Preconcentration techniques - Determination of Anions in Natural and Treated Waters - page 754

15.1.11 Sulphide
Sulphide has been preconcentrated on a column of Amberlite IRA 400 [90]. The sulphide
is removed from the column with 4M sodium hydroxide and determined
spectrophotometrically by the N,N' -dimethyl p-phenylene diamine method. Down to
0.1µg L −1 sulphide can be deter-mined by this procedure.
Desalvo and Street [91] preconcentrated sulphide from well water on a column of
cadmium exchanged zeolite sorbent prior to determination by visible spectrophotometry.
This method had an appreciably lower detection limit than conventional sulphide
methods.
Bain [92] preconcentrated sulphide in non saline waters at levels of 1-1000µg L −1 at
0.20V (vs Ag/AgCl) and determined by cathodic stripping voltammetry. Over the
concentration range of 0-40µg L −1 the stripping peak height is linearly proportional to
sulphide concentration; over the range of 40-10000µg L −1 concentrations are obtained
from the calibration plot. The detection limits are 0.20 and 0.10µg L −1 for dc and
differential pulse stripping analysis, respectively, and the coefficients of variation are
3.7% and 2.6% for 10 and 100µg L −1 sulphide.
A flow injection gas diffusion method has been described [93] for the preconcentration
and determination of traces of sulphide. Accumulation and preconcentration of the
analyte is accomplished using the acceptor stream of a diffusion unit in a closed-loop
recirculating mode. The detection limit from a 5ml sample is 0.15µg L −1 level is 4%.
15.1.12 Thiosulphate
Chakraborty and Das [94] have described an indirect atomic absorption spectrometric
method for determining thiosulphate in photographic processing effluents based on the
formation of a stable ion association complex between lead, thiourea and thiosulphate in
alkaline medium. The complex is extracted into n -butylacetate:n-butanol 2:1 and
analysed directly by flame atomic absorption spectrometry. Down to 0.2µg L −1
thiosulphate were determined.
15.1.13 Multianion analysis
Fung and Dao [2] used Chelex-100 resin to preconcentrate selenate, arsenate, molybdate,
vanadate, tungstate and chromate from non saline waters. The anions were subsequently
desorbed from the Chelex 100 with suitable reagents and determined in sub µg L −1
quantities.
15.2 Potable water
15.2.1 Molybdate
Hidalgo et al. [95] preconcentrated molybdenum(VI) in potable water and seawater by
co-flotation on iron(III) hydroxide. Co-flotation was achieved by means of surfactants;