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
KMnO4 (aq) reacts with oxalic acid, H2C2O4, in acidic solution by
the reaction:
5 H2C2O4 (aq) + 2 MnO4-1 (aq) + 6 H+1 (aq) 
10 CO2 (g) + 2 Mn+2 (aq) + 8 H2O (l)
0.2585 g of oxalic acid is dissolved in approximately 100 mL of
0.5 M H2SO4. Upon titration with KMnO4 and adding 22.35 mL,
a faint purple color persists indicating the reaction is complete.
What is the concentration of the KMnO4 solution?
?M
0.2585 g
22.35 mL
5 H2C2O4(aq) + 2 MnO4-1(aq) + 6 H+1(aq)10 CO2(g) + 2 Mn+2(aq) + 8 H2O (l)
𝟎. 𝟐𝟓𝟖𝟓 𝒈 𝑯𝟐 𝑪𝟐 𝑶𝟒
𝟎. 𝟎𝟐𝟐𝟑𝟓 𝑳 𝑴𝒏𝑶𝟒
−𝟏
𝟏 𝒎𝒐𝒍𝒆𝑯𝟐 𝑪𝟐 𝑶𝟒 𝟐 𝒎𝒐𝒍𝒆𝑴𝒏𝑶𝟒 −𝟏
∗
∗
𝟗𝟎. 𝟎 𝒈𝑯𝟐 𝑪𝟐 𝑶𝟒
𝟓 𝒎𝒐𝒍𝒆𝑯𝟐 𝑪𝟐 𝑶𝟒
= 0.0514 M MnO4-1
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
Example #2
The concentration of aqueous I3-1 solution can be
determined by titration with aqueous sodium thiosulfate,
Na2S2O3. We can use a starch indicator, which turns from
deep blue to colorless when all the I3-1 has reacted.
What is the molar concentration of I3-1 if 24.55 mL of 0.102
M Na2S2O3 is needed for complete reaction with 10.00 mL
of the I3-1 solution?
0.102 M
?M
24.55 mL
10.00 mL
2 S2O3-2 (aq) + I3-1 (aq)
0.02455 𝐿 𝑆2 𝑶𝟑 −𝟐
0.01000 𝐿 𝑰𝟑 −𝟏
∗

S4O6-2 (aq) + 3 I-1 (aq)
0.102 𝑚𝑜𝑙 𝑆2 𝑶𝟑 −𝟐
1 𝐿 𝑆2 𝑶𝟑 −𝟐
*
1 𝑚𝑜𝑙 𝑰𝟑 −𝟏
2 𝑚𝑜𝑙𝑒 𝑆2 𝑶𝟑 −𝟐
= 0.125 M I3-
Example #3
Normal range of Ca+2 ion in human blood is 9.0 - 11.5 mg of Ca+2 in
100 mL of blood. Abnormal levels (high and low) are indicative of
several diseases. 1.00 mL of blood is treated with Na2C2O4 to
precipitate calcium as CaC2O4. The precipitate is isolated and
dissolved in H2SO4. This solution is titrated with 4.88 x 10-4 M
KMnO4. 2.05 mL KMnO4 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 1.060 g, what is the mass % of
the Ca?
(a) How many mg calcium (Ca+2) are in 100 mL of this blood?
.000488 M
2.05 mL
? mg
2 KMnO4(aq) + 5 CaC2O4(aq) + 8 H2SO4(aq)  2 MnSO4(aq) + K2SO4(aq) +
5 CaSO4(aq) + 10 CO2(g) + 8 H2O(l)
0.000488 mole KMnO4 5 mole CaC2O4 1 mole Ca+2
40.1 g Ca+2 1000 mg
.00205 L KmnO4 x ---------------------------- x ------------------- x ------------------ x -------------- x ----------- = 0.100 mg Ca+2
1 L KMnO4
2 mole KMnO4 1 mole CaC2O4 1 mole Ca+2 1 g
100 mL x
0.100 mg Ca+2
----------------- = 10.0 mg
1 mL blood
(b) If the 1 mL of blood has a mass of 1.060 g, what is the mass % of the Ca?
% Ca+2
0.100 mg Ca+2
= -------------------- x 100% = 0.00943 %
1060. mg blood