Anions in trade effluents - Determination of Anions in Natural and Treated Waters - page 598

670nm whose intensity is proportional to the sulphide content of the sample. This method
is applicable to sulphide levels up to 0.5mg L −1 . Down to 0.0027mg L −1 sulphide can be
determined. Standard deviations range from 0.0023mg L −1 sulphide at the 0.048mg L −1
sulphide level to 0.0042mg L −1 sulphide at the 0.4mg L −1 sulphide level. Noteworthy is
the fact that heavy metal sulphides may be only partly recovered, also thiosulphates
above 10mg L −1 cause temporary loss of colour.
Sulphites cause a permanent loss of colour as do zinc and iodides. Cyanides do not
retard colour formation unless present in concentrations exceeding 500mg L −1 .

10.18.3 Spectrofluorometric methods
Fluorometric methods are based on substitution reactions in spite of the redox character
of the sulphide ion. The sensitivity of these methods is extremely high; for example, the
method with di(acetoxymercuri)-fluorescin [61-65] is sensitive to 1µg L −1 , and with the
mercury(II)-2,2'-pyridylbenzimidazole system [66] 0.03µg L −1 can be detected.
None of the fluorometric methods for determining sulphide is selective and often they
are subject to interference from cyanide and thiocyanate ions.

10.18.4 Polarography
Kakiyama and Komasu [67] evaluated sulphide in industrial effluents by measurement of
the peak at − 0.61V vs the mercury pool anode in a sodium hydroxide base electrolyte
containing EDTA as lead masking agent. Sulphite, thiosulphate, sulphate, thiocyanate,
iodide, bromide or chloride do not interfere.

10.18.5 Ion selective electrodes
An ion selective electrode can be used for the selective analysis of sulphide especially in
rivers and other relatively clean waters. It can be used as a direct measure of the sulphide
ion activity or as an end point detector for potentiometric titration of sulphide with silver
nitrate solutions.
There is only one type of sulphide ion selective electrode, based on a compressed disc
of silver sulphide. It should be noted that under the correctly buffered operating
conditions, the ion selective electrode will detect only the S = ion and does not respond to
other sulphur ions or molecules, eg HS − , H 2 S.
The electrode has a linear Nernstian response between 0.1-300ppm S - and can be used
down to about 0.01mg L −1

with additional calibration standards at closely spaced

concentration intervals.
In common with most ion selective electrodes the sulphide ion selective electrode is
very dependent on pH and exhibits a linear dependence on pH over the range 9-12. To be
operated successfully the electrode should be used only in samples buffered to pH 13.
The electrode is calibrated against standard solutions. Full calibration is only required
if the electrode is used below the limit of Nernstian response.
Since the response of the sulphide ion selective electrode is dependent of the
H 2 S/HS − /S - equilibrium and in common with all ion selective electrodes is dependent on