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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
Oxford, OH 45056
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.
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-
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