Determination of residual chlorine

AIM:- To determine the amount of residual chlorine in the chlorinated water sample.

APPARATUS:-

(i) Glass ware            (ii) Coloroscope

THEORY:-

            Chlorination is a common and widely used method of disinfection of water. Water is normally chlorinated to an extent beyond a little beyond the break point. This ensures the availability of free residual in the treated water. The dosage of chlorine should be such that residual chlorine of 0.1 to 0.2 ppm appears in water at the point of its entry into the distribution system. Free residual chlorine (i.e. Hocl and ocl^-) is more effective than combined residual (i.e. chloramaines). Hence water should be treated beyond breaking point to ensure the presence of free residual chlorine.

            When chlorine is added to water, it reacts with water forming Hypochlorous acid. This is indicated by the following reation.

                        Cl2 + H₂O ↔  Hocl + H⁺ + cl^-

            The above reaction if reversible. The hypochlorous acid (Hocl) dissociated into hydrogen ions (H⁺) and hypochlorite ions (ocl^-) as indicated below.

                        Hocl ↔ H⁺ + ocl^-

            It is the hypochlorous acid (Hocl) and the hypochlorite ions, which accomplish disinfection of water. The hypochlorous acid (Hocl) is about 80 times more powerful as disinfectant than the hypochlorite ions (ocl^-). The dissociation of hypochlorous acid is a function of PH value of chlorinated water. As the PH value of water increases, more and more Hocl dissociates to form ocl^- ions. At a PH of about 5.5, hypochlorous acid is 100% unionized while at PH above 9.5 it is all ocl- ions. Therefore chlorination should be carried out at low PH so as to ensure more percentage of hypochlorous acid for effective chlorination.

Residual chlorine is estimated by two methods.

(i) Starch – Iodide method     (ii) O – Tolidine method.

            In starch- iodide method, chlorine displaces an equivalent amount of iodine from iodide (i.e. KI) which combines with starch to give blue colour. The liberated iodine is titrated against standard sodium thiosulphate until the blue colour disappears.

            O – Tolidine method is most commonly used to determine the residual chlorine in small concentrations (i.e. upto 2 ppm). O- Tolidine is an organic compound, which is oxidized by chlorine in a yellow coloured compound called “Holoquinone”. Thus when O – Tolidine is added to water containing chlorine a yellow colour develops, the intensity of which is proportional to the amount of residual chlorine present in water. The colour so produced is matched or compared with standards.

RELEVANCE:- Presence of residual chlorine (i.e. free residual) in the desired concentration ensures the safety or purity of water during its conveyance. It safeguards the water against any contamination which is likely to occur during conveyance through a network of distribution system. The permissible limit of residual chlorine in the treated water is upto 0.2 ppm.

REAGENTS:-

i)                    Potassium Iodide Solution: Dissolve 20.0 grams KI in 100ml distilled water.

ii)                  Sodium thiosulphate titrant: Dissolve 6.205 gram sodium thiosulphate in distilled water and make it to 1 lit. Standardize it using potassium dichromate.

iii)                Potassium dichromate solution of 0.1N: Dissolve 1.226 gram K₂CR₂O₇ in distilled water and make it to 1 lit.

iv)                Starch: Dissolve 5 grams starch in 1 lit boiling distilled water.

v)                  Conc.: Acetic Acid.

vi)                O – Tolidine reagent: (for O – Tolidine method)

PROCEDURE:- 1. Starch – Iodide method.

Standardization of sodium thiosulphate using standard 0.1N potassium dichromate solution

a)      To 80ml distilled water, add 1ml conc. H₂SO₄, 10ml 0.1N K₂CR₂O₇ and 10ml KI solution.

b)      Keep the flaks in the dark place for about 5 min.

c)      Titrate with the given sodium thiosulphate till a pale yellow colour is observed. At this juncture, add 1ml starch and continue titration until blue colour disappears. Note down the volume of sodium thiosulphate consumed.

Normality of sodium thiosulphate = _________________1____________

                                                           ml of sodium thiosulphate consumed

Adjust the normality of sodium thiosulphate to 0.0.25 using the relation

N₁V₁ = N₂V₂

Determination of residual chlorine in the sample water:-

a)      To 100ml chlorinated water samples add 5ml. concentrated acetic acid and 10ml KI.

b)      Titrate with 0.025N sodium thiosulphate till a pale yellow colour is reached. At this juncture, add 1ml starch and continue titration till blue colour is discharged.

c)      Note down the volume of 0.025N sodium thiosulphate consumed.

Determination of Residual chlorine (O – Tolidine method)

            Take 10ml chlorinated water sample in a glass tube and add to this 0.1ml O-Tolidine solution. The sample turns to yellow. Determine the concentration of residual chlorine in the sample water by comparing the colour produced with the standards (i.e. coloured glass standards tinted for each concentration).

OBSERVATION:- Starch Iodide Method

S.No	Burette Readings		Vol. of Titrant used
	Initial	Final

CALCULATIONS:-

Starch-Iodide method:-

Normality of given sodium thiosulphate titrant = __________________1___________

                                                                         Ml of sodium thiosulphate consumed

Adjusting the normality of sodium thiosulphate to 0.025, using the relation

                        N₁V₁ = N₂V₂

Residual chlorine in mg/l = vol. of sodium thiosulphate consumed *N* 35450

                                                      ml of sample water

 RESULT:-

By Starch – Iodide Method
Residual chlorine in mg/l = ___________

By O – Tolidine method

Residual chlorine in mg/l = ___________

INTERPRETATION OF RESULTS:-