Preconcentration techniques - Determination of Anions in Natural and Treated Waters - page 741

[14].
Parkow [15] acidified water samples containing chromate to pH 5 and passed them
upwards through an anion exchange resin, so that the chromate is adsorbed in a narrow
zone at the lower end of the resin bed. The chromate is eluted rapidly with small volumes
of an acidic reluctant solution which reacts with chromate on the column to form trivalent
chromium during elution, thus producing very high concentration factors.
Parkow and Janer [16] acidified water samples containing chromate to pH 5 and then
passed them upwards through a Dowex Ag 1-X4 anion exchange resin, so that the
chromate was adsorbed in a narrow zone at the lower end of the resin bed. The chromate
was eluted rapidly with small volumes of an acidic reluctant solution which reacts with
chromate on the column to form trivalent chromium during elution, thus producing very
high concentration factors. Amberlite LA liquid anion exchange resin has also been used
for this purpose [17].
Farag et al. [18] determined chromium(VI) (chromate) in water using 1, 5-
diphenylcarbazide-loaded polyurethane foam.
A particularly recent innovation that has been used for metals and also for chromate
preconcentration consists of a modification of the flow injection analysis technique
whereby the samples pass through a micro-column containing an adsorbent for the ion of
interest, thereby achieving a concentration factor. Following an automatic switch to an
acidic reagent, the adsorbed anion is desorbed in a sharp pulse of the flowing reagent and
then passes on to the detection system. Systems such as this, therefore, combine
preconcentration and automation. Syty et al. [19] separated chromium(III) from
chromium(VI) on a microcolumn of alumina which preconcentrated chromium(VI).
Chromium(VI) was then flushed from the column with a small volume of acid before
determination by inductively coupled plasma atomic emission spectrometry in amounts
down to 0.2µg L −1 .
Hexavalent chromium is reduced by diethyldithiocarbamate to trivalent chromium
[20,21], with which it forms an isobutyl methyl ketone soluble complex. Hexavalent
chromium is then determined in the solvent extract by atomic absorption spectrometry at
357.9nm.
Music et al. [22] used hydrous iron oxides to preconcentrate chromate.
Chromium(VI) has been preconcentrated by procedures based on coprecipitation with
ferric hydroxide [23]. The mechanism of the sorption of chromate ions onto amorphous
ferric hydroxide, ferric oxide and magnetite particles has been examined and the effects
of several variables including the pH of the solution examined and the presence of
competing ions (chloride, sulphate, molybdate) determined. The sorption of hexavalent
chromium onto hydrous ferric oxide can be explained in terms of ligand exchange
hydroxyl ions being released from the surface of the hydrated oxide particles. For
magnetic particles, however, the sorption effect was explained by the reduction of traces
of hexavalent chromium at the magnetite/water interface, as a result of which a very fine
ferric hydroxide/chromium hydroxide coating was produced which exhibited further
reduction of hexavalent chromium.
Fung and Dao [2] used Chelex-100 resin to remove anions from non saline waters.
They retained chromate. The chromate could then be stripped from the column and
determined in sub µg L −1 quantities.