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46 CRABTREE, EATON, AND MUEHLER Vol 42, No. 1 The prints were soaked for 1 to 3 min in a 1 per cent solution of silver nitrate, rinsed briefly in water, treated for a short time in a 5 per cent solution of common salt, fixed, and washed again. The density of the brown stain which varies with the quantity of residual hypo was determined by transmitted light. It was stated that prints washed for 10 min produced a dark brown color with silver nitrate which was equivalent to 0.6 mg of hypo per sq in., and prints washed for 40 min gave a weak yellow color equivalent to 0.06 mg per sq in. It was also stated that silver nitrate produced no visible color in prints below 0.006 mg of hypo per sq in. Unfortunately, Weyde did not describe the method of stand- ardization whereby the above figures were obtained, and consequently it is not known if the method is quantitative. However, the adoption of a bathing technique and the measure- ment of the brown stain by transmitted light were the most im- portant improvements toward attaining quantitative estimations of the residual hypo in prints. The silver nitrate test, in contrast to the diffusion methods previously described, is dependent upon the conversion of the residual hypo to silver sulfide in situ according to the equation 2AgNO 3 + Na 2 S 2 O 3 + H 2 O —-> Ag 2 S + 2NaNO 3 + H 2 SO 4 Obviously, if a drop of silver nitrate is placed on either the emulsion side or the base side, the quantity of hypo affected by the reagent is dependent upon the extent of diffusion of the drop which, in turn, depends upon the condition of the print with respect to water con- tent. Therefore, a bathing technique is the most suitable because the reagent enters the print from all sides to react with the residual hypo and deposit silver sulfide in situ. Before a bathing test could be used to give quantitative results it was necessary to standardize the method by placing known quan- tities of hypo in samples of photographic paper of definite area. This was accomplished by soaking photographic paper samples in dilute solutions of thiosulfate (Na 2 S 2 O 3 • 5H 2 O) for approximately 5 min. 25 The quantity of hypo introduced into the sample was calculated from the volume of solution absorbed which was the difference in weight of the dry sample before soaking and the carefully blotted wet sample after soaking.* The series of samples thus obtained were air dried at room temperature and the hypo converted to silver * Sec Reference 25 for further details.