Reducing agents sodium thiosulphate

Sodium thiosulphate is an important reducing agent used in volumetric analysis for the estimation of iodine ... 

Discussion. Sodium thiosulphate (Na2S203,5H20) is readily obtainable in a state of high purity, but there is always some uncertainty as to the exact water content because of the efflorescent nature of the salt and for other reasons. The substance is therefore unsuitable as a primary standard. It is a reducing agent... 

Sodium thiosulphate is a reducing agent. In the presence of sodium nitrate it leads to explosive mixtures, which are sensitive if they are heated. Under the same conditions the detonation is immediate with sodium nitrite. 

In another situation, a known excess quantity of standard iodine solution is added in the substance (a reducing agent) to be assayed and then the excess iodine may be titrated with the help of standard sodium thiosulphate solution, such as the estimation of sodium bisulphite ... 

Other reducing agents, e.g. tin(II) chloride, sodium sulphite, sodium thiosulphate, etc. will reduce hexacyanoferrate(III) ions to hexacyanoferrate(II), so that this reaction is not always a reliable test. 

The 8204 ion exists in sodium dithionite, the reducing agent, hydrosulphite . This salt is prepared from the Zn salt, which results from the reduction of SO2 by Zn dust in aqueous suspension. In solution it rapidly decomposes to disulphite and thiosulphate 2S2O4" -> 8205 + 8203 The structure of this ion is rather extraordinary (Fig. 16.6(a)). It has an eclipsed configuration with the planes of... 

Transfer 5 ml of the test solution to a 50 ml beaker. Add 1 ml of N hydrochloric acid and then 0.5 ml of sodium acetate buffer solution and 1 drop of fluorescein solution. Mix thoroughly and then add 1 drop of chloramine T solution. Mix by swirling and set aside for 30 seconds, then stop the reaction by adding 2 drops of alkaline thiosulphate reducing agent. Treat the blank solution in a similar manner. When bromine is present in the sample, a rose-pink colour will be developed in the test solution compared with the yellow-green blank solution, note This test is also given by iodine. 

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