ABSTRACT

A nitrate ion-selective electrode, Orion model 93-07, has been examined to determine in situ tri- to hexathionate in an active crater lake. The electrode (ISE) showed a linear response to aqueous polythionates within a range of 10^-5-10^-2M. The slope in the plot of log concentration vs electrode potential was slightly larger than the Nernst slope for S₃O₆^2-, S₄O₆^2- and S₅O^2-₆, while a considerably larger one was observed for S₆O^2-₆. Selectivity coefficients determined by the fixed interference method were k₃₄^pot=23, k₄₅^pot=10 and k₅₆^pot=7.0 for a pair of S₃O^2-₆--S₄O^2-₆, S₄O^2-₆-S₅O^2-₆ and S₅O^2-₆-S₆O^2-₆, respectively. The observed variation in the electrode potential for polythionate solution was best explained in terms of the Nicholsky-Eisenman equation. Chloride and sulfite ions less than 2500 ppm and 500 ppm, respectively, did not interfere with the potential measurement. The sum of three polythionates (tetra-plus penta- plus hexathionate) of volcanic origin determined by the ISE method has agreed well with those by the microbore column HPLC.