Although the commercial strength of NaOCl is 12.5% to 15%, the actual delivered product is usually weaker since hypochlorite is unstable and degrades as much as half every 100 days (at 70°F). This degradation accelerates in higher temperatures and in the presence of sunlight. Dilution greatly reduces degradation, especially for solutions delivered in concentrations less than 7% to 8%. The pH of sodium hypochlorite is high because sodium hydroxide is used in its manufacturing process.
NaOCl produces gas as a natural by-product during its decomposition, as much as 1% per day at room temperature. This gas is primarily oxygen; however chlorine gas can also be released at lower pHs. The gas by-product of decomposition can be hazardous if not properly vented off or kept moving through the piping system. If the gas becomes trapped in a piping system or in a pump liquid end, over time it can build up enough pressure to rupture the piping or pump head. Installing vent valves, keeping the liquid moving at high velocities and operating at cooler liquid and/or ambient temperatures can help reduce this problem. Commercial strength hypochlorite is often diluted prior to being injected into a water stream, in order to provide proper mixing and disinfection. When an insufficient amount of dilution water is used the hypochlorite can cause the pH to rise. If the dilution water is hard water, the rise in pH will result in calcium carbonate precipitation which will coat the inside of piping, valves and pumps. This scale deposit will tend to be greatest in areas of high turbulence, such as pump heads, valves and rotometers. In order to prevent scaling, use soft water. If soft water is not available use enough hardwater to keep the pH below 9.
The decomposition (loss of O2) follows the following reaction:
2NaOCl --> O2 + 2NaCl
which results in a solution with a lower pH that the original solution of bleach. Off-gassing is a problem with commercial hypo feed systems as it effects shelf life. The commercial hypo systems that I have designed in the past 5 years have all diluted commercial strength to at or below 7.5% to reduce off-gassing (which greatly increases “shelf life”) and also to reduce the area classification, under IBC 2006 or later, to something outside the requiremebts of an “H” classification.
Hope this helps without going into far more detailed chemistry of the reactions involved and hazardous substance rules