Potassium Permanganate, 158.—Dark red and pur ple crystals, of needle shape, from which a deep red liquid flows on immersion in water. Known in solution as Condy's Fluid. Solubility, I in 16. Used to intensify negatives or carbon prints, and in very minute solutions as a hypo eliminator. To test for the presence of hypo make up a solution of 2 gr. in 20 oz. of water, and add 15 gr. caustic soda. Soak the negatives or prints for a few moments in water, and then add a few drops of the test solution. The pink colour will change to green if hypo is present.
N.B.—Permanganate stains are readily removed by a solution of oxalic acid.
Potassium PersulpIzate, 270.—Often used as a hypo eliminator, as it converts the hypo into the harmless sodium tetrathionate. Must be employed very cautiously in solutions, not exceeding per cent. All hypo eliminators have a tendency to bleach, by oxidising the silver image.
Silver Bromide, AgBr, i88.—An insoluble, light-yellow salt, usually prepared in emulsions by double decomposition, between nitrate of silver and one of the soluble bromide salts. Soluble in solutions of certain alkaline and cyanide salts. Becomes a grey colour by prolonged exposure to light with liberation of bromine.
Silver Chloride, AgC1, I43•5.—A white insoluble salt, prepared in a• similar manner to the bromide, by direct admixture of nitrate of silver with one of the soluble chloride salts. Soluble in ammonia, and the same solutions as silver bromide. On exposure to light remains unchanged in the pure dry state ; but in the presence of organic matter becomes gradually dark in colour with liberation of chloride, and is partially converted into metallic silver, together with, probably, a subchloride and oxychloride. Forms the principal sensi tive salt in many emulsions and printing-out papers.
Silver Iodide, AgI, 235.—A white or light-yellow insoluble salt, prepared by precipitation in emulsions in a similar way to the silver salts mentioned above. Less important in gelatine dry-plates than in wet-plate collodion, but in small quantities adds sensitivity and density-giving properties to bromide emulsions.
Silver Nitrate, iyo.—White tabular crystals which, when pure, are neutral or very slightly acid. Darkens on exposure to light in contact with organic matter. Solubility, in o•44 distilled cold water. Precipitates in common water.
Used in preparing nearly all emulsions, sensitive silver papers, etc., etc. An irritant poison. Antidote : common salt, followed by emetics. Produces black stains on the skin, removable only with potassium cyanide.
Sodium Acetate, 136.—White crystals, slightly hygroscopic. Solubility, I in 3 cold water. A favourite alkali in gold toning baths.
Sodium Carbonate, o6.—Solubility (anhydrous), i in 6. Usually met with in the monohydrated form of large white crystals, which whiten on exposure to air, losing part of their water of crystallisation. Some formulx suggest the commercial form of common washing soda, but this varies much in quality, often contains sulphate, and is liable to give stains. Care must be taken when purchasing from ordinary chemists not to mistake this salt for the bicarbonate, of little value to the compounder of developers. Sodium carbonate is the most valuable alkali, being of moderate energy and reliable action. When substituting for potassium carbonate, the following are the relative quantities required according to Bolton's table : Sodium Carbonate (anhydrous) . . . '76 „ (monohydrate) . . . . . •85 ,, (crystallised) . . . 2'07 Potassium Carbonate (anhydrous) . . . .
. . . '26 Ammonium Sesquicarbonate . . . 2•1 Caustic Soda . . . . . . . . •29 Caustic Potash . . . . . . . '40 Sodium Chloride, salt. The ordinary block quality often contains sulphates and magnesia salts. Solubility, I in 3 cold water. Used for chloride emulsions and papers ; also in first washing water of prints to convert free nitrate, which might otherwise cause an insoluble yellow stain after fixing.
Sodium Thiosulphate, 248.—Large, irregular crystals, or, in purer form, small white crystals. The yellow 3fflorescent crystals should be rejected. Known commercially hyposulphite of soda, or in photography as ” hypo." Manufactured by passing sulphurous acid gas through sodium sulphide. Solubility,' in Its discovery as a fixing agent, Dissolving out the superfluous silver salts, was discovered by Sir William Herschel.
Sodium Phosphate, 358.—Large, bright, :olourless crystals. Efflorescent on exposure to air. Solu )ility, about t in 6. Used as an alkali in gold toning baths.