Introduction - Determination of Anions in Natural and Treated Waters - page 80

1.2.3 Spectroscopic techniques
Spectrophotometric procedures have been employed for the determination of 35 anions.
However, in general, these procedures lack in sensitivity. Additionally, interference
effects can be a problem and it is necessary to exhaustively examine the effect of
potential interferants which consider using these methods.
In many cases spectrofluorimetric (19 anions) are surprisingly sensitive.
Chemiluminescence techniques have been applied to the determination of some seven
anions (cyanide, iodide, nitrite, nitrate, silicate, sulphite and vanadate).
The application of ultraviolet spectroscopy is extremely limited viz nitrite, nitrate,
phosphate and nitriloacetate, while the application of infrared spectroscopy and Raman
spectroscopy are limited to the determination of one anion (cyanide) and two anions
(nitrite and nitrate) respectively.
1.2.4 Flow analysis techniques
Other techniques which are frequently coupled with Spectrophoto-metric or atomic
procedures include absorption spectrometric flow injection analysis (26 anions),
segmented flow analysis (seven anions) and continuous flow analysis (nine anions).
These techniques all have the advantage of automation but suffer the same limitations as
direct spectrophotometry as regards sensitivity and selectivity.
1.2.5 Spectrometric techniques
The application of atomic absorption spectrometry and, more recently, inductively
coupled plasma atomic emission spectrometry to the deter-mination of anions has
involved a variety of techniques. Some 16 anions have been determined by atomic
absorption techniques listed below.
(a) Formation of ion pair complexes with malalite green and subsequent atomic
absorption spectrometry of the complex, phosphate, arsenate and arsenite.
(b) Formation of complex with benzoin oxime (tungstate) or diethyldithiocarbamate
(selenate, selenite, chromate and dichromate) followed by extraction of complex with
methyl isobutyl ketone and analysis of extract by atomic absorption spectrometry.
(c) Reaction of chloride ions with phenyl mercuric nitrite to produced phenyl mercuric
chloride and chloroform extraction of this compound, followed by atomic absorption
spectrometry of complex.
(d) Preconcentration columns of anion exchange resin for tungstate, or cupric sulphide
for cyanide, followed by desorption from the column of the tungstate or cuprocyanide
complexes and analysis by atomic absorption spectrometry.
(e) Formation of volatile derivative of anion eg reaction of borate with methanol to form
methyl borate and subsequent atomic absorption spectrometry of the solution. To
determine nitrate this anion is reduced to ammonia using Ti lll sulphate and the
ammonia is swept by an air current prior to gas phase molecular absorbance
measurements with an atomic absorption spectrometer.