pHYSICAL cHEMISTRY - Ii

Experiment 6

Aim of the Experiment

Determination of free chlorine in a given water sample .

Principle

Various kinds of undesirable microorganism like algae, bacteria and fungi may present in surface water. The presence of these microorganisms in water may lead to serious health hazards. Thus drinking water should sterilized and disinfected before domestic use. Since, chlorine is powerful oxidizing agent nd is cheaply available. It is widely used for disinfection of potable and municipal water supplies.

Chlorination is done with the help of bleaching powder or chlorine gas or chlorine dissolved in water in the form of concentrated solution or with chloramines. The sterilizing action of chlorine is supposed to be due to its reaction with water producing hypochlorous acid and nascent oxygen, both of which have powerful germicidal properties.

CaOCl2 + H2O ----> Ca(OH)2 + Cl2

Cl2 + H2O ------> HOCl + HCl

HOCl -------> HCl + (O) (nascent oxygen)

(hypochlorous acid)

HOCl + Germs ------> Germs are destroyed

However, excess of free chlorine in drinking water is undesirable as it is not only unpleasant for drinking but is also injurious for human metabolism. Hence, the amount of free chlorine in municipal water is estimated prior to the domestic supply so as to make necessary adjustments in dose.

The principle involved in the estimation of free chlorine in water is that when a measures quantity of water is treated with excess of potassium iodide, the free chlorine present in the water oxidizes the corresponding amount of potassium iodide to iodine. The liberated iodine is estimated by titrating against standard hypo solution using starch as indicator.

Cl2 + 2KI ------- > 2KCl + I2

I2 + 2Na2S2O3 ------> NaS2O6 + 2NaI

( Sodium thiosulfate) (Sodium tetra thionate)

Materials required

Apparatus: Pipette, Burette, Stopper, conical flask, Beaker, Funnel, Filter paper, Stand

Chemicals required: Standard N/20 K2Cr2O7 solution, Potassium iodide solution (10%), Conc. HCl, N/20 hypo solution, starch indicator

Procedure

(A) Standardization of hypo solution:

Transfer 100 mL of distilled water into 250 mL flask and add about 2 g of KI and shake until the it gets dissolved. Add 6 mL conc. HCl slowly while gently rotating the flask. Add 25 mL of N/20 K2Cr2O7 solution and mixt the flask with a little distilled water and stopper of the flask. Allow it to standby for 5 minutes in dark for the completion of the reaction.

Rinse the stopper with distilled water and titrate the liberated iodine with the hypo solution from the burette. When most of the iodine has reacted, the solution acquires a greenish-yellow colour. At this stage, add 1 mL of starch solution when the solution turns violet due to the formation of starch iodine complex. Continue the titration until the violet colour changes to light green by the addition of a single drop of hypo at the end point.

(B) Determination of free chlorine in the water sample:

Transfer 10 % of 10 mL KI solution (OR 5 mL KI) into a 250 mL conical flask (or stoppered conical flask) and add 100 mL or( 50 mL of water) into it. Replace the stopper to the flask and shake is vigorously. Remove the stopper, wash the solution adhering in the flask with 5-10 mL distilled water .Titrate the solution wit N/20 hypo solution taken in the burette until the solution is pale yellow. Then add 1 mL of starch and continue the titration until the solution becomes just colourless. Note the titrate value.

Observation Table

Calculations

A) Standardization of hypo solution

Vol.of K2Cr2O7 (V1) = 25 mL

Normality of K2Cr2O7 (N1) = N/20

Vol. of Hypo solution (V2) =

Normality of hypo solution (N2) = ?

Applying formula,

N1V1 = N2V2

SO, N2 = N1V1 / V2

Normality of hypo solution = N

B) Calculation with the water sample:

Vol.of water sample taken (V1) = 100 mL

Normality of water sample taken (N1) = ?

Vol. of Hypo solution (V2) =

Normality of hypo solution (N2) =

Applying formula,

N1V1 = N2V2

Normality of the water sample is N1 = _________ with respect to free chlorine present.

Now strength of free chlorine in the sample

= Normality * Eq.wt. of Cl

= N1 * 35.45 g/l

= ________ mg /lit = ____________ ppm

Result

Free Chlorine present in the water sample __________ ppm

Reference Material

1. Practical Physical Chemistry Google EBOOK Link.

2.Adavanced Practical Physical Chemistry by J.B.Yadav

Questions

1. What is idometric titration?

2. How do you standardize hypo solution?