Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 148

(Cr(VI)); 3 Cr(S 2 CN(C 2 H 5 ) 2 ) 3 ,(CR(III))
Source: Reproduced with permission from Frederick Wieweg and Son,
Wiesbaden [230]

room temperature with 5% sodium diethyldithiocarbamate. It is then extracted with
chloroform and analysed by reversed phase high performance liquid chromatography,
after the residue from evaporation of the chloroform has been dissolved in acetonitrile,
using methanol:water (65:35) as mobile phase. Detection was by ultraviolet at 254nm.
Corrections were made for interference between chromium(III) and chromium(VI) and
concentrations of 2-10mg L −1 of each ion were determined with accuracies of the order
of ±5%.
In this method chromium(II) chelates at pH 5.8 but not at pH 4.0.
Evaluation of the peaks shown in Fig. 2.16 produced with chromium(III) and
chromium(VI) standard solutions enables the concentration of these two species in
unknown water samples to be deduced.
The application of this technique is also discussed under multianion analysis in section
13.1.1.5.

2.19.8 Preservation of chromate containing samples

Stollenwirk and Grove [231] investigated the effect on the concentration of hexavalent
chromium of sample preservation techniques. Acidification of samples could reduce
hexavalent chromium to the trivalent form. The rate of such reduction was increased by
impurities such as nitric oxide present in nitric acid. Organic matter could also accelerate
reduction of hexavalent chromium in acidified samples, and could also reduce the
chromium even in unacidified samples. The best method for preserving hexavalent
chromium involved filtration and cooling of the sample. Storing the sample at 4°C might
preserve the concentration of hexavalent chromium for some time, depending on the
reactivity of other constituents in the sample, but should not be relied on. Measurement of
total chromium is recommended as a method of checking on chromium reduction.

2.19.9 Preconcentration

The preconcentration of chromium is discussed in sections 15.1.4 and 15.1.13.

2.19.10 Miscellaneous

Differential pulse voltammetry has been used to determine chromate and chromium(III)
[227,228].
C rosmun and Mueller [228] achieved a detection limit of 10µg L −1 chromate in non
saline waters. Rajendrababu and Nanda Kumar [232] have described a simple paper
chromatographic method for the rapid detection of chromate in non saline water in
amounts down to 2µg. Chemiluminescence with luminol peroxide [233] and with lophine
[234] have been used to determine low concentrations of chromate in water. The former
technique achieved a detection limit of 0.05mg L −1 and the latter 0.3µg L −1 without

preconcentration and 0.015µg L −1 with preconcentration.