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Interference of Humic Substances on Chloride Analysis Using the Chloride Ion-selective Electrode¹

ABSTRACT

Humic substances interfered with the ion-selective electrode determination of Cl^- by complexing Ag⁺ and thereby reducing the activity of the potential determining ion. Binding of Ag⁺ was proven with a technique that involved equilibrium dialysis and Ag⁺ determination by atomic absorption spectroscopy. Competition between Ag⁺ and H⁺ for binding sites was evident from an increase in millivolt response of the Cl^- ion-selective electrode with addition of HNO₃ and release of protons upon Ag⁺ binding in the dialysis experiment. Selectivity coefficients (k_Cl,HA) for 11 humic and fulvic acids ranged from 0.59 x 10^–4 to 1.3 x 10^–4 mol g^–1 at pH 6.5. Interference is negligible in solutions containing <0.1 g L^–1 humic substances, but is serious in samples with low concentrations of Cl^- (< 10^–4.5 M) and high amounts of organic matter (>0.1 g L^–1).

¹ Contribution from the Dep. of Agronomy, Univ. of Illinois, Urbana, IL 61801. Supported in part by a grant from the United States-Israel Agricultural Research and Development Fund (BARD I-500-82).

² Research Assistant and Professor of Soil Chemistry, respectively. The senior author is now Research Chemist, F242 NFDC, Tennessee Valley Authority, Muscle Shoals, AL 35660.

Received for publication November 17, 1986.