Sulphite solutions oxidize rapidly in air, and this change takes place more rapidly the weaker the solutions. As far as possible the preparation of stock solutions of sulphite should be avoided. If such stock solutions are absolutely necessary, and providing they are used as quickly as possible, a 2o per cent solution of the anhydrous salt may be made up and preserved by addition of hydroquinone or paraminophenol to the amount of about 5 gr. per 20 oz.
The oxidation of sulphite solutions is more rapid when they are alkaline. In making up a developing solution from several stock solutions it is necessary to avoid, as far as possible, dis solving the sulphite and the alkali together. Solutions of sulphite keep better if slightly acid, e.g. with the addition of a little bisulphite.
Strong acids decompose the sulphites, pro ducing first bisulphite and then liberating sul phurous gas ; this liberation does not take place when acetic acid is added to a solution of sulphite.
Sodium Bisulphite. Sodimn bisulphite is usually obtainable commercially in the form of a concentrated solution sold as 35° Baume (density I•320 at F.), containing about oz. of bisulphite in 20 Oz. (455 grin. per litre). This product is fairly constant in strength and contains few impurities. It de teriorates in the course of time, losing sulphur ous acid (recognizable by its smell) and leaving a crystalline deposit of sodium sulphite and acid sulphate.
It keeps better when less concentrated, and the occasional user will do well to dilute it with an equal quantity of water, being careful to take twice the amount for use.
Solutions of bisulphite and baths containing bisulphite must never be left in contact with zinc or with galvanized iron, for the bisulphite would be reduced to hydrosulphite, which may give rise to fog.
In the case of developing solutions made up from two stock solutions, the bisulphite is usually added to the stock containing the developer in order to enhance its keeping quality. Some of the alkali introduced when the two solutions are mixed is then employed in changing the bisulphite into neutral sulphite, 1 and this must be borne in mind when reckoning the quantity of alkali. Some formulae for one solution developers, evolved by combining all the ingredients of two-solution formulae, include neutral sulphite, bisulphite, and an alkali (caustic alkali or carbonate), a practice which is without any logical There is also commercially obtainable a pow dered product which is a mixture of sodium metabisulphite and various impurities and is known as dry sodium bisulphite. This
rapidly changes, especially in damp air, to a mixture of acid sulphate and neutral sulphite, which, when dissolved, re-form the sodium bisulphite, but in a quantity smaller than that originally present. Potassium metabisulphite is to be preferred to this product in spite of its higher price, unless it is possible to ascertain the amount of sulphurous acid contained in it at the time it is being used.
Potassium Metabisilphi/e. Potassium meta bisulphite occurs in large, colourless crystals, which keep fairly and are very soluble in cold water (about 30 per cent). The solution is partially decomposed when hot, and should therefore always be prepared with cold water.
The equivalence between sodium bisulphite lye and potass metabisulphite is as follows— oz potassium metabisulphite ---ii oz. 61 drams (a.) sodium bisulphite solution of 35 Be.
i oz. (ti.) bisulphite solution 525 gr. potassium metabisulphite.
The table on page 247 shows the quantities of the usual alkalis required to neutralize sodium bisulphite and potassium metabisulphite, all pro ducts being assumed to be pure.
Under these conditions ioo volumes of soda bisulphite lye produce about fro parts by weight of anhydrous neutral sulphite ; and Too parts by weight of potassium metabisulphite produce 142 parts of neutral potassium sulphite, equal to 113 parts of anhydrous sodium sulphite.
365. Alkalis and their Substitutes. Caustic Soda. Caustic soda is sold in the form of plates, sticks, or lumps, or as a concentrated solution or lye. In the solid state 2 it is available in two qualities, common or pure (called " spirit " soda, because alcohol is used to separate the carbonate forming its chief impurity). The white opaque pieces of soda are deliquescent, and in moist air absorb carbonic acid, being partly converted into carbonate. Caustic soda is extremely corrosive ; the fingers should be rinsed after handling it, and it should not be crushed unless the eyes are protected by goggles or the soda covered with a cloth. The soda and its solutions must not be kept in glass-stoppered bottles ; corks soaked in paraffin-wax must be used.