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Redox Behavior of the Latent Image in Silver Halide Films.

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not attacked by boiling sodium methoxide solution (but is
attacked by NH3) and is mereiy converted into the perchlorate by an excess of anhydrous HC104.
The hydrophobicity and the considerable size resulting from
their structure render these cations excellently suited for
extraction of weakly hydrated anions from the aqueous
phase. When shaken with (31, M = Sn, Y = NO3, in CH2C12
an aqueous solution of KMn04 is completely decolorized.
Addition of ether to the methylene dichloride solution then
precipitates the permanganate (3), M = Sn, Y = MnO4, as
violet crystals. Cations with M = Si or G e behave similarly
but are slowly hydrolyzed in the process. The size of the
cation may be responsible for the fact that with an excess of
iodine ( 3 ) , M = Sn, Y = I, gives, not only a triiodide, but
also an octaiodide corresponding to Cs2I8[31.
Experimental:
( 3 ) , M = Si, Ge: A suspension of compound ( I ) (12.5 g) i n
100 ml of a suitable solvent (CH2C12 for M = Si, s-C~H2C14
for M = Ge) is treated with 0.01 mole of MX4. X = C1, Br.
HX is liberated and clear solution results on standing or
after several hours’ refluxing, respectively. Compounds ( 3 ) ,
Y = X, crystallize out quantitatively from this solution
o n addition of ether.
( 3 ) , M = Sn: Compound (2j (X = C1,I [21) and AgNO3 is
each dissolved in acetonitrile in a molar ratio of 1:2. The hot
solutions are combined, precipitated AgX is filtered off, and
the filtrate is evaporated to dryness. The residue is dissolved
in CH2C12 and addition of ether to the resulting solution
causes crystallization of ( 3 ) , X = N03.
Received: May 6, 1968
[Z 782 IE]
German version: Angew. Chem. 80,558 (1968)
Shielding of the phosphorus in (3) decreases with increase in
the acceptor capacity of the central atom:
[*I Dr. A. Schmidpeter and Dip1.-Chem. K. Stoll
( 3 ) , Y = CI, M =
831P
Si
Ge
Sn
-29.4
-31.3
-35.7 ppm ref. to 85%
H3P04
The resonance signals are shifted strongly downfield from
those for the imidodiphosphinate anion (-12.7 ppm Ed]);
they lie in the same region as that of the dimethylated cation
[N(Ph2POCH&]’ (-35.0 ppm [41), thus indicating strong
involvement of the ligands.
Institut fur Anorganische Chemie der Universitat
8 Munchen 2, Meiserstr. 1 (Germany)
[ l ] Part 19 of Phosphazenes. - Part 18: A. Schmidpeter and 1.
Ebeling, Chem. Ber., in press.
[2] A. Schmidpeter and K . Stoll, Angew. Chem. 79, 242 (1967);
Angew. Chem. internat. Edit. 6, 252 (1967).
[3] E. E. Havinga, K . H . Boswijk, and E. H. Wiebenga, Acta
crystall3gr. 7 , 487 (1954); cf. also G. H. Cheeseman and E. K .
Nunn, J. chem. SOC.(London) 1964,2265.
[4] A . Schmidpeter and H . Brechr, Angew. Chem. 79, 946 (1967);
Angew. Chem. internat. Edit. 6, 945 (1967).
CONFERENCE REPORTS
Redox Behavior of the Latent Image in Silver
Halide Films
By E. A . Frei [ *I
According to the “silver nucleus theory” latent image specks
in photographic silver halide films are small centers consisting
of silver atoms. This being so, the question arises: Is the
latent image silver more noble or less noble than metallic
silver, or are the two equivalent?
To provide an answer, photographic films were treated for
24 h with buffered solutions at variable redox potentials
and a t a fixed silver equilibrium potential. After development the degrees of blackening were compared with a control
(no redox treatment). In such a system only the redox
potential can be measured directly with a Pt electrode. The
potential measured with a Ag electrode is not an equilibrium
potential since a mixed potential is set up at this electrode.
The silver equilibrium potential can, however, be measured
directly in the special circumstance where the silver potential
and the redox potential are equal. It was thus possible to
determine the effect of the ions used for buffering on the
silver equilibrium potential.
By measuring the redox potential of a Pt electrode coated
with gelatine in comparison with a n ordinary Pt electrode it
was shown that there is no Donnan potential between the
swollen gelatine phase and the pure electrolyte solution.
Surprisingly, appreciably more negative redox potentials
were obtained in the gelatine phase shortly after its immersion
in the buffered solution than existed in the surrounding solution. This temporary depression of the potential shows that
in the initial stage of the redox treatment the gelatine phase
is more strongly reducing than the pure electrolyte phase, and
this was particularly marked for formalin-hardened films.
A series of commercial films (including one with gold-sensitized emulsion) was studied. One that was successively fixed,
treated with redox buffers, and physically developed gave
the same results as a chemically developed film.
-
550
1. For all the emulsions except the gold-sensitized one, no
latent image can any longer be developed after redox treatment in a solution whose redox potential equals the silver
equilibrium potential.
2. Latent image nuclei formed by longer illumination at
correspondingly weaker light intensities exhibit increased
stability; also they are partly developed by redox solutions
that completely bleach the latent image formed by short
exposure.
3. The results with the gold-sensitized emulsion agree with
those reported by Bourdon and Bonnerot 111 and by Matejec
and Moisar[21: the nuclei were bleached only at redox potentials that were more positive than the silver equilibrium
potential.
4. The results described in 1 and 2 can be understood o n
Reinders’ [31 and Hillson’s [41 assumption that the free enthalpy
of the latent image is greater than that of massive silver. The
difference in free enthalpies lies between 0.04 and 0.07 eV
according to the amount of illumination. The stability of the
latent image nucleus against external influences, which is
surprising in view of its size, can be explained thermodynamically if we assume that the latent image nucleus contains not
only silver atoms but also impurities in the form of the components of the sensitivity centers. In this way the free surface
enthalpy of the nuclei is strongly reduced and the nuclei
themselves are thus protected against external attack.
Photographic Colloquium at Zurich (Switzerland), February 8. 1968
[VB 151 IEI
German version: Angew. Chem. 80,537 (1968)
[*] Dipl.-1ng.-Chem. E. A. Frei
Photographisches Institut der Eidgenossischen
[l]
[2]
[3]
[4]
Technischen Hochschule
CH-8006 Zurich, Clausiusstr. 25 (Switzerland)
J. Bourdon and A . Bonnerot, Sci. Ind. photogr. 30,205 (1959).
R. Matejec and E. Moisar, Photogr. Korresp. 100, 39 (1964).
W . Reinders, J. physic. Chem. 38, 783 (1934).
P. J . HiNson, J. photogr. Sci. 6, 97 (1958).
Angew. Chem. internat. Edit. 1 Vol. 7 (1968) 1 No. 7
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