Question

To determine content of chloride in cheese, a cheese sample is treated with 5.00ml of 0.1 mol/L silver nitrate solution, several milliters of concentrated nitric acid and potassiu permanganate solution are added. The filtrate is then diluted to the mark in a volumetric flask and titrated by a back titration with standard solution of potassium thiocyanate (Volhard method) (a). What is the role of potassium permanganate solution? Could we add potassium permanganate solution before adding silver nitrate solution? Why? (b). Write out the equation used to calculate the content of chloride in a cheese sample.

Answer 1

Answer A)

KMnO₄ is added to the reaction mixture so as to oxidize all the organic matter present in the cheese sample. Nitric acid also serves same purpose but the presence of two ensures the oxidation of organic matter.

KMnO₄ should not be added before AgNO₃ because it will oxidize Cl^- to Cl₂. This Cl₂will release as gas and hence, the reaction with AgNO₃ will not take place.

Answer B)

2AgNO₃ (excess) + Cl^- ---> AgCl (ppt) + AgNO₃ (unreacted) + NO₃^- (aq.)

AgNO₃ (unreacted) + KSCN -----> AgSCN + KNO₃

Fe³⁺ + KSCN -----> Fe(SCN)²⁺ + K⁺

In reaction 1, AgNO₃ reacts with chloride ions and precipitates of AgCl are formed.

Reaction 2, On back titration with KSCN, extra AgNO₃ reacts with KSCN to form AgSCN.

Reaction 3, Slight excess of SCN^- reacts with Fe⁺³ (indicator) to form Fe(SCN)⁺² which gives red coloration and indicates end point.