Sodium Hypochlorite 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 ClO3
- 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.
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- 6 ClO3
- + 2Cl-
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