Free Residual Bromine
Free Bromine residual means the Bromine concentration, in parts per million (ppm) of water, available for rapid and effective biocidal action.
Under certain circumstances neither chlorine nor iodine is entirely satisfactory as a viricidal and germicidal agent in the preparation of potable water. Many water sources contain ammonia in such quantity that chlorination results in the formation of chloramines which are far less effective as disinfectants than free chlorine; and iodine effectiveness decreases with decreasing pH and with increasing concentration of iodide ion. It appeared possible that bromine had properties which would avoid these problems. Although free bromine is usually approximately equivalent on a molar basis to free chlorine in disinfecting ability, the bromamines are reportedly far more effective than chloramines, approaching free bromine and free chlorine in effectiveness. A greater knowledge of the chemistry of bromine and bromamines in dilute solutions was necessary for a thorough evaluation. In order to evaluate the effectiveness of bromine as a disinfectant, analytical methods which could easily be adapted for field use were necessary for determining bromine and the bromamines.
The chemistry of bromination
Just like with chlorine, when any of these bromine types dissolve in water, they all produce Hypobromous acid (HOBr), the active form of bromine. This is bromine's equivalent to Hypochlorous acid (HOCl). So when bromine dissolves in water, it looks like this:
Br2 + H2O → HOBr + HBr
bromine + water → Hypobromous acid + Hydrobromic acid
Then the Hypobromous acid (HOBr) dissociates into hydrogen and Hypobromite ion, just like HOCl dissociates into hydrogen and Hypochlorite ion:
HOBr ⥄ H+ + OBr-
Hypobromous acid dissociates into Hydrogen ion and Hypobromite ion
The reason for the (⥄) instead of equal dissociation is that this equilibrium happens at a higher pH than pools and spas normally operate under. So most of the active bromine in water will be HOBr relative to OBr-.