As chlorine is added, it is consumed by chemical reaction with the net effect of a rise in chlorine concentration. The slope will depend on addition rate and reaction rate. For the usual rates of addition, the reaction rate will suddenly speed up so that the concentration of chlorine falls. This is explained by the ease with which chlorinated compounds accept more chlorine. In other words, the rate of addition of the first atom of chlorine is relatively slow, but rates are greater for further reaction because chlorinating potentiates reactivity. When most of the reactions with chlorine are complete, the addition of more chlorine results in a permanent residual. A reasonable time for the experiment is 30 minutes. The point at which the concentration returns to an upward slope is termed the breakpoint. There may be no breakpoint observed for certain waters because different organic compounds react at various rates.
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