Anions in seawater - Determination of Anions in Natural and Treated Waters - page 411

water and wiped dry. A starting potential of 0.2V was used and the solutions were
degassed with dry nitrogen for 2min prior to recording. A full-scale sensitivity of 10µA
was used.
As an indication of the accuracy of the technique the correlation coefficients for the
calibration curves indicate a precision of 0.2% or less. The standard deviation of the
measured samples is 0.70%.
The precision of the technique indicates its suitability for sulphate analysis. The
technique appears not to be subject to any interferences normally present in seawater
samples.
3.29.5 Ion chromatography
Singh et al. [220] have determined sulphate in deep sub-surface waters by suppressed ion
chromatography.
3.30 Sulphide
3.30.1 Capillary isotachoelectrophoresis
Fukishi and Hiiro [221] determined sulphide in seawater by this technique. The method is
based on the generation of hydrogen sulphide by the addition of sulphuric acid to the
water sample. The gas permeated through a microporous PTFE tube, and was collected in
a sodium hydroxide solution. The carbon dioxide in the permeate was removed by the
addition of a barium cation-exchange resin to the sodium hydroxide solution. Injection
into the isotachophoresis apparatus followed. A linear graph for sulphide concentrations
of up to 2mg L −1 was obtained.
3.30.2 Gas chromatography
Cutter and Oatts [222] determined dissolved sulphide and studied sedimentary sulphur
speciation using gas chromatography in conjunction with a photoionisation detector. The
determination of dissolved sulphide and sedimentary sulphur is important to studies of
trace element cycling in the aquatic environment. A method employing selective
generation of hydrogen sulphide, liquid-nitrogen-cooled trapping, and subsequent gas
chromatographic separation/photoionisation detection has been developed for such
studies. Dissolved sulphide is determined via acidification and gas stripping of a water
sample, with a detection limit of 12.7nM and a precision of 1% (relative standard
deviation). With preconcentration steps the detection limit is 0.13nM. Hydrogen sulphide
is generated from sedimentary acid volatile sulphides via acidification, from greigite
using sodium borohydride and potassium iodide, and from pyrite using acidic chromium
(II). The detection limit for these sulphur species is 6.1µg of S/g, with the precision not
exceeding 7% (relative standard deviation). This method is rapid and free of chemical
interferences, and field determinations are possible.
Lech and Bagander [223] determined reduced sulphur compounds (hydrogen sulphide,