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Nov., 1940] ELIMINATION OF HYPO FROM IMAGES 501
image to silver sulfide and the quantities actually required to produce fading under accelerated conditions in practice.
The "photometric equivalent"12 may be defined as the number of grams of silver in a 100 square-centimeter area of emulsion which are necessary to give a density of 1.00. Its magnitude varies with the exposure, the degree of development, and the grain size of the emulsion, ranging from approximately 0.005 to 0.035, depending upon the type of the emulsion.
Hickman and Spencer13 calculated the quantity of thiosulfate required to react with the silver in an image having a density of 0.10 and made the assumption that only one-tenth of the silver need be sulfided to produce just visible fading, which seems reasonable. The reaction may be expressed by the equation
2Ag+ + So03— — > Ag2S + S03—
Silver Thiosulfate Silver
Ion Sulfide
To express the quantity of thiosulfate in milligrams per square-inch which is required to fade an image of density 0.10, the following formula was employed :
Molecular Weight Photometric of Sodium Thiosulfate Conversion Factor
v ir)Cw v ni
Equivalent 2X Atomic Weight Metric to Avoir.
of Silver
The sulfiding reaction first occurs visibly in the low densities having a value of approximately 0.10. The photometric equivalents at low densities are given in Table IV and also the theoretical quantity of thiosulfate required to produce fading at a density of 0.10. Included in the same table are limiting concentrations of hypo above which fading occurred under the conditions of the accelerated fading test.
In general, the faded images with the intermediate and high-speed materials possessed a purplish-black coloration not greatly different in appearance from the original and often of similar printing density. The images of motion picture positive emulsions became more yellowish-brown than those of the high-speed materials but only when the hypo content was in considerable excess of 0.07 milligram per squareinch. With the finest-grained materials, a definite yellowing occurred at concentrations of thiosulfate greater than 0.02 milligram per squareinch.