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Bleaching powder is not a true compound but it is a mixed salt of calcium hypochlorite [Ca (OCl) 2].3H2O and basic calcium chloride [CaCl2Ca (OH) 2H2O]. [Simply it is represented by CaOCl2.
The bleaching action of [CaOCl2] is due to liberation of oxygen with limited quantity of dilute acid.                                                                                                        

Whereas disinfectant action available of Cl2 on reaction with excess of acid.
The Chlorine liberates is called available Chlorine, and calculation of available Chlorine is called IODOMETRY.

IODOMETRIC-CALCULATION OF AVAILABLE % CHLORINE: Take a W (gm) sample of bleaching powder; and  when bleaching powder treated with dilute acid or water then liberates chlorine gas.
The chlorine produced in above reaction is titrated with KI solution which produced (I2) Iodine which is further completely titrated with hypo solution.                                          

If number of millimoles of hypo solution consume is M V then millimoles of %available chlorine is calculated as ;
                            Millimoles of Iodine is =1/2 Millimoles Hypo solution
                            Millimoles of Iodine is = Millimoles chlorine
Weight of available chlorine is = Number of moles x molecular Wt of Chlorine 
ILLUSTRATIVE EXAMPLE(1): If trace of chlorine are not remove from pulp used for paper manufacturing , then on the long standing it weaken the paper and makes it yellowish , which of following antichlor can be used to remove chlorine from pulp ?.
(A) Na2S2O3        (B) conc. HCl            (C) NaHCO3           (D) Both (A) and (B)
An antichlor  is a substance used to decompose residual hypochlorite or chlorine after use of chlorine based bleaching , in order to prevent ongoing reactions .example s of antichlor – Sodiumbisulphite (NaHSO3), Pottassiumbisulphite (KHSO3) ,  Sodiummetasulphite (Na2S2O3), Sodiumthiosulphate  (Na2S2O3) and hydrogen peroxide (H2O2).
ILLUSTRATIVE EXAMPLE (2): Available chlorine in sample of bleaching powder can be calculated as per the following reactions.
If 4 gm of bleaching powder dissolved to give 100 ml solution 25 ml of it react with excess of CH3COOH and KI .The Iodine liberation required 10 ml of 0.125 N Hypo solutions. Calculate % of available chlorine in the sample.
(A)  21%                       (B) 35 %                     (C) 45 %             (D) 4.4%
ILLUSTRATIVE EXAMPLE (3): Calculate the % of available chlorine in the sample in a sample of 3.35 gm of bleaching powder which was dissolved to 100 ml water. 25 ml of this solution on treatment react with KI and dilute acid. Required  20 ml 0.125 N Hypo solution (Sodiumthiosulphate- Na2S2O3). 
ILLUSTRATIVE EXAMPLE (4): 25 ml of household bleach solution was mixed with 30 ml of 0.50 M KI and 10 ml of 4.0N acetic acid. In this titration of the liberated iodine 48 ml of 0.25 N Na2S2O3 was used to reach the end point. The Molarity of household bleach solution is?

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