Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 257

There are interferences from some ions such as arsenates, nitrites and hexavalent
chromium. Downes [613] gives details of an automated method for determination of
reactive phosphorus in concentrations lower than 0.5mg L −1 in non saline waters which
also contained arsenic, silicon and mercuric chloride preserving agent. The addition of
thiosulphate in acid solution removes various interferences. Potman and Lijkema [614]
have discussed the determination of phosphate in small samples of water.
Lei et al. [615] give details of a procedure for the determination of traces of
phosphorus in non saline waters by absorption spectrometry, using a long capillary cell to
lower the limit of detection.
Nurnberg [616] found that iron and hydrogen sulphide, often present in anoxic lake
waters, interfere with the determination of soluble reactive phosphorus. Methods for
reducing the interference are indicated. They propose an alternative to more complicated
procedures for soluble reactive phosphorus in anoxic waters for determination of total
(unfiltered reactive phosphorus. Results are on average only 2% lower, and the method
can be automated if required.
Hydrogen sulphide concentrations higher than 1mg L −1 and ferrous iron concentration
above 0.20mg L −1 produce interferences in the soluble reactive phosphate analysis (eg
80% underestimation of soluble reactive phosphorus (Fig. 2.37).
Ferrous iron is not problematic if the sample is kept anoxic before and during filtration.
On the other hand vigorous aeration is obligatory if hydrogen sulphide is present.
Fig. 2.37 Removal of soluble phosphorus during aeration in water from the
anoxic hypolimnion of Lake Maga, 16m 11 August 1981. The iron
concentration is 3.15mg L −1 . Circles represent soluble reactive
phosphorus (SRP), triangles soluble radioactive phosphorus
Source: Reproduced with permission from Elsevier Science [616]
Worsfold et al. [617] give a detailed illustrated description of equipment for
automatically monitoring phosphate in the field by the molybdenum blue method. It
incorporated a flow-injection manifold for injecting the reagent into the sample stream