Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 213

limit and dynamic range for nitrate were very similar to those for nitrite. The detection
limit was defined as the signal level corresponding to twice the standard deviation of the
blank signals.
A s can be seen in Table 2.30, the N 2 + main system bandhead at 391.4nm provided the
lowest detection limit, while the N 2 'second positive' system bandhead at 337.1nm and
the nitrogen atomic line at 746.8nm were observed to have wide dynamic ranges (more
than three orders of magnitude). The emission signal of the NH band maximum ('A'
system) at 336.0nm was the largest of four analytical wave lengths but the detection
limits of this band, especially for ammonium nitrogen were not so good because of the
significantly large blank signals at 336.0nm. The relative standard deviation of eight
replicate measurements was about 7% at the 10-fold concentration of the detection limit
and was about 4% at the 100-fold concentration for all the analytical wavelengths.
The effect of various concomitants possibly coexisting in water samples was
investigated. The samples containing 0.4mg L −1 ammonium nitrogen or 0.3mg L −1 nitrate
nitrogen were analysed in the presence of 100-20000mg L −1 of the inorganic ions. Of the
organic compounds, amino acids and urea were examined where the analysis was done in
the presence of 0.5-100-times concentration (in molar concentration) of the
concomitants. The results, summarised in Table 2.31, show that only a few inorganic ions
and organic compounds interfere with nitrogen analysis by helium microwave induced
plasma emission spectrometry.
The results of nitrate determinations in pond water quoted in Table 2.32 show that
results obtained by this procedure are consistent with those obtained by
spectrophotometry. No nitrite was found in the sample by either method.
2.62.8 Isotachoelectrophoresis
The application of this technique is discussed under multi anion analysis in section
14.6.1.1.
2.62.9 Ion selective electrodes
Schechter and Gruener [452] developed a nitrate-selective electrode and applied it to
highly mineralised borehole waters. Results obtained by this procedure were compared
with those obtained by the standard phenoldisulphonic acid spectrophotometric method.
These workers concluded that the electrode method is satisfactory under laboratory
conditions but not as a field method because of the detailed calibration and complex
controls required.
To evaluate the net influence of most common anions occurring in water, Schechter
and Gruener [452] made measurements in a variety of synthetic solutions containing
1.50mg L −1 nitrate-nitrogen and varying amounts of nitrite, hydrogen carbonate,
carbonate, chloride and sulphate. The results are presented in Table 2.33.
It is observed that the level of interfering ion, the total ionic strength and the level of
nitrate do influence the measured potential. For most anions occurring in water, there are
major additive influences from nitrite and chloride.