379. Exhaustion and Maintenance of the Developer. When a developer has been in continuous service a certain portion of it is carried away with the plates and films removed for rinsing ; its content of active products decreases, and it becomes gradually loaded with soluble bromide and with the oxidation products of the developer. Also, when the emulsion is wetted before development, the developing solution is diluted by the water conveyed to it by the plates and films to be developed.
The oxidation products of the developer usually limit the use of old baths long before they are exhausted, so that it is advisable, from the point of view of economy, to avoid employing too concentrated developing solu tions. For development with continuous machines, in which the duration of each treat ment is limited, it is best to choose a developer (or mixture of developers) only slightly sensitive to the action of bromides, and to avoid caustic alkalis, with which exhaustion of the bath is always more rapid.
Generally, a certain portion of old bath is replaced by fresh solution after developing a, certain area of plates or films. 2 The strength ening bath should not contain bromide, as the latter is present in more than abundant quantity in the used bath. In all cases where a bath has to be used to exhaustion, the partial change of carbonate into bicarbonate in the old bath is compensated by replacing a small portion of the carbonate in the strengthening bath by a chemically equivalent quantity of the corre sponding caustic alkali.
Finally, in metol-hydroquinone developers, in which the content in metol is much smaller than that of hydroquinone, the oxidation is about the same for both these developing agents, so that more metol is destroyed relatively to hydroquinone. For this reason, the strengthen ing developer should contain a greater amount Of me t ol.
380. Developer Stains on Hands and Clothes. As a rule, rinsing the hands in clean water before and after each contact with the developer suffices to prevent stains. Stains on clothes are best prevented by wearing a blouse, or apron, or an overall.
Developer stains on the hands 1 can generally he removed as follows : Scrub the hands with a little solution of potassium permanganate, about I per cent strength, acidified by adding a drop of sulphuric acid. The skin and nails will be stained a dark brown. After a few moments, rinse with clean water and then remove the colouration by means of a very small amount of a fairly strong solution of sodium bisulphite (the commercial solution diluted with an equal quantity of water) or solution of potassium metabisiilphite.
The same process may be used with white linen. With coloured fabrics there is a risk that this treatment may, by bleaching the fabric, cause a spot more apparent than the one it aims to remove ; it is well to make a test on a hidden edge to ascertain if the dye will resist these reagents.