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Jan., 1944 REVIEW OF HYPO'TESTING METHODS 47 sulfide with acidified silver nitrate as described below. The trans- mission densities of the sulfide stains were obtained by subtracting the transmission density of the untreated paper from that of the paper plus silver sulfide. These transmission densities were then plotted against the corresponding hypo contents to obtain a standard curve shown in Fig. 1. A series of single- and double-weight photographic prints was then washed for increasing times and analyzed by Ballard and Hutchins of these laboratories by a method shown to be quantitative for resid- ual hypo in photographic paper.* A 1-sq in. sample of each O.O2 O.O6 O.IO 0.14 0.18 O.22 O.2fe O 3O 0.34 u.-so CONCENTRATION OP HYPO MILLIGRAMS PER SQ INCH FIG. 1. Standardization curve showing relation between concentration of hypo and transmission density of silver sulfide deposit after treatment of print with silver nitrate. print was digested for l /% hr at 65 °F in approximately 100 cc of acidified thousandth normal silver nitrate solution, an excess of stand- ard potassium iodide added, and then back titrated amperometrically with silver nitrate solution. Corresponding samples were treated with silver nitrate, as described below, to produce silver sulfide in situ, the transmission densities obtained, and then plotted against the residual hypo contents. The resultant curve coincided practically with the standard curve in Fig. 1, thereby confirming the semi- quantitative technique used by the authors. A quantitative estimation of the residual hypo in photographic prints is obtained, therefore, in the following manner. The test solutions required are: * BALLARD, A., and HUTCHINS, B,: Unpublished results,