Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 204

maximum is at about 211nm and it absorbs less strongly than nitrate at a similar
concentration. Solutions containing 0-5mg L −1 of nitrate-nitrogen and up to 100µg L −1 of
nitrate-nitrogen were prepared and the interference from nitrite is significant only at
concentrations about 20µg L −1 . Interference by higher concentrations of nitrite can be
removed by the addition of 0.1% m/v of sulphamic acid.
Iron does not interfere at concentrations below 100µg L −1

and chloride is without

effect at concentrations below 100mg L −1 .
In Table 2.26 are compared results obtained for nitrate by ultraviolet spectroscopy and
spectrophotometry. Good agreement was obtained for all groundwater samples analysed.
Reduction with hydrazine has also been used to convert nitrate to nitrate as an
alternative to cadmium reduction [406,407].
N-phenylnthranilic acid forms an intensely coloured oxidation product by reaction with
nitrate in a concentrated sulphuric acid medium and this has been used for determining
nitrate concentrations in non saline waters[408]. The colour is stabilised by addition of
phosphoric acid. Beer's law is obeyed in the concentration range of 0.0-0.35µg of nitrate
mL −1 and Sandell's sensitivity is 0.0003µg/cm 2 .
A spectrophotometric determination of nitrate in non saline waters has been reported
by Harada [409]. The water sample is first treated with alum and sulphamic acid solution
to remove iron and manganese; the supernate is then acidified with hydrochloric acid and
treated with sulphamic acid solution to decompose nitrite. The nitrate in the sample is
reduced to nitrite with hydrazine sulphate and copper-zinc catalyst; the nitrite is then
reacted with sulphamic acid and N -1-naphthylethylenediamine and measured
colourmetrically. The relative standard deviation was 6.0-94% for nitrogen
concentrations in the range of 0.1-1.0mg L −1 .
Garcia Mellian and Cangas Rancaro [410] and Ferrer Saliente [411] described other
colourmetric determinations of nitrate in non saline waters. The standard deviation of the
latter method is 0.017 and the relative error is 2.16%.
Automated cadmium reduction methods for the determination of nitrate have been
reviewed [412],
A study has been carried out of the accuracy of determination of total oxidised nitrogen
(nitrate plus nitrate and nitrite in river waters [413].
Airey et al. [414] have described a method for the removal of sulphide prior to the
determination of nitrate in anoxic estuarine waters. Mercury(II) chloride was used to
precipitate sulphide from samples of anoxic water. The sulphide free supernatant liquid
was used to estimate sulphide by measuring the concentration of mercury(II). Nitrate was
determined by a spectrophometric method in amounts down to 1µgL −1 .

Direct spectrophotometric methods
Apart from cadmium reduction of nitrate to nitrite, other direct spectrophotometric
methods for the direct determination of nitrate have been described and are reviewed in
Table 2.27. None of these methods, however, has gained the popularity enjoyed by
reduction procedures especially those employing cadmium.
The application of this technique is also discussed under multianion analysis in section
14.2.1.1.