Analytical Sciences

Oxidation of Chromium(III) by Free Chlorine in Tap Water during the Chlorination Process Studied by an Improved Solid-Phase Spectrometry

Sulistyo SAPUTRO,*1 Kazuhisa YOSHIMURA,*2 Kô TAKEHARA,*2 Shiro MATSUOKA,*3 and NARSITO*4

*1 Department of Mathematics and Natural Science Education, Faculty of Teacher Training and Education Science, Sebelas Maret University, Surakarta-57126, Indonesia
*2 Department of Chemistry, Faculty of Sciences, Kyushu University, Hakozaki, Higashi, Fukuoka 812-8581, Japan
*3 Department of Environmental Science, Faculty of Science, Niigata University, Ikarashi, Niigata 950-2181, Japan
*4 Department of Chemistry, Faculty of Mathematics and Natural Sciences, Gadjah Mada University, Yogyakarta-55281, Indonesia

The oxidation of Cr(III) at naturally-occurring concentration levels, i.e., μg dm⁻³ or lower levels, by free chlorine during the chlorination process of tap water was studied using an improved solid-phase spectrophotometric method, which can be directly applicable to the specific determination of Cr(VI) at μg dm⁻³ or lower levels. The effect of the pH on the oxidation kinetics was investigated under three different pH conditions. The results showed that free chlorine oxidized the Cr(III) to Cr(VI), following the pseudo-first-order kinetics with half lifetimes of 3.0, 3.3 and 14.4 h at pH 5.0, 7.0 and 8.0, respectively, if the hypochlorite concentration was maintained at 4 mg Cl dm⁻³.

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