This method uses a back titration with potassium thiocyanate to determine the concentration of chloride ions in a solution. Before the titration an excess volume of a silver nitrate solution is added to the solution containing chloride ions, forming a precipitate of silver chloride. The term ‘excess‘ is used as the moles of silver nitrate added are known to exceed the moles of sodium chloride present in the sample so that all the chloride ions present will react.
Ag+(aq) + Cl-(aq) --> AgCl(s)
The indicator Fe3+ (ferric ion) is then added and the solution is titrated with the potassium thiocyanate solution. The titrate remains pale yellow as the excess (unreacted) silver ions react with the thiocyanate ions to form a silver thiocyanate precipitate.
Ag+(aq) + SCN -(aq) --> AgSCN(s)
Once all the silver ions have reacted, the slightest excess of thiocyanate reacts with Fe3+ to form a dark red complex.
Fe3+(aq) + SCN -(aq) --> [FeSCN]2+(aq)
The concentration of chloride ions is determined by subtracting the titration findings of the moles of silver ions that reacted with the thiocyanate from the total moles of silver nitrate added to the solution.
This method is used when the pH of the solution after the sample has been prepared is acidic. If the pH is neutral or basic, Mohr’s method or the gravimetric method should be used. The method is illustrated below by using the procedure to determine the concentration of chloride (from sodium chloride) in cheese.
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Chloride Volhard (PDF 77Kb)