A highly selective technique to determine hexavalent chromium in electronic and electrical products for RoHS compliance

With the increasing concern about the toxicity of hexavalent Chromium (Cr⁶⁺) in electrical and electronic equipment under the restriction range of EU 2002/95/EC Directive, which is known as RoHS coming into force from 1^st in July 2006, a combined technique to determination of the special hexavalent chromium was studied in this paper. Firstly, by EDXRF screening, the presence of the Cr element or not was detected distinctly. The spot test as an easy and cheap method was subsequently adopted to identify the hexavalent state of chromium compounds because Cr⁶⁺ and Cr³⁺ normally coexist in the same component. After alkali digestion, Cr⁶⁺ was selected and separated from other compounds. By acidity adjustment with pH meter, Cr⁶⁺ was purified further and retained stability during the time of analysis. With a color reagent to chelated with Chromium (VI), then solution was directly introduced into UV-visible spectrophotometer. At the wavelength of 540 nm, a sharp absorption peak was displayed which is the basis of analyzing chromium (VI) quantitatively. Because of the combined application of these techniques, the probability of Cr⁶⁺ turning into Cr³⁺ was minimized and the sensitivity for Cr⁶⁺ was improved significantly. Some parameters affecting the absorbance intensity of spectrometry and the spiked recovery in the proposed procedure were studied. It was found that when pH is 2.0, extraction time is 60 min, mixture buffers of K₂HPO₄ and KH₂PO₄ are added up to 0.5M 1ml, and the temperature is 90degC, it is optimal for extraction. The volume of colorimetric reagent is 2 ml, the ligand efficiency with Cr⁶⁺ is ideal. Although there was interference of coexisting ions such as Fe³⁺, Pb²⁺ etc., it can be minimized by adding MgCI₂ in process of digestion. It was also found that the correlation coeff- icient of calibration curves is up to 0.999907 and the recovery rates are more than 85% in these optimal conditions. Then it can be concluded that the proposed method has a high selectivity and sensitivity to Cr⁶⁺ determination. It is more convincible and reliable method due to its relative standard deviation (R.S.D) being less than 1%.