Decomposition of Sodium Hypochlorite (NaOCl) and the Formation of Sodium Chlorate

The presence of transition metal ions (copper and nickel) is known to catalyze the decomposition of liquid sodium hypochlorite, contributing to the loss of bleach strength and the formation of oxygen. Loss of bleach strength means more product will be needed when the bleach is used as a disinfectant. Oxygen build-up can pose problems when storing sodium hypochlorite in storage containers or bleach transport piping due to pressure build-up.

All bleach decomposition is dependant on temperature. For any given temperature, the higher the strength, the faster it decomposes and the higher the level of oxygen and chlorate formation. In summary, for every 10° C increase in storage temperature, the sodium hypochlorite will decompose at an increased rate factor of approximately 3.5.

Powell offers two free interactive tools showing the rate of decomposition of both filtered and non-filtered sodium hypochlorite at varying strengths and temperature as selected by the user. They also calculate the formation of oxygen and sodium chlorate in sodium hypochlorite over time.

How Temperature and the Strength Affect the Rate of Decomposition

All sodium hypochlorite decomposition is dependant on temperature. For any given temperature, the higher the strength the faster it decomposes. In order to completely understand the decomposition of bleach with respect to strength versus temperature, please refer to the AWWA research document "Minimizing Chlorate when Hypochlorite is the Chlorinating Ion." In summary, for every 10°C increase in storage temperature, the sodium hypochlorite will decompose at an increased rate factor of approximately 3.5.

Sodium hypochlorite filtered with the Powell Bleach Filter System contains only trace amounts of transition metal ions, significantly reducing the rate of decomposition and virtually eliminating the formation of oxygen.

More Information:

For more information about sodium hypochlorite, visit the Technical Information section.