Anions in aqueous precipitation - Determination of Anions in Natural and Treated Waters - page 447

5.2.3 Hydroxymethane sulphonate
5.2.3.1 Ion chromatography
Davies and Ivey [49] describe the ion chromatographic determination of the
hydroxymethane-sulphonate form of sulphur(IV) after stabilisation by a stoichiometric
excess of formaldehyde. The latter method gave a detection limit of 0.1µg L −1 and was
used for determining sulphur(IV) in rainwater and Antarctic ice.
5.2.4 Nitrate
5.2.4.1 Spectrophotometric method
Parker et al. [50] have evaluated a Spectrophotometric method for the determination of
nitrate in snow and ice.
5.2.4.2 Polarography
Hemmi et al. [51] used differential pulse polarography to determine down to 8×10 −7 mol
L −1 nitrate in fresh snow. This method utilises the catalytic reaction between nitrate and
uranyl ion in the presence of potassium sulphate. The differential pulse peak is
proportional to nitrate concentration from 1 to 50µmol L −1 . Between 350 and 700µg g −1
nitrate was found in snow samples by this method. Nitrite and phosphate interfere
seriously in this method and should be absent if accurate results are to be obtained.
5.2.5 Sulphate
5.2.5.1 Spectrophotometric method
Suzuki et al. [52] have described a simple procedure for the Spectrophotometric
determination of sulphate in snow. It involves reaction of sulphate with amino perimidine
to form perimidinyl ammonium sulphate, and collection of this precipitate on a teflon
filter followed by dissolution in nitric acid. The resulting solution is made alkaline and
the absorbance of the violet colour is measured at 550nm. Both phosphate and nitrate
interfere in the procedure.
A 5ml portion of sample solution containing up to 50µg of sulphate is placed in a
funnel, followed by 3ml of 0.3% fresh aqueous 2-perimidinyl ammonium bromide
solution. After 2min, the solution is removed by gentle suction. The funnel and the
precipitate are washed with fine water droplets from a spray to remove excess of reagent.
About 2ml of concentrated nitric acid is added dropwise to the precipitate until the
washings become colourless. All the washings are collected in a 50ml beaker and the
solution made basic with the addition of 3ml of 10mol L −1 caesium hydroxide,
transferred to a 25ml volumetric flask and diluted to the mark with deionised water. The