The shelf-life of sodium hypochlorite irrigating solutions

Background: Sodium hypochlorite (bleach) solutions are widely used as irrigants in endodontics. Their tendency to deteriorate is worsened by environmental influences which could cause loss of available chlorine content. Methods: This study measured the loss of available chlorine concentration in a range of sodium hypochlorite solutions under conditions which mimic clinical usage and storage. Domestic bleach, both undiluted and diluted with demineralised water or hard water, was tested, along with Milton, for initial concentration of available chlorine, and then retested after varying periods and modes of storage to measure loss of chlorine concentration. The types of storage were: (1) in closed plastic bottles which were opened daily and agitated; (2) in open plastic bottles; (3) in syringes exposed to sunlight; (4) in syringes kept in the dark; (5) in open stainless steel bowls; (6) in closed stainless steel bowls; and (7) when heated to 50 degreesC. Not all solutions were tested under all storage conditions. Results: Of the solutions opened daily, undiluted domestic bleach was the most stable and Milton was the least stable. Initially, diluted bleach left open deteriorated rapidly but deterioration slowed with time. Solutions in syringes exposed to sunlight showed the most rapid loss of chlorine content. Heated bleach lost nearly 5 per cent of its strength in six hours. Diluted bleach surprisingly, increased its chlorine concentration in open bowls probably due to evaporation of water. Dilution of bleach with hard tap water did not significantly affect shelf-life. Preloading of diluted bleach into syringes appears to be a sound technique if the syringes are stored away from light. Conclusions: This study reinforces the need for sodium hypochlorite to be stored in closed opaque containers. Constant opening of containers appears to cause greater loss in chlorine concentration of diluted bleach solutions, perhaps because a lower concentration of sodium hydroxide allows the pH to drop more rapidly.