Elimination of equivalence point errors in the potentiometric titration of chloride, bromide and iodide mixtures with silver nitrate

When two or more halides are determined in solution by precipitation titration with silver nitrate as the titrant, significant errors can occur at the first equivalence point as a result of coprecipitation. Errors of up to 33% were found for the first equivalence point for solutions containing mixtures of halides at micromolar levels. The addition of a flocculating agent to the solution reduced coprecipitation by increasing the rate of exchange between the precipitated silver halide and the halide ion remaining in solution. A logarithmic relationship was observed between the charge of the flocculating agent and the logarithmic concentration of the agent needed to minimise coprecipitation. Although flocculating agents reduced coprecipitation, they do not, however, completely eliminate equivalence point errors. Here a new method is presented which effectively eliminates the problem of coprecipitation during precipitation titrations for solutions containing two halides. In order to decrease the possibility of coprecipitation, we used selective complexation of the precipitation ion Ag+ in order to control the AgX solubility. For example, in the case of CF(-)plus X- (X=Br- or I-), we added sufficient NH3 to form Ag(NH3)(+) so that the free Ag+ activity was reduced below that required for theoretical AgCl precipitation in the absence of the other halides. Once the titration of the less soluble halide was completed and the first equivalence point determined, the Ag(NH3)(+) complex was destroyed by acidification of the solution to a pH less than 6. The titration is then continued and the second equivalence point determined. Equivalence point errors were reduced to less than 1.5% with careful application of the method. (C) 2003 Elsevier Science B.V. All rights reserved.