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1949 COLOR FILM DEVELOPERS AND BLEACH 29
Determination of Potassium Bromide
Since developer A-502 contains thiocyanate which behaves very similarly to bromide, a separation of the two must be effected. To determine bromide in the presence of chloride or thiocyanate, Atkinson and Shaner5 recommended a rather lengthy iodometric procedure. To determine bromide in the presence of chloride, Stott4 used the method of Evans, Hanson, and Glasoe7 in which the developer was boiled, acidified, and boiled again, cooled, and titrated potentiometrically with silver nitrate using silver and calomel electrodes. This method cannot be used in the presence of thiocyanate because the boiling removes some but not all of the thiocyanate.
Potassium bromide may be determined in developer A-502 in the range 1.0 to 3.5 grams per liter with an accuracy of 100.0 =»= 0.2 per cent by oxidation with 30 per cent hydrogen peroxide followed by the standard Volhard bromide procedure.
Developer A-605 does not contain thiocyanate but does contain chloride ions from the color-developing agent which is added in the form of its hydrochloride. The method of Evans, Hanson, and Glasoe7 cited above gives satisfactory results, but it has been found that boiling, either before or after acidification, is unnecessary. After acidification, the bromide may be determined by potentiometric titration with silver nitrate. Experience has shown that no advantage is gained by the addition of barium nitrate, sodium acetate, or aluminum sulfate, as is sometimes recommended. The above procedure gave an accuracy of 100.0 =*= 0.5 per cent for concentrations of 1.0 to 3.5 grams of potassium bromide per liter.
Determination of Sodium Thiocyanate
No method has been reported for the determination of thiocyanate in developers. Separation of thiocyanate from bromide is a lengthy procedure. Since the solubilities of silver thiocyanate and bromide are about the same, they are precipitated together and cannot be differentiated by the usual potentiometric titration. However, the sum of bromide and thiocyanate may be determined and, by deduction of the titer caused by the bromide, the thiocyanate concentration calculated. The Volhard method cannot be used because the developing agents present reduce the ferric ion added as indicator.
The sum of bromide and thiocyanate may be conveniently determined by acidification of the developer and direct potentiometric titration with standard silver nitrate to the inflection point using