Reduction of silver chloride

The reduction of silver chloride, precipitated in the presence of excess chloride ion, yielded the S-shaped curve typical of an autocatalyzed reaction (James, 25). The initial reaction rate, measured in terms of the reciprocal of the time required to complete 5 % of the total reaction, varied directly as the hydroxylamine concentration and inversely as the chloride ion concentration when the latter was relatively large. The specific surface of the freshly prepared precipitate, as measured by dye adsorption, decreased with aging, and the reaction rate decreased proportionately. 

The cyanine dye, 3,3 -diethyl-9-methylthiacarbocyanine chloride, had a much greater effect than gelatin in decreasing the reaction rate of the silver chloride. The rate of reduction of silver chloride varied linearly with the amount of silver chloride surface not covered by the dye, and the rate attained at complete coverage was of the order of one-thousandth that for the undyed precipitate. The dye exerted scarcely any effect upon the reduction of silver ions from silver sulfite complex solution. 

The kinetics of the silver-catalyzed reduction of silver chloride were studied on two types of preparations. In the first the nuclei were... 

Fig. 4. Effect of age of exposure on reduction of silver chloride by hydroxylamine A, no exposure B, exposure 17 hours old C, 6 hours old D, 2 hours old E, 20 minutes old.

The effect of the cyanine dye and of gelatin on the reaction rate shows that reduction of silver ions from solution is not the rate-controlling process. These influences of adsorbed components on the reaction rate speak against the concept that solution of the silver halide is the rate controlling process. Hence, the silver catalyzed reduction of silver chloride by hydroxylamine takes place substantially at the solid silver/ silver halide interface. 

Temperature Coefficients of the Reduction of Silver Chloride by Hydroxylamine... 

The reduction of silver chloride by hydrazine shows some points of similarity to the action of hydroxylamine, but also some important points of difference (James, 34). An induction period was obtained with the unnucleated precipitates which, under some conditions, was relatively large. However, exposure of the precipitate to actinic light had only a small effect upon the induction period and upon the subsequent course of the reaction. Previous nucleation of the precipitate by the action of hydroxylamine decreased the induction period without eliminating it, and produced little or no effect upon the subsequent course of the reaction. Addition of the dye, 3,3 -diethyl-9-methylthiacarbocyanine chloride, produced no effect until the surface of the precipitate was more than half covered. Further increase in the amount of dye added produced an irregular decrease in the reaction rate. Gelatin decreased the reaction rate, but to a smaller extent than in the hydroxylamine reaction, and a minimum rate was not attained. As the gelatin concentration increased, more and more reduced silver appeared in colloidal form in the solution. 

A large part of the reduction of silver chloride by hydrazine evidently takes place by a different mechanism from that of the reduction by hydroxylamine. The effect of gelatin and dye on the process, together with the appearance of colloidal silver in the solution when gelatin is present to stabilize it, shows that the reaction involves dissolved silver chloride to a greater degree than the hydroxylamine reaction. Indeed, if the reaction rate is plotted against a silver ion concentration calculated on the assumption that a saturated solution of silver chloride is maintained, the same relation is obtained as is found for the reduction of silver ions from a solution of the sulfite ion complex. 

Cupric sulfate exerts an effect on the silver chloride-hydroxylamine reaction similar in kind to that which it exerts on the hydrazine reaction, but in a smaller degree. If sufficient cupric sulfate is added to the hydroxylamine solution, the character of the reduction of silver chloride shifts towards that shown by the hydrazine reaction, e.g., the effect of gelatin becomes less pronounced, a minimum rate at a small gelatin addition is not obtained, and significant amounts of colloidal silver appear in the solution. 

The pH dependence of the rate of development by hydroxylamine indicates that the monovalent ion is the active species. The rate varies as about the 0.65 power of the hydroxylamine concentration at pH 12.7 and the 0.75 power at pH 10.8. These results suggest adsorption of the hydroxylamine ion, and are in complete agreement with previous findings for the catalyzed reduction of silver chloride precipitates. 

The relative rate of fog formation compared to image development increases with increasing pH of the hydroxylamine solution. This is to be expected from analogy with the studies of the reduction of silver chloride and silver bromide precipitates, where the change in nitrogen yield shows that the uncatalyzed reaction becomes more and more prominent as the pH is increased. 

The titration medium is also simple to prepare. It contains gelatin to give a smooth indicator current and prevent reduction of silver chloride. With a preservative added, the reagent may be made up in quantity and stored for some time when refrigerated. 

Half a century later, Carl Wilhelm Scheele, more famous for his discovery of oxygen, described the reduction of silver chloride to silver metal by light in 1779 ... 

---