The Determination of Phosphates in Natural Waters

Abstract

The method for determining phosphates as it is outlined (p. 86) in the eighth edition (1936) of the Standard Methods of Water Analysis is a gravimetric one. It states that accurate results are obtained but that when low concentrations of phosphates are encountered, the large samples which are required cause some inconvenience. Snell states, although the gravimetric method of analysis for phosphorus by precipitation of the phosphomolybdate is very inaccurate for small amounts of phosphorus, the solutions of these small amounts of phosphomolybdate may be determined titrimetrically or compared colorimetrically with a very satisfactory degree of accuracy (1). The colorimetrie method for the determination of phosphates as outlined by Scott (2) in the seventh edition (1933) of the Standard Methods of Water Analysis has not been found satisfactory for determining small quantities of phosphates such as are found in ground and surface waters. Scott's method would be satisfactory for boiler waters or waters containing at least one part per million of phosphates, but the yellow phosphomolybdate color developed is not intense enough to enable a differentiation between small quantities of phosphate (1 p.p.m. or less). A colorimetrie method is also given (p. 109) in the eighth edition (1936) of the Standard Methods of Water Analysis, but it is given in the section on Stationary Boiler Waters. It states that this method may be employed in routine analysis of boiler water for control purposes where results within about three parts per million of the correct amount are satisfactory. The aminonaphthol sulfonic acid solution used for reducing the phosphomolybdate has to be very pure and has to be made up fresh at least every two weeks. The Deniges colori-