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10 CRABTREE, EATON, AND MUEHLER Vol 42, No. 1 tion, the less the quantity of residual hypo in the material but, in general, especially in the case of prints, when the test solution has indicated no hypo in the wash water or drippings there may be a considerable quantity of hypo in the material. Therefore, it is apparent that tests made to determine hypo in the wash water or drippings from films and prints do not indicate quantitatively the residual hypo content of the photographic material. TABLE I Relative Sensitivities Test Solution Sensitivity Potassium permanganate 1 part hypo per 100,000 parts water lodine-azide-starch 1 part hypo per 167,000 parts water Silver nitrate 1 part hypo per 1,000,000 parts water Mercuric chloride 1 part hypo per 2,000,000 parts water On the other hand, this method of testing can be used for control purposes in order to be certain that the films or prints have received the proper degree of washing in a system previously adjusted for washing the material to a definite residual hypo content as deter- mined by a quantitative test method. TESTS FOR HYPO RETAINED BY PHOTOGRAPHIC MATERIALS It is obvious thus far that the hypo test should be made on the photographic material in order to determine the total residual hypo of the film or print. This may be accomplished with the commonly used test reagents by bathing, or by spot tests, but it is important that the testing methods be standardized against a quan- titative method for the estimation of residual hypo before the test can be said to be "quantitative." (a) Potassium Permanganate Test This particular reagent has not been found satisfactory for use as a spot test primarily because the test solution is too weak to permit accurate judgment of color changes. The test solution may also produce a brown stain of manganese dioxide. It is possible to employ a bathing technique with this reagent, but it is not very practical because (1) a brown stain of manganese dioxide is formed over the film or print area, and (2) only a fraction of the residual hypo in prints is tested.