Separation and preconcentration of Cr(Vi) as ion associate using solid phase extraction

A method of separation and preconcentration of Cr(VI) was developed based o n sorption on modified silica gel (C18) of an ion associate of Cr(VI) anion with a quaternary base. The study was performed with the following quatern ary onium salts: [1-(ethoxycarbonyl)-pentadecyl]trimethylammonium bromide, 1-hexadecylpyridinium chloride, benzyl(dodecyl)dimethylammonium bromide, bu tyl(triphenyl)phosphonium bromide and tetraphenylarsonium chloride. Benzyl( dodecyl)dimethylammonium bromide was found the optimum ion-pairing reagent. Sample containing Cr(III) and Cr(VI) in the presence of 0.005 mol/l of ben zyl(dodecyl)dimethylammonium bromide was pumped with a peristaltic pump thr ough the column containing the sorbent. The optimum pH range 4-5 was mainta ined with 0.05 mol/l phosphate buffer. Elution was accomplished using 95 vo l.% ethanol and the recovery of Cr(VI) was (96 +/- 6)% in the concentration range 0.005-1 mg/l of Cr(VI); even in a tenfold excess of Cr(III) the reco very of Cr(VI) was 99.8% with the relative standard deviation of repeatabil ity about 2.4% Cr in the eluate was determined by emission flame spectromet ry (Cr I 425.435 nm) in an air-acetylene or N2O-acetylene flame with the li mits of detection 10 or 2 ng/ml, respectively. Hence, with a typical precon centration factor of 200, the limits of detection in natural aqueous sample s were 50 and 10 pg/ml, respectively. Calibrations were linear at least up to 10 mg/l.