Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 236

2.63.6 Polarography
Nitrites have been determined by oxidation at a glassy carbon electrode [526,534]
electrocatalytically using a molybdenum catalyst [535] and polarographically using
differential pulse polarography using the reaction between nitrite ion and diphenylamine
to produce diphenyl-nitrosamine [536].
Differential pulse absorptive volumetric analysis has been used to determine µg L −1
levels of nitrate in water [537].
Barsotti et al. [538] determined nitrite in amounts down to 0.4µg L −1 N and nitrite by
differential pulse polarography. Nitrate is reduced on a cadmium column to
diphenylnitrosamine and then determined polarographically. Cadmium interference is
removed by adjustment of pH and complexing with EDTA. Nitrite is determined directly
as the diphenylnitrosamine.
In order to separate the cadmium(II) peak from that of diphenyl nitrosamine, advantage
was taken of the fact that organic polarograms are usually pH dependent, while those of
reversible inorganic ions are not. Changing the pH from 1.0 to 4.0 shifts the diphenyl
nitrosamine peak, without loss of peak height, hence sensitivity, but causes no change in
the cadmium(II) peak. A pH of 1.0 is used for the formation of diphenyl nitrosamine
since the rate is fast at that pH with no decomposition. At a pH of 4.0 a peak separation
of 140m V occurs which is sufficient to determine nitrite ion in the presence of cadmium
(II) to a level of approximately 200µg L −1 . At this pH the EDTA complex of cadmium(II)
has a stability constant of approximately 10 16 . Thus, the addition of EDTA at a pH of 4.0
results in the complete disappearance of the cadmium(II) peak and total removal of any
interference. Therefore, by an adjustment of pH and the addition of EDTA to the nitrite
ion procedure, nitrate could be determined.
Liu et al. [539] have described a polarographic method for the determination of nitrite
in non saline waters.
2.63.7 Isotachoelectrophoresis
The application of this technique is discussed under multianion analysis in section
14.6,.1.1.
2.63.8 Ultraviolet spectroscopy
Espinola [540] has described a direct ultraviolet spectroscopic method for the
determination of nitrite in non saline waters. By using an addition technique and a
reference nitrate/nitrite solution, he compensated for the interference caused by the
overlapping of the nitrate and nitrite bands, which is normally a limiting factor in the
analysis of mixtures of nitrite with large excesses of nitrate. The detection limit was
5×10 −5 mol L −1 nitrite, which corresponded to a minimal detectable amount of 2.3mg L −1
nitrite in the presence of up to a 20,000 times greater amount of nitrate.