Redox Titration

Redox Titration: The Standardization of Potassium Permanganate Lab Objectives: Titration Using a Primary Standard, Oxidation/Reduction Titration Equipment and Materials: magnetic stirrer / hotplate, stir bar, 50cc buret with stand and clamp, 400mL beaker, weigh boat, analytical balance, ~0.02M KMnO4 , reagent grade Na2C2O4 , 100mL graduated cylinder, 6.0M H2SO4 Sodium oxalate is commonly used as a primary standard for determining the concentration of many strong oxidizers used in oxidation/reduction analyses. In this experiment, sodium oxalate will be used to determine the concentration of the strong oxidizer potassium permanganate. Sodium oxalate, Na2C2O4, is a strong electrolyte that dissociates completely in water according to the following equation: Na2C2O4 → 2 Na+ (aq) + C2O42- (aq) The oxalate ion, C2O42-, will react quantitatively with permanganate ion, MnO4-, in the presence of strong acid according to the following equation: 2 MnO4- (aq) + 5 C2O42- (aq) + 16 H+ (aq) ↔ 2Mn2+ (aq) + 10 CO2 (g) + 8 H2O The numerical value of Keq for the equilibrium is very large. When reaction conditions are anywhere near optimum (in terms of pH and temperature) the reaction can be considered to be quantitative. Procedure 1.

Transfer 200mL of water to a 400mL (or larger) beaker. Place the beaker on a magnetic stirrer/hotplate, add a stir bar, and start the stirrer on slow speed. If a magnetic stirrer is not available the solution can be stirred by hand with a stirring rod.

2.

Add 40mL of 6.0M sulfuric acid to the water. CAUTION: Sulfuric acid is extremely corrosive.

3.

Turn on the heater (full on). Monitor the temperature with a thermometer. You will need to heat the acid solution to about 80oC to 90oC. Continue with the procedure while the acid solution is heating.

4.

Use an analytical balance to mass 0.20g to 0.25g (NO MORE!) of reagent grade sodium oxalate in a weigh boat. Record the mass of sodium oxalate on the data sheet. Record the value to the nearest 0.0001g.

5.

Transfer the sodium oxalate sample to the acid solution. You may use a water bottle to facilitate the transfer if necessary.

6.

Rinse a buret with a few milliliters of the potassium permanganate solution. Dispose of the permanganate waste as directed by your teacher.

7.

Fill the buret with the permanganate solution above the zero mark. Drain the buret until the liquid level reads exactly zero. Make sure all the air is expelled from the tip of the buret. Mount the buret in a buret stand and position the buret over the acid/oxalate solution.

8.

Once the temperature of the acid/oxalate solution has reached the desired 80oC to 90oC range turn off the heater.

9.

Rapidly add ~5mL of the permanganate titrant and allow the solution to stir until the purple color disappears.

10. Continue the titration dropwise. Position the buret’s stopcock so that the permanganate titrant drips at a moderate pace. Do not allow the titrant to pour rapidly beyond the first five mLs. 11. As the titration approaches it’s equivalence point the purple color from the permanganate will persist longer and longer before it disappears. 12. Slow the rate of titration as the equivalence point is approached.

UAH Science in Motion

1

13. Immediately stop the titration when you believe the equivalence point is reached. The equivalence point is defined as the point in the titration where the color from the titrant persists for 30 seconds. A perfect titration will yield a faint pink solution that persists for a few minutes. 14. If the color fades before the 30 second requirement add more titrant one drop at a time until the 30 second requirement is reached. 15. Once you are satisfied the equivalence point has been reached record the volume of titrant (to the nearest 0.1mL) on the data sheet .

Calculations: 1.

Calculate the moles of Na2C2O4 in your sample. Record this value on the data sheet. The molar mass of Na2C2O4 is 134.00g.

2.

The stoichiometric ratio between oxalate and permanganate is 5:2. Use this to find the moles of MnO 4- need to react with the moles of C2O42- in your sample. Record this value on the data sheet.

3.

Calculate the molarity of the permanganate solution. Record this value on the data sheet.

UAH Science in Motion

2

Data Sheet A. Mass of Na2C2O4 = _____________________g (Record to nearest 0.0001g) B. Volume of KMnO4 used = ___________________mL (Record to nearest 0.1mL) C. Moles of Na2C2O4 = _____________________mol D. Moles of KMnO4 = _____________________mol

E. Molarity of KMnO4 =____________________M Questions: 1.

Why is it necessary to perform the titration in strong acid?

2.

Speculate on what would happen if the titration were attempted in neutral or basic conditions.

3.

Why is it necessary to heat the solution?

4.

Why is it necessary to make sure all air is removed from the tip of the buret?

5.

Write an expression for the equilibrium constant of the reaction.

6.

Define the term “primary standard.”

UAH Science in Motion

3

Questions: 1.

Why is it necessary to perform the titration in strong acid? A: The equation tells us that 16 moles of H+ are consumed for every 5 moles of sodium oxalate and 2 moles of potassium permanganate. Without an excess of strong acid the equilibrium positions of the reactants would remain very large relative to the equilibrium positions of the products.

2.

Speculate on what would happen if the titration were attempted in neutral or basic conditions. A: The rate of the forward reaction would be near zero. Essentially no apparent reaction would occur.

3.

Why is it necessary to heat the solution? A: To facilitate the removal of the gaseous CO2 product. At room temperature buildup of CO2 in the solution would tend to retard the forward rate of the reaction.

4.

Why is it necessary to make sure all air is removed from the tip of the buret? A: To prevent volume measurement error in the experiment. If the tip is not cleared of air in the beginning but gets cleared during the experiment Veq will be overstated.

5.

Write an expression for the equilibrium constant of the reaction.

6.

Define the term “primary standard.” A: A primary standard is a reagent grade solid that reacts quantitatively with some component of a solution. This allows for the direct computation of the concentration of the solution component given the mass of the primary standard and the volume of the solution required for complete reaction.

UAH Science in Motion

4