Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 306

titration with standard solutions of caesium octacyano-molybdate (Cs 3 Mo (CN) 8 ) or
caesium octacyanotungstate (Cs 3 W(CN) 8 ) ions. With the former titrant, the reaction is
carried out in 0.05mol L −1 borate buffer at pH 9.5. With the latter titrant the reaction is
carried out in 0.05mol L −1 phosphate buffer at pH 11.5. Methylene blue is used as
indicator in the visual titration procedure. In the determination of 1-40mg of sulphide the
average error was ±0.14%. Dithionite and sulphite also react but there is no interference
from sulphate or thiosulphate ions.
A standard UK method [806] describes the determination of sulphide hydrogen
sulphide liberated from the sample by acidification and carried by a stream of carbon
dioxide into zinc acetate solution. Standard 0.01417M potassium iodate-iodine is added
and excess iodine titrated with 0.0125M or 0.25M sodium thiosulphate. Down to 0.26mg
L −1 sulphide can be determined with a standard deviation ranging from 0.19mg L −1 at the
4.75mg L −1 sulphide level to 0.7mg L −1 at the 26.4mg L −1 sulphide level.
2.94.2 Spectrophotometric methods
Spectrophotometric methods for sulphide have been developed based on the use of the
following chromogenic reagents; NN-dimethyl-p-phenylene diamine (methylene blue
methods) [807,808] sodium nitro-prusside [809,810], 1, 10-phenanthroline-ferric chloride
[811], methylene blue [812,876], bis-2,9-dimethyl-1, 10-phenanthroline copper ion [813],
and copper quinolin-8-olate [814].The latter two methods are based on solvent extraction
of the coloured produced formed with sulphide followed by spectrophotometry.
Caspieri et al. [810] automated the nitroprusside procedure using an AutoAnalyser.
The samples are treated in a reproducible manner, and the colour is measured after a
definite time from addition of the reagents. The inclusion of a distillation step increases
the selectivity of the procedure and eliminates the necessity for filtration. The precision is
satisfactory in the concentration range 1-10µg L −1 of sulphide and is slightly better than
that for the manual procedure. The coefficient of variation at the 10mg L −1 level for the
AutoAnalyzer and manual procedures are 4.2% and 3.6%, respectively. If high selectivity
is not required, the distillation of hydrogen sulphide may be omitted. At a sampling rate
of 40 per h, 250 samples can be analysed daily.
Bethea and Bethea [809] overcame some of the problems encountered in this method
by lengthening the mixing coils and alternating reagent flow rates. A detection limit of
0.05mg L −1 sulphide was achieved.
In the methylene blue procedure [808] ferric chloride and NN -dimethylphenylene
diamine is added to the sample to form methylene blue. After 20min the extinction is
measured at 670nm. The method is applicable to waters containing between 0.03 and
32mg L −1 of sulphide. Within this range, Beer's law is obeyed and the method is free
from salt effects and temperature dependence. The precision is ±2% at the 95%
confidence level.
Grasshoff and Chau [807] automated the methylene blue procedure using an
AutoAnalyser.
Ceba et al. [815] developed a method which is claimed to be a simple, rapid, sensitive
and selective means of determining microgram levels of sulphide in water. The technique
involves the use of chloroform solutions of copper quinolin-8-olate, which are re-