Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 198

buffer solution.
Koupparis et al. [399] have described an automated method for the determination of
nitrate in river waters with a microcomputer-based stopped-flow mixing system. Nitrate
is reduced to nitrite with a copperised cadmium-sliver alloy or cadmium tube column
fitted to the stopped-flow system, nitrite is determined using fast kinetic, multi-point or
single point procedures with N-(1-naphthyl)ethylenediamine dihydrochloride as the
colour reagent. Water samples in the range of 0.025-3mg L −1 NO 3 − -N can be processed
with a throughput of up to 100 samples per hour and a detection limit of 0.013mg L −1 .
Interference studies showed that cyanide, dichromate, iodide, sulphide, copper and tin
ions cause negative results.
Using this method Koupparis et al. [399] carried out a recovery study on river and lake
water samples by measuring the nitrite concentration before and after standard addition.
Samples were also assayed for nitrite and the equation for the determination of nitrite and
nitrate in the mixture was used. The nitrite content was found to be equivalent to less than
0.2mg L −1 . Samples higher than 3mg L −1 were diluted with distilled water. The results
obtained show an average recovery of 100.2% (Table 2.22).

Table 2.22 Recovery data for the determination of nitrate in lake and river water samples
with the multipoint kinetic procedure and cadmium alloy column

Nitrate content (mg L −1 NO 3 − -N)

Sample Recovery %

Before std After std. addition Expected Determined

1 1.90 6.90 7.05 102.2

2 10.50 13.00 13.00 100.0

3 11.95 14.45 14.25 98.6

4 0.10 1.10 1.11 100.9

5 0.40 1.40 1.41 100.7

6 10.70 13.20 13.15 99.6

7 10.90 13.40 13.55 101.1

8 12.90 14.15 14.20 100.4

9 5.20 7.70 7.65 99.4

10 0.39 1.39 1.37 98.6

Average 100.2

Source: Reproduced with permission from Elsevier Science [399]

The effect of various potential interferences was also investigated. Most of the common
ions present in water samples do not interfere even in high concentrations. Those ions
that showed serious interference rarely are found in water samples at the highest
concentrations examined.