Applications of conductimetric titrations

Some typical conductimetric titration curves are collected in Fig. 13.2 (a)-(d). 

Strong acid with a weak base. The titration of a strong acid with a moderately weak base (K sslO-5) may be illustrated by the neutralisation of dilute sulphuric acid by dilute ammonia solution [curves 1 and 3, Fig. 13.2(a)]. The first branch of the graph reflects the disappearance of the hydrogen ions during the neutralisation, but after the end point has been reached the graph becomes almost horizontal, since the excess aqueous ammonia is not appreciably ionised in the presence of ammonium sulphate. 

Weak acid with a strong base. In the titration of a weak acid with a strong base, the shape of the curve will depend upon the concentration and the dissociation constant Ka of the acid. Thus in the neutralisation of acetic acid (Ka— 1.8 x 10-5) with sodium hydroxide solution, the salt (sodium acetate) which is formed during the first part of the titration tends to repress the ionisation of the acetic acid still present so that its conductance decreases. The rising salt concentration will, however, tend to produce an increase in conductance. In consequence of these opposing influences the titration curves may have minima, the position of which will depend upon the concentration and upon the strength of the weak acid. As the titration proceeds, a somewhat indefinite break will occur at the end point, and the graph will become linear after all the acid has been neutralised. Some curves for acetic acid-sodium hydroxide titrations are shown in Fig. 13.2(h) clearly it is not possible to fix an accurate end point. 

For moderately strong acids (Ka ca 10-3) the influence of the rising salt concentration is less pronounced, but, nevertheless, difficulty is also experienced in locating the end point accurately and generally titrations of weak and moderately strong acids with a strong base are not suitable for conductimetric techniques. 

For very weak acids however, e.g. boric acid [trioxoboric(III) acid], the initial conductance is very small but increases as the neutralisation proceeds owing to the salt formed. The conductance values near the equivalence point are high because of hydrolysis beyond the equivalence point the hydrolysis is considerably reduced by the excess alkali. To determine the end point, values of the conductance considerably removed from the equivalence point must therefore be used for extrapolation. 

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