Estimation Of Free CO2 in Given Water Sample

 Aim of the experiment: Estimation of free CO2 in given water sample

Theory

Carbon Dioxide is present in water in the form of a dissolved gas. Surface waters normally contain less than 10 ppm free carbon dioxide, while some ground waters may easily exceed that concentration. Carbon dioxide is readily soluble in water. Over the ordinary temperature range (0-30 C) the solubility is about 200 times that of oxygen. Calcium and magnesium combine with carbon dioxide to form carbonates and bicarbonates. Aquatic plant life depends upon carbon dioxide and bicarbonates in water for growth. Microscopic plant life suspended in the water, phytoplankton, as well as large rooted plants, utilize carbon dioxide in the photosynthesis of plant materials; starches, sugars, oils, proteins. The carbon in all these materials comes from the carbon dioxide in water. Dissolved CO2 can show marked effects on the properties of water when it forms a weak carbonic acid solution that decreases the pH. Decrease in pH increases alkalinity and hardness of water by dissolving minerals.

Principle

Free CO2 reacts with NaOH or Na2CO3 to form Na(HCO3 )2 , the completion of the reaction is indicated by the appearance of pink color in the presence with phenolphthalein indicator.

Chemical reactions involved

Requirements

Apparatus

• 500 mL conical flask
• 250 mL graduated cylinder 
• Burette with stand Pipette with elongated tips 
• Wash bottle 

Chemicals

• NaOH solution (0.02 N): Dissolve 200 ml of stock solution to 1000 ml with distilled water. 
• Phenolphthalein indicator: Dissolve 0.5 g of phenolphthalein powder in 50 mL of 95% ethanol and add 50 mL distilled water

Procedure

• Collect water sample from a source (pond). 
• Take 100 mL of water sample in a 500 mL conical flask. Label it as test. 
• Take 100 mL of distilled water in another 500 mL conical flask and label it as control. 
• Add 4 to 5 drops of phenolphthalein indicator. 
• If the sample is pink color does not appear, titrate the sample against standard 0.02 N NaOH and until the pale pink color develop and remain for 30 sec. 
• Note down the burette reading.

Test sample

Volume of titrant used (V1 ) = Free CO2 = (V1 x 0.02 x 44 x 1000) / 100 = X mg/L

Control sample

Volume of titrant used (V2 ) = Free CO2 = (V2 x 0.02 x 44 x 1000) / 100 = Y mg/L 

Calculation

Free CO2 (mg / L) = (Volume of 0.02 N NaOH x Normality of NaOH x Equivalent weight of CO2 x 1000) / Volume of sample taken = (V x 0.02 x 44 x 1000) / 100

Result and Inference:

The free CO2 of given water sample 1 is X mg/L. The free CO2 of control water sample is Y mg/L. It is evident that X > Y Thus, free CO2 of pond water was higher than that of distilled water 

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