REDOX TITRATION REACTIONS
ACID-BASE TITRATION
- neutralization between _____________and _________
REDOX TITRATION
- Redox reaction between ______________ agent and
___________________ agent
- ________________ are transferred from acid to base
- _________________ are transferred from
______________ agent to _________________ agent
EXAMPLES: REDOX TITRATION PROBLEMS
1. ANALYSIS OF IRON WITH MANGANATE(VII)  All the iron in a 2.000 g tablet was dissolved in an acidic
solution and converted to Fe2+. This was then titrated with KMnO4. The titration required 27.50 cm3 of 0.100 mol dm-3
KMnO4. Calculate the total mass of iron in the tablet and its percentage by mass. Describe what would be observed during
the reaction, and how the equivalence point can be detected.
2. IODINE-THIOSULFATE REACTION  Household bleach is an oxidizing agent that contains sodium
hypochlorite, NaOCl, as the active ingredient. It reacts with iodide ions in acidic solution as follows:
OCl-(aq) + 2I-(aq) + 2H+(aq)  I2(aq) + Cl-(aq) + H2O(l)
A 10.00 cm3 sample of bleach was reacted with a solution of excess iodide ions, and the liberated iodine was then titrated
with Na2S2O3. The titration required 38.65 cm3 of 0.0200 mol dm-3 Na2S2O3. Determine the concentration of OCl- in the
bleach.
3. AN ENVIROMENTAL APPLICATION OF REDOX CHEMISTRY: USING THE WINKLER METHOD TO
CALCULATE DISSOLVED OXYGEN
*The WINKLER METHOD  can be used to measure biochemical oxygen demand (BOD), used as a measure of the
degree of _______________________ in a water sample (Dissolved oxygen content is one of the most important
indicators of water quality, as oxygen is essential to the survival of aquatic life.)
*BOD
 the amount of _________________ used to decompose the organic matter in a sample of water over a specified
 the BOD is expressed in ______ (1 mg dm-3 = 1 ppm)
 [BOD = *______ ppm – (concentration of dissolved O2 in sample, in ppm)]
*Max solubility of O2 in water at 293 K = 9.00 ppm.
SUMMARY OF IMPORTANT CONNECTIONS:
___POLLUTION = ___ BIODEGRADABLE ORGANIC WASTE = ___ OXYGEN USE BY BACTERIA IN
DECOMPOSITION REACTIONS = ___ DISSOLVED OXYGEN CONTENT = ___ BOD
THE WINKLER METHOD  THE SPECIFICS
*Based on a sequence of redox reactions as follows:
(i)
2Mn2+(aq) + O2(g) + 4OH-(aq)  2MnO2(s) + 2H2O(l)
 dissolve O2 ‘fixed’ by addition of a manganese(II) salt such as MnSO4 in basic solution
(ii)
MnO2(s) + 2I-(aq) + 4H+(aq)  Mn2+(aq) + I2(aq) + 2H2O(l)
 MnO2 [from step (i)] reacts with acidified iodide ions to produce I2
(iii)
2S2O32-(aq) + I2(aq)  2I-(aq) + S4O62-(aq)
 I2 produced [in step (ii)] is then titrated with sodium thiosulfate
**For every ONE mole of O2 in the water, _____ mol of S2O32- are used.
EXAMPLE PROBLEM (WINKLER METHOD): A 500 cm3 sample of water was collected and tested for dissolved
oxygen by the addition of MnSO4 in basic solution, followed by the addition of acidified KI. It was found that 12.50 cm3
of 0.0500 mol dm-3 Na2S2O3(aq) was required to react with the iodine produced.
(a) Calculate the dissolved oxygen content of the water in g dm-3, using the equations given above. (Ans.: 0.0102 g dm-3)
(b) Deduce the BOD, in ppm, of the water sample, assuming that the maximum solubility of oxygen is 9.00 ppm at 293 K. (Ans. -1.2)
(c) Comment on the BOD value obtained with reference to the following data:
Example Source
Pure water
Untreated domestic sewage
Effluent from a brewery
Water from an abbatoir
BOD (ppm)
Less than 1
350
500
3000
MORE PRACTICE (REDOX TITRATIONS)
1. A bag of ‘road salt’, used to melt snow from roads, contains a mixture of calcium chloride, CaCl2, and sodium chloride,
NaCl. A 2.765 g sample of the mixture was analyzed by first converting all the calcium into calcium oxalate, CaC2O4.
This was then dissolved in H2SO4, and titrated with 0.100 mol dm-3 KMnO4 solution. The titration required 24.65 cm3 of
KMnO4(aq) and produced Mn2+(aq), CO2(g) and H2O(l).
(a) What would be observed at the equivalence point of the titration?
(b) Write the half-equation for the oxidation reaction, starting with C2O42-.
(c) Write the half-equation for the reduction reaction, starting with MnO4-.
(d) Write the overall equation for the redox reaction.
(e) Determine the number of moles of C2O42-.
(f) Deduce the number of moles of Ca2+ in the original sample.
(g) What was the percentage by mass of CaCl2 in the road salt?
2. Alcohol levels in blood can be determined by a redox titration with potassium dichromate, K 2Cr2O7, according to the
following equation.
C2H5OH(aq) + 2Cr2O72-(aq) + H+(aq)  2CO2(g) + 4Cr3+(aq) + 11H2O(l)
(a) Determine the alcohol percentage in the blood by mass if a 10.000 g sample of blood requires 9.25 cm3 of
0.0550 mol dm-3 K2Cr2O7 solution to reach equivalence.
(b) Describe the change in colour that would be observed during the titration.