Anions in sewage effluent - Determination of Anions in Natural and Treated Waters - page 580

assay, unlike the colorimetric molybdate blue technique, was essentially unaffected by
complex matrices such as polyphosphates and phosphoesters.
Fig. 9.3 shows a typical chromatogram for a standard 42µg L −1 phosphate and
hypoxanthine peaks resulting from various phosphate samples after reaction with the
enzyme. The calibration curve had a slope of 0.043± 0.002 and an intercept of
0.124±0.033 with a correlation coefficient of 0.9981. Linearity up to 30mg L −1 was
observed. Relative standard deviation of triplicate runs was 10% or less. The detection
limit, twice the signal of the blank, was determined to be 1.5mg L −1 . Dialysis of the
commercial enzyme in 0.1mol L −1 . Tris buffer, pH 7.4, for 24h was found to reduce the
blank and therefore the detection limit by at least a factor of two.
A plot of phosphate concentration given by the enzymatic method vs that found by the
molybdate blue method had a slope of 0.999±0.046 and an intercept of 0.421±1.222 with
a correlation coefficient of 0.996.
To compare the selectivity of the enzymatic method with that of the molybdate blue
method, interference studies were conducted for both methods. The first study was the
effect of tripolyphosphate in the sample. The molybdate blue method showed a positive
deviation with increasing amounts of tripolyphosphate. The reason for this positive
deviation can be attributed to the acid hydrolysis of tripolyphosphate to orthophosphate.
The enzymatic method also showed a slight positive deviation, probably due to
orthophosphate impurity in the tripolyphosphate used.
9.8.4 Miscellaneous
See section 14.12.6.1.
9.9 Sulphide
9.9.1 Draeger tube method
Ballinger and Lloyd [16] have described a detailed procedure for the rapid determination
of down to 0.06mg L −1 sulphides in sewage samples. Hydrogen sulphide is brought to
solution/vapour equilibrium in a closed flask under controlled conditions. The
concentration of hydrogen sulphide vapour is determined by means of Draeger tubes, and
related to the concentration in solution by means of a calibration graph.
In aqueous solution, hydrogen sulphide dissociates as a weak diprotic acid:
At a given pH, the degree of dissociation depends upon the temperature and activity of
the sulphide species. At pH 5.0, dissolved sulphides are present almost entirely as
undissociated hydrogen sulphide. It follows from Henry's law that the vapour pressure of
hydrogen sulphide above its aqueous solution is a function of the mole fraction of
dissolved hydrogen sulphide. In this method, hydrogen sulphide is brought to solution-