CONCENTRATED SOLUTION A solution made up very strong, chiefly in the form of a stock developing solution, and needing to be diluted with water before use ; it may or may not be saturated. It is not synonymous with saturated solution (which see), although sometimes thought to be so. Rodinal, certinal, azol, and similar developers are con centrated solutions. The single-solution devel oper given under the heading " Adurol " is a good example of a concentrated home-made developer. Concentrated solutions are handy for storing ; they do not take up so much room and as a rule keep better. Almost any developer with which the worker is instructed to take equal parts of Nos. r and 2 (or A and B) may be made in a more concentrated form by simply using half the water given in the formula, and adding the other half at the time of using. Hydroquinone cannot in the ordinary way be made up in a highly concentrated form, but for a " ten per cent." formula see " Hydroquinone."
Toning baths are not, as a rule, made up in a highly concentrated form, because of the danger of the gold precipitating, particularly when sul phocyanide is used ; the following, however, has been recommended for the use of workers whose dark-room space is limited and who wish to keep a highly concentrated bath : Gold •thloride . 8i grs. 4'8 g.
Strontium chloride . 85 „ ), Distilled water . r oz. 25o ccs.
Heat the water to 200° F. (93° C.), add the gold, and then the strontium. Next add 7 drms. of water in which 25 grs. of potassium sulpho cyanide have been dissolved. Heat again to the same temperature as before, filter, and make up to 2i oz. with water. This is highly concentrated, and keeps well ; when a toning bath is to be made up for use, add • oz. of the concentrated solution to 5 oz. of water.