Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 270

2.75.13 Gas chromatography
Hashimoto et al. [653] converted phosphates to phosphine prior to gas chromatographic
analysis in their method for the determination of phosphates in non saline waters.
2.75.14 Preconcentration
The application of this technique is discussed under multianion analysis in section 15.1.8.
2.75.15 Miscellaneous
Various other methods for the determination of phosphate in water include atomic
absorption inhibition titration of magnesium absorption [437,654], neutron activation
analysis of the tungstophosphate [437,654, 655], X-ray fluorescence spectrometry
following adsorption of phosphate onto silica gel modified with N-substituted diamine
functional groups [656], radio-activation analysis as the 185W
tungstomolybdophosphoric acid complex [657] and differential pulse anodic voltammetry
[643].
Herve and Kauppi [658] studied the effects of filtration on the determination of
phosphate phosphorus in run off waters. The effect of filtration was to reduce the amount
of phosphorus reacting with molybdenum in turbid water samples. The GF/C and 0.45µm
membrane filters gave similar results but a much smaller reduction occurred with the
black ribbon filter (pore size 7.4µm). The total reactive phosphorus concentration method
was thought to over-estimate the level of available phosphorus in turbid waters because
of the severity of the acid treatment embodied in the method.
Ryden et al. [659] have studied the adsorption of inorganic phosphates by laboratory
ware, including glass, polypropylene and polycarbonate. They conclude that the use of
acid washed glassware should be discouraged and a polycarbonate which has been
previously 'phosphated' by treatment with hot hydrochloric acid followed by potassium
dihydrogen phosphate is the most suitable sample container material.
Nelson and Romkous [660] have shown that, after freezing of water samples followed
by storage at −20°C for 3d, the soluble phosphate concen-tration decreased by 2-27%.
Rapid freezing with liquid nitrogen produced about half the effect of slow freezing. No
decrease in the phosphate concentration was found when samples were stored at 2°C (for
3d) or when they were frozen slowly after removal of sediments by centrifuging.
Lindquist and Cox [661] determined phosphate in lakewater by cathodic stripping
chronopotentiometry at a copper electrode. Good agreement was obtained with results
obtained by spectrophotometric procedures.
Shapiro [662] fixed lakewater samples for phosphate determination by adding aqueous
ammonium molybdate to the sample immediately after collection. As soon as convenient
the samples were extracted with isobutyl alcohol and the organic layers stored for up to
two weeks before analysis. During this period the determined phosphate content does not
decrease by more than 20%.