Chemistry Reference
In-Depth Information
More recent spectroscopic methods for measurement of nitrate are based on
measurements in the ultraviolet region.
Spectrophotometric methods based on the use of resorcinol [45] and 2, 4-xylenol [46]
have been described for the determination of nitrate in mineral waters and ground waters.
Miles and Espejoi [46] compared results for nitrate in ground waters of low salinity
obtained by the 2, 4-xylenol Spectrophotometric method and by ultraviolet spectroscopy.
The ultraviolet procedure is not subject to interference from the major cations present in
typical ground waters, and interference by high levels of nitrite can be overcome by
addition of sulphamic acid. There was good agreement between nitrate levels determined
by both methods on a number of ground water samples.
The samples are filtered through 0.45µm membrane filters on collection and acidified
by addition of 2ml of 50 vol% Arister sulphuric acid per L of sample.
Ultraviolet spectrophotometric procedure
Dilute the samples as necessary to bring their concentrations of nitrate into the range 0-
1.00mg L
−1
of nitrate-nitrogen and add sufficient 1% sulphamic acid solution to give a
final sulphamic acid concentration of 0.1% w/v if the concentration of nitrite in the final
solution is likely to exceed 20µg L
−1
of nitrite-nitrogen. Measure absorbances at 200nm
in 1.0cm cells.
2, 4-xylenol method
To 10ml of sample, or an aliquot containing not more than 300µg of nitrate-nitrogen, in a
100ml conical flask, 2ml of 2% mercury(II) sulphate and 0.5ml of 1% sulphamic acid
solutions are added; and the flask allowed to stand for 5min, then 1ml of 1% 2, 4-xylenol
is added to the solution and mixed. 24.5ml of 85 vol% sulphuric acid is added, while
cooling the flask in running water. The flask is placed in a water bath at 30°C for 30min,
then the contents are transferred to a 125ml separating funnel and rinsed in the flask and
the funnel shaken with two successive 25ml volumes of water. 10ml of toluene phases
are left for 3min. The phases are left to separate and the aqueous layer run to waste. 10ml
of sodium hydroxide (40gL
−1
) is added to the toluene extract, measured and shaken for
1min to separate. The sodium hydroxide layer is run into a 1.0cm cell and the absorbance
measured at 445nm. A set of standards in the range 1-20mg L
−1
of nitrate-nitrogen are
run in parallel.
Nitrate, which absorbs in a similar part of the ultraviolet region, is a potential source of
interference in this method. A solution containing 1mg L
−1
of nitrite-nitrogen was
scanned between 190 and 260nm and the spectrum is shown in Fig. 7.9 together with that
of a 1mg L
−1
solution of nitrate-nitrogen for comparison. The nitrite absorption
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
nitrite-nitrogen were prepared and the interference fromnitrite is significant only at
concentrations above 20µg L
−1
Interference by higher concentrations of nitrite can be
removed by the addition of 0.1% w/v of sulphamic acid.
Iron does not interfere at concentrations below 1000µg L
−1
and chloride is without
effect at concentrations below 100mg L
−1
.
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