Preconcentration of Heavy Metal Ions onto Metals with High Melting Points for Graphite Furnace Atomic Absorption Spectrometry

Abstract

For direct determination of heavy metal ions at ppb level by graphite furnace atomic absorption spectrometry (GFAAS), spontaneous preconcentration of these ions onto the surface of metals with high melting points such as tungsten, molybdenum, tantalum and rhenium ha s been studied. Effect of experimental parameters such as soaking time, chemical pretreatment of metal wires, pH and temperature of the sample solution on the absorbance gignal was investigated. This approach was found to satisfy required conditions for an analytical metho d, because a quite good linear relationship was obtained between the absorbance signal an d soaking time of metal wire or the concentration of sample solution in a limited concentration r a nge peculiar to the analyte ions.
Taking the preconcentration ability of heavy metal ions, chemical stability against acids and bases, durability and melting points into consideration, tungsten, molybdenum and rheniu m wire showed a good property in this method and tantalum wire was inferior to other t hree in the preconcentration ability. The main mechanism of preconcentration of heavy metal ion s onto tungsten and molybdenum was supposed to be cation exchange reaction with hydrat e d oxides present on the metal surface. As a result of investigation by X-ray diffracto m etry, however, Ag⁺ and Cu⁺² were preconcentrated by the deposition of these ions in metallic state in addition to cation-exchange reaction. Preconcentration of Pb" occurs by the formation of compound such as PbWO₄ or PbMoO₄.
As an example of the applica t i o n of this analytical technique to practical samples the determination of copper in sodium nitrate was carried out. The results obtained agreed well with those obtained by the solvent extraction method. This method is considered to be suitable for the determination of trace amount of heavy metal ions in simple solutions such as river wat e r or alkali salt solutions.