Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 230

reaction and interferences due to foreign ions were examined. Concentration curves were
linear over the range 1-20µg nitrite per 25ml final volume.
There are numerous other reports dealing with the spectrophotometric determination of
nitrite in non saline waters. The reagent N,N- bis(2-hydroxyproypl)anlinine reacts with
nitrite in an acidic medium to form a reddish product which gradually
changes to a yellowish product The reddish product was used for
determining nitrite in river water using a flow-injection method [510]. The calibration
graph was linear from 3 to 64µg L − 1 of nitrite nitrogen and the relative standard
deviation for nine determinations was 0.7% for a solution containing 16µg L −1 of nitrite
nitrogen; sampling rate was 50 samples/h. The solid reagents p-amino-benzenesulphonic
acid and N -1-naphthylethylenedimine react with nitrite to generate a product which is
measured photometrically [511]. The method has recoveries of 95-105%, and relative
standard deviations of 0.5-7.9% for eight measurements of nitrite concentration at <1
ppm; the detection limits at 544nm were 0.002 and 0.0005mg L −1 nitrite nitrogen for
cuvette pathlengths of 1 and 10cm respectively. Reaction of nitrite with 5,7-dihydroxy-4-
amino-2-oxochroman in acid medium produces a complex which is extracted into a
solvent mixture of butanol and ethyl acetate for spectrometric determination at 361nm
[512]. This method gives a relative standard deviation of 0.4% for seven measurements
of nitrite nitrogen at the 0.140ppm level. Other spectrophotometric determinations of
nitrite in waters can be found in the following references [501,508,513-518]. Nitrite in 10
to 50mL volumes of water was injected into an anion exchange resin column and then
eluted with 4M sodium chloride. Nitrite in the eluate was measured
spectrophotometrically at 210nm. The detection limit was 0.1µg L −1 nitrite and the anions
nitrate, chloride and sulphate did not interfere over a considerable range of
concentrations.
2.63.2 Spectrofluorometric methods
The method using p -chloroaniline and 2,6-diaminopyridine [519] is highly sensitive
(limit of detection 2µg L −1 ) and apparently selective, as the authors did not investigate
the effect of diverse ions on the method. They consider that interferences can be expected
from those ions which either are not compatible with the nitrite ion or can effect the
diazotisation and coupling reactions. 2,3-diaminonaphthalene [497] is also a sensitive
fluorometric reagent for nitrite (0-850µg L −1 ), but all these methods have the
disadvantage of being complicated as they rely on solvent extraction. The method
developed with 5-aminofluorescencein is simple and is sensitive to 0.05µg L −1 .
Benzidine has been used for the determination of microgram amounts of nitrite [520].
This anion oxidises the benzidine and produces a shift in its emission maximum from 387
to 348nm and in its excitation maximum from 295 to 275nm. Also, the intensity
decreases by about two-thirds.
Rubio et al. [521] have described a spectrofluorometric method for determining nitrite
whereby the nitrite ion oxidises pyridoxal-5-phosphate-2-pyridylhydrazone in acidic
media to give a fluorescent product. Although less sensitive than other methods, it was
selective, simple and rapid and could be applied in the 0.1-1.0mg L −1 nitrite range.