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In the determination of the percentage purity of a sample of soda ash by back titration,...

In the determination of the percentage purity of a sample of soda ash by back titration, (Na2CO3(aq) + H2SO4(aq) = Na2SO4(aq) + CO2(g) + H2O(l))

Excess acid was back-titrated (H2SO4 + 2NaOH = Na2SO4 + 2H2O​). Why could this not have been done with a straight acid-base titration?

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Answer #1

A back Titration is necessary in situation where the reaction occurs very slowly.

If you treat a sample of unknown mass of carbonate, the carbonate containing solid can't be put in the flask and titrate in forward Titration with acid in burrete. Your indicator will be showing that the mixture has become acidic and endpoint has been reached but the slowish reaction between solid carbonate and the added acid will continue and you will find out that you have not reached the endpoint.

The back Titration works as you add excess acid , enough to react with all of carbonate and more. You then titrate a sample of resulting mixture of excess acid . Subtraction enable you to find out how much acid reacted in the initial the reaction with carbonate . So determine its mass

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