Anions in potable waters - Determination of Anions in Natural and Treated Waters - page 501

technique show that the method is precise.

Table 7.11 Nitrate concentrations determined by ultraviolet and 2, 4-xylenol methods

Absorbances measured in 1cm cells

Nitrate-nitrogen /mg L −1

Type of water

Ultraviolet method 2, 4-Xylenol method

0.04 a

Bunter sandstone 0.04

Bunter sandstone 8.3 8.2

Bunter sandstone

0.005 a 0.04 a

Bunter sandstone 0.54 0.55

Bunter 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 [46]

Rennie et al. [48] utilised absorbances at 210 and 275nm to indicate the presence of
nitrate and organic matter respectively. The absorbance at 275nm was used as an
indicator of the presence or absence of organic matter that could absorb also at 210nm.
Although some workers have corrected measurements of nitrate made at 210nm for
organic absorbance based on measurements at or near 275nm, this method of correction
becomes decreasingly reliable when low nitrate contents are being measured. Hence, in
the method described by Rennie et al. [48], organic matter is completely removed from
the sample prior to ultraviolet spectroscopy. This method is based on the observation that
at pH values above 12 one particular grade of carbon black ADC 33 (Sutcliffe Speakman
Co) will adsorb 98-100% of organic compounds from the water sample and, unlike many
other grades of carbon black, will retain no nitrate. An analytical grade carbon filter paper
(Schleicher and Schill No. 58) was also satisfactory in this respect and permitted organics
removal from the alkaline sample to be conducted in a filtration assembly. Nitrite
interference is overcome by the presence of sulphamic acid in the mixed reagent.
The effects of certain concentrations of the adventitious contaminants are summarised
in Table 7.12 and the concentration that gave an absorbance at 210nm equivalent to
0.02mg L −1 N is defined as the interference limit for the method (denoted by a
superscript 'a') in Table 7.12. The lack of an interference effect, compared with that
found by direct ultraviolet spectrophotometry, of the sodium salt of dodecylbenzene-
sulphonic acid is due to the effectiveness of removal of organic matter by the filter.
Whereas iron(II) and iron(III) interfere significantly in