Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 219

Crumpton et al. [460] give details of equipment and procedure for determination of
nitrate, based on alkaline reduction with titanous chloride in gas-tight reaction vials and
determination of the ammonia produced by means of gas-sensing ammonia probes. The
method was simple and reliable and could be applied to very small sample volumes.
Results obtained by this method and by the cadmium reduction method are compared.
Various other workers have studied the application of ion selective electrodes to the
determination of nitrate in non saline waters [451,461-466].
2.62.10 Ultraviolet spectroscopy
Munoz [467] adapted the ultraviolet procedure for the determination of nitrate to an
automatic discrete-sample system, the AMA-40 system for the determination of nitrate in
waters at low mg L −1 levels. Results indicate that the automated system is simple to set
up and is easily applicable to routine determinations of nitrate in water samples, giving
excellent repeatability of results.
Brown and Bellinger [468] have proposed an ultraviolet technique which is applicable
to both polluted and unpolluted fresh waters and some estuarine waters. Humic acid and
other organics are removed on an ion exchange resin. Bromide interference in sea water
samples can be minimised by suitable dilution of the sample but this raises the lower
limit of detection such that only on relatively rich (0.5mg L −1 nitrate) estuarine and
inshore waters could the method be used. Chloride at concentrations in excess of
10,000mg L −1 does not interfere.
The method is either not affected by, or can allow for, interference from phosphate,
sulphate, carbonate, bicarbonate, nitrite, coloured metal complexes, ammonia, dyes,
detergents, phenols and other ultraviolet absorbing substances.
The method incorporates three features designed to reduce interferences.
(a) Humic acid interference is reduced by carrying out measurements at 225nm, a higher
wavelength than that used by previous workers (210-229nm).
(b) Removal of inorganic interferences, particularly the removal of bromide interference
in sea water, by diluting the sample fivefold, the removal of nitrite by the addition of
sulphamic acid and the removal of metals by passage through Amberlite IR120 cation
exchange resin.
(c) Removal of ultraviolet absorbing organics by passage through a specific ion exchange
resin such as Amberlite XAD-2.
In Table 2.36 nitrate determination by this method in the presence of various interfering
substances is compared with two alternate methods, namely phenoldisulphonic acid and
ion selective electrode methods. It is seen that, in general, the method proposed by Brown
and Bellinger [468] is less subject to interference.
Saito et al. [469] used ultraviolet spectrophotometry to determine nitrate in non saline
waters, interference by nitrite was eliminated by ion chromatography.
2.62.11 Mass spectrometry
Heumann and Unger [470] have described a mass spectrometric determination of nitrate