Elimination of ionic interferences in the determination of sulphates in water using the lead-sensitive ion-selective electrode

Abstract

In the titration of sulphate ions with lead(II) perchlorate solution using a lead-sensitive electrode for end-point determination, the addition of 75%V/V of methanol permits titration of 10^–5M samples. Under these conditions, a 200-fold excess of nitrate does not interfere, but chloride at this concentration introduces a small systematic error of 4%. Significant errors are caused by the presence of calcium, which co-precipitates as calcium sulphate with lead sulphate. In titrations carried out rapidly, the negative error increases with the amount of calcium and the true equilibrium may be obtained after a relatively long time. Increasing the ionic strength of the solution by addition of sodium perchlorate eliminates the interference by calcium, but for concentrations of sulphate below 10^–4M it is advantageous to remove calcium by using a cation-exchange resin in the hydrogen form. The method has been applied successfully to the determination of sulphate in water.