Anions in potable waters - Determination of Anions in Natural and Treated Waters - page 511

aeration of anaerobically digested sewage sludge are shown in Fig. 7.11. The sludge was
centrifuged at 40000g for 1h, then the supernatant was diluted 1:100 with distilled water
and injected into the column. Potable water was injected undiluted. Without dilution,
samples with concentrations of up to 20mg L −1 each (as nitrogen) of nitrite and nitrate
could be injected.
Davenport and Johnson [72] used ion exchange chromatography on Amberlite IRA-
900 strongly basic resin to determine nitrate and nitrite in water. Perchloric acid (0.01mol
L −1 ) was used as eluent and an electrochemical cadmium electrode detector was used.
7.20.5 Gas chromatography
A method has been described [73,74] for determining aqueous nitrates by conversion to
nitrobenzene followed by electron capture gas chromatography. The procedure can also
be used to determine aqueous nitrites and gaseous oxides of nitrogen if they are first
converted to nitrates. The procedure was evaluated by analysing drinking water for
nitrate over a period of one month. The method is sensitive and capable of measuring
typical environmental levels of nitrogen compounds. The detection limit for nitrobenzene
is about 10 −12 g.
For the determination of nitrate, use a 1 dram vial with a polyethylene stopper (Kimble
No. 60975-L) as a reaction vessel. Introduce a 0.20ml aliquot of aqueous sample into the
vial, followed by 1.0ml of thiophen free benzene. Catalyse the reaction by addition of
1.0ml of concentrated sulphuric acid. Shake the vial for 10min. Remove the benzene
layer immediately from the reaction vial with a Pasteur pipette, place it in a separate vial
and analyse by gas chromatography with electron capture detection for the nitrobenzene
concentration generated. Treat standard solutions of potassium nitrate in the same manner
to generate a standard calibration plot relating nitrobenzene concentration to peak height.
If higher precision is desired (approximately 4% relative standard deviation), add 2, 5-
dimethylnitrobenzene to the benzene prior to reaction as an internal standard
(concentration 5×10 −7 gm L −1 to correct for differences in injection size, detector
sensitivity, fluctuations, etc. When this internal standard is used, normalise the peak
heights of nitrobenzene to the peak heights of the standard before further data reduction.
Determine nitrite by difference, treating a second identical sample with an oxidant to
convert nitrite to nitrate. Consequently, the concentration of nitrite plus nitrate is
subsequently measured, so nitrite must be determined by difference. Specifically, the
procedure used was to introduce 0.15ml of the sample into the reaction vial, followed by
addition of 0.05ml of 0.1N hydrogen peroxide. The rest of the procedure is the same as
that for nitrate.
The results of applying the microtechnique to the analysis of nitrate in several samples
of drinking waters are summarised in Table 7.17. In no cases were detectable amounts of
nitrite found (the limit of detection for nitrite depends on the nitrate concentration, since
nitrite is determined by