CO, + SO, = Na,S0, + CO,.
It takes the form of prismatic crystals, soluble in four times its weight of cold and twice its weight of hot water.
Much difficulty has been experienced in getting pure sodium sulphite as necessary for pho tographic purposes, but recently, however, manufacturers have discovered the necessity of putting this on the market, and it is now easily obtainable.
The crystals should be quite transparent and free from a powdery appearance on the out side. They should not smell of sulphur dioxide, denoting the absence of sodium bisulphite. To test samples of sodium sulphite dissolve io grains in a drachm of strong hydrochloric acid, free from chlorine, and dilute with half an ounce of water; add to this a few drops of a solution of barium nitrate. Only a very slight precipitate should be formed. If this is considerable, however, it denotes the presence of large quantities of sodium sulphate. Mr. J. C. Belcher, on referring to the purity of sulphites for photographic purposes, says: "The presence of an excess of sodium carbonate in the sulphite causes great annoyance to photographers, as by its varying amount, they are unable to make any allowance in their formula, which demands an addition of a definite amount of the former in conjunction with the latter for developing. The presence of carbonate in sulphite of sodium is easily detected. The method depends upon the fact that carbonic acid gives a beautiful red color with an alcoholic solution of phenolphthalein, while sulphurous acid produces no action in this respect.
" If, then, we dissolve some of the sulphite above referred to containing only one or two per cent. of carbonate, add a little alcoholic phenolphthalein, we shall have a red color developed, due to the carbonic acid, and on adding carefully a solution of the meta-sulphite—preferably potassium meta-sulphite, as this crystallizes much more freely than the sodium salt, and is more stable—till the color just disappears, the result is a pure solution of neutral or normal sodium (potassium) sulphite, the SO, radical combining with the sodium which was previously combined with the CO, radical. The question, therefore, of obtaining a pure solution of normal or neutral
sulphite of sodium need no longer perplex photographers.
The same writer refers to the fact that sulphites oxidize more or less rapidly than sulphates, and suggests as a means of preserving that they should be kept in well stoppered bottles, with the crystals covered with pure ether. When required for use they could be laid for a few seconds on blotting paper to dry, preparatory to dissolving.
Its use in photography is principally as a so-called preservative of pyro, as it absorbs oxygen, and is converted into sulphate,. preventing the pyro from becoming discolored.
It is also used in mercurial intensification.
In 1885 Captain Abney pointed out that sodium sulphite is an excellent fixing agent. It is, however, considerably more expensive than hypo. It is used in the proportion of about four ounces to the pint of water. Sodium sulphite has also been recommended as an addition to the hyposulphite bath, to prevent it from discoloring negatives after it has been used for some time. See PHOTOGRAPHIC TIMES, page 306, Vol. XXIV.