Electrochemical methods for nitrate, silicate and phosphate analysis in waters

Nitrate can be determined by its electroreduction at a copper electrode formed in situ by the electrodeposition of Cu2+ from solution onto a carbon disc substrate. A simple amperometric method for determining nitrate, involving the measurement of the difference in current at the end of two potential steps for copper deposition and nitrate reduction, has been developed. Involving the addition of only a small volume of an acidified Cu2+/SO42- solution to a sample, this has been applied to the determination of nitrate in Southampton drinking water. In 1995, the nitrate concentration was determined as ≈0.46 mM, a value which is in excellent agreement with data published by the water company. This method can tolerate chloride concentrations of up to ≈ 10mM, a level which approximates to the maximum allowable limit for water supplies intended for human consumption.Silicate can be electrochemically determined following its reaction with an acidified molybdate reagent to form a yellow electroactive Si-Mo complex, most probably the Keggin anion β-[SiMo12O40]4-. A rate constant for the formation of this complex is approximately 3.8 x 10-3 s-1, as determined electrochemically at both a carbon rotating disc electrode and a gold microdisc electrode. Once quantitatively formed over a period of about half an hour, the Si-Mo complex can be reduced at either of these electrodes, such that the current measured at a predetermined potential can be used to determine silicate in the range 10-100 μM, even in the presence of 0.1 M chloride.Under the same solution conditions as for the silicate determination, phosphate can also be determined as an electroactive P-Mo complex. This complex forms rapidly (<15 s), thus permitting more rapid analysis than for the Si-Mo analogue. The steady-state current for the reduction of this species at a fixed potential on a carbon RDE provides an excellent measure of phosphate in the concentration range 10-100 μM, whilst analysis for the range 1-10 μM appears very promising.