Known addition-dilution titration potentiometry: A method for the determination of trace chloride in borax/nitrite solution

Abstract

Chloride concentrations of 75–250 μg liter −1 have been determined in simulated PWR auxiliary coolant containing 1000 mg liter −1 each of sodium tetraborate and sodium nitrite. Sodium tetraborate posed no problem except through its effect on the pH, which is easily adjusted. The high concentration of nitrite affected the emfs of all the chloride-selective electrodes tested and caused marked tarnishing of the electroactive membranes. Sulfamic acid was used to remove nitrite, and silver chloride electrodes were preferred over mercury(I) chloride electrodes because of their greater robustness under the conditions. At these chloride concentrations, the electrodes are non-Nernstian and direct potentiometry is imprecise. Known addition-dilution titration potentiometry was adopted, in which the spiked sample is titrated with a diluent until the original emf and, implicitly, sample condition are restored. This may be considered a variant of null-point potentiometry and provides internal calibration for both slope factor and standard potential; although the apparent slope factor is grossly non-Nernstian, it compensates for all the sources of error involved, provided that the diluent adequately matches the treated sample matrix. Recoveries were 100–106% for 100–250 μg liter −1 chloride and 117% at 75 μg liter −1, with within-batch Standard deviations of 6–11%.