Chlorine – Hypochlorite Equilibria Calculations

Blog provides calculations of chlorine released on adding acid to a bleach solution. 

Every year people are injured from inhaling chlorine gas accidentally formed by mixing acids with bleach. Chlorine is a strong oxidizing agent, and its hazards are well known. It has an OSHA PEL of 1 ppm (ceiling) and was used for a time as a chemical weapon in WW1.

Bleach is a solution of sodium or calcium hypochlorite and there is an equilibrium between the chlorine in the air, the hypochlorite concentration and the pH. The question that is often asked is what concentration of chlorine in equilibrium with a given concentration of hypochlorite at a known pH.

ChemDAQ Inc. supports consensus standards development through Association for the Advancement of Medical Instrumentation (AAMI) and this document was initially written in response to a question raised during a standards committee meeting. The linked document describes the known equilibria and solves for the chlorine concentration

If there is the possibility that chlorine may be formed at hazardous concentrations (> 1 ppm) in the workplace, then appropriate measures should be taken to protect workers. These measures include engineering controls such as increased air turnover, exhaust hoods etc. continuous gas monitors  to provide a warning to personnel if the chlorine concentration approaches dangerous levels, good work practices and training so that personnel know how to avoid a release of chlorine, how to recognize if there is a release and what to do in the event that chlorine is released.