Anions in potable waters - Determination of Anions in Natural and Treated Waters - page 509

7.20.2 Differential pulse polarography
Barsotti et al. [66] determined nitrate in amounts down to 0.4µg L −1 N and nitrite by this
technique. 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 140mV 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.

7.20.3 Ion chromatography
Various workers [67-70] have discussed the application of this technique to the
determination of nitrite and nitrate (Table 7.16).

7.20.4 Ion exchange chromatography

Gerritse [71] separated nitrate and nitrite on a cellulose anion-exchanger column and
detected them in a spectrophotometric flow through cell at 210 nm. The detection limit
was 1-5µg L −1 as nitrogen with a sample volume of 100-200µL and the linear range
extends up to 20mg L −1 as nitrogen.

Table 7.16 Determination of nitrite and nitrate in rain water by ion chromatography

Type of water sample Co-determined onions Ref

Cl, PO 4 , SO 4 Rain [67]

Cl, PO 4 Rain [68]

Cl, F, Br, SO 4 , SO 3 , PO 4

Rain [69]

Cl, Br, F, PO 4 , BrO 3 , SO 4

Rain [70]

Source: Own files