Preconcentration techniques - Determination of Anions in Natural and Treated Waters - page 740

hydrochloric acid and redistilled water. The coprecipitation step for chromium(VI) was
carried out as soon as possible after sampling to minimise the sample storage problem.
Cranston and Murray [7,8] took the samples in polyethylene bottles that had been
precleaned at 20°C for 4 days with 1% hydrochloric acid. This acid had been previously
distilled to reduce metal impurity levels. Total chromium, Cr(IV)+Cr(III)+Cr p
(particulate chromium) was coprecipitated with iron(II) hydroxide and reduced chromium
(Cr(III)+ Cr p ) was coprecipitated with iron(III) hydroxide. These coprecipitation steps
were completed within minutes of sample collection to minimise storage problems. The
iron hydroxide precipitates were filtered through 0.4µm Nuclepore filters and stored in
polyethylene vials for later analyses in the laboratory. Particulate chromium, was also
obtained by filtering unaltered samples through 0.4µm filters. In the laboratory the iron
hydroxide coprecipitates were dissolved in 6mol L −1 hydrochloric acid and analysed by
flameless atomic absorption. The limit of detection of this method is about 0.1-0.2nmol
L -1 . Precision is about 5%.
Alumina has been used to preconcentrate chromate before determination by
inductively coupled plasma atomic emission spectrometry [9]. Trivalent chromium is not
adsorbed under these conditions. A detection limit of 0.2µg L −1

was achieved by this

procedure.
Use of immobilised chelating agents for sequestering trace metals from aqueous and
saline media presents several significant advantages over chelation-solvent extraction
approaches to this problem [10,11]. With little sample manipulation, large
preconcentration factors can generally be realised in relatively short times with low
analytical blanks. As a consequence of these considerations Willie et al. [12] developed a
new approach to the determination of total chromium. This involves preliminary
concentration of dissolved chromium from sea water by means of an immobilised
diphenyl-carbazone chelating agent, prior to determination by atomic absorption
spectrometry A Perkin-Elmer Model 500 atomic absorption spectrometer fitted with a
HGA-500 furnace with Zeeman background correction capability was used for chromium
determinations. Chromium was first reduced to Cr(III) by addition of 0.5ml aqueous
sulphur dioxide and allowing the sample to stand for several minutes. Aliquots of sea
water were then adjusted to pH 9.0 ±0.2 using high-purity ammonium hydroxide and
gravity fed through a column of silica at a nominal flow rate of 10ml min−1.
The sequestered chromium was then eluted from the column with 10.0ml 0.2mol L −1
nitric acid. More than 93% of chromium was recovered in the first 5ml of eluate by this
method. Extraction of 80ng spikes of Cr(III) from 200ml aliquots of sea water was semi-
quantitative.
Schaller and Neeb [13] extracted di(trifluoroethyl) dithiocarbamate chelates of
hexavalent and trivalent chromium and cobalt from aqueous solution at pH 3 using a
carbon-18 column. Adsorbed chelates were eluted with toluene before gas
chromatographic analysis with electron capture detection. Broadening of the peaks of
cobalt and chromium was reduced by arranging that the end of the capillary was 1cm
within the detector body, the base of which was isolated with glass wool and aluminium
foil. The last 0.2cm of the column was also protected with a sleeve of braided wire. The
detection limit for chromium(VI) was 0.05µg L −1. Various ion exchange resins have been
used to preconcentrate chromate ions including Dowex AG 1-X4 anion exchange resin