then equation (5.25) gives
which is the criterion for a coincident end point and equivalence point, and an indicator must be
selected to satisfy
In general, for the observation of a sharp end point the indicator complex must be reasonably strong,
KMIn > 10^4 , but less than KMY × 10 –^1.
Potentiometric EDTA titrations are best carried out with a mercury pool electrode (Figure 5.6) or a gold
amalgam electrode. When this electrode dips into a solution containing the analyte together with a small
amount of added Hg-EDTA complex, three interdependent reactions occur. For example, at pH = 8 the
half cell reaction (a) which determines the electrode potential is related to the solution equilibrium by
(b) and (c).
(a) Hg2+ + 2e– = Hg (1)
(b) Hg2+ + HY^3 – = Hg (1)(c) Mn+ + HY^3 – = MY(n–4)+ + H+Figure 5.6
A 'J-type' mercury pool electrode.As the titration proceeds, the analyte ion, Mn+ is increasingly complexed and eventually the equilibria
involving Hg2+ are displaced and the potential of the electrode varies according to
In overall form this equation resembles that for the glass electrode (Chapter 6) and a pM-EDTA curve
resembles an acid–base titration curve. The mercury electrode is most usefully employed when
coloured or turbid solutions are being titrated, or when dilute solutions and weak complexes lead to
poor colour changes.