The influence of metal ions on polyelectrolyte titration

Abstract The influence of sodium, calcium, lanthanum and aluminium ions on the adsorption of the cationic dye ortho-toluidine blue (OTB) onto the polymer potassium polyvinylsulphate (KPVS) was studied in a spectrophotometer, measuring the metachromasy of ortho-toluidine blue. Spectra between 400 and 800 nm were recorded and the absorption at 628 nm was taken as a measure of the amount of dye adsorbed. The experiments show that there is a considerable interference from the metal ions already at very low concentrations, in the case of aluminium and lanthanum below 10 −4 M. The interference is strongly dependent on the valency of the cation. The adsorption of ortho-toluidine blue is used for the determination of the endpoint in polyelectrolyte titrations and there is a critical concentration above which titration can no longer be performed. The critical concentration is approximately proportional to 10 z , where z is the valency of the cation. the interference of metal ions can be described as an ion-exchange process with KPVS as the ion-exchanger. The thermodynamic equilibrium constant K a for the ion-exchange reaction between OTB + and Ca 2+ was found to be 10 6.3 . Since the adsorption can be regarded as an ion-exchange process, the accuracy of polyelectrolyte titrations in salt solutions can be increased by increasing the concentration of the indicator OTB. When the interfering ion is Al 3+ , a fluoride buffer can be used to form complexes with the cation, thereby reducing the negative effect on the titration.