Anions in wastewaters - Determination of Anions in Natural and Treated Waters - page 554

spectrophotometer. In order to improve the S/N ratios of the spectra, the data were
accumulated and the number of repetitive scans was determined according to the noise
levels for each of the samples. The sample solutions were illuminated with the 488nm
line at 200mW output. A mirror served to double pass the laser beam through a
cylindrical sample cell (0.3ml) Pyrex glass. In order to obtain the largest and most
reproducible intensities, it is important that the mirror system is adjusted carefully in such
a way that the scattering lights focus into a clear image on the entrance slit. As a key
band, the most intense Raman line at 1045cm −1 was used.
In the case of waste and treated waters, the detection limit becomes about an order of
magnitude higher than that in the case of pure water because of the strong luminescence
of water samples. Furaya et al. [71] demonstrated that the addition of potassium iodide as
a quencher makes the background markedly smaller. By this procedure, the sensitivity
becomes comparable to that obtained in the case of pure water samples.
A plot of the background intensifies against the time of laser beam irradiation after the
addition of various inorganic sails shows the effectiveness of potassium iodide.
Furaya et al. [71] recommended that Raman spectra were measured on sample
solutions which were then exposed to the laser beam for about 30min.

8.15 Nitrite

8.15.1 Spectrophotometric method
Various Spectrophotometric procedures have been described for the determination of
nitrite in waste waters [73-75]. In the Spectrophotometric method [73] based on the
reaction between Griess Romijn reagent (ie the reaction of nitrite with a primary aromatic
amine to form a diazonium salt, which is then coupled with another aromatic compound
to form an azo dye whose absorbance is measured) and nitrite, interference by iodide,
sulphide thiosulphate, sulphite or tetrathionate is avoided by the addition of mercuric
chloride solution.
Nakamura and Mazuka [74] extracted nitrite in an ethyl acetate solution of 4, 5-
dihydroxycoumarin. Beer's law was obeyed up to 0.75mg L −1 . Only iodide, sulphide,
iron(III) and chromium ions interfered seriously at the 10µg L −1

level. Results agreed

well with those obtained using the diazotising coupling method.
Koupparis et al. [75] have described procedures for the determination of nitrite using
an automatic microprocessor-based stopped-flow analyser. The reaction is based on
diazotisation of sulphanilamide, the product being coupled with N -(1-naphthyl)
ethylenediamine dihydrochloride to form a coloured azo dye which is measured at
540nm. The analysis should be carried out on fresh samples to avoid bacterial conversion
of nitrite to nitrate or ammonia. However, samples can be preserved for 1-2 days, either
by freezing at −20°C or by addition of mercuric chloride and storage at 4°C. The methods
are fast, sensitive, accurate and precise, and without serious interference. The sample
throughput for routine analysis can be up to 360 samples per h in the range 0.025-2.00mg
L −1 of nitrite-nitrogen.
The automated microprocessor-based stopped flow analyser has been described by