Double Indicator Titration of Carbonate-Bicarbonate Mixture

Using two indicators to analyze a carbonate mixture

Titrations Advanced (University) 55 min ~$14.00

Objective

Analyze a mixture of sodium carbonate and sodium bicarbonate by titrating with HCl using two indicators sequentially: phenolphthalein and methyl orange.

Background

A mixture of Na₂CO₃ and NaHCO₃ has two equivalence points when titrated with acid. Phenolphthalein detects the first (CO₂³⁻ → HCO₃⁻), and methyl orange detects the second (HCO₃⁻ → H₂CO₃). By comparing the volumes at each endpoint, the individual concentrations can be calculated.

Safety Warnings

  • HCl is corrosive
  • Wear safety goggles throughout
  • Handle indicators carefully — they stain

Required PPE

goggles lab_coat

Materials

  • Sodium carbonate/bicarbonate mixture (100 mL)
    Unknown composition
  • Hydrochloric acid (HCl) (100 mL)
    0.1M standardized
  • Phenolphthalein indicator (5 mL)
  • Methyl orange indicator (5 mL)
  • Distilled water (200 mL)

Equipment

50 mL burette Burette stand and clamp 250 mL Erlenmeyer flask 25 mL pipette White tile

Procedure

1

Pipette 25.0 mL of the carbonate/bicarbonate mixture into the flask.

2 min
2

Add 2-3 drops of phenolphthalein. The solution should turn pink.

1 min
3

Fill the burette with 0.1M HCl. Record the initial volume.

3 min
4

Titrate until the pink color disappears (first endpoint). Record volume V₁.

10 min
5

Without refilling the burette, add 2-3 drops of methyl orange to the same flask.

1 min
6

Continue titrating until the color changes from yellow to orange/salmon (second endpoint). Record total volume V₂.

10 min
7

Repeat the entire process for concordant results.

18 min
8

Calculate: V(phenolphthalein) = V₁, V(methyl orange only) = V₂ - V₁. Use these to find the individual concentrations of Na₂CO₃ and NaHCO₃.

7 min

Expected Results

V₁ corresponds to the carbonate content, and (V₂ - V₁) includes both the remaining carbonate reaction and the original bicarbonate. For pure carbonate, V₂ - V₁ = V₁. Any excess indicates bicarbonate present.

Cleanup

Neutralize waste solutions before disposal. Rinse all glassware thoroughly.

Details

Category
Titrations
Difficulty
Advanced (University)
Duration
55 min
Est. Cost
$14.00
Steps
8
Materials
5

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Frequently Asked Questions

What is the objective of Double Indicator Titration of Carbonate-Bicarbonate Mixture?
Analyze a mixture of sodium carbonate and sodium bicarbonate by titrating with HCl using two indicators sequentially: phenolphthalein and methyl orange.
How difficult is Double Indicator Titration of Carbonate-Bicarbonate Mixture?
This experiment is rated as Advanced (University). It takes approximately 55 minutes to complete.
What safety precautions are needed for Double Indicator Titration of Carbonate-Bicarbonate Mixture?
Key safety precautions include: HCl is corrosive; Wear safety goggles throughout; Handle indicators carefully — they stain.
What materials are needed for Double Indicator Titration of Carbonate-Bicarbonate Mixture?
The main materials required are: Sodium carbonate/bicarbonate mixture, Hydrochloric acid (HCl), Phenolphthalein indicator, Methyl orange indicator, Distilled water.
What results should I expect from Double Indicator Titration of Carbonate-Bicarbonate Mixture?
V₁ corresponds to the carbonate content, and (V₂ - V₁) includes both the remaining carbonate reaction and the original bicarbonate. For pure carbonate, V₂ - V₁ = V₁. Any excess indicates bicarbonate present.