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1949 COLOR FILM DEVELOPERS AND BLEACH 27
procedure. Even if the procedure is changed and 0.05 normal acid is used, the volumes needed are still too small for reasonable accuracy with ordinary equipment. Moreover, with a sample of this size, the amount of metol present is too little to give a usable inflection in the titration curve.
The disadvantage of the iodine titration may be avoided by oxidizing the developers with eerie sulfate. Since this oxidation is performed in a strongly acid solution, highly colored oxidation products are not formed, and the end-point is easily observed. Stott4 described a eerie sulfate titration to determine developing agents, but he determined the end-point potentiometrically. Use of the ortho-phenanthroline ferrous complex (ferroin) as indicator makes the titration simpler and faster since the color change is easily discernible.
Equally satisfactory results have been obtained with methyl acetate, ethyl acetate, or isopropyl acetate as extracting solvent. With the methyl acetate used, a slight dark color appeared during the eerie sulfate titration which may be objectionable, but no such darkening occurred with the other two acetates. Methyl ethyl ketone, the solvent used by Shaner and Sparks, is itself oxidized by eerie sulfate, and therefore cannot be used.
This modified Baumbach procedure involving the acid titration of metol and eerie sulfate oxidation of both metol and hydroquinone has been tested on developer A-502 with variation in the concentration of the developing agents from 50 per cent less to 20 per cent more than normal. Within this range, the metol determination was found to be accurate to 100.7 =*= 2.4 per cent and the hydroquinone to 99.5 =*= 1.5 per cent. Occasional values above 100 per cent are probably due to mechanical carry-over of traces of developing solution with the solvent.
Determination of Diethyl-p-phenylenediamine Hydrochloride
Diethyl-p-phenylenediamine hydrochloride or its derivatives may be extracted in exactly the same manner as metol and hydroquinone. It may be titrated with acid the same as metol but since it is more basic and no hydroquinone is present, a better inflection point is obtained in the titration curve. It may also be oxidized with eerie sulfate, and this oxidation using ferroin indicator is preferred to the potentiometric acid titration because it is faster and simpler. Immediately on addition of the eerie sulfate, a bright cherry red color is produced which is an intermediate oxidation product. The intensity of this color soon reaches a maximum, and then begins to fade until