Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 217

graph with the horizontal axis represents the volume of standard solution containing a
quantity of nitrate equal to that in analysed solution. Table 2.35 shows the experimental
results for eight different samples.
Cox and Litwinski [448] described a voltammetric ion selective electrode for the
determination of nitrate down to 6.7×10 −6 M. The electrolysis cell used was equipped
with anion-exchange membrane sheath. An instrument has been described [455] which is
based on ion selective electrode measurements, with an acidic buffer solution which
ensures that alkaline earth carbonates do not deposit, and variations in sample ionic
strength are decreased. A feature is the automatic checking and autocorrection for sensor
drift, carried out at two concentrations at preset time intervals. Monitoring trials have
shown that the instrument can produce accurate and consistent records without manual
intervention almost indefinitely.
Nitrate has been determined by a nitrate-selective electrode [441] consisting of a
porous polymer wick sealed with liquid ion-exchanger (tris (4,7-diphenyl-1, 10-
phenanthrolinato) nickel(II) in 2-nitro- p -cymene).

Table 2.35 Nitrate-nitrogen content (mg L −1 ) in lake water samples analysed by Gran
method

S. No. Gran

1 (standard)
1.06

1.06

2 1.87

3 1.93

4 2.80

5 2.94

6 2.89

7 2.05

8 1.82

9 2.35

Source: Reproduced with permission from American Water Works Association [452]

Chloride and bicarbonate interferences are overcome by the addition of silver sulphate
and 0.5mol L −1 phosphate pH buffer respectively.
Synott et al. [456] compared ion selective electrodes and gas-sensing electrode
techniques for the measurement of nitrate in non saline waters. The gas electrode method
is based on the reduction of nitrate to ammonia by titanium trichloride and detection of
the ammonia by an ammonia electrode. The results indicated satisfactory reproducibility
and correlation between the two methods, except for sea water where the high
concentration of dissolved salts made the membrane electrode method unreliable. The