Determination of orthophosphate in highly saline waters

Abstract

Standard methods for the determination of phosphorus as phosphate ion are now well established for fresh and marine waters. In highly saline waters, however, salt effects due to ionic strength, or to particular ions present, may result in method interferences. Three methods of analysis of phosphate based on the formation of phosphomolybdenum blue complexes have been evaluated here for hypersaline waters. Stannous chloride reduction in aqueous media exhibits a substantial salt effect and its use is not recommended. Stannous chloride reduction following extraction into non aqueous solvents shows a significant salt effect (up to 30 per cent) in solutions of salinity >100 g L−1. Dilution of hypersaline waters to below this salinity may overcome the salt effect but the method suffers from other disadvantages involving resource constraints and health and safety considerations. Ascorbic acid reduction, catalysed by antimony (III) ions, appears to offer the most promise for hypersaline waters. Turbidity in samples having high salinity (> 100 g L−1) and high phosphorus concentrations (> 500 μg P L−1) changes the spectral characteristics of solutions but linear calibration curves still result for concentrations in the range 400 to 1,000 μg P L−1. The occurrence of turbidity is also affected by the ionic composition of hypersaline waters since solutions made from sea salt give different results to those made from sodium chloride. Dilution of samples, to give salinities less than 100 g L−1 prior to reduction is recommended to avoid turbidity. The salt effect in these lower salinity waters is less than 3 per cent up to 100 g L−1.