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Bromine and chlorine are highly reactive halogens which form hypobromous or hypochlorous acid, the active sanitisers in a swimming pool or spa. Organic bromine or chlorine and the inorganic forms such as bromide or the hypochlorites, all release bromine or chlorine which, in water, form these oxidizing acids under specific conditions. Although bromine and chlorine are compatible with each other, some points need to be addressed if a pool is currently on bromine and the pool owner wants to use chlorine as a disinfectant. After hypobromous acid has done its job, it is considered "spent" and bromide salts remain.


Similarly, spent chlorine produces chloride. This is where the difference exists. A pool which has used organic bromine for a period of time builds up a bromide "bank." In the same way, the two step bromine system used by some pool owners creates a bromide bank by adding bromide salts to the pool. The major effect of these bromides in a pool using chlorine is disruption of stabilisation. Bromide interferes with the chlorine/cyanuric acid stabilisation and will not permit stabilisation to occur. Economically, it would be like running an unstabilised chlorine pool where much larger amounts of chlorine are required to maintain residuals.

These bromides are also inactive as a sanitiser until they are regenerated by another oxidant. Any chlorine which is added to a pool which has used bromine in the past reacts with these bromides to form hypobromous acid in water. There is no particular problem with this, but you must be aware that the addition of chlorine to a pool does not mean you a running a chlorine pool. In fact, you are still running a bromine pool. Since the DPD test measures both bromine and chlorine residual, there is no way to distinguish between the two. Chloride remains an inactive salt under most circumstances in the swimming pool.


There is no easy test for bromide in water, but if bromine has been used in the past, bromide is in the water. Of course, just as with any chemical, splash-out and periodic draining reduce the level of bromide present, but this can be a slow process in which you never know how much bromide remains. If you want to switch over to chlorine from bromine you must drain your pool to eliminate the bromides. If you simply start chlorinating with no regard for the bromide bank, over a period of time, usually one to two seasons of normal use and maintenance, the level of bromide will be reduced significantly. In the interim, much more chlorine (and the money needed to purchase this chlorine) will be required due to stabilisation interference.


Points to Remember

  • "Spent" bromine is called bromide. "Spent" chlorine is called chloride. Bromide is present if bromine has been used in the past.
  • Bromide interferes with chlorine/cyanuric acid stabilisation, requiring an increase in the amount of chlorine required to properly disinfect the pool.
  • Bromide is regenerated by an oxidant such as chlorine or potassium monopersulfate to form bromine. This can occur with normal chlorination, not just "shocking".
  • Bromide is only eliminated through draining the pool. Normally two seasons of use may be required to eliminate bromides if the pool has not been drained. In this period, much more chlorine will be required.
  • If you convert from chlorine to bromine, a partial drain may be necessary to reduce the cyanuric acid level below 25 ppm. Higher levels of cyanuric acid will destabilize the bromine system.


The above information is supplied by Bio-Lab and represents its best interpretation of available technical information at the time of preparation. The sole purpose is to supply factual information to Bio-Lab customers. It is not to be taken out of context nor used as support for any other claim not made herein.