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Sodium Sulphite                          Solutions and Solubility
soda). Na2HP04i2H20 = 358. Prepared by neutralising phos-phoric acid with soda. Solubility: 1 in 4 of cold, 1 in 2 of boiling water; insoluble in alcohol. On exposure to the air the crystals effloresce - that is, give up some molecules of water - and it is questionable whether a complex phosphate is not formed. It is used for toning.
Sodium Sulphite (Ger., Schwefligstiiires Natron, Natrium-sulfit; Fr., Sulfite de sonde; Ital., Solfitodi soda). Na2S03,7H20 = 252. Is prepared by passing sulphurous acid gas through carbonate of soda in concentrated solution. Solubility: 1 in 4 of cold water, 1 in 1 of hot water; insoluble in alcohol. It is used as a preservative of pyrogallol, as it absorbs oxygen, and is converted into sulphate. This salt is difficult to keep as pure sulphite, but keeps better in concentrated solutions than weak ones. It has also been recommended as a fixing agent; but as its powers are considerably less, and its price greater than hyposulphite, it is hardly likely to come into general use in spite of its advantages. (See Fixing.)
Sodium Tungstate (Ger., Wolframsciures Natron; Fr., Tungstate de sonde; Ital., Tungstato di soda). Na2W04, 2H20 = 317. A combination of tungstic acid and soda, crystallising in rhombic plates. Solubility 55 per cent, in cold water, 124 per cent, in hot water; insoluble in alcohol. It is used in Toning (q.v.).
Solarisation. See Reversal
Solutions and Solubility. In preparing solutions, especially saturated solutions, it is often a very great saving of time to support the salt to be dissolved near the surface of the liquid, so that the heavy solution formed may always tend downwards, and fresh liquid rise up to exert its solvent power. Such salts as hyposulphite of soda, carbonate of soda, or oxalate of potas-sium can be tied*up in canvas or muslin "blue-bag" fashion, and the bag can be suspended from a stick placed across the vessel. For small quantities or corrosive salts, the strainer of a French coffee-pot (earthenware) may be used. The following tabular matter relating to solutions and solubilities requires no explanation : -