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Thiosulphates

The salts of thiosulphuric acid, with the exception of those of the alkali metals, are sparingly soluble in water but are commonly much more soluble in an aqueous solution of an alkali thiosulphate, soluble double salts being formed in which the heavier metal is probably situated in a complex acidic radical; hence, on the addition of a solution of alkali thiosulphate to a salt of a heavy metal, the precipitate of the thiosulphate of the metal is generally soluble in excess.

The formation of these relatively stable complex salts explains the solubility of the silver halides in sodium thiosulphate solution and the value of such a solution for "fixing" photographic prints. In many cases the complex salts have been isolated in the solid state, for example, from solutions containing sodium thiosulphate and a silver salt, well-defined crystalline compounds variously formulated as Na2S2O3.Ag2S2O3. 2H2O, 2Na2S2O3.Ag2S2O3.2H2O and Na2S2O3.Ag2S2O3.H2O, have been obtained. According to Baines the first of these may best be obtained in a pure condition by the addition of silver carbonate in aqueous suspension to a solution of sodium thiosulphate, when the double salt separates as a colourless crystalline powder; or it may be obtained in larger crystals by saturating with sulphur dioxide a dilute solution of silver carbonate and sodium sulphite in aqueous sodium thiosulphate. According to the same investigator the salt should be considered as sodium monoargentomonothiosulphate, Na[AgS2O3].H2O. When pure it darkens only very slightly in bright light, and in solution it is more stable than sodium thiosulphate towards dilute acids. The free acid, monoargentomonothiosulphuric acid, H[AgS2O3].H2O, may be obtained as a silky white precipitate by the addition of concentrated nitric acid to the sodium double salt in ammoniacal solution. It is less stable than the salt. A solution of the latter precipitated with alcohol yields a salt of composition Na5[Ag3(S2O3)4].2H2O, and there is evidence that a third salt, Na3[Ag(S2O3)2], exists in solution.

Many of the heavier thiosulphates when heated with water give rise to the corresponding sulphides. To this ready decomposition of the thiosulphate is due the precipitation of sulphides when hot acid-containing solutions of various metals are treated with sodium thiosulphate solution. When a solution of alkali thiosulphate is boiled with a copper salt, the yellow precipitate of sodium cuprous thiosulphate first formed undergoes decomposition to produce cuprous and cupric sulphides and free sulphur, the relative amounts of these products depending on the thiosulphate concentration, the duration of boiling, and the acidity of the solution.

When heated strongly with exclusion of air, thiosulphates undergo decomposition, giving sulphate and polysulphide or the further decomposition products of the latter, namely, sulphide and free sulphur:

4Na2S2O3 = 3Na2SO4 + Na2S + 4S.

When heated in the presence of air, slow oxidation to sulphate and sulphur dioxide occurs:

2Na2S2O3 + 3O2 = 2Na2SO4 + 2SO2.

In the presence of a reducing agent, for instance hydrogen, carbon or sulphur, the corresponding sulphide may be the almost exclusive product. A similar reduction to sulphide may be effected in aqueous solution with nascent hydrogen, or even by in situ fermentation of sugar with yeast.

Electrolysis of a thiosulphate in neutral solution causes the formation of tetrathionate at the anode:

2S2O3'' + 2+ = S4O6''.

A similar effect is produced by treatment with iodine, but in the presence of alkali there is also some sulphate formed:

2Na2S2O3 + I2 = Na2S4O6 + 2NaI,

or in ionic terms:

2S2O3'' + I2 = S4O6'' + 2I'.

Chlorine and bromine exert a more vigorous oxidising action than iodine, and although some tetrathionate may be formed, this is accompanied by much sulphate, the latter being the main product:

Na2S2O3 + 4Cl2 + 5H2O = 2NaHSO4 + 8HCl.

With sodium hypochlorite in the presence of acid, or even in the presence of sodium hydrogen carbonate, the reaction proceeds according to the foregoing equation, but if the thiosulphate and hypochlorite are allowed to react in dilute solution, the course followed is according to the equation:

3Na2S2O8 + 5NaOCl = 2Na2SO4 + Na2S4O6 + 5NaCl.

As might be expected, treatment with vigorous oxidising agents such as nitric acid or permanganic acid, converts a thiosulphate into a sulphate and sulphuric acid (or an acid sulphate). A mixture of alkali thiosulphate and nitrate heated in a dry tube is liable to explode. With milder oxidising agents, for example iodic acid or ferric chloride, intermediate products are obtainable, especially tetrathionate, and sometimes dithionate. The oxidation of sodium thiosulphate by a solution of iron alum is of interest as presenting an example of a quadrimolecular reaction:

2Fe••• + 2S2O3'' = 2Fe•• + S4O6''.

In alkaline solution hydrogen peroxide effects oxidation to sulphate, dithionate being a probable intermediate product; sulphate is also obtained with hydrogen peroxide in the presence of molybdic acid as catalyst:

Na2S2O3 + 4H2O2 = 2NaHSO4 + 3H2O,

but in the presence of alkali the peroxide produces trithionate, whilst in the presence of a feeble acid like acetic acid the product is tetrathionate; iodine catalytically accelerates the last change:

2Na2S2O3 + H2O2 + 2CH3.CO2H = Na2S4O6 + 2CH3.CO2Na + 2H2O.

Applications of the Thiosulphates

Sodium thiosulphate is the salt manufactured in largest quantity and it finds application for a variety of purposes, for example, as an "antichlor" for the removal of traces of chlorine from bleached linen, cotton, paper, etc., and also mixed with sodium carbonate for the absorption of chlorine fumes. For this latter purpose it was used in the earlier types of respirators during the Great War. On account of its ability to dissolve silver halides, sodium thiosulphate in aqueous solution is used as a " fixer " in photography, and also may be used for the removal of silver chloride from the accompanying mineral matter in the extraction of silver from its ores. The aurothiosulphates of organic bases, for example of ethylene diamine, have been suggested for medicinal use.

In chemical analysis, sodium thiosulphate is applicable in various directions. It is frequently of value for the precipitation of metals in the form of sulphides, and occasionally provides a convenient method for the separation of two metals, e.g. copper from zinc; more especially, however, it finds use as a standard volumetric reagent for iodometric processes, but its use as a standard can even be extended to acidimetry and alkalimetry, the reaction

2Na2S2O3 + 3HgCl2 + 2H2O = 2Na2SO4 + 4HCl + HgCl2.2HgS

allowing a standard solution of sodium thiosulphate to be used for checking alkali solutions for volumetric analysis.

Fused hydrated sodium thiosulphate may be used as a cryoscopic solvent.

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