SODIUM HYPOSULPHITE (Formula, + 511,0 ; molecular weight, 248 ; synonyms, sodium thiosulphate, hyposulphite of soda, by passing sulphurous acid gas through sodium sulphide until sulphur ceases to precipitate, or by heating sodium sulphite with excess of sulphur. It takes the form of irregular crystaline masses more or less transparent. If quite pure it should not effloresce.
Sodium hyposulphite is very soluble in water and insoluble in alcohol. It does not keep very well when in the form of solution. Its kgeping properties are, however, improved by the addition of sodium carbonate till alkaline. The addition of salicylic acid has also been recom mended for this purpose. It should also be noted that solutions of this salt keep better in the dark than in the light.
Commercial sodium hyposulphite should be tested for sodium sulphate, which is often con tained in it. Leaper's instructions are to mix an aqueous solution of it with an excess of hydro chlorfe acid free from chlorine, filtered off the precipated sulphur, and adding barium nitrate to the filtrate, when a white precipitate will occur, if sodium sulphate is present.
Hyposulphite of soda was first prepared by Chaussier, as far back as 1799. In 1869 Schut zenberger showed that the so-called "hyposulphurous acid " was really thiosulphuric acid,* from which it resulted that the salts, previously termed hyposulphites, were in reality, thiosulphates.t The correct name for this substance is therefore sodium thiosulphate, although the familiar name of hypo will not be easily or soon discarded.
The principal use of hyposulphite of soda in photography is as a fixing agent owing to the fact of its being a solvent of the silver haloids. This property was first discovered by Sir William Herschel about the year 1819. If a silver salt be added to sodium hyposulphite two salts are formed, thus, for instance, with silver chloride in silver printing • 2AgC1+3NagS203=Ag,Na43(S203)+2NaC1 The first salt is a double hyposulphite of silver and sodium, which is with diffi culty soluble in water, but soluble in sodium hyposulphite. It is for this reason that it is impor tant to use an excess of sodium hyposulphite, so as to completely eliminate the insoluble salt, as upon this rests, to a considerable extent, the permanency of the image. Many substances have been recommended to get rid of the excess of sodium salt (see Eliminator, Hypo,), but these, as a rule, are as injurious as the hypo itself. After thoroughly well washing the prints in many changes of water, a test for hypo should be used. This is usually made up as follows: Potassium permanganate i grain Potassium zu grains Distilled water 20 ounces This will form a pinkish purple liquid, and a few drops are added to the last washing water. If hypo be present a slightly greenish color will appear. The prints should be made to receive a further washing, and the test again applied. Another method is to make a little starch paste by boiling a little starch in distilled water, and adding thereto a few drops of an alcoholic solution of iodine. A deep blue color will be formed, due to the formation of iodide of starch. One or two drops of this deep blue liquid are added to the last washing water, and if hypo is present the blue color will disappear. Mercuric chloride is also used as a test for hypo. A few drops of a
solution of it added to the washing water will cause a cloudiness if any hypo be present. A very minute quality of a dilute hyposulphite solution added to the ferrous oxalate developer has a very powerful accelerating action. Hypo solution is also an excellent cleanser of silver articles.
If an acid be added to a solution of sodium hyposulphite, sulphurous acid will be evolved and sulphur deposited. It is for this reason that the fixing bath for printing should be made dis tinctly alkaline.
The photographer who desires good and ,permanent results should at all times attend to the purity of the chemicals he is using, and with his hypo no less than with the others. The two principal impurities to be found in hyposulphite are sodium chloride and sodium sulphate, two substances that are cheaper than hypo, and consequently often added for the sake of extra profit to the manufacturer. Every suspected sample of hypo should be tested for sodium sulphate, the principal and most undesirable adulteration. This may be roughly done by exposing a few lumps to the air in a warm place for a week or so, when the presence of any considerable quantity will be apparent from an increased dryness, and by the efflorescence of the sulphate due to the discharge of the water of crystallization.
If a more rapid means of testing is desired, Leaper's instructions may be followed. These are to mix an aqueous solution of the hyposulphite to be tested with an excess of hydrochloric acid, free from chlorine, filtering off the precipitated sulphur and adding barium nitrate to the filtrate, when a white precipitate will occur if sodium sulphate is present.
The British Journal of Photography says : " Should any one desire to ascertain with exactitude whether any sample is pure, it can easily he done by a solution of pure dry iodine; 24.8 grains of clean dry hypo ought to be entirely decomposed by x2.7 grains of iodine; any defect in the quantity of the latter required to effect complete decolorization of its solution indi cates the proportion of impurity present." Sodium hyposulphite is very soluble in water, but insoluble in alcohol. It is a common belief that an aqueous solution will keep indefinitely, and in many photographic establishments large quantities are made up to last for some time, no precautions being taken to prevent decom position. This is a great mistake. Watts, in his Dictionary of Chemistry, states that "an aque ous solution cannot be preserved unaltered even in closed vessels, but deposits sulphur, and is partly converted into sodium sulphate; in contact with the air it is gradually but completely oxidized into sulphate." It is evident, therefore, that in making up large quantities of hypo solution for fixing purposes and using when required, the photographer runs no inconsiderable risk of destroying the permanency of his prints. ' Decomposition can, however, be prevented to a very great extent, although not totally, by the addition of sesquicarbonate of ammonia (Mohr) until alkaline. Caustic soda has been recommended for this purpose, and also salicylic acid. Another point that should be observed is that solutions of hyposulphite of soda will keep better in the dark than when exposed to light.