pH-stat techniques in titrimetric analysis: Part 3. The principles of pH-stat chelatometric titrations

Chelatometric titrations can be effectively monitored by means of the pH-stat technique. During the addition of the primary titrant (the chelating agent), the pH of the titrated solution is kept at a pre-selected value by adding a strong base as the auxiliary titrant. The equivalence point is characterised by a sudden change in the ratio between the added volumes of auxiliary and primary titrant solutions, i.e. in the slope of the linearly segmented titration plot representing the volume of the base against the volume of the chelating agent. The slope of the linear segments depends on the reactions involving proton exchange, which accompany the primary complexation reaction. The dependence of the shape of the plots on the imposed pH for typical titrations (magnesium(II) and copper(II)) is examined and the conditions for the feasibility of pH-stat chelatometric titrations are discussed.