Developpement of a simple method for rapid determination of As(III) and total As in drinking water
Résumé
Inorganic arsenic, a term which encompasses both As(III) and As(V) species, constitutes the highest toxicological risk associated with arsenic in water opposite to organic arsenic species. Besides, Arsenic standard in drinking water has been lowered from 50 to 10 µg.L-1. Nevertheless, the degrees of toxicity for As(III) and As(V) are so different that it is not sufficient to determine the total content of arsenic in a given sample in order to estimate its physiological or environmental risks. Therefore, speciation of arsenic in water sample shows an increasing interest. In this work, a simple and rapid procedure was developed for the direct determination of As(III) in drinking water by Hydride Generation Atomic Absorption Spectrometry (HG-AAS) without pre-reduction of As(V). The principle of this method is to volatilise arsenic from As(III) to arsine AsH3(g) according to an apparatus equipped with a gas-liquid separator and a quartz cell. Nevertheless, if determination of total arsenic by HG-AAS technique using sodium borohydride (NaBH4) and hydrochloric acid (HCl) is well known, the application of the method for As(III) estimation is more difficult. Indeed, it is important to define a pH range in the tri-junction point of sample borohydride-acid. So, in order to overcome this problem, citric acid has been used for selective hydride formation of As(III). The following parameters were evaluated: first, the needed quantity of citric acid and NaBH4 to achieve complete reduction of As(III) without reduction of As(V); next, the carrier flow rate and the time necessary to perform the best arsine formation. Finally, measurements were performed in de-ionised water and artificial water but also in natural water. The optimised protocol for As(III) determination in drinking water was the next one: the sample was mixed with 0.2 M citric acid and 0.1% (w/v) NaBH4, this last one prepared in a 0.2% (w/v) NaOH solution. The flow rates were of 6.5, 1.0 and 1.0 mL.min-1for sample, NaBH4 and citric acid respectively. The optimised pH range obtained in the tri-junction point was of 2.5 - 3.5. This method was shown to be satisfactory for determination of traces arsenic in water samples. Thus, a calibration range was obtained for As(III) concentration between 0.5 and 20 µg.L-1 with a detection limit of 0.1 µg.L-1 and a quantification limit of 0.3 µg.L-1. A relative standard deviation value, based on three replicate measurements, below 3% was attained. Recovery studies of different added concentrations of As(III) in artificial and natural water samples were between 96 and 105%. The essay of a certified reference material solution was 5.2 ± 0.3 µg.L-1 (certified value 5.0 ± 0.1 µg.L-1).