Strategies for Meeting Sodium Hypochlorite Specifications in the Drinking Water Industry

Strategies for Meeting Sodium Hypochlorite Specifications in the Drinking Water Industry

Gilbert Gordon
Department of Chemistry
Miami University
Oxford, OH 45056

Abstract

Sodium Hypochlorite (bleach) manufacturers are now frequently required to provide high quality sodium hypochlorite with limits on chlorate ion and transition metal ions. Sodium hypochlorite "decomposes" by two mechanisms. The first is the 2^nd order process that forms chlorate ion

3OCl^- → ClO₃^- + 2 Cl^-

In the presence of transition metal ions, decomposing bleach forms oxygen

3OC^- + OCl^- → O₂ + 2Cl^-

Large municipalities are requiring that delivered sodium hypochlorite (9 to 16 wt% NaOCl) have between 0.1 - 0.4 wt% excess caustic, <1,500 mg/L ClO₃^-, <0.5 mg/L iron and <0.05 mg/L nickel and copper. Important considerations for minimizing ClO₃ formation include: pH (i.e. excess caustic), dilution (decomposition is 2^nd order with respect to OCl^-), and temperature control.

Sodium Hypochlorite filtering produces stable, high quality bleach by removing inert sediments, which impart off-color and turbidity to the bleach. Filtration with the proper filter-aid materials can be used to precipitate soluble metal ion species of Fe, Ni, and Cu and helps to reduce the coating of pumps/piping and the accumulation of heavy metal sludge on tank bottoms. It also reduces the potential for the catalytic formation of oxygen and pressure build-up in bleach containers. The analysis of 15% bleach before and after filtration shows a reduction of iron from 2-3 ppm to <0.3 mg/L. The same kind of reduction was observed for nickel ion (0.5 to <0.01 ppm).

Introduction

Sodium Hypochlorite loses its strength by two decomposition pathways. The more dominant pathway leads to the formation of chlorate ion. A slower, second bleach decomposition pathway leads to oxygen formation.

Chlorate Ion (ClO₃) Formation:

The rate of bleach decomposition between pH 11 and 13 is: rate = k₂[OCl^-]² with the following stoichiometry:

3OCl^-→ ClO₃^- + 2Cl^-

The decomposition of OCl^- involves chlorite ion (ClO₂^-) as an intermediate in the following generally accepted mechanism:

OCl^- + OCl^- → ClO₂^- + Cl^- OCl^- + ClO₂ → ClO₃^- + Cl^-

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