Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 258

and a double-beam photometric detector incorporating light-emitting diodes and photo-
diodes. The response was linear over the range 0-2000µg phosphorus and the limit of
detection was 12µg L −1 . There was no interference by silicate at a concentration of 10mg
L −1 .
The Water Research Association UK [618] have issued a standard method for the
determination of phosphorus in waters, effluents and sewages. In this method, which is
capable of determining 3-6µg L −1 phosphate a 200ml sample is reacted with acidic
molybdate reagent and ascorbic acid to form a reduced molybdenum blue complex.
The complex is extracted into 10ml n -hexanol (ie preconcentration factor of 20) and
the colour of the reduced molybdenum blue complex measured spectrophotometrically at
680nm.
The Water Research Association UK [619] has issued a method, based on similar
principles, which is capable of determining down to 0.5-0.7µg L −1 .
Interferences of silicate, germanate, chromate and nitrate are negligible. Arsenate at the
0.05-0.1mg L −1 level and nitrite at 0.2-0.5mg L −1 level do interfere.
In both these methods precautions are used when preparing sampling bottles involving
the use of iodised low density polyethylene or nylon sampling bottles.

2.75.1.2 Methods based on malachite green

The only alternative spectrophotometric methods for the determination of phosphate to
those based on molybdenum blue appear to be those which utilise malachite green as the
chromogenic reagent [620,621]. Fernandez et al. [620] have discussed an automated
version of the malachite green micromethod for determining inorganic phosphate in non
saline waters. The molar absorptivity of the technique was six times more sensitive than
that of molybdenum blue methods. Reproducibility was very good and interferences from
silicate and arsenate were easily removed.
Motomizu et al. [622] have described a spectrophotometric method for the
determination of phosphate in river waters based on the measurement at 650nm of the
colour produced with molybdate and malachite green. The limit of detection is in the µg
L −1 range. In this method the sample solution was acidified with sulphuric acid and
heated in a water bath above 90°C for 40min and subsequently it was coloured with
molybdate and malachite green. The colour was stabilised by adding poly (vinyl alcohol).
The interference of diverse ions was examined and the results obtained are shown in
Table 2.43. The amounts of ions, except silicate, generally present in river water are
much smaller than those listed in Table 2.43. Silicate ion at concentrations above
5×10 −5 M reacts gradually with molybdate to form heteropolyacid and this results in
positive errors, but large amounts of silicate ion were eliminated by acidifying the sample
solution and filtering it through a membrane filter (pore size 0.45mµ).

Table 2.43 Effect of diverse ions

Tolerance limit a

Phosphorus taken: 3.72µg