Estimation of Carbonate and Bicarbonate Ions in Water | Chemistry Practical

Estimation of Carbonate and Bicarbonate Ions in Water

Water Analysis Practical | Acid-Base Titration | By Aniket Petrotech

1. Aim

To estimate the amount of carbonate and bicarbonate ions present in the given water sample by titration.

2. Apparatus Required

Burette (50 mL)

Used to fill and deliver the acid solution accurately during titration.

Pipette (20 mL)

Used to accurately measure and transfer the water sample.

Conical Flask (250 mL)

Where the titration reaction takes place.

Volumetric Flask

Used for preparing standard solutions.

Beakers and Funnel

Used for transferring liquids safely.

Burette Stand

Used to hold the burette securely during titration.

3. Chemicals Required and Indicators

1. Water Sample

The water sample to be tested. It may be municipal tap water or natural surface water.

2. Standard Hydrochloric Acid (HCl) Solution

Usually, 0.01 N or 0.1 N HCl solution is used as the standard acid for titration.

3. Phenolphthalein Indicator

This is the first indicator used to detect the first end point of the titration.

Pink in alkaline solution Colourless at the end point

4. Methyl Orange Indicator

This is the second indicator used to detect the final end point.

Yellow initially Orange-red at the end point

4. Principle

Friends, understanding the principle or basic concept of this practical is crucial. This experiment is based on an acid-base titration, which is a neutralization reaction.

When hydrochloric acid (HCl) is added to the water sample, the reaction occurs in two distinct steps.

Step 1: Conversion of Carbonate to Bicarbonate

First, the HCl converts the carbonate ions present in the water into bicarbonate ions.

CO₃²⁻ + H⁺ → HCO₃⁻

At this specific point, the pH of the water drops to approximately 8.3. To detect this point, we use the phenolphthalein indicator.

As soon as all the carbonate ions are converted into bicarbonate ions, the solution changes from pink to colourless.

First End Point (P): The burette reading at the moment the pink colour disappears completely is recorded as P.

Step 2: Neutralization of Bicarbonate

Now, only bicarbonate ions remain in the solution. Some were already present in the water, while others were formed from carbonate ions.

At this stage, methyl orange indicator is used. As more HCl is added, the bicarbonate ions are neutralized to form carbon dioxide and water.

HCO₃⁻ + H⁺ → H₂O + CO₂

At the final end point, the solution changes from yellow to orange or reddish-orange, depending on the endpoint colour and experimental conditions.

Final End Point (M): Record the total volume of acid used from the beginning of the titration up to this point as M.

5. Procedure

Step 1: Preparation of the Burette

First, rinse the burette properly and fill it with the 0.1 N HCl solution up to the zero (0) mark.

• Ensure that the burette tip is filled with solution and contains no air bubbles.
• Remove any funnel before taking the initial reading.
• Read the burette at eye level.

Step 2: Sampling

Using a pipette, take exactly 20 mL of the water sample and transfer it into a clean 250 mL conical flask.

Step 3: First Titration

Add 2 to 3 drops of phenolphthalein indicator to the flask.

If carbonate ions are present, the solution turns pink. If it does not turn pink, carbonate ions are considered absent under the usual alkalinity interpretation, and bicarbonate alkalinity may be present.

Step 4: First End Point (P)

Slowly add HCl from the burette while constantly swirling the flask.

The moment the pink colour completely disappears and the solution becomes colourless, stop the titration.

Record the burette reading as P. This is the phenolphthalein end point.

Colour change: Pink → Colourless.

Step 5: Second Titration

In the same colourless flask, add 2 to 3 drops of methyl orange indicator.

The solution will turn yellow. Continue the titration by adding HCl slowly from the same burette without resetting the reading.

Step 6: Second End Point (M)

Continue adding HCl until the yellow colour changes to orange or reddish-orange at the final end point.

Close the burette tap immediately and note the final burette reading.

Record the total acid volume from the beginning as M. This is the methyl orange end point.

Colour change: Yellow → Orange or reddish-orange.

6. Observation Summary

Indicator and End Point Changes

• Phenolphthalein: Pink to colourless.
• First end point: Record the acid volume as P.
• Methyl orange: Yellow initially, changing to orange or reddish-orange at the final end point.
• Final end point: Record the total acid volume from the beginning as M.

Important: P is the volume delivered up to the first end point. M is the cumulative volume delivered up to the final end point. Therefore, the additional acid used after the first end point is M − P, not M itself.

7. Precautions

• Use clean glassware for the experiment.
• Rinse the burette with the acid solution before filling it.
• Use a pipette to measure the water sample accurately.
• Remove air bubbles from the burette tip.
• Add the acid slowly near the end point while continuously swirling the flask.
• Observe colour changes against a white background.
• Read the burette at eye level to avoid parallax error.
• Record both end points carefully and take concordant readings.
• Wear suitable laboratory PPE and handle hydrochloric acid carefully.

Learn Chemistry Through Practical Experiments

Aniket Petrotech

Water Analysis | Acid-Base Titration | Laboratory Techniques