The Stability Of Chlorine Bleach Solutions
Sodium hypochlorite (chlorine bleach) solutions are produced by percolating chlorine gas in controlled amounts through a solution of sodium hydroxide in water. When the reaction is completed, there is a 1% to 2% excess of sodium hydroxide present in the solution. The chlorine gas reacts with sodium hydroxide to yield sodium hypochlorite. This reaction product is not stable—tending to decompose over time and releasing chlorine gas. Factors that promote this decomposition are heat, light, metal ions (including water hardness), low pH (less than 11), and soil. This decomposition rate is exponential, not linear, so a significant amount of the decomposition occurs in the first week after production. The best storage conditions are a cool dark room in a closed opaque container.
Sodium hypochlorite solutions containing less than 7.5% available chlorine are the most stable. If the intended shelf life for the product is greater than 30 days (such as household laundry bleach), we recommend purchasing a product containing less than 7.5% available chlorine. Typical household bleach is 5.25% available chlorine, which is initially produced at circa 6% available chlorine to insure 5.25% in the consumer product. The higher the reported available chlorine level, the faster the initial decomposition rate of the product. Typically a product made at 12.5% available chlorine will have decomposed to circa 10% within a week of manufacture under the best of storage and handling conditions.
Birko sells this product as an EPA registered sanitizer/disinfectant labeled at 10% because we need to have a realistic number that the EPA can test for when they inspect a facility. If the chlorine bleach label makes no sanitizing/disinfecting claims, companies can label their product to reflect the chlorine concentration at the time of manufacture. These products should be used within 30 days of manufacture. At 30 days, the chlorine concentration will probably be in the 7%-8% range.
The strongest sodium hypochlorite solutions commercially available are 15%. It is possible to make stronger solutions of sodium hypochlorite but they tend to precipitate sodium chloride (salt) as an additional reaction residue at these higher concentrations. Because of this and the very short shelf life, there is little effort put into commercializing stronger sodium hypochlorite solutions.