Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 139

13.2.1.3.

2.17.16 Ion exchange chromatography
Akaiwa et al. [140] have used ion exchange chromatography on hydrous zirconium oxide
combined with a detection based on direct potentiometry with an ion selective electrode
for the simultaneous determination of chloride and bromide in non saline waters.

2.17.17 Gas chromatography
Belcher et al. [204] determined chloride by a method based on reaction with phenyl
mercuric nitrate to produce phenyl mercury chloride, followed by carbon tetrachloride
extraction of the latter and gas chromatography. Down to 0.4 mg L −1 chloride in the

sample could be determined.

2.18 Chlorobenzoate

2.18.1 Ion chromatography
The application of this technique is discussed under multianion analysis in section 12.2.5.

2.19 Chromate and dichromate

Differentiating between the two most frequent oxidation states of chromium is
fundamental in speciation studies of this element on account of the toxicity of hexavalent
chromium. In fact, most of the works on this element published so far deal solely with
this aspect [205]. Flow injection analysis has been used several times for determining this
element in one [206,207] or in its two most common oxidation states [208-214].
Nevertheless, it is known that there is a large variety of chemical forms of Cr(III) and
Cr(VI), which show different properties depending on the groups or elements to which
they are bound. Their relative abundance is a function of the type of medium. Since the
pH is a major parameter in any type of solution (especially in aqueous ones), and taking
into account that its value dictates the predominance of a particular species of Cr(III) and
Cr(VI) (hydroxylated, dimer etc), the following species of chromium must be considered
to be present in waters. Chromium (III): Cr(H 2 O) 6 3+ , Cr(OH) 2+ , Cr(OH) 2 + , Cr(OH) 3 , Cr
(OH) 4 − . Chromium(VI): H2 C rO4, HCrO 4 − , Cr 2 O 7 2− , CrO 4 2− . Among these species, Cr
(OH) 3 is very insoluble, its concentration in water being negligible, and H 2 CrO 4 is a
relatively strong acid (pK a =−1.0), which is unlikely to occur in water. Consequently the
following equilibria, with their respective constants, must be considered:
C hromium(VI)