Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 255

siemen) was used for standards, reagents and rinsing of glassware. All sample tubes and
glassware were washed with 'BRIJ-35' detergent each time before use and soaked in 5N
hydrochloric acid at frequent intervals. All samples were filtered through a Millipore
0.45µm filter before analysis.
Their determination of phosphate utilised the manifold and flow sequence are
illustrated in Fig. 2.36.
Harmsen [601] used derivative spectrophotometry to determine phosphate in turbid
water samples using the conventional molybdenum blue spectrophotometric procedure.
Turbidities less than 0.8 did not interfere with adsorbance. Precautions were needed for
phosphorus concentrations higher than 3mg L −1 . Nevertheless recovery tests were
accurate to within 4%.
Stainton [602] has discussed errors in molybdenum blue methods for determining
orthophosphate in non saline waters. He suspected that the release of orthophosphate
from colloidal phosphorus was the source of such errors as had been shown by previous
workers, and he therefore studied the effect of a variety of the more popular molybdenum
blue methods on the colloidal phosphorus fraction. All variations on the method caused a
significant loss of phosphorus from the fraction. Phosphorus originating from the
colloidal fraction tested as orthophosphate, gave an overestimate of the true
orthophosphate present.
Sulphide interference in the determination of phosphate by molybdenum blue methods
has been discussed by De Jonge et al. [603].
Tarapchak et al. [604] have shown that when the molybdenum blue method was
applied to samples of Lake Michigan water, differences in the period of exposure of
samples to acid molybdate and differences in molybdate concentration, can affect the
accuracy of the results.
Studies by Tarapchak [605] have indicated that methods using molybdenum blue to
determine orthophosphate in surface waters may result in an over-estimation of the
amount of available phosphorus. Evidence was obtained that molybdenum accelerates
hydrolysis of organic phosphorus in the presence of acid and also either causes hydrolysis
or forms complexes with organic phosphorus before acidification. Possible ways for
limiting these effects are discussed.
Yoshimura et al. [606] have carried out microdeterminations of phosphate by gel-
phase colorimetry with molybdenum blue. In this method the blue species of
molybdophosphate are strongly adsorbed on Sephadex gels. Almost all the blue species
in a 50cm 3 sample solution are concentrated in 0.20g of Sephadex G-25 (fine) within
10min. Direct absorptiometry of the heteropoly acid concentrated in the gel phase was
developed for the determination of the phosphate at parts per billion levels in non saline
waters. The coloured gel beads, on which the blue species reduced by ascorbic acid in the
presence of antimonyl ions were adsorbed, are packed into a 10mm cell; the attenuances
at 836 and 416nm are measured; and the attenuance difference is used for the
determination of traces of phosphate. The use of cells of other length (5, 2 and 1mm) give
a wide concentration range for calibration from parts per billion to parts per million
levels. This method is simple in operation and has a high reproducibility of
measurements.
Towns [607] determined phosphate in non saline waters by ascorbic acid reduction of