Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 191

2.62 Nitrate
The quantification of nitrate is important for water quality evaluation in terms of
agricultural, industrial and biological pollution. Moreover, the potential health risk from
nitrate, especially from its chemically and biologically active toxic metabolite nitrite,
leads to increased stringency in nitrate monitoring of waters [379-381]. Owing to the
toxicity (risk of anemia, formation of carcinogenic nitrosamines), the nitrate content of
drinking waters has been regulated by national authorities and by the European
Community Directive 80/778/EEC, which states a maximum admissible concentration
(MAC) of 50gm of NO 3 − L −1 [382]. This directive gives also a guide level of 25mg of
NO 3 − L −1 . The US Environmental Protection Agency (USEAP), based in the World
Health Organization (WHO) guideline (1984), adopted a MAC value of 45mg of NO 3
− L −1 . The WHO states also a guideline value of 10mg of NO 3 − L −1 for water intended for
bottle fed infants [381].
Achieving good analytical measurements capability for water analysis seems to be a
necessity with 'Europe 93' and the ratification of the Maastrict Treaty [383]. It is of
political importance that quality and camparability of measurement results be improved.
Political and legal decisions, such as whether or not water is too polluted for
consumption, are based on chemical measurements.
The Institute for Reference Materials and Measurements' (IRMM) International
Measurement Evaluation Programme (IMEP) [383,384] is intended to provide pictures of
the 'state-of-the-practice' in the assay of toxic or life-essential elements of species (in this
case nitrate) in the assay of toxic or life-essential elements or species (in this case nitrate)
in different matrices compared to a 'certified' reference value obtained by an
isotopespecific method, namely isotope dilution mass spectrometry (IDMS).
2.62.1 Spectrophotometric methods
Reduction of nitrate to nitrate by copper activated cadmium followed by estimation of
nitrite by one of the methods discussed in this section, usually diazotisation coupling
methods, is one of the most widely used methods for the determination of nitrite [385-
387].
When it is necessary to analyse a large number of low volume, highly coloured pore
waters, the usefulness of the cadmium reduction method is limited because:
(1) either 100ml of sample is required (Strickland and Parsons [388]), or nitrate levels
must be high enough to allow for dilution of 25ml of sample with75 ml of ammonium
chloride solution [385],
(2) analyst time is excessive for single sample manipulation and maintence of
recommended flow rate (7-10ml min −1 ) and
(3) interferences progressively decreased column efficiency [386].
Jones [389] has described a cadmium reduction method which avoids the foregoing
problems by shaking the sample with spongy cadmium as an alternative to the use of
cadmium columns.