Characterizing the Lead-Complexing Properties of Polyphosphate Water Treatment Products by Competing-Ligand Spectrophotometry Using 4-(2-Pyridylazo)Resorcinol

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Abstract
A spectrophotometric titration method for characterizing Pb complexation by drinking-water-treatment chemicals has been developed. The colorimetric reagent is 4-(2-pyridylazo)resorcinol (PAR). The total PAR:total Pb ratio is kept low to allow measurable competition for Pb2+ between PAR and the polyphosphates. The data are analyzed by nonlinear regression involving the solution of the chemical equilibrium problem for each experimental point. Accurate results were obtained for both the concentration of nitrilotriacetic acid (NTA) (a test ligand) and the stability constant of PbNTA. The accuracy of the method is a function of the ligand concentrations and the stability constants. Estimates of the accuracy of the experimentally determined values of Log K and Log (total ligand concentration) as functions of the values of the parameters are presented.