t s
F t |(H2 Q»Q) |h + e l e c tr o d e v a rie d by _+ 0 . 4 m i l l i v o l t s
This kind of comparison in d ic a te s t h a t the f i r s t two e le c tro d e s l i s t e d
a re the most r e l i a b l e ,
sin ce th e y give more c o n s is te n t r e s u l t s .
Of these
two, the Hg| Hg2 S0^|S0^= e le c t r o d e gave r e s u l t s n e a r e r to the c o r r e c t E°
v a lu e .
F u r th e r th e s e two e l e c t r o d e s are much e a s i e r to p re p a re .
The
quinhydrone e le c t r o d e is not d i f f i c u l t to p r e p a r e , but i t s use involves
changing f r e q u e n t l y because of decomposition of the quinhydrone.
Although th e calomel e le c tro d e does not reach e q u ilib riu m f o r a day
or so , a f t e r once a t t a i n i n g e q u ilib riu m i t remains tr u e f o r a long time and
does not need to be changed.
The Cr04~jPbCr04 |pb E le c tro d e
An attem pt
was made t o determine th e E°
Cr04 |pbCr04 |pb
through measurement of c e l l s
value f o r the e le c tro d e
of th e ty p e:
Hg|Hga C12 1HC1 (. In)(Glass | H2Cr04 |PbCr04 |pb.
The g la s s e le c tr o d e was found to behave q u ite as expected, but th e c e l l s
s tu d ie d d id n o t
re a ch , or approach e q u ilib riu m .
i d e n t i c a l l y did
not show agreement w ith each
F u r th e r , c e l l s
made up
o th e r.
Both platin u m p la t e d w ith lead ( e l e c t r o l y t i c a l l y ) and lead wires
were used, but n e i t h e r type showed any con siste n c y .
Because o f th e t o t a l la c k of agreement among d u p lic a te s th e r e s u l t s
are n o t inclu d ed in t h i s t h e s i s .
The author b e lie v e s the d i f f i c u l t y to
l i e i n th e chrom ate-acid chromate-diohromate e q u ilib riu m system* (2 3 )
The Glass E le c tr o d e .
I n t h e study of t h e s e e le c tro d e s s e v e ra l m atters of i n t e r e s t have
been le a rn e d about th e g la s s e le c tro d e i t s e l f .
The e le c t r o d e employed in t h i s re se a rc h was about th r e e years old.
Except f o r a c o n s id e ra b le in c re a s e in asymmetry p o t e n t i a l , th e e le c tro d e
behaved as w ell as when i t was new.
The asymmetry p o t e n t i a l was u su a lly
c o n s ta n t though in s e v e ra l months i t v aried between
and 9 m illiv o lts *
The values are l i s t e d in t a b l e s I to XI,
I t was observed t h a t i f th e asymmetry p o t e n t i a l varied more then a
few t e n t h s of a m i l l i v o l t before and a f t e r measurement of a given c e l l ,
th e E° c alcu ls.ted from th e measurement could not be t r u s t e d .
of measurements had t o be d isc a rd e d f o r t h i s reason.
One s e r i e s
Although some values
showing t h i s v a r i a t i o n a re included th e y are not r e l i a b l e .
I t was observed, a lso , t h a t i f t h e r e was a change of asymmetry
p o t e n t i a l , th e change must have been immediate f o r th e r e was no corresrondi?.
d r i f t in th e E. M. F. of th e c e l l being measured.
In every case th e curved o u te r su rface of th e c y l i n d r i c a l g la ss
membrane was found to be p o s i t i v e w ith re s p e c t t o the in n er surface when
th e same s o l u t i o n was on each s id e .
e l e c t r o d e o f s i m i l a r c o n s tr u c tio n .
The same was found t r u e of another
Although i t cannot be s t a te d as
proved, i t seems t r u e t h a t th e convex su rface in a g la s s e le c tro d e is
p o s i t i v e w ith r e s p e c t t o th e concave su rfa c e .
H a u g a a r d ^ 4 )
observed
t h a t t h e r e i s a d e f i n i t e d if f e r e n c e in behavior e x h ib ite d by the two
s id e s of th e membrane.
He s t a t e s ,
“th e e le c tro d e fu n c tio n of the concave
sid e of th e g la s s membrane i s more l i k e the r e v e r s ib le hydrogen e le c tro d e
th a n t h a t o f th e convex side')
Haugaard a lso found a change in th e asymmetry p o t e n t i a l with a
change i n t h e pH of th e s o l u t i o n .
re s e a rc h on ly in p a r t .
This was confirmed in th e present
In t a b l e VII, f o r in s ta n c e , the asymmetry p o t e n t i a l
remained c o n s ta n t f o r c o n c e n tra tio n s of hy d ro chlo ric acid from 0 . 2 molal
to 0*01 m olal.
A statem en t more c o n s is te n t with t h i s re search would be
t h a t th e asymmetry p o t e n t i a l changes w ith the n atu re of the li q u i d ,
although t h i s change may n ot always be in th e same d ir e c t i o n .
V.
In
a ll
c e lls
o f th e
sam e
acid
in
a c tiv ity
th e
duced
b y th is
e x p re sse d
m easured,
on
each
o f th e
excep t th o se
sid e
o f th e
w ater.
d iffe re n c e
by th e
DISCUSSION OF ERRORS
in
h av $ .in g th e
g lass
A cco rd in g to
a c tiv itie s .
m em brane,
D ole^2-^
ssm e c o n c e n tra tio n
th e re
an
The m ag n itu d e
is
e rro r
of th e
a d iffere n c e
is
in tro ­
e rro r
is
e q u a tio n
e
-
gr
■T
m
Q fe.p./S a.o
r
8)
f II
tl
c HgO J H20
where R, T, and F have t h e i r usual sig n ific a n c e.
c = c o n c e n tra tio n
of w ater.
j[ =• a c t i v i t y c o e f f i c i e n t .
This eq u a tio n reduces to
E = .05915 log
# HoO
39)
^"HsO
Using th e e q u a tio n i n t h i s form some c a lc u la t io n s were made f o r
c e l l s u s in g 0 . 1 m HCl in the r e fe re n c e e le c tro d e and varying con cen tratio n s
of H^SO^ i n th e unknown c e l l .
The a c t i v i t y of water i n 0.1 m HCl i s
ta k e n as 1 as a f i r s t approximation (since a c t i v i t i e s were not a v a i l a b l e ) .
This w i l l give th e maximum e r r o r , f o r th e t r u e a c t i v i t y w i l l be le s s than
1 and make th e a c t i v i t y r a t i o more n e a r l y 1 giving th e log more n e a rly
equal to zero.
The a c t i v i t y values f o r water in s u l f u r i c acid s o lu tio n
are th o se o f Hamed and Hamer^2^ .
C a lc u la tio n gave f o r 0*3 m H2S04 and
e r r o r o f 0 . 3 m i l l i v o l t , f o r 0*2 m H3S04 an e r r o r of 0.2 m i l l i v o l t , f o r
0 .1 m H2 S04 an e r r o r of 0 .1 m i l l i v o l t which i s w ith in experim ental e r r o r .
The lower c o n c e n tra tio n s give in c o n s e q u e n tia l e r r o r s .
-Application of
th e s e c o r r e c t io n s t o th e measured values would in c re a s e the E° ^alu e.
33.
The in c r e a s e would, however, s c a r c e ly change th e average value f o r a l l
c o n c e n tr a tio n s measured.
For c e l l s employing th e same a c id on each sid e of the membrane the
e r r o r s would n o t be la r g e f o r th e range of c o n c e n tra tio n s i s not great
and th e a c t i v i t y o f w ater would be n e a r ly th e same.
The quinhydrone used in th e vario u s e le c tro d e s was prepared by the
Eastman Kodak Company.
This product was assumed to be s u f f i c i e n t l y pure
f o r use and was n o t r e c r y s t a l l i z e d .
in t h i s
reagent as used.
There may have been im p u ritie s present
However, e le c tr o d e s measured ag ainst the
quinhydrone re f e re n c e gave in rr.Gst cases as good or b e t t e r E° values
th a n th e same e le c tr o d e s measured a g a in s t o th e r re fe re n c e s .
hand,
On the other
in th e d e te rm in a tio n of th e E° value f o r th e quinhydrone e le c tro d e
th e r e s u l t s were th e p o o re st of any ob tain ed , in one case being in e r r o r
T°y 3* 5 m i l l i v o l t s .
A p o s s i b l e a d d i t i o n a l source of e r r o r in the quinhydrone measurements
i s th e f a c t t h a t th e r e fe re n c e e le c tro d e s used did not have as long to
come t o e q u ilib r iu m as in measurements of o th er e le c tro d e s .
The rapid
e s ta b lis h m e n t o f e q u ilib riu m i n the quinhydrone e le c tro d e made i t p o ssib le
to measure a s e r i e s of th e s e e le c tr o d e s in a much s h o r te r time.
As a
r u l e , th e re f e re n c e e l e c t r o d e s were used th e day follow ing t h e i r assembly,
a complete s e r i e s of c e l l s being measured in one day.
For other c e l l s ,
th e r e fe re n c e e le c tr o d e s were in use f o r a week or more without changing.
Hence, th e e sta b lish m e n t of e q u ilib riu m was more c e r t a i n .
r e f e r e n c e , e s p e c i a l l y , r e q u ir e s time to become c o n s ta n t.
The calomel
I t w i l l be
noted t h a t th e r e s u l t s were po o re st f o r quinhydrone measured a g a in st
calomel ( s e e t a b l e XIV).
In th e use of th e Ag|AgCl | c i ” e l e c tr o d e th e ex clusion of a i r was
n o t attem pted.
This was b e lie v e d to be unnecessary since according to
Taylor th e p o t e n t i a l i s not a f f e c t e d .
Taylor^-2?) s t a t e s :
"After the
experim ents on aging of t h e r m a l - e l e c t r o l y t i c e le c tro d e s were f in is h e d ,
a supplementary experiment was conducted to show t h a t oxygen was not
r e s p o n s ib le f o r th e aging e f i e c t .
Several of th e se e q u ilib r a te d
e le c t r o d e s were ta k e n out o f th e c e l l and placed, f o r four hours in an
a i r s a t u r a t e d s o l u t i o n of th e same composition as in th e c e l l .
They
were th e n re tu r n e d t o the c e l l and compared with e le c tro d e s which had
not been th u s exposed.
No s i g n i f i c a n t changes in p o t e n t i a l were o b s e rv e d .“
The e le c t r o d e s used in t h i s re se a rc h were of d i f f e r e n t typ e, but would
be expected to show the same behavior with re sp e c t to oxygen.
F u rth e r,
in th e p re lim in a ry r e se a rc h a i r was removed from th e acid so lu tio n s but
th e E° values found were not as good as the p re se n t ones.
By r e f e re n c e t o t a b l e s XII to XV i t can be seen th a t th e r e s u l t s
found f o r E° o f th e e le c tr o d e s stud ied are below the c o rre c t values in
p r a c t i c a l l y every case.
Having compared four d i f f e r e n t e le c tro d e s using
each one as unknown and as re f e re n c e , the w r i te r f e e l s t h a t the e r r o r must
l i e somewhere in the g la s s e le c tro d e i t s e l f since i t is the only p a r t of
th e c e l l used in a l l measurements.
The -e r r o r i s believ ed to be in th e
asymmetry p o t e n t i a l f o r t h i s p o t e n t i a l was found to vary u np red ictably
a t tim e s .
In th e valu es used f o r c a l c u l a t i o n , the lowest recorded value
f o r th e asymmetry p o t e n t i a l i s 0 . 7 m i l l i v o l t s and tne h ig h e s t, 9*4
m illiv o lts.
The average v tlu e i s about 7 m i l l i v o l t s fo r most of the
measurements.
I t seems much more l i k e l y t h a t th e e r r o r is here, r a th e r th an in
impure r e a g e n ts , or measuring equipment; otherw ise the r e s u l t s would not
be uniform ly below th e c o r r e c t v a lu e s.
That e r r o r s were intro d u ced by tem perature v a r i a ti o n s in the a i r
b a th i s n o t 1 ik e ly .
Except t o r a few tim es when i t was necessary to
p la c e ic e in s id e th e b a th , the tem perature was maintained at 25 ° C + 0 *1°.
The g r e a t e s t v a r i a t i o n even under th e worst co nd itio n s was + 0.15° C.
Since th e c o n tr o l was in an a i r bath , th e c e l l would not undergo as great
v a r i a t i o n but remain c lo se to 2 5 ° a f t e r once reaching t h i s temperature.
Taking v a lu e s from a Leeds and Northrup b u l l e t i n ^ ® ^ f o r voltages for
g la s s e le c t r o d e and a s a tu r a t e d calomel c e l l , f o r 0 . 1 molal s o lu tio n the
E. M. F. a t 20° C i s 0.3952 V and a t 25° C is 0*3943 V.
of .0009 v o l t s f o r 5° change in tem perature.
change in th e a i r b a th ,
This is a change
For the maximum temperature
0 .1 5°, th e change would amount to .00002? V,
which i s w e ll w ith in th e experim ental e r r o r o f measurement.
41.
v i.
Co n c l u s i o n
As a r e s u l t of t h i s r e s e a rc h se v e ra l f a c t s have been e s ta b lis h e d .
F i r s t , th e g la s s e le c tro d e has been shown to be ap p lic a b le to the
measurement o f sta n d a rd e l e c tr o d e p o t e n t i a l s to an accuracy of one
m illiv o lt.
The r e s u l t s are re p c o d u c ib le , but c o n s is t e n t l y below the
t r u e va lu e .
Second, th e g la s s e l e c tr o d e i s s t i l l usable a f t e r a long period of
time although i t s asymmetry p o t e n t i a l is co nsiderably higher than when
th e e le c tr o d e *i s new.
The asymmetry p o t e n t i a l changes w ith the n a tu re of the s o lu tio n ,
although th e change i s not
always in th e samed i r e c t i o n .
I f , during
given c e l l measurement th e
asymmetry p o t e n t i a l changes moreth an a few
t e n t h s o f a m i l l i v o l t , th e
measurement i s not r e l i a b l e .
a
Of th e e le c tr o d e s s t u d ie d , th e two found most s u ita b le f o r t h i s type
of measurements were th e Hg|Hg2Clg |C l” e le c tro d e and th e ^gJ^-ggSO^ JsO^
e le c t r o d e .
They are more re p ro d u c ib le and also e a s i e r t o prepare and use.
The g la ss e l e c tr o d e should prove u s e f u l as a r e fe re n c e e le c tro d e in
measuring stan d ard p o t e n t i a l s f o r e le c tr o d e s not y et determined, and
thrtou^i
avoided.
us® th e troublesome c a l c u l a t i o n o f ju n c ti o n p o t e n t i a l s may be
h2.
The
g la ss
e s p e c ia lly
em p lo y ed
e le c tro d e
fo r m easu rin g
e lim in a te s
has
g lass
b o th
as
e le c tro d e .
S e v era l
The
been
show n u se fu l
c a lc u la tio n
unknow ns,
The m ost
fe a tu re s
p o ssib le
SUMMARY
stan d ard
A num ber o f w ell-k n o w n
e le c tro d e s
VI*
o f th e
so u rces
o f
a re fere n c e
e le c tro d e
p o te n tia ls .
ju n c tio n
p o te n tia ls .
o f
e le c tro d e s
and
as
as
have
been
re fe re n c e s
su ita b le
o f th e se
g la ss
e le c tro d e
e rro r
in
th e
have
have
m eth o d
The m eth od
stu d ie d ,
w ith
c e lls
been
been
have
e le c tro d e ,
u sin g
th e
in v o lv in g th e
p o in ted
o u t.
stu d ie d .
been
d isc u sse d .
43
V III.
( )
BIBLIOGRAPHY
Brooks, P. S . , The Use of t h e Glass E le c tro d e in th e D etermination
o f Standard E le c tro d e P o t e n t i a l s , ^ a s t e r , s T h e sis, U n iv e r s ity of
Maryland, 1 9 3 8 .
(2 )
Dole, M., Experim ental and T h e o r e tic a l E le c tro c h e m is try , p. 435 BP*
McGraw-Hill C o ., 1935.
(3)
Haugaard, G ., Chem. A b s t . , ^ 1 , 6976 (193?)
(4)
Haugaard, G., Chem. A b s t., ^ 2 * 6160 (193B)
(5)
Dole, M., Measuring pH w ith th e Glass E le c tr o d e , B u l l e t i n p u b lish e d
by th e Coleman E l e c t r i c C o ., 1937*
(6)
Dole and Gabbard, Chem. A b s t., ^ 2 , 2032 (1938)
(7 )
Getman and D a n ie ls, O u tlin e s o f T h e o r e tic a l Chemistry, p. 443
John Wiley and Sons, 1931*
(8)
T aylor, H. S ., T r e a t i s e on P h y s ic a l Chemistry, p. 827* D. Van
Nostrand C o., 1931*
(9)
Getman and D aniels, O u tlin e s o f T h e o r e tic a l Chemistry, p. 431. 1931»
(1 0 )
Getman and D a n ie ls, O u tlin e s of T h e o r e tic a l
Chemistry, p. 432* 1931*
(11)
Getman and D a n ie ls, O u tlin e s o f T h e o r e t ic a l
Chemistry, p. 435 BB,
(12)
Cherry, R. H ., The Measurement of D ire c t P o t e n t i a l s O r ig in a tin g in
1931*
C i r c u i t s of High R e s is ta n c e , Pamphlet by Leeds and Northrup Company,
1937.
(13)
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(14)
H u le t t and Minchin, Phys. Rev., 21, 388 (1905)
(15)
Blum and H o g a b o o m ,
P rin c ip le s
of E le c tro p la tin g
and E le c tro fo rm in g ,
p. 299* McGraw-Hill Co., 1924*
(1 6 )
H arn ed ,
H. S . , J . Am. Chem. S0 o . . £1, b 2 5 (1929).
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Harned and Hamer, J . Aiji. Chem. S o c.,
30 (1935)
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I n t e r n a t i o n a l C r i t i c a l Tables, Vol. VI, p. 332, 1929.
(19)
Harned and Hamer, J . Am. Chem. S o c . , 5 7 , 31 (1935)
(20)
Dole, M.,
Exp e rim e n ta l and. T h e o r e tic a l E le c tro c h e m is try , p. 2 6 2 ,
McGraw-Hill C o., 1935.
(21)
I n t e r n a t i o n a l C r i t i c a l T ables, Vol. VI, p. 334, 1929.
(2 2 )
Brooks, I 1. S . , The ^se o f th e Glass E le c tro d e in th e D eterm ination
of Standard E le o tro d e P o t e n t i a l s , p. 27, M a s te r 's T h e sis, U n iv e r s it y
of Maryland, 1938*
(23)
Dehn, W. M., J . Am. Chem. S o c ., 26, 8 2 9 - 4 7
(1 9 1 4 )
(24)
Haugaard, G, , Chem. A b s t . , jll* 930,
(25)
Dole, M., Experim ental and T h e o r e tic a l E le c tr o c h e m is tr y , p. 437,
(1937)
McGraw-Hill Co., 1935*
(2 6 )
Harned and Hamer, J . Am. Chem. So c.,
(27)
T ay lo r,
(28)
J . K ., R e p r o d u c i b i l i t y
E le c tro d e ,
p.
D ire c tio n s
fo r
22, M a s t e r ' s
7685, D i r e c t i o n
P re p a ra tio n
Book
S td.
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T h esis,
29 (1935)
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U n iv e rsity
and M a i n t e n a n c e
1203, L e e d s
and
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C h lo rid e
o f M aryland,
G lass
N o rth ru p
1938*
E le c tro d e
Com pany.
Ho.
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