Lab Experience n. 3 Redox Titration

Tuesday 22nd January 2013

Lab Experience n. 3 Redox Titration

Objective/Task:

Investigate the amount of hydrogen peroxide in commercial oxygenated water.

Theoretical Background: 

Reaction between hydrogen peroxide and permanganate in acidic media is as follows: 

2 MnO₄^- + 5 H₂O₂ + 6 H⁺ à 2 Mn⁺ + 5 O₂ + 8 H₂O

For more information refer to: 

San Francisco de Paula (2012); Laboratory Handbook 10º FyQ - ByG, Year 2012-2013; Page 42. Experiment 5: REDOX TITRATION.

Materials:

- Stand and clamp

- Burette

- Erlenmeyer's flask

- Pipette

- 100 mL volumetric flask

- Beaker with Potassium permanganate 0.05 M dissolved in water

- Commercial oxygenated water

- Sulfuric acid 2M

Procedure:

1. Arrange the burette in the stand with the clamp. Fill it with the permanganate solution, assuring that no bubbles remain anywhere, and adjust the level to 0. 

2. Make a solution of commercial oxygenated water taking 10 mL of the one provided and diluting it to 100mL in a volumetric flask with distilled water.

3. Take 25 mL of this solution with a pipette and pour it in the Erlenmeyer. Add 25 mL of 2M sulfuric acid and 25 mL of distilled water.

4. Titrate the mix with the permanganate solution until a slight violet colour remains in the Erlenmeyer (that means that you have put just one drop more than needed).

5. Record the volume needed and repeat the whole procedure to verify the result.

6. Calculate the hydrogen peroxide concentration of the commercial oxygenated water remembering the dilution performed at the beginning and compare your result with the one on the commercial bottle.

Notes and changes on the method (evaluation of the procedure):

1. You might need to open the tap so that no air is left and return the liquid to the burette to adjust it at 0. If you cannot see if the meniscus is just in the line, meaning that you have gone beyond the scale, take out liquid and leave it a a lower level but take note of the level at which you have started so that at the end you can calculate the difference from what has remained and with what you started.

2. We didn't perform step 2 and 3 as stated, instead we added 4mL of H₂O₂ with a pipette to the Erlenmeyer and we went to the teacher who poured in our Erlenmeyer's a small amount of 4M H₂SO₄ (sulfuric acid), that amount wasn't measured with accuracy, he just poured the approximated amount. This might be the source of error of our final result.

3. We were just asked to calculate the difference in volume of the permanganate solution inside the burette between the inicial and final volume.

4. We placed the paper we normally put on the table, under the Erlenmeyer while doing the titration to see the reaction clearly, it is a way in which we can be more precise while obtaining the exact higher volume of permanganate that can be dissolved, meaning the volume of the limiting reactant.

Pictures:

These three first images represent how your burette should look like. It should have any bubbles of air. As you can see in two first pictures, we had to open the tap to let the solution get into the whole burette, some of the solution went out, but it was returned to the burette as well as more amount of this solution which was needed to adjust the volume to a determine number.            
                                                                                                                            

                                                                                              

1.

2.

3.

4. 

This is the picture we took of how we adjusted the volume, although you cannot see it clear, we adjusted it at 0,5 mL, because it was the only way to be sure that we had adjusted it correctly and we didn't went beyond the scale on the burette. What we did wasn't wrong as long as we record the volume at which we had adjusted the volume, for future calculations. 

Results: 

In order to calculate the volume of O₂ gas was produced by the oxygenated water:

Step 1: Calculate the volume consumed of  all the reactants.
H₂O₂ : 4 mL
KMnO:
V₀ = 49,5 mL 
Our initial volume was 49,5 mL, because the burette's maximum capacity was of 25 mL however we adjusted it to 0,5 which leads us to have as initial volume 49,5 mL.
V_f = 26,3 mL.
Which means that the amount of potassium permanganate (KMnO₄) needed so that the solution in the Enrlenmeyer stayed pink was: 49,5 - 26,3 = 23,2 mL of KMnO4.
Step 2: Balance the equation of the reaction:

Step 3: Calculate the number of moles of potassium permanganate consumed during the reaction.

When we see the excess of one of the reactants, this means that there is a limiting reactant and that the reaction has stopped.

Step 4: Determine the number of moles of H₂O_{2  consumed in the reaction.}

Step 5: If we had used 4 mL of H₂O₂, calculate the molarity (M) (concentration of solute per litre of solution) of H₂O_{2 .}

The concentration of oxygenated water is measured in 'volumes'. We have been told tal the decomposition of any volume gives ten times that amount of O₂. Volume 'O₂' gas produced by a given volume of liquid H₂O₂. Commercial H₂O₂: 10 volumes of O₂.
Step 6: If we have used 4 mL of H₂O₂, determine the volume of O₂ produced at standard temperature and pressure. Use the state equation or law of ideal gases.

Step 7: 

Videos:

Isabel Caro