Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 203

As an alternative to cadmium reduction, Zhang [404] showed that nitrate is reduced with
93% efficiency to nitrite in 8min by ultraviolet light under alkaline conditions. He applied
a method based on these principles to the analysis of river water samples. A cadmium
reduction column was used to convert the remaining nitrate present in solution to nitrite.
Nitrite determinations involved adjustment to pH 7 and sequential addition of
sulphanilamide solution (0.6%), alpha- naphthylamine solution (0.6%) and sodium
acetate solution (2mol L −1 ). Adsorbances at 520nm were used to determine nitrite
concentrations against a calibration graph. For total nitrogen determinations, river water
samples were digested with hydrochloric acid prior to analysis. Addition of potassium
persulphate (4.0%) and sodium hydroxide (6mol L −1 ) solutions and subsequent
irradiation for 17min facilitated total nitrogen determination. With this method the
coefficient of variation was 1.94% for river waters for a total nitrogen concentration of
1.8mg L −1 .

Table 2.26 Nitrate concentrations determined by ultraviolet and 2,4 xylenol methods

Absorbonces measured in 1cm cells
Nitrate-nitrogen (mg L −1 )

Type of water Ultraviolet method 2,4- Xylenol method

0.04 a

Butter Sandstone 0.04

Butter Sandstone 8.3 8.2

Butter Sandstone

0.005 a 0.04 a

Butter Sandstone 0.54 0.55

Butter Sandstone 4.5 4.6

Lincolnshire Limestone 7.9 7.9

Lincolnshire Limestone 5.8 5.5

River Thames, Chiswick 11.0 10.9

a 4cm cell
Source: Reproduced with permission from the Royal Society of Chemistry [405]

Miles and Espejoi [405] compared results for nitrate in ground water of low salinity by
the 2,4-xylenol spectrophotometric method and by ultraviolet spectroscopy. The
ultraviolet procedure is not subject to interference from the major cations present in
typical ground waters, and interference by high levels of nitrite can be overcome by
addition of sulphamic acid. There was good agreement between nitrate levels determined
by both methods on a number of ground water samples.
Nitrite, which absorbs in a similar part of the ultraviolet region is a potential source of
interference in this method. A solution containing 1mg L −1 of nitrite-nitrogen was
scanned between 190 and 260nm and the spectrum is shown in Fig. 2.26 together with
that of a 1mg L −1 solution of nitrate-nitrogen for comparison. The nitrite absorption