Bleaching powder is a mixture of calcium hypochlorite $(C a O C l_{2})$ and the basic chloride $C a C l_{2} . C a {(O H)}_{2}$ . Water free slaked lime is also present in addition.
Available chlorine in bleaching powder is the amount of chlorine liberated by the action of dilute acids on bleaching powder according to the equation
$O C l^{-} + C l^{-} + 2 H^{+} \to \underset{A v a i l a b l e c h l o r i n e}{C l_{2}} + H_{2} O$
The available chlorine is expressed as the percentage by weight of the bleaching powder. It is measured by iodometric titration. The bleaching powder solution or suspension is treated with an excess of KI and then acidified with acetic acid where $I_{2}$ is liberated.
$O C l^{-} + 2 I^{-} + 2 H^{+} \to C l^{-} + I_{2} + H_{2} O$

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