Anions in trade effluents - Determination of Anions in Natural and Treated Waters - page 599

temperature, ionic strength and pH, standards used for calibration should have a similar
temperature, ionic strength and pH. The usual way of achieving these conditions and
preventing air oxidation at the same time is to use a sulphide anti-oxidant buffer.
A sulphide ion selective electrode has been used [68] as an indicator electrode in the
potentiometric titration of sulphide in paper mill effluents with standard mercuric
chloride. In 0.1N sodium hydroxide medium, sulphide and polysulphides are titrated. As
the electrode responds to mercuric ions, thiosulphate and sulphite can also be titrated at
pH 7.0-7-5. The poly sulphide plus sulphite can be converted to thiosulphate and can be
titrated. Sulphite can be decomposed with formaldehyde and, consequently, thiosulphate
can be titrated alone.
The construction, characteristics and application of a custom-built coated tubular solid-
state sulphide-selective electrode are described by Van Staden [69]. The tubular flow-
through sensor is made from silver foil that is treated with a sodium sulphide solution.
The electrode was used in association with flow-injection analysis and gave a working
range up to 1000mg L −1 with a detection limit of 0.4mg L −1 when 30µL L −1 samples
were injected. The system was suitable for determining soluble sulphide in effluent water
samples in the range 0.4-24mg L −1 with a relative standard deviation better than 1.8% at
a sampling rate of 60 samples L −1 .
Jovanovic and Jovanovic [70] determined low sulphide and cyanide levels using a
deposited on wire silver-silver sulphide electrode.
10.18.6 Ion chromatography
The application of this technique is discussed under multianion analysis in section 12.8.2.
10.18.7 Gas chromatography
Hawke et al. [71] described a method for the determination of sulphides in effluents
using gas chromatography with a flame photometric detector. Samples were acidified and
the hydrogen sulphide brought to solution vapour equilibrium. Following analysis by gas
chromatography the concentration of hydrogen sulphide was determined by interpolation
on a calibration graph. The precision, accuracy, bias and effect of potentially interfering
substances were determined. The samples could be processed in 1h by this method.
Knoery and Cutter [72] have described a method for the determination of sulphide and
carbonyl sulphide in non saline waters using specialised collection procedures followed
by gas chromatography using flame photometric detector. The following species were
determined: H 2 S aq , HS − , S 2− and particulate metal sulphides. The sample was stripped
with helium and the evolved gases analysed by gas chromatography. Down to 0.2pmol
L −1 of total dissolved sulphide could be determined.