Anions in seawater - Determination of Anions in Natural and Treated Waters - page 395

molybdophosphate. The spectrophotometric determination of phosphate by solvent
extraction of molybdophosphate with ethyl violet is more sensitive than those previously
reported [163,171-174], and less troublesome; a single extraction is adequate. However,
in the determination of phosphorus at ng mL −1 levels in waters it has certain
disadvantages. First, the absorbance of the reagent blank becomes too large for the
concentration effect achieved by the solvent extraction to be of much use; for example,
when 20ml of sample solution and 5ml organic solvent were used, the absorbance of the
reagent blank was 0.14. Secondly, the shaking time needed was long and the colour of the
extract faded gradually if the shaking lasted more than 30min.
However malachite green [172] gave a stable dark yellow species in 1.5M sulphuric
acid (probably a protonated one), whereas ethyl violet became colourless within 30min
even in only 0.5M sulphuric acid.
In this procedure 10ml of the sample solution containing up to 0.7µg phosphorus as
orthophosphate, was transferred into a 25ml test tube. To this solution was added 1ml
each of 4.5M sulphuric acid and the reagent solution. The solution was shaken with 5ml
of a 1:3 v/v mixture of toluene and 4-methylpentan-2-one for 5min. After phase
separation, the absorbance of the organic phase was measured at 630nm against a reagent
blank, in 1cm cells.
The absorption spectra of the ion-pair and the reagent blank are shown in Fig. 3.14.
Linear calibration graphs are obtained even when the aqueous phase volume is increased
to 100ml (and 7ml of extracting solvent are used). The results obtained by varying the
volume of sample solution are shown in Table 3.11, which shows that when 50ml of
sample solution are used, 0.1µg L −1 of phosphorus can be detected.
Arsenic(V) causes large positive errors; arsenic(V) at a concentration of 10µg L −1
p r oduces an absorbance of 0.07 but can be masked with tartaric acid (added in the reagent
solution). When arsenic(V) was present at concentrations of 50µg L −1 it was masked with
0.1ml 10 −4 M sodium thiosulphate added after the sulphuric acid.
For a 25ml sample, almost all of the foreign ions tested, when present at the
concentrations listed in Table 3.12, produced an absorbance of less than 0.005.
Table 3.13 shows the results for determination of phosphorus in waters. Less than
15ml of seawater was taken as sample and was diluted to 25ml with distilled water
because of its high chloride content (>0.5M).
Recovery tests were done by adding known amounts of phosphate. The results are also
shown in Table 3.13. The recovery of phosphorus was good; 99-103%. The relative
standard deviation for phosphorus was 0.6% for 21.0µg L −1 in seawater.