Orthophosphate in seawater

OrthoPhosphate in seawater

(Lachat QuickChem method #31-115-01-1-G; http://www.lachatinstruments.com/)

Principle

Ammonium molybdate and antimony potassium tartrate react in an acid medium with phosphate to form an antimony-phospho-molybdate complex.  This complex is reduced to an intensely blue-colored complex by ascorbic acid.  The color produced is proportional to the phosphate concentration in the sample.  Though there is a density difference between seawater and reagent water the bias is less than 2%.

Though the method is written for seawater and brackish water it is also applicable to non-saline sample matrixes.  The method is calibrated using standards prepared in deionized water.  Once calibrated, samples of varying salinities (0 to 35 ppt) may be analyzed.  The determination of background absorbance is necessary only for samples, which have color absorbing at 880 nm.

Procedure

Orthophosphate will be determined manually or on autoanalyser/flow injection analyser following standard colorimetric methods (Grasshoff et al. 1983).

References.

Grasshoff, K., Ehrhardt, K. and kremling, K., (1983). Methods of seawater analysis. Verlag Chemie, Weinheim, New York.