REDOX TITRATIONS

REDOX Titrations A redox titration is used to determine the concentration of an oxidizing or reducing agent. The reaction needs to have a 100% yield (go to completion – no excess of either reactant) and there needs to be a color change to indicate when the reaction is complete. Indicators can be used but often there is a color change when one of the reactants undergoes oxidation or reduction (and that serves as the indicator).

Example #1 KMnO 4 (aq) reacts with oxalic acid, H 2 C 2 O 4, in acidic solution by the reaction: 5 H 2 C 2 O 4 (aq) + 2 MnO 4 -1 (aq) + 6 H +1 (aq)  10 CO 2 (g) + 2 Mn +2 (aq) + 8 H 2 O (l) g of oxalic acid is dissolved in approximately 100 mL of 0.5 M H 2 SO 4. Upon titration with KMnO 4 and adding mL, a faint purple color persists indicating the reaction is complete. What is the concentration of the KMnO 4 solution?

? M g mL 5 H 2 C 2 O 4 (aq) + 2 MnO 4 -1 (aq) + 6 H +1 (aq)  10 CO 2 (g) + 2 Mn +2 (aq) + 8 H 2 O (l) whenever you are finding an M value, you must start with a fraction (because M is a fraction) AND the bottom of the fraction needs to be the volume in liters of THAT solution = M MnO 4 -1

Example #2 The concentration of aqueous I 3 -1 solution can be determined by titration with aqueous sodium thiosulfate, Na 2 S 2 O 3. We can use a starch indicator, which turns from deep blue to colorless when all the I 3 -1 has reacted. What is the molar concentration of I 3 -1 if mL of M Na 2 S 2 O 3 is needed for complete reaction with mL of the I 3 -1 solution?

0.102 M ? M mL mL 2 S 2 O 3 -2 (aq) + I 3 -1 (aq)  S 4 O 6 -2 (aq) + 3 I -1 (aq) = M I 3 -

Example #3 Normal range of Ca +2 ion in human blood is mg of Ca +2 in 100 mL of blood. Abnormal levels (high and low) are indicative of several diseases mL of blood is treated with Na 2 C 2 O 4 to precipitate calcium as CaC 2 O 4. The precipitate is isolated and dissolved in H 2 SO 4. This solution is titrated with 4.88 x M KMnO mL KMnO 4 required to reach the end point (faint violet color remains). (a) How many mg calcium (Ca +2 ) are in 100 mL of this blood? (b) If the 1 mL of blood has a mass of g, what is the mass % of the Ca?

(a) How many mg calcium (Ca +2 ) are in 100 mL of this blood? M 2.05 mL ? mg 2 KMnO 4 (aq) + 5 CaC 2 O 4 (aq) + 8 H 2 SO 4 (aq)  2 MnSO 4 (aq) + K 2 SO 4 (aq) + 5 CaSO 4 (aq) + 10 CO 2 (g) + 8 H 2 O(l) mole KMnO 4 5 mole CaC 2 O 4 1 mole Ca g Ca mg L KmnO 4 x x x x x = mg Ca +2 1 L KMnO 4 2 mole KMnO 4 1 mole CaC 2 O 4 1 mole Ca +2 1 g mg Ca mL x = 10.0 mg 1 mL blood (b)If the 1 mL of blood has a mass of g, what is the mass % of the Ca? mg Ca +2 % Ca +2 = x 100% = % mg blood