The Dictionary Of Photography

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Sodium Citrate
Sodium Hyposulphite
cuisine; Ital., Cloruro di sodio, Sale comune). NaCl = 58*5. Synonyms: Common Salt, Sea Salt. This, the most abundant source of sodium, occurs native in very large deposits in Cheshire, Galicia, and Eastern Russia, also in sea water, which contains about 3 per cent. Solubility: 35 per cent, in cold, 396 per cent, in hot water, insoluble in absolute, but sparingly soluble in dilute, alcohol, insoluble in ether. Ordinary salt is usually contaminated with traces of sulphate of soda and chloride of magnesium, which make it hygroscopic. Salt is used for precipitating silver from print washings, for making gelatino-chloride emulsions, and salting positive papers.
Sodium Citrate (Ger., Natriumcitrat, Citronensaures Natron ; Fr., Citrate de sodium; Ital., Citrato di soda). Na3C6H507H20 = 276. Made by neutralising citric acid with carbonate, bi-carbonate, or hydrate of sodium, evaporating, and crystallising the resulting solution. Solubility: 1 in 1 of water; sparingly soluble in alcohol. It is used sometimes as a Restrainer (q.v.), and also in preparing some printing-out emulsions.
Sodium Hyposulphite (Ger., Natriumthiosulfat, Fixirnatron, Unterschwefligsaures iVatron, Fixirsalz; Yx.^Hyposulfite de soude, Thiosulfate de soude; Ital., Iposolfite di soda). Na2S2035H20 = 248. Synonym: Thiosulphate of Soda. This important salt may be formed by passing sulphurous acid gas through sulphide of sodium until no further precipitation of sulphur occurs, or it may be made by heating - not boiling - sulphite of sodium with excess of sulphur, and commercially by treating tank waste or calcium-sulphide. It is met with in commerce as large watery crystals, which should be entirely free from acid or any yellow tinge. It is soluble 1 in 2 of cold water, 1 in 1 of boiling water, and insoluble in alcohol. Its importance in a photographic sense, as a solvent for the unacted-upon silver salts, was discovered by Sir William Herschel. When a salt of silver is added to hypo-sulphite, two salts are formed, as shown in the following equations: -
AgCl + Na2S203 = AgNaS2Os +      NaCl.
Silver Sodium Double hypo-         Sodium
chloride hyposulphite " sulphite of           chloride.
silver and sodium