The Detection of Hexavalent Chromium by Organically Doped Sol-Gels
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ABSTRACT The sol-gel process can be used to produce porous inorganic matrices that are doped with organic molecules. These doped gels can be used as a quantitative method for the spectrophotometric determination of trace concentrations of metallic ions. For the detection of hexavalent chromium, malachite green was used as the dopant. Preliminary results indicate concentrations on the order of 5 ppb are detectable using this method.
INTRODUCTION Hexavalent chromium [Cr(Vl)] along with many of its compounds are known to be highly toxic. Chromium compounds that are inhaled have been linked with lung cancer, while those ingested can cause kidney and liver damage [1]. Other studies have indicated that Cr(VI) can cause genetic mutations and modifications to the structure of DNA [1-2]. Concern over the toxicity of Cr(VI) has led the Federal Water Pollution Prevention Act to identify it as a Priority Pollutant and the U.S. Environmental Protection Agency to limit emissions to 5 parts per million (ppm) [3]. A number of local water districts, however, have imposed even lower limits, creating a need for more sensitive methods of detection. Two of the most common methods of Cr(VI) detection in water are atomic absorption spectrometry (AAS) and spectrophotometry using the organic reagent diphenylcarbazide. AAS has the disadvantage of being unable to differentiate between the different chromium oxidation states. Although diphenylcarbazide is specific for Cr(VI), this method can detect in the low ppb range only when coupled with a separation step like ion chromatography [4]. Recently, the use of organically doped sol-gels as a new technique for the spectrophotometric determination of trace elements has been reported [5]. The solgel procedure allows for the incorporation of organic molecules at room temperature into a porous, glass-like inorganic matrix. The sensors have achieved levels of detection down to the 50 ppt range for iron [6-7], which is two to three orders of magnitude higher than what is achievable by solution techniques alone. The high level of sensitivity is achieved because the ions are allowed to diffuse into the matrix of the gel where they react with the organic and remain trapped. This results in a concentration of the ions into the matrix of the gel.
329 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
Malachite green has been utilized -s a reagent for the detection of hexavalent chromium (in the form of dichromate, Cr2 0 7 2 -) by Parkash et al [8]. In the presence of acetic acid, malachite green (A+) and Cr(VI) react to form a colored complex according to the following proposed reaction: 2A+ + Cr2 0 7 2
--
A2 Cr 2 07
In addition to forming an unstable colored complex, the reaction results in a decrease of the malachite green absorbance peak. The amount of the absorbance change is dependent on the concentration of Cr(VI). This paper explores the use of malachite green doped sol-gels as a method for Cr(VI) detection.
EXPERIMENTAL DETAILS Apparatus and Chemicals Absorbance measureme
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