Chemistry Reference
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extracted by aqueous solutions of sulphide, for the indirect spectrophotometric
determination of sulphide. The effects of variables on the decolouration of the coloured
complex by the aqueous sulphide were established, and the presence of other sulphur
anions (sulphite, thiosulphate, and thiocyanate) did not interfere. Results compared
favourably with those from the methylene blue method.
A study of the effect of other ions was carried out by preparing a series of synthetic
samples containing 40.0µg of sulphide and variable amounts of foreign ions. The results
obtained for binary mixtures are summarised below.
Ion tested sulphide mg L
−1
Relative amount tolerated
Na, K, Cl, NO
3
, SO
4
, Ca, Mg, Ba, Sr
25-400
NO
2
, S
2
O
3
, CNS, SO
3
100-125
tartrate, citrate, PO
4
, F, BO
3
25-50
Al, EDTA, CN
1-8
As can be seen, the procedure is highly selective and some results, especially for the
sulphide mixtures with thiosulphate, have analytical interest, eg the relative amount of
thiosulphate tolerated is greater than that in the methylene blue method.
The recommended method was tested by applying it to the determination of sulphide in
water samples. A sample was prepared by adding a known amount of sodium sulphide to
a sample of Guadiana River water. The decrease in the sulphide concentration in this
sample was studied as a function of time in order to establish the evolution of a possible
accidental spill. The results obtained are given in Table 2.53 together with those obtained
by using the methylene blue method. These results agree for concentrations of sulphide in
the range 0.5-1.0mg L
−1
whereas for sulphide concentrations greater than 1.0mg L
−1
the
methylene blue method gives values higher than the method described by Ceba
et al.
[815]. To confirm the above results, two synthetic samples of water were prepared with
the following composition: 5mgL
−1
of aluminium, 10mg L
−1
of sodium, 10mg L
−1
of
potassium, 30mgL
−1
of magnesium, 30mgL
−1
of calcium, 185mg L
−1
of chloride, and
1.25 or 0.9mg L
−1
of sulphide. The results obtained as a function of time are given in
Table 2.54.
Broderius and Smith [816] have developed a procedure for the determination of traces
of sulphide in non saline waters in which compressed nitrogen is bubbled through the
sample to displace hydrogen sulphide which is collected in a glass bead column and
measured colorimetrically.
Table 2.53
Evolution of the sulphide concentration in a sample of Guadiana river water
Time/min
Re-extraction method
Methylene blue method
Decoloration value
S
2−
found/
mg L
−1
Absorbance
S
2−
found/
mg L
−1
10
0.853
1.15
0.605
1.28
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