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10.26 CHAPTER TEN

tion of sodium hydroxide, whereas the reaction of chlorine gas with water increases the
hydrogen ion concentration (pH decrease), forming hydrochloric acid.
In most waters, these differences are not significant, but when high chlorine doses are
used in poorly buffered waters, these effects should be considered. They can be evaluated
by calculation or by simple laboratory tests.
In the commercial trade, the concentration of sodium hypochlorite solutions is usually
expressed as a percentage. The trade percent is actually a measure of weight per unit vol-
ume, with 1% corresponding to a weight of 10 g of available chlorine per liter. Common
household bleach, at a trade concentration of 5.25%, has approximately 5.25 g/100 mL,
or 52.5 g of available chlorine per liter. Sodium hypochlorite available for municipal use
usually has a trade concentration of 12% to 17%. These are approximate concentrations,
and they should always be confirmed for a particular shipment by laboratory procedures.
When one is purchasing bulk sodium hypochlorite, purchasing specifications should
be used delineating the acceptable ranges for available chlorine (12% to 17%) and pH (11
to 11.2), as well as maximum contaminant limits for iron (2 mg/L) and copper (1 mg/L).
Specifications should also require that shipments be free of sediment and other deleteri-
ous particulate material. Shipments should be analyzed for the concentration of chlorine,
the pH, and the concentration of metal contaminants.

Hypochlorite Degradation
Sodium hypochlorite solutions are subject to degradation during storage. The rate of degra-
dation is accelerated by high hypochlorite concentrations, high temperature, the presence
of light (UV radiation), low pH, and the presence of heavy metal cations such as iron,
copper, nickel, and cobalt.
During storage, sodium hypochlorite continually degrades to salt and oxygen through
the following reaction pathway:
2NaOCI ~ 2NaC1 + O2
Because of the generation of diatomic oxygen, sodium hypochlorite continually effer-
vesces. This causes several distinct problems in sodium hypochlorite systems. The first
problem is that metering pumps will air-bind (strategies to limit or eliminate this prob-
lem are discussed later). The second problem is that if sodium hypochlorite is trapped in
a section of piping (between two valves, for instance), the chemical will degrade and at-
tempt to effervesce. Because the air bubbles do not have anywhere to go, the pressure in
the trapped section increases. The pressure will continue to increase until the pipe rup-
tures. Cases of pipe rupture (and valve failure) have been reported in the literature.
Several strategies should be followed in the design of a sodium hypochlorite system
to limit the danger inherent in the system. First, valves should be kept to a minimum in
the system so that there are limited sections of pipe that can be isolated. Second, any sec-
tion of pipe that can be isolated must be provided with a relief valve. Third, operations
and maintenance procedures that limit the possibility of sodium hypochlorite being trapped
within piping should be put in place. These procedures can include lockout/tagout of valves
and flushing of systems prior to shutdown and isolation.

Hypochlorite Storage and Pipe and Valve Materials
The storage of sodium hypochlorite must be carefully managed to limit degradation and
the formation of chlorate. The quicker that the chemical is used, the less time it will have