Sodium thiosulphate, Na₂S₂O₃

The Sodium thiosulphate is manufactured from the alkali-waste of the Le Blanc process by atmospheric oxidation in presence of sodium sulphate, which reacts with the calcium thiosulphate first formed to yield sodium thiosulphate:

Ca(SH)₂+4O=CaS₂O₃+H₂O;

CaS₂O₃+Na₂SO₄ =Na₂S₂O₃+CaSO₄.

The salt crystallizes from the solution as the monoclinic pentahydrate, the ordinary commercial form. It is obtained in the anhydrous state by oxidizing dry sodium hydrogen sulphide with air at 100° to 150° C.:

2NaSH+4O =Na₂S₂O₃+H₂O.

The thiosulphate is also produced by addition of iodine to an aqueous solution containing sodium sulphite and sodium sulphide:

Na₂SO₃+Na₂S+2I = Na₂S₂O₃+2NaI.

A convenient method for preparing sodium thiosulphate in the laboratory consists in dividing a hot saturated solution of sodium carbonate into two equal portions, passing sulphur dioxide through one until absorption ceases, and then adding the other half. The solution of sodium sulphite thus produced is boiled with sulphur until no more dissolves, filtered, and allowed to crystallize. The reactions involved correspond with the equations

1. Na₂CO₃ + 2SO₂+H₂O=2NaHSO₃+CO₂;
2. 2NaHSO₃+Na₂CO₃ = 2Na₂SO₃+CO₂+H₂O;
3. Na₂SO₃+S=Na₂S₂O₃.

The constitution of sodium thiosulphate can be represented by two formulae:

I. ; II

The preference is given to the first formula, as Schorlemmer has pointed out that the second is not in good accord with some of the reactions of the salt:

(1) Sodium thiosulphate in solution is converted by sodium-amalgam into sodium sulphite and sodium sulphide:



(2) On warming with water, silver thiosulphate decomposes, precipitating silver sulphide and forming sulphuric acid:



For the density of the anhydrous salt Gerlach gives 1.667 at mean temperature compared with water at 4° C.; for the pentahydrate Kopp gives 1.736, Dewar 1.729 at 17° C. and 1.7635 at the temperature of liquid air. For the specific heat of the anhydrous salt between 25° and 100° C. Pape gives 0.221; for the pentahydrate between 11° and 44° C. Trentinagali 0.4447, and for the liquid between 13° and 98° C., 0.569. The molecular heat of the anhydrous salt is 34.91, and of the pentahydrate 86.22. Berthelot gives the heat of formation of the anhydrous salt from its elements as 256.3 Cal. Or 262.6 Cal., and Thomsen that of the pentahydrate as 265.07 Cal. Berthelot found for the heat of solution of the anhydrous salt 1.7 Cal. at 15° C.; his value for that of the pentahydrate at 11° C. is -10.8 Cal., and Thomsen's –11.37 Cal., so that the heat of hydration of the anhydrous salt to pentahydrate is about 13 Cal.

Nine hydrates, including thirteen crystalline forms, have been described. The pentahydrate melts at 48.45° C., and the dihydrate at 50.3° C. Each exists in two isomeric forms. According to Muller, the unstable form of the pentahydrate crystallizes at -20° C. and melts at 33° C.; the stable modification crystallizes at - 40° C. and melts at 48° C. The transition-point of the pentahydrate to the dihydrate is 48.17° C., and of the dihydrate to the anhydrous salt 68.5° C. The solubility of the pentahydrate, the dihydrate, and the anhydrous salt, expressed in grams of Na₂S₂O₃ per 100 grams of solution, are given in the table.

Temperature, °C.	0	5	10	15	20	25	30	35	40	45	50	55	60	65	70	75	80
Pentahydrate	33.4	35.3	37.4	39.1	41.2	43.2	45.2	47.7	50.8	55.3
Dihydrate.	52.7	53.4	53.9	54.6	55.2	56.0	57.1	58.2	59.4	60.7	62.3	63.9	65.7	68.0
Anhydrous salt									67.4	67.6	67.8	68.2	68.5	68.8	69.1	69.4	69.9

In contact with the solid the saturated solution boils at 126° C., and contains 348 grams of the anhydrous salt in 100 grams of water.

Concentrated solutions of sodium thiosulphate are moderately stable, but in dilute solution atmospheric carbon dioxide tends to liberate the unstable thiosulphuric acid, a substance readily changed into sulphurous acid and sulphur. Siebenschuh states that, after being kept for fourteen days, protection from light ensures the stability of the solution. Hampshire and Pratt kept a solution for eight months without alteration in strength.

When a mineral acid is added to a dilute solution of sodium thiosulphate, the liquid remains clear for a time, and then becomes turbid owing to deposition of sulphur which at first was in solution:

H₂S₂O₃=H₂O+SO₂+S.

The logarithms of the times elapsing before appearance of turbidity are proportional to the concentration of the hydrogen ions in the mineral acids added, so that for isohydric solutions the time of reaction is independent of the acid employed. When hydrochloric acid is added slowly to a boiling solution of sodium thiosulphate, the sulphur is oxidized to sulphur dioxide:

SO₂+S+2H₂O=H₂SO₄+H₂S.

The salt reacts with ozone in accordance with the equation 3Na₂S₂O₃+2O₃=2Na₂SO₄+Na₂SO₃+2O₂+3S.

With hydrogen peroxide the first product is sodium tetrathionate, and the solution becomes alkaline:

2Na₂S₂O₃ + H₂O₂=Na₂S₄O₆ + 2NaOH.

The subsequent stages of the reaction are more complex.

With silver halides sodium thiosulphate forms a soluble sodium silver thiosulphate, and is employed as a fixing material for photographic plates and paper to remove the portion of silver salt unaffected by light:

AgBr+Na₂S₂O₃ =NaAgS₂O₃+NaBr.

In the bleaching industry it finds application as an "anti-chlor," the excess of chlorine in the bleached material being removed by its aid in accordance with the equation

Na₂S₂O₃+4Cl₂+5H₂O =2NaCl+2H₂SO₄+6HCl.

Sodium thiosulphate is employed in iodometry, being converted by the action of iodine into sodium tetrathionate:

2Na₂S₂O₃+2I =Na₂S₄O₆+2NaI.

According to Ivolthoff, the reaction represented by the equation takes place in neutral or slightly acidic solution, and also in presence of strong acid. When the solution is strongly alkaline, all the thiosulphate is oxidized directly to sulphate without the intermediate formation of tetra-thionate, weak alkali having a similar, though only partial, effect.

The action of hypochlorite solutions on sodium thiosulphate is in accordance with the equations

3Na₂S₂O₃+5Cl₂+5H₂O =Na₂SO₄+Na₂S₄O₆+8HCl+H₂SO₄+2NaCl;
or
3Na₂S₂O₃+5NaOCl+5H₂O=2Na₂SO₄+Na₂S₄O₆ + 5NaCl+5H₂O.

In presence of acids or sodium hydrogen carbonate, the reaction accords with the equation

Na₂S₂O₃+4Cl₂+5H₂O =2NaHSO₄+8HCl.

Kurtenacker found that cyanogen bromide and iodide react with sodium thiosulphate in neutral solution in accordance with the equation

3CNBr+5S₂O₃''+H₂O = 3Br'+2HCN+CNS' + SO₄'' + 2S₄O₆''.

In acidic solution the thiosulphate is converted into tetrathionate:

CNBr+2S₂O₃''+H^• = Br' +HCN+S₄O₆''.

The reaction in neutral solution probably involves two stages, the solution becoming temporarily alkaline through the formation of sodium cyanide, which reacts with the generated tetrathionate in accordance with the equation

Na₂S₄O₆+3NaCN+H₂O=NaCNS+Na₂SO₄+Na₂S₂O₃+2HCN.

The thiosulphate produced then reacts with the halogen cyanide.

A mixture of crystallized sodium thiosulphate and ammonium nitrate finds application as a freezing-mixture.

References are appended to investigation of the refractivity of the solid; to properties of solutions such as the transition-points of the hydrates, super saturation, electric conductivity, density, vapour-pressure, boiling-point, molecular depression of the freezing-point, refractivity, solubility in alcohol, and electrolysis; and to the formation of mixed thiosulphates of sodium and potassium and their isomerism.