The weight % of available Cl2 from the given sample of bleaching powder on reaction with dil acids or CO2 is called available chlorine.

CaOCl2 + H2SO4 → CaSO4 + H2O + Cl2
CaOCl2 + 2HCl → CaCl2 + H2O + Cl2
CaOCl2 + 2CH3COOH → Ca (CH3COO) 2 + H2O + Cl2
CaCOl2 + CO2 → CaCO3 + Cl2

Method of determination:

Bleaching powder + CH3COOH + KI (starch/Hypo)end point
Meq. Of I2 = Meq. Of Cl2 = Meq. Of Hypo solution
% of Cl2 = (3.55 × x × V (mL)/W (g))
Where x=Molarity of hypo solution
V =mL of hypo solution used in titration.

Ex.1.   10 gm sample of bleaching powder was dissolved into water to make the solution on liter. To this solution 35 mL of 1.0 M Mohr salt solution was added containing enough H2SO4. After the reaction was complete, the excess Mohr salt required 30 mL of 0.1 M KMnO4 for oxidation. The % of available Cl2 approximately is (mol wt=71)

Sol.2.  Meq. Of Mohr’s salt =35×1×1=35
Meq. Of KMnO4=Meq. Of excess Mohr salt=30×0.1×5=15
Meq. Of Mohr salt reacted with bleaching powder =35-15=20
Meq. Of Cl2=20
Wt. of Cl2 = 20 × 10-3 × (71/2) = 0.71 gm
% of Cl2 = (0.71/10) × 100 = 7.1% Ans.