Anions in wastewaters - Determination of Anions in Natural and Treated Waters - page 559

8.17.1 Spectrophotometric method
Buchanan and Easty [84] showed that the official methods [85,86] for determining
orthophosphate and total phosphorus (ie phosphate after digestion with persulphate) in
waste water as reduced molybdophosulphate are subject to interference when applied to
waste samples. Thus sulphide interferes in the determination of orthophosphate but not in
that of total phosphorus because they are oxidised by the persulphate. Ligninsulphonates
interfere in the determination of total phosphorus and somewhat less in the determination
of orthophosphate. Addition of excess of aqueous bromine to the sample prevents
interference from sulphide and greatly reduces that from lignin-sulphonates; in the
determination of orthophosphate the unconsumed bromine is removed by a stream of air
before further steps in the determination. It is also shown that the precipitation of alkali
lignin and other substances which occurs when the solution, after the persulphate
oxidation is neutralised according to the official procedure, can be eliminated by, instead,
filtering the solution while it is still acidic (the amount of acid used in the reagent
solution being reduced to compensate for that already present).

8.17.2 Ultraviolet spectroscopy
The automated ultraviolet Spectrophotometric method [87] for the determination of ortho
and ortho plus hydrolysable or total phosphate involves the use of two independent flow
systems, which are flow-charted, concurrently allowing these phosphates to be measured
colorimetrically as orthophosphate. Features of the method include compensation of the
bias effect of turbidity during orthophosphate determination, and the elimination of the
need to neutralise acidified samples in hydrolysable and total phosphate determinations
prior to colour development. The method and equipment both allow continuous operation
for up to 24h.
Standard deviations over the ranges 0-3mg of L −1 phosphate and 3-30mg L −1
phosphate were 0.03 and 0.2 respectively. Up to 100mg of silicate, iron or nitrate −1 did
not interfere in the determination of 30mg L −1 phosphate.

8.17.3 Atomic absorption spectrometry

In the atomic absorption inhibition titration method [88] the phosphate in the sample is
treated with incremental amounts of magnesium solution

Table 8.20 Major environmental factors affecting the hydrolytic degradation of
condensed phosphates

Factor Approximate effect on rate

10 5 -10 6 faster from freezing to boiling

Temperature

10 3 -10 4 slower from strong acid to base

pH