Anions in aqueous precipitation - Determination of Anions in Natural and Treated Waters - page 433

with mercuric ions and 4-(2-pyridylazo) resorcinol and then absorbance at 500nm
recorded automat-ically as a function of time. The extinction after 30s is measured and
compared with iodide standards which have been treated in the same way.

5.1.6.2 Ion chromatography

The application of this technique is discussed under multianion analysis in section 12.4.2.

5.1.6.3 High performance liquid chromatography

The application of this technique is discussed under multianion analysis in sections
13.1.3.1 and 13.1.3.3.

5.1.6.4 X-ray fluorescence spectroscopy

The application of this technique is discussed under multianion analysis in section
14.7.1.1.

5.1.7 Nitrate

5.1.7.1 Spectrophotometric methods

A requirement of methods for the determination of nitrate in rainwater is that it has
sufficient sensitivity to determine the very low levels of this anion encountered in this
type of sample. With this in mind, Osibanjo and Ajaya [20] have modified the usual
distillation procedure for the determination of low levels of nitrate using 3, 4 xylenol as a
chromogenic reagent. In this method the nitration product is extracted with toluene. The
method is relatively free from interferences, is rapid and is highly sensitive, being
capable of determining down to 5µg L −1 nitrate in unpolluted surface and ground waters
and rain with a recovery rate of 96-108%.
Nitration of 3, 4-xylenol by nitrate is carried out instantaneously at 0°C in 80%
sulphuric acid and the nitration layer extracted into toluene. The toluene layer is treated
with sodium hydroxide to form a coloured product in the aqueous layer, the absorbance
of which is measured at 432nm. Between 0.5 and 2.2mg L −1

of nitrate was found in

rainwater samples by this procedure.
Spectrophotometric methods, based on the use of diphenylamine or diphenylbenzidine
[21], and xylenol [20] have been used for the determination of nitrate in rainwater. The
methods using amines are insufficiently sensitive for rainwater analysis (LD 0.5mg L −1 )
whereas the 3, 4-xylenol method is capable of analysing at the 5µg L −1 nitrate level. In
this method, nitration of 3, 4-xylenol by nitrate in the sample is carried out
instantaneously at about 0°C in 80% sulphuric acid and the nitration product is extracted
into toluene, the excess of the reagent remaining in the aqueous layer. The toluene layer
is then treated with sodium hydroxide solution to form a coloured product (the sodium
salt of the nitrophenol) in the aqueous layer, the absorbance of which is measured at
432nm.