Soil and Water Chemistry (1+2)

Estimation of free carbon dioxide

Introduction: Dissolved CO₂ has marked effects on the properties of water, if forms a weak carbonic acid solution that changes the 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 as pH of 8.3.

Reagents:

• Std. NaOH solution (0.02 N): Dissolve 200 ml of stock solution to 1000 ml with distilled water.
• Stock NaOH solution: Dissolved 4 g NaOH in 1000 ml distilled water.
  
• Phenolphthalein indicator: Dissolve 0.5 g of phenolphthalein powder in 50ml of 95% C₂H₂OH and add 50ml distilled water.

Procedure:

• Take 50 ml of sample in a conical flask.
• 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.

Calculation:

V x N x 44 x 1000

Free CO₂ (mg/l) = ---------------------------

Volume of sample

V = Volume of 0.02N NaOH
N= Normality of NaOH