Chemical elements
  Sodium
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      Sodium hydride
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      Sodium bromide
      Sodium iodide
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      Sodium chlorate
      Sodium hypobromite
      Sodium bromate
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      Sodium periodates
      Sodium monoxide
      Sodium peroxide
      Sodium hydroxide
      Sodium perhydroxide
      Sodium monosulphide
      Sodium polysulphides
      Sodium hydrogen sulphide
      Sodium sulphite
      Sodium hydrogen sulphite
      Sodium potassium sulphite
      Sodium pyrosulphite
      Sodium sulphate
      Sodium hydrogen sulphate
      Sodium monopersulphate
      Sodium pyrosulphate
      Sodium persulphate
      Sodium thiosulphate
      Sodium dithionate
      Sodium trithionate
      Sodium tetrathionate
      Sodium pentathionate
      Sodium hyposulphite
      Sodium selenides
      Sodium selenite
      Sodium selenate
      Sodium sulphodiselenide
      Sodium tellurides
      Sodium tellurate
      Sodium nitride
      Sodium hydrazoate
      Sodamide
      Sodium hydrazide
      Sodium hyponitrite
      Sodium nitrite
      Disodium nitrite
      Sodium nitrate
      Sodium phosphides
      Sodium dihydrophosphide
      Sodium hypophosphite
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      Sodium dihydrogen phosphite
      Sodium hypophosphates
      Sodium orthophosphates
      Disodium hydrogen orthophosphate
      Sodium pyrophosphate
      Disodium dihydrogen pyrophosphate
      Sodium metaphosphate
      Sodium polyphosphate
      Sodium arsenites
      Sodium arsenates
      Sodium antimonate
      Sodium carbide
      Sodium carbonate
      Sodium hydrogen carbonate
      Sodium percarbonate
      Sodium cyanide
      Sodium thiocyanate
      Sodium silicates
      Sodium borates
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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.05101520253035404550556065707580
Pentahydrate33.435.337.439.141.243.245.247.750.855.3
Dihydrate.52.753.453.954.655.256.057.158.259.460.762.363.965.768.0
Anhydrous salt67.467.667.868.268.568.869.169.469.9


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.


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