The effect of sodium hypochlorite (NaOCl) on the decolorization of azo dye orange II (4-[(2-hydroxy-1-naphthalenyl) azo]-benzenesulfonic acid, monosodium salt) in aqueous solution was studied as a function of pH. The first-order rate constant of color reduction (k) was evaluated from the curve of color reduction in the initial stage of decolorization. The k values increased with increasing the pH from 5.4 to 9.3 and the NaOCl concentration from 2.82 × 10(-4) M to 1.13 × 10(-3) M. It was found that k depended on the concentration of OCl(-), but not on HOCl. On the other hand, k decreased markedly at pHs more than 10, depending on pH. This suggested that the reactivity of the orange II molecule with OCl(-) decreased probably due to the competitive action of the hydroxide ion under alkaline conditions. It could be concluded that controlling the solution pH and the OCl(-) concentration was indeed a key factor determining the rate of decolorization of orange II in aqueous solution.
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