Volumetric Determination of Nitrites – Chemistry Lab Simulation

Volumetric Determination of Nitrites

Redox Titration: Sodium Nitrite with Potassium Permanganate

Objective

To determine the concentration of a sodium nitrite solution using a standard solution of potassium permanganate through redox titration in an acidic medium.

Safety Precautions

  • Wear protective gloves and goggles throughout the experiment.
  • Handle KMnO4 with care as it is a strong oxidizer and can cause stains.
  • Sodium nitrite can be toxic; avoid ingestion and inhalation.
  • Work in a well-ventilated area.
  • Dispose of chemical waste properly according to safety guidelines.

Principle

The experiment is based on the redox reaction between sodium nitrite and potassium permanganate in an acidic medium. KMnO4 (a strong oxidizing agent) is reduced, while NaNO2 is oxidized. The endpoint is indicated by the appearance of a faint pink colour that persists for about 30 seconds.

2KMnO4 + 5NaNO2 + 3H2SO4 → K2SO4 + 2MnSO4 + 5NaNO3 + 3H2O

In this reaction, Mn7+ (in MnO4–) is reduced to Mn2+ (in MnSO4), while N3+ (in NO2–) is oxidized to N5+ (in NO3–).

Materials Required

Chemicals

  • 0.02M KMnO4 solution
  • Sodium nitrite solution (unknown concentration)
  • Dilute sulphuric acid (H2SO4)
  • Distilled water

Apparatus

  • Burette (50 mL)
  • Pipette (10 mL)
  • Conical flask (250 mL)
  • White tile
  • Burette stand
  • Funnel
  • Wash bottle

Experimental Procedure

  1. Rinse the burette with the 0.02M KMnO4 solution and then fill it with the same solution, ensuring no air bubbles are present in the nozzle.
  2. Pipette out 10 mL of the given sodium nitrite solution into a clean conical flask.
  3. Add about 20 mL of dilute sulphuric acid to the conical flask to provide an acidic medium for the reaction.
  4. Place the conical flask on a white tile to observe the colour change clearly during titration.
  5. Start adding KMnO4 solution from the burette to the conical flask while swirling continuously.
  6. Initially, the purple colour of KMnO4 will be decolorized as it reacts with nitrite ions.
  7. Continue adding KMnO4 solution dropwise near the endpoint.
  8. The endpoint is reached when a light pink colour persists for about 30 seconds.
  9. Record the initial and final readings of the burette.
  10. Repeat the titration at least three times to obtain consistent readings.

Observations

Sr. No. Initial Reading (mL) Final Reading (mL) Volume of KMnO4 Used (mL)
1 0.00 10.10 10.10
2 10.10 20.21 10.11
3 20.21 30.31 10.11
Mean Volume of KMnO4 10.11 mL

Calculations

Given:

Molarity of KMnO4 solution, M1 = 0.02 M

Volume of KMnO4 solution used, V1 = 10.11 mL

Volume of NaNO2 solution used, V2 = 10 mL

From the balanced equation: 2 moles of KMnO4 react with 5 moles of NaNO2

Therefore, n1 = 2, n2 = 5

Using the formula:

M1V1/n1 = M2V2/n2

Calculation:

(0.02 × 10.11) / 2 = (M2 × 10) / 5

0.1011 / 2 = 10M2 / 5

0.05055 = 2M2

M2 = 0.025275 M ≈ 0.0253 M

Amount of NaNO2 per dm3 of solution:

= Molarity × Molar mass of NaNO2

= 0.0253 mol/L × 69 g/mol

= 1.7457 g ≈ 1.75 g

Result

The concentration of the given sodium nitrite solution is 0.0253 M.

The amount of NaNO2 dissolved per dm3 of solution is 1.75 g.

Post-Lab Points

  • Clean all glassware thoroughly after use.
  • Dispose of chemical waste according to safety guidelines.
  • Record the final concentration of the sodium nitrite solution.
  • Discuss any discrepancies between trials and potential sources of error.
  • Potential errors: Incorrect burette readings, incomplete transfer of solutions, over-titration, or impurities in reagents.

15 Short Questions (1 mark each)

1. What is the purpose of adding dilute sulphuric acid in this titration?
To provide an acidic medium for the redox reaction between KMnO4 and NaNO2.
2. Why is KMnO4 considered a self-indicator?
Because it has a distinct purple color that disappears when reduced and reappears at the endpoint.
3. What is the color change observed at the endpoint of this titration?
Colorless to persistent light pink color.
4. What is the oxidation state of manganese in KMnO4?
+7
5. What is the oxidation state of manganese in MnSO4?
+2
6. What is the oxidation state of nitrogen in NaNO2?
+3
7. What is the oxidation state of nitrogen in NaNO3?
+5
8. Why is a white tile placed under the conical flask during titration?
To clearly observe the color change at the endpoint.
9. What safety precaution is essential when handling KMnO4?
Wear protective gloves and goggles as it is a strong oxidizer.
10. How many moles of NaNO2 react with 2 moles of KMnO4 according to the balanced equation?
5 moles
11. What is the molar mass of NaNO2?
69 g/mol
12. Why should air bubbles be removed from the burette nozzle?
To ensure accurate volume measurement of the titrant.
13. What is the purpose of repeating the titration multiple times?
To obtain consistent and accurate results.
14. What would happen if insufficient acid is added to the reaction mixture?
The reaction would be incomplete or slow, leading to inaccurate results.
15. What is the name of the technique used in this experiment?
Redox titration or volumetric analysis.

Interactive Quiz: Nitrite Determination

Test your knowledge with this 10-question multiple-choice quiz. Select your answer to see immediate feedback.

Quiz Results

You scored 0 out of 10 (0%)

Concept Assessment Exercise

Design an experiment to determine the % age purity of KNO2, 5.0 g of which has been dissolved per dm3 of solution. You are provided 0.02M KMnO4 solution.

Solution Approach:

  1. Dissolve 5.0 g of the given KNO2 sample in distilled water and make up to 1 L to prepare the solution.
  2. Take 10 mL of this solution in a conical flask and add about 20 mL of dilute H2SO4.
  3. Titrate against 0.02M KMnO4 solution until a permanent light pink color appears.
  4. Record the volume of KMnO4 used (V mL).
  5. Calculate the concentration of KNO2 using the formula: M1V1/n1 = M2V2/n2
  6. Calculate the theoretical molarity of pure KNO2 (Molar mass = 85 g/mol):
    M = (5.0 g/L) / (85 g/mol) = 0.0588 M
  7. Calculate the actual molarity from titration data.
  8. Percentage purity = (Actual molarity / Theoretical molarity) × 100