Sodium Phosphate. This salt • 12 H,0) must not be confused with ordinary! sodium phosphate 12 or cliso die phosphate, which occurs in almost the same form of colourless, prismatic, efflorescent crystals. It is partially hydrolyzed in an aqueous solution and thus supplies a permanent reserve of caustic soda without the same conse quences as regards the disintegration of the gelatine (A. and L. Lumiere, 1894).
Borates of Soda. Among the numerous varieties of borates of soda,' use is made of borax and metaborate of soda.
Borax is marketed in colourless crystals slightly efflorescent, and spar ingly soluble in cold water (7 per cent at 59° F.) ; its activity in developing baths is intermediate between that of the carbonates and of the bicar bonates. It is chiefly used in preparing slow acting developers.' Metaborate of soda occurs in the form of efflorescent, colourless crystals very soluble (33 per cent at 66° F.) in water, melting at F. in their water of crystalliza tion. When acted on by atmospheric carbonic acid, this salt, either in the solid state or in solution, gradually changes into a mixture of borax and of sodium carbonate. Its photo graphic activity is slightly superior to that of the carbonates. 3 The addition of a caustic alkali to a solution of borax changes this salt into metaborate.
Sodium Al etasilicate. Sodium metasilicate which is used in the U.S.A. as a household detergent under the name of Metso, has been suggested as a substitute for trisodic phosphate and for sodium metaborate.
Acetone. Acetone is sometimes used to liberate a part of the soda of the sulphite, and thus to dispense with the use of an alkali. It is a colour less liquid with a peculiar smell, very volatile, inflammable, miscible in all proportions with water, alcohol, and ether.
Acetone is an excellent solvent of many resins and is one of the best solvents of celluloid. When a photographic film is immersed in a mixture of water and acetone, the celluloid swells without dissolving, and the gelatine emulsion coating tends to detach itself (Lumiere, 1902). For this reason developing solutions containing acetone should not be used for films.
366. Practical Equivalence of the Usual Alkalis. The normal amount of the various caustic alkalis required for various developing agents is easily calculated (assuming pure products to be used) on the basis of formation, in all cases, of the monophenolate, and, in the case of the aminophenols, of saturating the acid of the salt employed. This method of calculation
fails with paraphenylene diamine ; the quantity indicated for this particular case is one ascer tained experimentally (J. Desalme, r9r r).
Considerations of chemical equivalence, based on the power to neutralize a given quantity of an acid, do not permit of equivalence being worked out between the caustic alkalis and their substitutes, particularly as the amounts of sodium carbonate and potassium carbonate equivalent to a certain strength do not seem to be equivalent at another strength of the same developer, and furthermore there is no propor tionality between the equivalent amounts as regards different developers.
It is only possible to consider as equivalent those quantities of alkalis that bring the developer to the same degree of alkalinity, i.e. to the same value (§ 354).
The following table (after Lumiere and Seyewetz, 1906) indicates the various amounts required with various developers as compared with ro grin, of caustic soda.' 367. Alkali Bromides. Potassium Bromide. Small anhydrous crystals (KBr), colourless or white, cubical in shape, very soluble in water (over 6o per cent at 60° F.), insoluble in alcohol ; the chief impurity is potassium chloride, which, however, does not interfere with the action of the bromide. Potassium bromide is a perfectly stable salt.
Sodium Bromide. Sodium bromide is a very deliquescent salt, of very variable composition. It is appreciably cheaper than potassium bromide, but can only be used where sufficient quantities warrant the trouble of ascertaining the quantities to be used, the salt being made up into stock solutions as soon as purchased.
Ammonium Bromide. This occurs in the form of small colourless crystals of anhydrous salt which is slightly deliquescent, and turns yellow on long exposure to light. It is even more soluble in water than potassium bromide and is slightly soluble in alcohol. It slowly decomposes at boiling point. It is used in some formulae for warm-tone developers.