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Table I. Metal-Ion Systems Which Will Reduce Silver Bromide in a Silver Bromide— Bromide-Ion System.
Ti4+ +
e
m
Ti3 +
E
=
0.
03 to 0 . 06 v
PH
=
Tto
3
W'"^~ 1
le
=
W3+
E
=
0.
0
PH
=
0
V3+ +
e
=
vs+
E
=
-0.
2
PH
=
0
Ti3 + +
e
=
Ti2"*"
E
=
-0.
4
PH
=
1
03 + +
e
=
Gr2 +
E
=
-0.
4
pH
=
0
Sn«+ +
2e
™
Sn2 +
E
=
-0.
7
pH
=
11
+0.120 v would be capable of reducing silver bromide in a silver bromidebromide-ion system. More accurately, he calculated that the metal-ion system would have to satisfy the equation:
-0.058 log (Br-)-£~°^-8 log (°X) ^
where
-0.006,
(Br~) = bromide-ion concentration, E = standard reduction potential, n = electron change in the reaction, (ox) = concentration of oxidized state
of metal ion, and
(red) = concentration of reduced state of metal ion.
Roman also decided to limit his investigation to systems which are not greatly affected in activity by reasonable changes in the concentration of the oxidized form of the active compound. The system should also meet the final requirement that both oxidized and reduced forms of the active agent be soluble and dissociated.
With these requirements in mind, Roman found that the systems listed in Table I were of possible interest. Tin was eliminated because of its nonselective development characteristics, and chromium for the same reason. The divalent-trivalent titanium system was too unstable for practical use, while the other titanium system was too inactive. Solubilizing the tungsten required the use of hydrofluoric acid which, for obvious reasons, was unsuitable.
The vanadium solution, however, could be readily prepared and was found to be an active developer. Vanadium
pentoxide was found to be commercially available and a suitable starting material for the preparation of the developer.
By using one of several possible methods described below, the vanadium pentoxide can be dissolved in a strongly acid solution. This solution is then electrolyzed to reduce the vanadium to the divalent state. Roman found that this solution was a very active developer, giving especially good results with motion-picture positive-type emulsions. When such a solution is used for development, the divalent vanadium is oxidized to the trivalent state, and, since the reaction is reversible, the divalent vanadium can be re-formed by further electrolysis which, as will be seen, is a feature of this metal-ion system.
Subsequent to this original work by Roman, an extensive investigation of the vanadium developer system has been made in the Kodak Research Laboratories to determine the solution composition for producing optimum results on motion-picture positive-type emulsions. An experimental machine has been designed for processing with the vanadium developer which includes a means of maintaining developer activity by electrolytic regeneration.
Preparation of Vanadium Developer
Although a number of vanadium compounds are available commercially, vanadium pentoxide is by far the cheapest but it is somewhat difficult to compound in a developer, being only slightly soluble in dilute sulfuric acid
January 1954 Journal of the SMPTE Vol.62