Determination of ammonia-, nitrate- and organic nitrogen in water and waste water with an ammonia gas-sensing electrode

Abstract

The use of the Orion ammonium ion selective electrode for the determination of ammonia-, nitrate- and organic nitrogen in a number of waters and effluents has been investigated. Adjustment of the pH to attain the optimum conditions for the determination was achieved by use of 2 M sodium hydroxide solution that was 1 M in ethylenediaminetetraacetic acid (EDTA) disodium salt. Evidence is produced which shows that the alkali present in excess of that required to give a pH value of 12 does not affect the calibration graphs or the recovery of ammonia from the water samples investigated. Standard nitrate solutions and samples were reduced and measured as ammonia-nitrogen. Reagents used to bring about this reduction are discussed. Recoveries at a level of 1 mg l^–1 of nitrate-nitrogen averaged 95 per cent., using the nascent hydrogen produced by Devarda's alloy in acidic conditions as the reductant. A contact time of 24 h was required for this percentage reduction. A coefficient of variation of ±6·8 per cent. was achieved. Under these reducing conditions, the percentage recovery decreased as the concentration of nitrate-nitrogen increased but acceptable recoveries were obtained for concentrations of nitrate-nitrogen up to 5 mg l^–1. However, when ammonia-nitrogen was present at levels significantly higher than those of nitrate-nitrogen, the reliability of the results decreased. The method is convenient for the determination of ammonia-, nitrate- and organic nitrogen in the same sample. Interference by metals that form insoluble hydroxides under the alkaline conditions of measurement of ammonia-nitrogen was studied. The high level of magnesium present in sea water is shown to interfere with electrode response, probably because of hydroxide formation and consequent precipitation. This potential interference is eliminated by the use of 1 M EDTA to complex magnesium, and other metals, present in sea water and estuarine waters. It is further shown that organic nitrogen can be determined with the ammonium ion selective electrode. The method involves removal of free ammonia followed by acid digestion and final release and measurement of the ammonia produced in alkaline solution. Recoveries are shown to be acceptable at the relatively low levels measured and show good agreement with published figures for similar environments.