Quantitative analysis of sodium bicarbonate in baking soda using acid-base titration with 0.1 M HCl
To determine the amount (percentage purity) of sodium bicarbonate (NaHCO₃) in a given sample of baking soda using 0.1 M HCl through acid-base titration.
Baking soda (sodium bicarbonate, NaHCO₃) reacts with acids to produce carbon dioxide gas, which is why it’s used as a leavening agent in baking. Commercial baking soda may contain impurities. Titration with standardized HCl allows quantitative determination of NaHCO₃ purity.
This is an acid-base neutralization reaction with gas evolution
Based on the procedure and sample data:
The percentage purity of Na₂CO₃ in the given washing soda sample
Note: There’s confusion in the original text between baking soda (NaHCO₃) and washing soda (Na₂CO₃). This activity actually determines Na₂CO₃ purity.
Important Distinction:
The calculations in this activity use 1:2 mole ratio, indicating it’s actually for washing soda (Na₂CO₃) analysis.
Titration is based on the concept of neutralization, which occurs when an acid reacts with a base to form water and a salt. In this case, the reaction also produces carbon dioxide gas.
Mole Ratio: 1 mole Na₂CO₃ : 2 moles HCl
Mole Ratio: 1 mole NaHCO₃ : 1 mole HCl
Important: The mole ratio difference is crucial for correct calculations. This activity uses 1:2 ratio, indicating Na₂CO₃ analysis.
Methyl orange is chosen because:
Colour Transition: Yellow (basic) → Orange-red (acidic) at end point
1 dm³ capacity for preparing solution
50 cm³ for HCl delivery
10 cm³ for transferring solution
250 cm³ for titration
For weighing baking soda (±0.001 g)
Methyl orange solution
Weigh exactly 10 g of baking soda using analytical balance. Dissolve in small amount of distilled water. Transfer quantitatively to 1 dm³ volumetric flask. Add distilled water up to the mark (1000 cm³). Shake well to ensure complete mixing.
Rinse burette with small amount of 0.1 M HCl, then fill it with the same solution. Ensure no air bubbles in the tip. Record initial reading at eye level.
Using clean, dry pipette, transfer exactly 10 cm³ of baking soda solution to clean conical flask.
Add 2-3 drops of methyl orange indicator. The solution will turn yellow due to basic nature of carbonate/bicarbonate.
Slowly add HCl from burette while swirling conical flask continuously. Continue until yellow colour changes to orange-red. This colour change should be permanent (lasts at least 30 seconds).
Record final burette reading. Calculate volume of HCl used (V₁ = Final – Initial).
Repeat steps 3-6 two more times until you obtain three concordant readings (within ±0.1 cm³). Calculate mean volume.
CO₂ Evolution: Reaction produces CO₂ gas. Swirl gently to release gas without splashing.
End Point Detection: Methyl orange changes from yellow to orange-red. Stop at first permanent orange-red colour.
Ventilation: Perform in well-ventilated area as CO₂ accumulates.
Concordant Readings: Require at least 2 readings within 0.1 cm³ difference.
| Sr. No. | Initial Reading (cm³) | Final Reading (cm³) | Volume of HCl used (cm³) |
|---|---|---|---|
| 1 | 0.0 | 9.9 | 9.9 |
| 2 | 9.9 | 19.9 | 10.0 |
| 3 | 19.9 | 29.9 | 10.0 |
| Mean Volume of HCl (V₁): | 10.0 cm³ | ||
Given Data:
Note: Calculations show 1:2 mole ratio, indicating analysis is for washing soda (Na₂CO₃), not baking soda (NaHCO₃).
Step 1: Calculate Molarity of Na₂CO₃ in solution (M₂)
Using formula: M₁V₁/n₁ = M₂V₂/n₂ (2:1 reaction for Na₂CO₃)
(0.1 × 10.0)/2 = (M₂ × 10.0)/1
0.5 = M₂ × 10.0
M₂ = 0.5/10.0 = 0.05 M
Step 2: Calculate Strength of Na₂CO₃ in solution
Strength = Molarity × Molar mass
Strength = 0.05 M × 106 g/mol
Strength = 5.3 g/dm³
This means 1 dm³ of solution contains 5.3 g of pure Na₂CO₃
Step 3: Calculate pure Na₂CO₃ in original sample
10 g of sample was dissolved in 1 dm³ solution
So, 10 g sample contains 5.3 g pure Na₂CO₃
Step 4: Calculate percentage purity
Percentage = (Mass of pure Na₂CO₃ / Mass of sample) × 100
Percentage = (5.3 g / 10 g) × 100
Percentage = 53%
The percentage purity of Na₂CO₃ in the given sample
Interpretation: The sample contains only 53% pure sodium carbonate, with 47% impurities.
Correction: If this were truly baking soda (NaHCO₃) analysis with 1:1 mole ratio, the percentage would be different.
Key Formulas:
Crucial: Determine correct mole ratio from balanced equation. For Na₂CO₃: n₁(HCl) = 2, n₂(Na₂CO₃) = 1. For NaHCO₃: both = 1.
Answer: Methyl orange is used because it changes colour in the acidic pH range (3.1-4.4). In the titration of carbonate/bicarbonate with strong acid (HCl), the equivalence point occurs in the acidic range due to the formation of carbonic acid (H₂CO₃) which partially dissociates. Phenolphthalein changes colour in the basic range (pH 8.2-10.0), which would occur before the equivalence point in this titration, leading to inaccurate results. Methyl orange’s colour change from yellow (basic) to orange-red (acidic) accurately signals the end point for carbonate/bicarbonate titrations.
Answer: The key difference is in their mole ratios with HCl:
Baking Soda (NaHCO₃):
NaHCO₃ + HCl → NaCl + H₂O + CO₂
Mole Ratio: 1:1 (1 mole NaHCO₃ : 1 mole HCl)
Washing Soda (Na₂CO₃):
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
Mole Ratio: 1:2 (1 mole Na₂CO₃ : 2 moles HCl)
Effect on Calculations: Using the wrong mole ratio leads to incorrect results:
In this activity, the calculations use 1:2 ratio, indicating the sample is actually washing soda (Na₂CO₃), not baking soda (NaHCO₃).
Answer: For NaHCO₃ (1:1 mole ratio with HCl):
Step 1: Calculate molarity of NaHCO₃ (M₂)
M₁V₁ = M₂V₂ (1:1 ratio)
0.1 × 12.0 = M₂ × 10.0
1.2 = M₂ × 10.0
M₂ = 0.12 M
Step 2: Calculate strength of NaHCO₃
Strength = M × Molar mass = 0.12 × 84 = 10.08 g/dm³
Step 3: Calculate pure NaHCO₃ in sample
8.0 g sample contains 10.08 g/dm³ × 1 dm³ = 10.08 g pure NaHCO₃ in the entire solution
This means 8.0 g sample actually contains 10.08 g pure NaHCO₃? Wait, this doesn’t make sense – the strength can’t be greater than the mass dissolved.
Let’s recalculate carefully:
If 10 cm³ of solution requires 12.0 cm³ of 0.1 M HCl, then moles HCl = 0.1 × 0.012 = 0.0012 moles
Moles NaHCO₃ in 10 cm³ = 0.0012 moles (1:1 ratio)
Moles NaHCO₃ in 1000 cm³ = 0.0012 × 100 = 0.12 moles
Mass of pure NaHCO₃ in 1 dm³ = 0.12 × 84 = 10.08 g
But only 8.0 g was dissolved, so this is impossible. There must be an error in the question data.
Assuming the data meant 12.0 cm³ for titration:
Percentage purity = (Mass pure / Mass sample) × 100
Since 8.0 g dissolved gave 10.08 g pure (impossible), the data is inconsistent.
With reasonable data: If 8.0 g sample gives strength less than 8.0 g/dm³, then % = (strength/8.0) × 100
Answer: Carbon dioxide gas evolves because the reaction between carbonate/bicarbonate and acid produces CO₂ as a product:
NaHCO₃ + HCl → NaCl + H₂O + CO₂↑
or
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑
Handling CO₂ evolution during titration:
The gas evolution doesn’t interfere with the accuracy of the titration as long as proper technique is followed, but it does make the titration more challenging than simple acid-base titrations without gas evolution.
Answer:
(a) Using phenolphthalein instead of methyl orange:
Phenolphthalein changes colour at pH 8.2-10.0, which corresponds to the first equivalence point in carbonate titration (conversion of carbonate to bicarbonate: CO₃²⁻ + H⁺ → HCO₃⁻). If you use phenolphthalein and stop at its colour change (colourless to pink), you would only titrate half of the carbonate. The calculated molarity would be half of the actual value, leading to a percentage purity result that is 50% of the true value.
(b) Assuming 1:1 mole ratio for Na₂CO₃ instead of 1:2:
This is a critical error. Using the formula M₁V₁/n₁ = M₂V₂/n₂ with n₁=1 instead of 2 would give:
M₂ = (M₁V₁ × n₂) / (V₂ × n₁) = (0.1 × 10.0 × 1) / (10.0 × 1) = 0.1 M (instead of 0.05 M)
Strength = 0.1 × 106 = 10.6 g/dm³ (instead of 5.3 g/dm³)
Percentage = (10.6/10) × 100 = 106% (instead of 53%)
This gives an impossible result (>100%) and clearly indicates the error. The calculated value would be exactly double the correct value when using the wrong mole ratio for Na₂CO₃.
Key Lesson: Always use the correct mole ratio from the balanced chemical equation, and choose the appropriate indicator for the specific titration.
Quality control of baking soda/powder for consistent leavening action in baked goods.
Purity testing of sodium carbonate/bicarbonate as raw materials for various industries.
Testing alkalinity of water using similar titration methods with acids.
Quality control of antacid preparations containing carbonates/bicarbonates.
Determining carbonate content in soils, sediments, and industrial wastes.
Developing new carbonate-based materials and processes requiring precise purity measurements.
Typical Purity Requirements:
Note: The 53% result in this experiment indicates very impure material that would not meet any commercial standards.
Test your understanding of carbonate/bicarbonate analysis with these 20 multiple choice questions. Select your answer and click submit to check your score.