Anions in surface, ground, mineral, borehole and pure waters - Determination of Anions in Natural and Treated Waters - page 464

as it consumes nitrogen.

6.4.2 Phosphate

6.4.2.1 Spectrophotometric method

Potman and Lijkema [31] have discussed the following determination of down to 0.4µg
L −1 phosphate in small samples of borehole and interstitial water.

Table 6.6 Interference of iron and silica in phosphate determination

µgP in sample 0.34 0.34 0.34 0.34 0.34

µgFe in sample 0 170 255 340 510

Abs. (corrected) 0.157 0.158 0.159 0.182 0.189

µgP recovered 0.34 0.34 0.35 0.40 0.41

µgP in sample 0.34 0.34 0.34 0.34 0.34

µg Si in sample 0 17 25.5 34 51

Abs. (corrected) 0.163 0.168 0.161 0.168 0.205

µgP recovered 0.34 0.35 0.34 0.35 0.43

Source: Reproduced with permission from Elsevier Science [31]

Procedure. 5ml 2N hydrochloric acid, 6ml molybdate solution (1.159g (NH 4 ) 6 Mo 7 4H 2 O
in 250ml water) and at most 25ml of the sample are mixed in a separating funnel. If
necessary fill up with water to 25ml, After 10min 25ml of 5:1(v/v) ether :isobutanol
solvent are added and the funnel shaken for 1min. The aqueous phase is removed. The
organic solution is washed twice with 20ml 0.4N hydrochloric acid for 1min and the
aqueous phase removed carefully. The ether solution is shaken for 1min with 10ml 0.1N
sodium hydroxide, the alkaline solution separated completely and the extraction repeated
with 5ml 0.1N sodium hydroxide, shaken for about 30s.
The combined alkaline solutions are mixed with 11ml of the copper sulphate solution
(0.04g CuSO 4 . 4H 2 O plus 1ml H 2 SO 4 per litre), 8.5ml concentrated hydrochloric acid,
7ml acetone, 10ml of 5% ammonium thiocyanate solution and filled up with 3N
hydrochloric acid to 50mL. The absorbance is measured after 45min at 470nm in a 1cm
cell, comparing with standards after correction for the blank.
Iron and silicate interfere in the procedure but only at higher concentrations (Table 6.6)
ie a 1000-fold excess of iron and a 100-fold excess of silica do not interfere.