The Determination of Silver in a Photographic Emulsion by a Sulfide Separation1 m .
SOL W M L E R University of Chicago, Chicago, Illinois
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SCHEME of qualitative analysis utilizes the precipitation of silver sulfide by HzSin the separation of silver from its halides. The method is based on the great difference in solubilities of the silver halides and the sulfide, the order of magnitude of the solubility products of the halides ranging from 10-lo to 10-18 while that of the sulfide is 10-62. The possibility of using this reaction quantitatively for such a separation in the analysis of silver in a photographic emulsion was considered experimentally by the author. Since the emulsion was on plates, a saturated solution of sodium sulfide was used instead of hydrogen sulfide. The silver sulfide was dissolved in nitric acid and determined volumetrically with standard KSCN. A potassium-cyanide-extraction method was employed as a comparison, the method being that of M a t s ~ m a y e ,excepting ~ that glacial acetic acid, as recommended by Whiteley and S l ~ a n e ,was ~ used instead of hydrochloric acid and carbon dioxide in driving off the HCN. The silver was extracted with KCN, reduced to metallic silver by boiling with granulated zinc, dissolved in nitric acid, and titrated by the Volhard method. An inconvenience due to the hydrogen cyanide produced by the hydrolysis of the KCN entered here, hut. the addition of a small amount of potassium hydroxide completely alleviated this undesirable action. Furthermore, the KOH seemed to promote disintegration of the emulsion, for without the KOH the emulsion held to the plates, while in its presence the emulsion completely disintegrated. Some remarks may be made concerning the experimental procedure used in the sulfide separation. The method effects both extraction from the emulsion and separation of the silver from the halides a t the same time. Since the entire emulsion disintegrated in the presence of the sodium sulfide, the extraction of the silver from the emulsion must have been complete. Moreover, the sulfur produced in the solution of the silver sulfide in the nitric acid does not interfere with the end point. When the silver sulfide was filtered free of the halides and excess sulfide, the
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Presented before the Division of Chemical Education of the American Chemical Society, lOlst meeting, St. Louis, Missouri, April 10, 1941. MATSUMAYE, J. SOC.Chcm. Ind., Japan, 32, 99-106 (1929); Suppl. "WHITELEY AND SLOANE, J . SOC.Chem. Ind., 55, 167-81' (1936).
filter paper was placed in the original beaker and thus remained in the solution titrated. The presence of the filter paper also did not interfere with the end point; as a matter of fact, it aided the determination of the end point by pulling the AgSCN precipitate to the bottom of the beaker, leaving a clear solution above. Thus the brownish tinge which marks the end point can be noticed easily. However, this tinge disappears after a period of about 10 minutes, a change which does not occur in the titration of the cyanide-separated samples. Eastman x-ray plates, 21/zV X 31/2",were used in the analysis. The weights obtained using the cyanide-separation method, per plate, are 101.4mg. and 100.4 mg. With the sulfide separation method weights of 98.77 mg. and 99.45 mg. were obtained. As a matter of interest, these results may be compared with that obtained by Lawrie,' using ordinary roll film. Using the Volhard method he obtained as an average 7.96 grains per square foot. Converting this weight of silver to milligrams and the weight in an area 2'/nU X 3/'2", 31.33 mg. are obtained. This difference of silver content is probably the reason for the greater sensitivity of the x-ray emulsion. EXPERIMENTAL
Individual plates were soaked one-half hour in 2M KCN to which a small amount of KOH had been added. The extraction was transferred to a beaker and the plate washed with three changes of 100 ml. water. The foregoing was carried out in a photographic dark room. Granulated zinc (2 to 3 g.) was added and the mixture boiled for one hour. Twenty milliliters of glacial acetic acid were then added and boiling was continued for 10 minutes to drive off HCN. The reduced silver and excess zinc were then separated from the halides by filtration, and dissolved in nitric acid (1:4). In this procedure the filter paper containing the silver was placed in a beaker and the HNOa added; 2 ml. ferric ammonium sulfate indicator solution were added and the silver was titrated with 0,0523 N KSCN. In the sulfide-separation method, individual plates containing the same emulsion as those used in the (Continued on page 154) -L A ~ R I EI., Soc. Chem. Ind., 29, 28T (1930).
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THE DETERMINATION OF SILVER IN A PHOTOGRAPHIC EMULSION (Continuedfront p a ~ e150) cyanide-separation method were soaked in 50 ml. saturated sodium sulfide solution for one-half hour. The disintegrated emulsion was transferred to beakers, and the mixture digested on a steam bath for one day. After separating the supernatant liquid by filtration, the silver sulfide residue was washed by decantation until washing was complete--until no brown
precipitate of silver sulfide' was formed on adding silver nitrate to the hot wash water used in the washing. The filter paper was returned to the beaker containing the bulk of the silver sulfide, and 100 ml. nitric acid (1:l) added. After boiling until solution of the sulfide was complete, the solution was cooled and titrated as before with 0.0523 N KSCN.
