Titrations of Permanganate
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.
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.
If you know how much of the reducing agent (H2O2 or Fe2+) was in the solution, you can calculate the mass of the reducing agent present, then calculate the mass% of species in the orginal sample ([mass Fe/mass of sample] for example)
0.1 M MnO4- solution
Prepare 250.0 mL of a "standard" 0.1xxx 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. You will need to know the exact mass that got into the flask, but the 250 mL flask is too heavy to put on the analytical scale. When diluting the solution in the flask, add about 125 mL of water and 30 mL of 3 M H2SO4, dissolve the solid completely, and then dilute to volume. Once made, put your solution in DRY water bottle you brought to class. Cap tightly and label with your name and concentration.
Titration of an Unknown Hydrogen Peroxide Solution
In a 125 mL place a known MASS (~5 g) of commercial hydrogen peroxide. To it add ~50 ml of DI water and 20 mL of 3 M H2SO4. Fill your buret (after 2x 5mL rinse, remember) with your MnO4- solution. 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. Determine how much MnO4- was added in the titration and then calculate the mass% of H2O2 in your sample. Repeat runs until you have 3 that are within 2% of each other. You will report average and standard deviation from 3 ‘good’ runs.
Titration of an Unknown Iron Salt
1. Obtain approx. 1-2 g of the unknown iron salt, add ~50 ml of DI water and 15 mL of 3 M H2SO4. Titrate as before, getting at least three ‘good’ runs. The unknown salt could be any one of the following: Iron (II) sulfate, iron (II) nitrate, iron (II) chloride, or iron (II) sulfate heptahydrate (7 waters for every 1 iron (II) sulfate). You will determine the mass % of Fe in your solid sample and it should match of of these choices.
2. 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 which will be provided by your instructor. 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 as it can be ‘recycled’
Lab report: Filled out data sheet (see class blog) complete with sample calculations sheet (seperate) and commentary.
Prelab Questions: (worth 8 points)
1. What mass of KMnO4 will you need to make 250 mL of a 0.1 M MnO4- solution?
2. In this lab, you will be reducing MnO4- to Mn2+ in the presence of H+. The reduction half reaction is
MnO4-(aq) + 8H+(aq) + 5e- → Mn2+(aq) + 4 H2O(l)
The H2O2 is being oxidized into H2O and O2. That half reaction is
H2O2(aq) → O2(g) + 2 H+(aq) + 2 e-
Write the overall (balanced) REDOX reaction that is occurring in H2O2 titration portion of this experiment.
3. In the other section of this experiment, you will be oxidizing Fe2+ into Fe3+. The oxidation half reaction for that should be easy to figure out. Write the overall (balanced) REDOX reaction that is occurring in the iron salt portion of this experiment.
4. 16.4 mL of a 0.104 M solution of KMnO4 were needed to titratite (to endpoint) a 5.013 g sample of H2O2 solution. What is teh mass % of H2O2 in the sample?
--- You will need to bring in an EMPTY water bottle
for storing your KMnO4 solution that you will make. This should be a
'disposable' bottle as you will not want to use it after lab.---
Last updated by MEO :25Feb08