Problems in the analysis of phosphorus compounds

Abstract

The analysis of all forms of phosphorus in waters rests essentially on measurement of free orthophosphate into which all combined forms may be converted. This measurement is made, with few exceptions, by absorptiometric methods in which 12-molybdophosphoric acid is formed and reduced to phosphomolybdenum blue. Many analytical problems thus centre on the precision, accuracy and sensitivity of these procedures. General chemical considerations and an examination of inter-laboratory and inter-method comparisons show that ascorbic acid, in the presence of antimonyl ions, is a particularly advantageous reductant for this purpose.Differentiations between operationally defined fractions of dissolved phosphorus are limited primarily by interfering effects between fractions. Under favourable circumstances it is possible to measure accurately the free orthophosphate, organically combined phosphorus which can be photo-oxidized to yield orthophosphate directly, and total condensed phosphates. The distinction within the combined fraction is made possible by the development of photo-oxidation procedures using ultra-violet radiation, in which hydrolysis can be minimized. In the use of any differentiation method, however, very critical attention must be given to the relative amounts of various types of phosphorus compounds present, and in the determination of orthophosphate the possible interferences of other elements, such as arsenic, must also be considered. For measurements of total dissolved phosphorus the use of oxidative digestion with potassium persulphate or photo-oxidation followed by hydrolysis has proved suitable for a wide range of samples.Improvements in methods of measurement may be nullified unless problems of filtration and, particularly, storage of samples, are given equal attention. Freezing is an effective method of preservation under most circumstances but in samples of very high biological activity supplementary preservatives may have to be added.An examination of some alternatives to the phosphomolybdenum blue procedures suggests that, while several raise interesting possibilities, they are unlikely to make an immediate, extensive contribution to the analysis of phosphorus in natural waters.