Back and blank titrations

In the example above, a reaction was chosen that was quick to carry out and was quantitative, i.e. it went to completion. In many pharmaceutical analyses this is not the case and a back titration has to be carried out. 

The best way to illustrate the procedures adopted for back and blank titrations is to consider an example, the determination of chalk, or calcium carbonate, CaC03. Chalk is used as an antacid and indigestion remedy, particularly in children, and is official in the British Pharmacopoeia as the powder and the mixture (Paediatric Chalk BP). 

The official assay is by the addition of a known excess of hydrochloric acid and back titration of the unreacted excess with sodium hydroxide. A blank determination is carried out since the sample is heated and cooled. The calculation will be carried out initially as a back titration without a blank and then compared with the answer obtained when the blank is taken into account. The calculation should be studied closely as there are subtle differences between the back and blank calculations. 

2HC1 (unreacted excess) + 2NaOH — 2NaCl + 2H20 The relative molecular mass of chalk is 100.1, so that 

1 mole CaCOs = 2 moles HC1 = 2 moles NaOH Therefore, 

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Sources of error can be introduced in each conversion from volume to moles and back to weight, although for simple examples such as the one above it does not really matter which method of calculation is employed as long as the correct answer for the purity of citric acid is obtained. However, for more complicated calculations, involving the use of back and blank titrations, this author believes that factors and equivalents simplify volumetric analysis and they will be used for that reason (rather than any reason of dogma) in the remainder of this book. 

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