Intrinsic End-Point Errors in Titration with Ion Selective Electrodes: Chelometric Titrations

Quantitative errors obtained in chelometric titrations using ion selective electrodes as end-point indicators are presented. The position of the inflection point depends strongly upon β, defined as the product of the equilibrium constant (K±') and the initial analyte concentration (C°'_A), and upon the magnitude of the ionic interference (5). For example, when β = 10⁶ and b = 0.01, an error of —0.38% occuss while under these conditions the analytical precision is 0.05%. The error increases as β decreases or b increases. Titration to a preset potential was examined and shown to be superior to the inflection-point method only under restricted conditions where b and β are both small. The method of Gran plots does not entail any serious determinate errors; however, as ionic interference increases, the precision deteriorates. Under titration conditions, where activity coefficients and liquid junction potentials are constant, Gran's method is preferable.