The determination of ammonia in boiler feed-water with an ammonium-selective glass electrode

Abstract

An investigation has been made into the accuracy of ammonium-selective (ammonium-responsive) glass electrodes for determining ammonia (10 to 1000 µg 1^–1) in boiler feed-water and similar high-purity water samples from power stations. The electrode potential follows the Nernst equation in samples containing up to 10 000 mg 1^–1 of ammonia, the pH of which is controlled between 8·0 and 8·4 by the addition of triethanolamine-hydrochloric acid buffer solution. However, interfering species in the buffer solution cause a detectable deviation from Nernstian response at low ammonia concentrations (less than 1000 µg 1^–1). By use of a calomel-0·1 N hydrochloric acid reference electrode, reproducible results have been obtained in static buffered solutions containing 10 to 1000 µg 1^–1 of ammonia. Of the other impurities likely to be present in power-station waters only sodium caused a serious effect (100 µg 1^–1 of sodium is equivalent to 25 µg 1^–1 of ammonia). The within-batch standard deviations of analytical results were 2, 7, 17 and 33 µg 1^–1 at concentrations of 10, 100, 500 and 1000 µg 1^–1 of ammonia, respectively. Details of a recommended analytical procedure for discrete samples are given, and the application of the ammonium-selective electrode to continuous on-stream analysis is briefly discussed.