Volumetric Determination of Persulphates

Persulphates (e.g. K2S2OS or (NH4)2S20g) may be determined in acidified aqueous solution by adding a known excess of an iron(ll) salt solution and back titrating the 

KMn04 acts as a self indicator in the standardisation and in the back titration. By adding H3PO4, the reduction is contplete in a few minutes. 

Using purest potassium or ammonium persulphate prepare 0.03 M solution. Pipette 25.0 cm of the solution into a 250 cm conical flask, add 3 cm of H3PO4 (syrupy), 10 cm dil. H2SO4 and 25.0 cm of 0.100 mol dm of iron(II) solution. After 5 minutes, titrate the excess Fe(II) with 0.02M KMn04 solution to the first permanent faint pink colour. Repeat to obtain concordant results. 

Calculate (a) the concentration of KMn04, from the average titre, in mol dm (b) the concentration of S20g in the solution, in mol dm .  

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