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(T/EH107) Automatic determination of Chromium in sewage by Flow Injection-Flame Atomic Absorption Spectrometry. Hernandez-Cordoba, Manuel1, Perez-Sirvent, Carmen 1, Lopez-Garcia, Ignacio1, Merino, Beatriz1, Aguinaga, Nerea1, 1 ABSTRACT- The concentration of total chromium in river water samples obtained in areas surrounding sites in which industrial wastes are discharged can reach a high value, even exceeding 150 micrograms/mL. For waters with a pH value in the 8-9 range, the content in soluble chromium, i.e., that obtained after filtering the sample through a 0.45 micrometer filter, is usually below 10 micrograms/mL. Taking into account that the organic matter content of these samples is high, the medium is reductor and, consequently, the state of oxidation of the chromium is the trivalent one, most of the metal being precipitated in colloidal form. The conventional determination of the total chromium content by FAAS involves a previous treatment of the sample with nitric and sulphuric acids or nitric acid and hydrogen peroxide until almost dryness. Next the residues are diluted up to volume with 1% nitric acid. The entire procedure is time-consuming. To determine the fraction of chromium considered as soluble, the sample must be first microfiltrated (a tedious stage) and then the liquid phase mineralized, being the entire procedure also time-consuming. The aim of the work here presented is to develop a procedure for chromium determination in a semi-automatic, continuous way, allowing a saving in time and efforts. To this purpose, the possibility of avoiding the tedious mineralization stage with acids has been studied. The flow injection manifold here optimized allows the sample to be directly injected, the serial determination of both soluble chromium and total chromium being obtained in a fast and reliable way. A manifold with two tubes for carrier has been proposed. As the contents for soluble and total chromium are very different, the system includes a device that allows important dilutions to be automatically obtained. A double calibration is carried out, one of them being referred to the sample filtered on-line and the other to the unfiltered sample. A number of samples corresponding to wastes from tanner industries of our region were obtained and analyzed following the on-line procedure. The results agreed with those obtained by applying the conventional mineralization-based tedious procedure. Key words: sewage, chromium, FAAS, AAS |
