Percentage of available chlorine in bleaching powder.

Chlorine and Chloride! What is the difference?

1. SHREE MALLIKARJUN COLLEGE CLASS: SYBSC
SAFETY: Refer to MSDS of sodium thiosulphate, starch, glacial acetic acid, potassium iodide and bleaching powder.
-Dr. Mithil S. Fal Desai

Aim: Percentage of available chlorine in bleaching powder.
Chemicals: Sodium thiosulphate, starch, glacial acetic acid, potassium iodide and bleaching powder.
Apparatus: Standard volumetric flask, pipette, burette, conical flask, beakers, funnel and glass rod.
Theory: Available chlorine is the amount of chlorine liberated by the action of dilute acid on bleaching powder.
Chlorine or hypochlorite can react with an excess of I
-
in acidic medium to liberate iodine, quantitatively.
(OCl)
-
+ Cl
-
+ 2H
+
 Cl2 + H2O ----- Req 1
(OCl)
-
+ 2I
-
+ 2H
+
 Cl
-
+ I2 + H2O ----- Req 2
The liberated iodine can be titrated against standardized Na2S2O3 using starch.
I2 + 2(S2O3)
2-
 (S4O6)
2-
+ 2I
-
----- Req 3
From the above reactions it is evident that
Cl2 ↔ (OCl)
-
↔ I2 ↔ 2(S2O3)
2-
↔ 2e
-

As sodium thiosulphate is not a primary standard it needs to be standardized against standard dichromate solution.
K2Cr2O7 + 7H2SO4 + 6KI  4K2SO4 + Cr2(SO4)3 + 7H2O + 3I2 ----- Req 4
The liberated iodine is titrated against thiosulphate solution using starch (eq 3) and the exact strength of the
thiosulphate solution is determined.
Procedure:
i) Standardization of sodium thiosulphate. Refer to Cu(II) estimation isometrically.

ii) Estimation of available chlorine
Rinse and fill the burette with sodium thiosulphate. Pipette out 50 mL of water sample in a conical flask and add 2
mL of acetic acid (pH 3-4) and 1 g of KI. Keep the flask in dark for 5 min. After 5 min, add about 2 mL of starch
solution to the flask. Titrate this solution from the flask against sodium thiosulphate from the burette. Colour change
will be observed from blue to colourless. Repeat the titration to get constant readings.
Results: i) Amount of available chlorine =____g/L.

Further reading (available in the library)
1) Experiments in Applied Chemistry, S. Rattan, S. K. Kataria and Sons.
2) Vogel’s textbook of quantitative chemical analysis, J. Mendham, R. C. Denney, J. D. Barns, M.
Thomas, B. Sivasankar, Pearson India Education, 6
th
Edition.
Questions:
1) Why chlorine is added in public water supply?
2) What is the advantage of addition of acetic acid in thetitration?
3) Why chlorine is harmful than the chloride ions?
4) How chloride can be determined volumetrically?