Preconcentration techniques - Determination of Anions in Natural and Treated Waters - page 752

15.1.8.3 Preconcentration on inorganic solids
Coprecipitation with aluminium hydroxide has been used to preconcentrate phosphate ion
in waters [75]. This method involves coprecipitation with aluminium hydroxide, flotation
of the precipitate which is then dissolved in sulphuric acid and determination of
phosphate by a conventional molybdenum blue method. Recoveries are better than 95%
and the relative standard deviation is 1% with samples in the range 5-150µg L −1 .
Khan and Chow [76] have described a method in which the phosphate is converted into
phosphomolybdate which is then extracted into polyurethane foam and analysed for
molybdenum by X-ray fluorescence spectrometry directly on the foam. A polyether type
polyurethane foam disc is squeezed for an hour in a mixture of phosphate solution,
sodium molybdate and hydrochloric acid, spiked with phosphorus 32. After washing and
drying, the foam disc is placed on plastic foam and stretched across the X-ray source
holder. The method is simple and rapid and the precision is 5% for 0.25µg L −1 and 2%
for 2.5mg L −1 phosphate. Equimolar amounts of silicon, germanium and arsenic(VI)
appear to interfere with the determination.
A preconcentration technique based on nitrocellulose or acetyl cellulose membranes
has been described for phosphate [77]. Phosphorus was collected as phosphomolybdate
on a nitrocellulose or acetylcellulose membrane in the presence of n -
dodecyltrimethylammonium bromide. The membrane was dissolved in
dimethylsulphoxide and the absorbance of dimethylsulphoxide solution measured.
Moderate concentrations of silicate, anionic and non-ionic surfactants and high
concentrations of sodium chloride did not interfere. Arsenate interference could be
eliminated by reducing arsenate to arsenite. Determination of condensed and organic
phosphates was possible following their conversion to orthophosphoric acid. The limit of
determination was 0.02µg L −1 phosphate.
15.1.8.4 Coprecipitation with ferric hydroxide
A solution of iron containing ferric chloride and acidified with hydrochloric acid is
treated with aqueous ammonia to precipitate ferric hydroxide coprecipitated with ferric
phosphate produced by any phosphate ions in the sample [78]. The precipitate is filtered
off and dissolved in dilute hydrochloric acid and ammonia and uranyl acetate added to
produce uranyl phosphate which is estimated polarographically. To determine phosphate
plus arsenate the ferric hydroxide precipitate, containing coprecipitated iron phosphate
and arsenite is made acid with hydrochloric acid and potassium iodide added. The arsenic
trichloride produced is extracted with carbon tetrachloride and this solution back
extracted into hydrochloric acid. Application of the ammonia ferric chloride precipitation
technique to this gives a preconcentrate containing arsenic only.
Hori et al. [78] also examined the adsorption behaviour of various phosphorus
containing anions on ferric hydroxide as a function of solution pH. Adsorbed phosphorus
compounds were determined spectrophotometrically and percent adsorption calculated
from adsorbed and initial amounts. This was plotted against solution pH. Orthophosphate
was adsorbed quantitatively at pH 4.0-8.0; triphosphate and pyrophosphate at pH 4.0-9.3