Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 245

2.64.11 Ultraviolet spectroscopy
Suzuki and Kuroda [574] and Koida et al. [575] used ultraviolet spectroscopy for the
direct simultaneous determination of nitrite and nitrate in non saline waters.
The sum of nitrite and nitrate concentrations in non saline waters have been determined
by measuring the absorbance of a solution at 219nm, the isosbestic point for these two
species [576]. Beer's law is obeyed over the concentration range of 0-2.0mg L −1 (nitrate
plus nitrite) and the coefficient of variation of the method is 4.2-7.5%.
2.64.12 Resonance Roman spectroscopy
Furaya et al. [577] investigated the simultaneous determination of nitrite and nitrate using
resonance Raman spectroscopy. Fig. 2.34 shows an example of the spectra of the sample
solutions which contain much larger quantities of nitrate relative to those of nitrite. This
spectrum demonstrates that the key band of nitrate can be measured almost independently
of the adjacent band of the coloured product corresponding to nitrite. This result shows
clearly the possibility of the simultaneous determination of nitrite and nitrate and also
proves the excellent selectivity of Raman spectrometry.
P lots of I NO3- /I NO2− against C NO3− /C NO2- give the most satisfactory result (I NO3− ,
I NO2− are peak heights of the nitrate band at 1045cm −1 and that of the band at 1056cm −1
corresponding to nitrite, respectively; C NO3− , C NO2- are concentrations of nitrate and
nitrite) I NO3- and I NO2- are estimated as shown in Fig. 2.34. The plots of peak heights or
areas of the 1045cm −1 band against C NO3- does not give such satisfactory results
regardless of the selections of base lines. Thus the process of the simultaneous
determination of nitrite and nitrate is as follows.
(1) Sample waters are treated to form the coloured product from nitrite by the colour
reaction
(2) From the resonance Raman band of the coloured product at 1250cm −1 , the
concentrations of nitrate are determined.
(3) From the spectra in the 1030-1080cm −1 region I NO3− /I NO2− is measured, and, using
the analytical curve shown in Fig. 2.34 C NO2 - is estimated by operation (2), and the
concentrations of nitrate (C NO3- ) are determined.
2.64.13 Helium microwave induced emission spectrometry
Ultratrace amounts (µg L −1 ) of nitrite, nitrate and ammonium have been determined
[451] in pond waters by atmospheric pressure helium microwave induced emission
spectrometry utilising a gas generation technique. The instrumental set up consists
primarily of the plasma system, the optical measurement system and the gas generation
system. Ammonium ion is oxidised to nitrogen gas by sodium hypochlorite in an alkaline
medium and nitrite ion by sulphonic acid in an acidic medium. Nitrate ion is reduced to
nitrite in a cadmium/copper column and then determined as above. The nitrogen
produced by the chemical reduction is introduced into the helium flow via a three-way
valve system connecting the reaction vessel. Plasma is generated by a microwave