Sodium Hypochlorite AWWA Specifications

Paper presented at AWWA 2000 Annual Conference & Expo Denver, Colorado

June 14, 2000

Gilbert Gordon* and Bernard Bubnis
Department of Chemistry
Miami University
Oxford, OH 45056

Introduction

With increasing frequency, the purchasers of bleach are requiring a higher quality product. This means that issues like low chlorate ion levels, minimal suspended solids, and negligible oxygen build-up are important. Specifically, water utilities are requiring the delivery of high quality bleach (NaOCl) with upper limits on chlorate ion (ClO3-) and transition metal ions. The amount of ClO3- present in liquid bleach is an indicator of bleach decomposition. The presence of transition metal ions also leads to bleach decomposition however oxygen is formed instead of ClO3-.

Large municipalities are requiring that delivered bleach (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. As a consequence, bleach manufacturers are making adjustments to their bleach production facilities. For example, important considerations for minimizing ClO33 - formation include:

  • pH (i.e. excess caustic)
  • dilution (decomposition is 2nd order with respect to OCl-)
  • and temperature control.

To minimize the problems caused by the presence of transition metal ions, manufacturers are filtering the bleach. This process not only reduces the concentration of transition metal ions in the bleach but also removes inert sediments that impart off-color and turbidity to the bleach. Filtration with the proper filter-aid materials can be used to remove submicron particles of the various species of Fe, Ni, and Cu and help to reduce the coating of pumps/piping and the accumulation of heavy metal sludge on tank bottoms.

Sodium Hypochlorite Decomposition

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

Chlorate Ion (ClO3-) Formation

Bleach (OCl-) decomposes between pH 11 and 13 behaves according to a second order rate law:

Rate = k2 [OCl-]2 with the following stoichiometry:

2 3OCl- 6ClO3- + 2Cl-

More Information:

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