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420 J. I. CRABTREE AND J. F. Ross [j. S. M. p. E.
example, if the tap water sample turned orange, and the wash water sample became yellow, it would indicate the presence of hypo; but if both samples remained the same shade, this would indicate the absence of hypo.
This method is subject to error inasmuch as the quantity of hypo in the water drained from the film constitutes only a small proportion of the quantity absorbed by the film, so that a negative result with the above permanganate test does not necessarily indicate the absence of hypo.
(2) A drop of a dilute potassium permanganate solution (0.5 gram per liter) is placed on the dry film. Any rapid change in the color from brown to yellow indicates an unsafe degree of washing, but since permanganate is likewise reduced by silver and gelatin, a slow discoloration occurs even in the absence of hypo. By carefully comparing the rates of discoloration of the permanganate, however, a satisfactory estimate of the relative quantities of hypo present can be made.
(3) Strips of the film being tested are soaked for several hours in a small volume of distilled water. A few drops of starch solution are added to this solution and then a drop of dilute iodine solution (1 gram of solid iodine dissolved in 20 cc. of water containing 2 grams of potassium iodide, and after the iodine is dissolved the solution is diluted to 100 cc.). If the solution turns blue immediately because of the formation of a starch iodide complex, the material is free from residual sodium thiosulfate. If, however, several drops of the iodine solution are required to produce a permanent blue coloration, the material is poorly washed. By using a standard iodine solution it is possible to determine quantitatively the sodium thiosulfate per unit area of film.
(4) The new method of determining the thiosulfate content of films consists in placing strips of these materials in a solution of mercuric chloride, preferably one which also contains potassium bromide, and observing any turbidity which may develop in the solution. If sodium thiosulfate is present, it reacts with the mercuric chloride to form mercurous chloride which is relatively insoluble and causes a turbidity to appear in the solution; while in the presence of potassium bromide, the precipitate consists of mercurous bromide. If no sodium thiosulfate or other reducing agent is present, the solution remains clear although the silver image is bleached white. This method is also subject to error inasmuch as it