Choice of chemical conditions in order to obtain linear titration curves in potentiometry.
In the titration of an acid HA with a strong base, [HA] diminishes, on the whole, in proportion to the addition of titrant. However, [HA] = K(HA)[H] [A], and if [A] is kept constant [HA] will be proportional to [H]. Therefore if [H] instead of [HA] is plotted against ml of titrant added, one obtains a linear titration curve. [A] is kept constant by addition of NaA before titration in such high concentration that the formation of A can be neglected during the titration. When an acid stronger than acetic acid is titrated, sodium acetate can be used as the added salt. Even mixtures of adds, mono- or polybasic, then yield'straight titration curves since one is in fact titrating acetic acid in the presence of excess of acetate. For weaker acids sodium sulphite is used. In the oxidation-reduction titration of, for example, ferrous iron with permanganate, the potential measured is dependent on the ratio [Fe(2+)]/[Fe(3+)]. If [Fe(3+)] is kept constant by the addition of ferric chloride before the titration, the potential win be dependent only on [Fe(2+)]. Since this diminishes in proportion to the added volume of titrant, straight titration curves can be obtained in this case also. No correction for dilution should be made.
Johansson
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Royal Institute of Technology, Stockholm, Sweden.