Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 243

reduction of nitrate to nitrite. Detection limits are 0.05-0.26mg L −1

(nitrate plus nitrite)

and 0.01-0.05mg L −1 (nitrite).

2.64.5 Segmented flow analysis
Marti and Hale [560] compared automated segmented flow and discrete analysers for the
determination of nitrite, nitrate, ammonia and phosphate. The discrete analyser provided
a 50% increase in the rate of analysis compared with segmented flow analysis. The
accuracy and precision were comparable. While both methods are in agreement, the rate
of analysis (60 samples per h) was faster by discrete analysis than segmented flow
analysis (40 samples per h).
This technique has been applied to the determination of nitrite and nitrate in
concentration ranges 0-0.5, 0-5, 0-20, 0-40 and 0-100mg L −1 (as N).

2.64.6 Flow injection analysis
This technique has been used to determine nitrite and nitrate in non saline waters [561].
Van Staden [561] converts nitrate to nitrite using a pre-valve in-valve reduction technique
before the sampling valve. Lerique and Lighetto [562] describe an analyser which
depends on three principles: injection of sample into a continuous, non-segmented flux of
reagent, a reproducible residence time for the samples, and controlled dispersion of the
sample. These objectives are achieved by independent peristaltic pumps for each of the
four analysis channels, a very precise injection system and a specially designed
replaceable manifold. Nitrite deter-mination is based on the sulphanilamide/naphthyl
ethylenediamine reaction with a colorimetric measurement at 540nm. Nitrates are
reduced to nitrites using a cadmium catalyst and the nitrites determined as above. The
apparatus is connected to a microprocessor, can analyse 300 samples per h, and results
for each sample are printed out within about 20s.
Workers at Tecator Ltd, UK in a series of application notes discuss the determination
of total nitrite plus nitrate by flow injection analysis at concentrations between 0.05 and
500mg L −1 [ 563-568].

2.64.7 Polarography
Polarography has been applied to the determination of nitrites and nitrates [569,570]. In
the polarographic technique, total nitrate and nitrite are determined. The nitrite is
decomposed by the addition of methyl alcohol, and nitrate only is determined on a second
polarogram. Boese et al. [570] described a method based on the enhancement of the
differential pulse polarographic peak current of ytterbium by nitrate and nitrite.
Holak and Specchio [571] determined nitrite and nitrate in non saline waters by
differential pulse polarography with simultaneous nitrogen purging.

2.64.8 Amperometry
Nygaard [572] has described a pneumatoamperometric technique for the determination of