Determination of Dissolved Oxygen

LAB EXPERCISE # 10 DETERMINATION OF RESIDUAL CHLORINE THEORY Residual chlorine is the free chlorine remaining in water after 15-30 minutes of contact between the water and the chlorine dose. The presence of residual chlorine ensures an effective check over the reduction of bacteria in water to the safe limit. The residual chlorine may be present in the free form which has a very rapid disinfecting action or it may be in the combined form with ammonia to form chloramines which is less active. Residual chlorine is determined by two methods- the Starch iodide and the orthotolidine method. The free and combined forms of chlorine residuals are determined by the Orthotolidine Arsenite method. The disinfection of water for the control of micro-organisms is an important phenomenon of water supply. Chlorine and chlorine release compounds used universally for the control of micro-organisms in water. Ozone is another alternative, but it is quite expensive. If chlorine is present in high concentration it adversely affects the taste and odour. The elemental form of chlorine such as hypochlorite is generally used which undergoes hydrolysis to form free available chlorine which kills the micro-organisms. The chlorine level can be expressed in terms of the free chlorine, combined chlorine or total residual chlorine. For the majority of applications free residual chlorine is used for disinfection.

MATERIALS REQUIRED Burrete, Burrete Stand, Pippete, Pippete Bulb, Wash Bottle, 250 ml Graduated Cylinder, 500 ml volumetric flask.

CHEMICALS REQUIRED Glacial Acetic Acid, Potassium iodide, Sodium thiosulphate, Starch Indicator, Distilled Water.

ENVIRONMENTAL POLLUTION & MONITORING-I (ENV 309) Naeem Samoon

PROCEDURE 1. Rinse the burrete with Sodium thiosulphate and then fill the burrete with Sodium thiosulphate. 2. Take 200 ml of the given sample in the conical flask. 3. Add 5 ml Acetic acid to acidify the sample to pH 3 to 4. 4. Add about 1 g Potassium iodide using the spatula and dissolve it by thoroughly mixing it with stirring rod. 5. Perform the titration quickly, since iodine liberates faster. 6. Titrate with standard Na2S2O3 until the yellow colour of liberated iodine is almost faded out (pale yellow colour). 7. Add 1 ml of starch solution and continue the titration until the blue colour disappears. 8. In many cases residual chlorine is very low and starch needs to be added before starting up the titration. 9. Note down the burrete reading (to know the volume of sodium thiosulphate added.

OBSERVATIONS S.No .

Volume of Sample (ml)

Burrete Reading Initial

Final

Volume of titrant (Na2S2O3) used (ml)

Concordant Reading

CALCULATIONS Volume of Sodium thiosulphate (V1) =

ENVIRONMENTAL POLLUTION & MONITORING-I (ENV 309) Naeem Samoon

Normality of Sodium thiosulphate (N1)= Volume of Sample (V2)= Equivalent Weight of Chlorine = Residual Chlorine=

Volume of Sodium thiosulphate x N x 35.45 x 1000 Volume of Sample taken

ENVIRONMENTAL POLLUTION & MONITORING-I (ENV 309) Naeem Samoon