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bath. If all its constituents are closely controlled, it should be possible to rely on it to produce a definite result.
Although colour sensitometry will be referred to later, the technique may be regarded as of interest principally to the stock manufacturer.
Analysis of Developing Agent
There are several ways of ascertaining the concentration of colour developing agent. One may proceed as for metol, either by the ether extraction method of Atkinson and Shaner13, or by an ethyl acetate extraction, followed by a potentiometric titration, as developed by Stott, by Gloyns, and by Levenson14. The titration is carried out with eerie sulphate, using calomel and platinum electrodes.
Alternatively, the diamine may be estimated colorimetrically. Briefly, the procedure is as follows :
A portion of the developing solution is acidified and boiled to get rid of the sulphite. It is then made alkaline again, and excess of a coupler and an oxidising agent, say ferricyanide, are added. The dye formed by this operation is extracted with a suitable solvent — preferably one of the less volatile ones — and estimated in a colorimeter.
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Fig. I . Gambrell Colorimeter.
While it is possible to do this visually, a photo-electric instrument is to be preferred, as Fig. 1. The sample is put in a cell or a test tube, and light from a lamp is passed through it, and strikes a barrier layer photo-cell at the end of the instrument ; the current from this is indicated by an external galvanometer. The intensity of the lamp may be varied with a resistance, or for fine adjustment by changing its position relative to the rest of the system ; provision is made for a filter which, for maximum sensitivity, should be of a colour complementary to that of the dye being examined.
It is necessary to calibrate the instrument with samples of known diamine content. When there is coupler in the solution, this may be estimated by the same method. In this case, excess developing agent is added, followed by the oxidising agent.
Hydroxylamine, if present, may be determined by a titration after interfering materials have been removed. The developing agent is removed by