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Sodium Hypochlorite - Bleach Stability and Filtration

By Gilbert Gordon, Miami University, Oxford, OH

Bernard Bubnis
Novatek, A Division of EBB, Inc.
Oxford, Ohio

Abstract

Liquid Sodium Hypochlorite (bleach) is an alternative to gaseous chlorine for water disinfection. Commercially produced liquid bleach typically contains excess caustic and possibly dissolved transition metal ions. In basic solution, bleach (OCL(^-) decomposition to form chlorate ion has been shown to be a second-order process (Rate = k₂[OCl^-]²). The formation of oxygen from decomposing bleach is a slow side reaction in solutions of very pure sodium hypochlorite. However, in the presence of transition metal ions, the rate of bleach decomposition by the oxygen pathway is increased.

The chemistry of the catalytic decomposition of bleach is complex. In general, nickel ion appears to effectively catalyze decomposition either alone or in combination with other transition metal ions (Ni > Co > Cu > Fe > Mn > Hg). The addition of Mn²⁺ and Ni²⁺ together produced the highest rate of decomposition when compared to other transition metal ion combinations. The initial rate of decomposition with 10.3 mg/L Cu²⁺ is increased by a factor of 11 relative to Sodium Hypochlorite (NaOCl) containing 1.03 mg/L Cu²⁺. This indicates that the decomposition of Sodium Hypochlorite is roughly first-order in Cu²⁺.

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