EXPERIMENT #9 PRELAB EXERCISES Redox Titration (Molarity Version) Name Section 1. Balance the following redox reaction under acidic conditions. C 2 O 2-4 (aq) + MnO - 4 (aq) CO 2 (g) + Mn 2+ (aq) 2. A 5.00 gram sample of impure sodium oxalate required 36.91 ml of 0.100 M KMnO 4 to reach the endpoint. What was the percent purity of the sample? You will need the balanced equation from question #1.
EXPERIMENT #9 A Redox Titration Using Potassium Permanganate In this experiment you will perform a REDOX TITRATION using potassium permanganate as the titrant. Potassium permanganate is a powerful oxidizing agent because of the high positive charge on manganese (see Questions section), and is used in a wide variety of chemical analyses of reducing agents such as the determination of iron in iron ore or nitrites in aquarium water. In this lab we will determine the percent purity of an impure sample of sodium oxalate, Na 2 C 2 O 4. Titrations involving permanganate are normally carried out in acidic solutions, and the half reaction for permanganate under these conditions is: MnO 4 - (aq) + 8H + (aq) + 5e - Mn 2+ (aq) + 4H 2 O( ) There is one problem with using permanganate ion in titrations, and that is it is able to autocatalyze it's own destruction. This is because as Mn 2+ is produced during the titration, the Mn 2+ can actually react with additional MnO 4 -, producing solid MnO 2 as a result: 3Mn 2+ (aq) + 2 MnO 4 - (aq) + 2H 2 O( ) 5 MnO 2 (s) + 4H + (aq) The unfortunate result of this side reaction is that you overestimate the amount of analyte that is being consumed because you must add more titrant to reach the endpoint. There are several methods that have been developed to overcome this difficulty, and we will use the McBride method where the titration is carried out at an elevated temperature. This speeds up the reaction with oxalate ion, and since the reaction of Mn 2+ with MnO 4 - is relatively slower, the titration error can be minimized. The oxidation half reaction in the titration occurs when oxalate ion, C 2 O 4 2-, is oxidized to carbon dioxide: C 2 O 4 2- (aq) 2CO 2 (g) + 2e - The combination of half-reactions yields the overall reaction - 5 C 2 O 4 2- (aq) + 2 MnO 4 - (aq) + 16 H + (aq) 10 CO 2 (g) + 2Mn 2+ (aq) + 8 H 2 O( ) An additional advantage of permanganate titrations is that it serves as its own indicator due to its deep purple color. The titration is stopped at the first persistent light pink color. In Part I of the procedure you will standardize the unknown concentration of your permanganate solution using a known quantity of primary standard sodium oxalate. In Part II you then use the standardized permanganate to titrate a sample of sodium oxalate with an unknown percent purity.
Supplies 50 ml buret and clamp KMnO 4 solution (unknown molarity - approx. 0.10 M) 20 ml pipet and pipetting aid 28.000 g/l Na 2 C 2 O 4 solution (to standardize the KMnO 4 ) two 250 ml erlenmeyer flasks (one is already in 3M H 2 SO 4 your drawer) hot plate or bunsen burner sodium oxalate with unknown percent purity thermometer Procedure Part I: Determining the Concentration of KMnO 4 Using Primary Standard Sodium Oxalate 1. Prime a clean buret with the permanganate solution by rinsing it with approx. 2-3 ml of KMnO 4 into a beaker. 2. Fill the buret with permanganate to approx. 0.0 ml (it doesn t have to be exact), and make sure there are no air bubbles in the buret tip by dispensing a small portion into a beaker. 3. Record the initial volume of permanganate in the data sheet. 4. Using a 20-mL pipet, transfer 20 ml of sodium oxalate solution into the two Erlenmeyer flasks. To each flask add 50 ml of distilled water and 20 ml of 3M H 2 SO 4. 5. Heat the contents of the first flask to about 80 o C, and then turn off the Bunsen burner. 6. Now titrate slowly while stirring until a permanent faint pink coloration is attained. You will see a brown solid forming that quickly disappears, which is completely normal. If the solution changes completely to brown, then you ll need to do a another sample. 7. Record the volume used in the first titration. 8. Repeat steps 5 and 6 for the second flask. 9. In the data sheet calculate the molarity of the permanganate and average the results.
Part II: Determination of the Percent Purity of an Unknown Sample of Sodium Oxalate 1. Record your unknown number on the data sheet. 2. Thoroughly clean your two Erlenmeyer flasks. 3. You have been given approx. 10-g of unknown. Accurately weigh out three 3 gram samples using weighing paper the third sample is an extra in case anything goes wrong. 4. Place two of the samples into the 250 ml Erlenmeyer flasks, and dissolve each sample in 75 ml of distilled water. This will take some vigorous stirring since sodium oxalate is hard to dissolve. You might have to gently heat. 5. CAREFULLY add 25 ml of 3M H 2 SO 4 to each flask. 6. Heat the first sample to about 80 o C again. Remove the heat, and titrate to a light pink color. Gently swirl the flask while adding titrant. 7. Repeat for the second sample. 8. Calculate the percent purity of your unknown using the data sheet.
DATA SHEET - Lab #9 Redox Titration Name Part I: Determining the Concentration of KMnO 4 Using Primary Standard Sodium Oxalate Observations Trial 1 Trial 2 Volume of Na 2 C 2 O 4 soln pipetted Buret, KMnO 4, 1 st reading Buret, KMnO 4, 2 nd reading Volume KMnO 4 used 20.00 ml 20.00 ml 1. Calculate the number of moles of Na 2 C 2 O 4 pipetted. Carry extra sig fig s, and note that the number of moles will be the same for each trial: 2. Calculate the number of moles of KMnO 4 that will react with the number of moles of oxalate you calculated in step 1 using the equation - 5 C 2 O 4 2- (aq) + 2 MnO 4 - (aq) + 16 H + (aq) 10 CO 2 (g) + 2Mn 2+ (aq) + 8 H 2 O( ) 3. Calculate the molarity of the KMnO 4 in each trial (the two trials will be different because the volume of permanganate will vary because of experimental error): 4. Average molarity of the KMnO 4 solution (round off to correct number of sig fig s) =
Part II: Determination of the Percent Purity of an Unknown Sample of Sodium Oxalate Unknown # Observations Trial 1 Trial 2 Weight of unknown sample Buret reading, KMnO 4, 1 st time Buret reading, KMnO 4, 2 nd time Volume of KmnO 4 solution used Average molarity of the KMnO 4 solution (from Part I of the data sheet, step 4) = 1. moles of KMnO 4 used (carry extra sig fig s): 2. grams of Na 2 C 2 O 4 in the sample (carry extra sig fig s): 3. percent purity = mass Na 2 C 2 O 4 X 100% mass sample 4. Average percent purity =