A roatating ring disc electrode (RRDE) was used to study the fast reactions of electrochemically produced bromine with three different model compounds and one natural water sample. One of the three test compounds, ammonia, exhibited slow, pseudo first-order kinetics while two of the test compounds, NaAsO2and phenyl-arsine oxide, exhibited rapid second-order kinetics. Interpretation of the pseudo first order data yielded a specific rate constant for the reaction of HOBr and NH3of 8.1 #xD7; 107M#x2212;1s#x2212;1, in agreement with published results. Interpretation of the second-order data showed that the minumum detectable concentration of substrate was on the order of 1 #xD7; 10#x2212;6M and the rate constant for bromine reacting with As(III) approached the upper limit of our RRDE system. The natural water sample collected from the Patuxent River exhibited a rapid second-order bromine demand, similar to As(III). Analysis of the RRDE data suggested a family of naturally occurring reactants with a concentration of 2 #xD7; 10#x2212;5M, and a reaction rate constant in excess of 107M#x2212;1s#x2212;1. This method provides a useful way of studying very fast reactions responsible for oxidant demand in chlorinated or brominated estuarine and marine waters.
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