Chemical Analysis by Redox Titration

 

Introduction:

Titration is a common method for determining the amount or concentration of an unknown substance. The method is easy to use if the quantitative relationship between two reacting substances is known. The method is particularly well-suited to acid-base and oxidation-reduction reactions. In this experiment, you will conduct two separate redox titrations using a standardized permanganate solution. In the first titration, you will be trying to find the % hydrogen peroxide in a commercially sold solution. In the second titration, you will be trying to find the % iron in an unknown iron salt.

Permanganate ion is a powerful oxidizing agent, especially in acidic solution, which can be used to analyze (by titration) solutions containing many different species. In these titration reactions, the intensely colored MnO4- ion is reduced to form the colorless Mn2+ ion.

 

Species Titrated Species Formed in Titration

H2O2 O2

Fe2+ Fe3+

H2C2O4 CO2

 

An advantage of using the permanganate ion in the titration of colorless unknown solutions is that it is "self-indicating". As long as the reducing agent remains present in the sample, the color of MnO4- quickly disappears as it is reduced to Mn2+. However, at the endpoint, all the reducing agent has been used up so the next drop of MnO4- solution is sufficient to cause an easily detected color change, colorless ( faint, permanent peach/pink. So we know that at the endpoint, the oxidizing agent (MnO4-) and reducing agent (H2O2 or Fe2+) have reacted in exactly in proportion to their stoichiometry in the balanced redox equation. If we know how much of the oxidizing agent we added, then we can figure out exactly how much reducing agent was present in the unknown!

 

Titration Notes:

1. Always rinse buret with water (from a beaker, not the faucet) first. Second, rinse with a small amount of the titrant and drain it through the tip.

2. Fill the buret with the titrant using a funnel.

3. Fill the buret tip by momentarily opening the stopcock.

4. Now you are ready to read the initial volume (bottom of the meniscus). Remember that burets are graduated in a downward direction. The first estimated digit will probably be the hundredths place.

5. Do not waste time trying to fill the buret to zero for each titration.

6. Do not start above the 0 mL mark or titrate past the 50 mL mark.

7. Always use white paper underneath your sample flask so that you will notice slight color changes.

8. Learn to swirl the flask without removing it from underneath the buret.

9. Use a drop, drop, drop pace until you see the color change becoming more than local (where the titrant meets the sample). Now proceed dropwise.

10. Second and third trial titrations should always be fast assuming the sample will be about the same because you now know approximately how much titrant is needed. If the first titration required 25 mL than you can add 22 mL all at once and then proceed cautiously.

11. Remember that the amount of water used to dilute the sample is not crucial because it does not affect "how many" of the sample molecules are present in the sample flask. Diluting with water allows you to see the color change easier.

12. Always rinse sample flasks before using.

13. Always label multiple burets and sample flasks.

14. Did you add your indicator?

A. Preparation of a Standard Permanganate Solution

Procedure:

1. Using a 250.0 mL volumetric flask, prepare 250.0 mL of a "standard" 0.1xxx M (xxx means that it does not have to be exactly 0.1000 M) solution of MnO4- using KMnO4(s) as the permanganate source. You should calculate the amount of KMnO4 you will need before coming to lab. After massing the

approximate (as long as you know the amount accurately than you will be able to calculate the concentration accurately) amount, transfer it to your flask, add about 125 mL of water and 30 mL of 3 M H2SO4, dissolve the solid completely, and then dilute to volume.

 

Data:

* mass of KMnO4 dissolved

 

Calculations and Questions:

1. Show how mass of KMnO4 was determined.

2. Calculate the concentration of your standard solution to the correct number of sig figs (consider the volumetric flask to be 250.0 mL).

3. Write equation for the dissociation of KMnO4 in water. Write the molarity of each species underneath.

 

B. Titration of an Unknown Hydrogen Peroxide Solution

Procedure:

1. Obtain approx. 10.xxx g (record its mass accurately) of the commercial hydrogen peroxide solution in a 100 mL plastic beaker. Transfer the solution to a clean 250 mL Erlenmeyer flask. Rinse the small beaker twice with a small amount of water and add the rinsings to the flask. Dilute the sample in the flask to about 75 mL with water, then add about 20 mL of 3 M H2SO4.

2. Obtain a 50 mL buret and rinse it with water. Do a final rinse with a small amount (5 mL) of the standard MnO4- solution. Fill the buret with the standard solution. Fill the buret tip by momentarily opening the stopcock. Record the initial reading.

3. Place a sheet of white paper under the sample flask. Now slowly begin titrating the H2O2 solution while it is continously being stirred by gently swirling the flask. Continue titrating until you see the color of MnO4- begin to persist locally in the solution, at which point, you should slow down to dropwise additions. Continue until one added drop of MnO4- solution produces a faint peach/pink color that lasts at least 30 seconds. This is the first excess MnO4- which is not being reduced by the H2O2. Record the final buret reading.

4. Complete a second trial. For excellent work, the calculated percents need to be within 1% of each other.

 

Data:

* mass of H2O2 sample for each trial

* concentration of standard solution

* initial buret reading for each trial

* final buret reading for each trial

* volume of MnO4- used in each trial

* observations of reaction

Calculations:

1. Write the balanced net ionic equation for the reaction (show the working equation as well). Identify the oxidizing and reducing agents.

2. Calculate the mg of H2O2 in the sample for each trial.

3. Calculate the %H2O2 by mass in the commercial sample for each trial.

4. Calculate the avg. %H2O2 in the commercial hydrogen peroxide solution.

5. How many oxygen molecules were produced in the average titration?

6. How many molecules of hydrogen peroxide were in the whole bottle (use a density that you think would be appropriate)?

7. Explain the effect on your calculated results for H2O2 if tap water had been used to make your standard solution and samples. Include two balanced equations.

 

C. Titration of an Unknown Iron Salt

Procedure:

1. Obtain approx. 2-3 g of an unknown iron salt. Record the number of your unknown. Transfer the sample to a 250 mL Erlenmeyer flask. Dilute the sample to about 75 mL with water then add about 20 mL of 3 M H2SO4.

2. Titrate as in Part B.

3. Complete a second trial.

4. When finished, clean and rinse both the volumetric flask and buret thoroughly because MnO4- solutions will cause bad glass stains. Rinse with an acidic solution of H2O2. Place about 50 mL of the solution in a flask. Swirl the solution until the flask is clean and then transfer it to another piece of stained glassware. When you are done using it, pass it on to another group.

 

Data:

* # of unknown iron salt

* mass of unknown iron salt for each trial

* concentration of standard solution

* initial buret reading for each trial

* final buret reading for each trial

* volume of MnO4- used for each trial

Calculations:

1. Write the balanced net ionic equation for the reaction (show the working equation as well). Identify the oxidizing and reducing agents.

2. Calculate the mg of Fe2+ in the sample for each trial.

3. Calculate the %Fe2+ by mass in the unknown iron salt for each trial.

4. Calculate the avg. %Fe2+ in the salt.

Lab Report #1.6:

* title page

* abstract

* procedure sheet

* data and calculations for part A (then part B, part C)

* results and discussion - separate paragraphs for each part (including A). Read about R + D on lab report format handout and consider the following: variation in duplicates, comparisons to class averages, and comparison to labels.

 


Questions? Comments??

W.G. Rushin, Cary Academy