PURPOSE
The purpose of this experiment is to analyze iron content of an unknown solution by reduction-oxidation titration with a standardized permanganate solution.
THEORY
Conditions of Redox Analysis
Redox titration is based on reduction and oxidation, it is similar to acid-bade titration if the four principles are met:
1 The reaction is thermodynamically spontaneous enough to be stoichiometric
2 The reaction is usually fast to produce an immediate result
3 There is no other reaction that occurs
4 There is an adequate indicator (Burk, 84)
Analysis of Fe2+ by Redox Titration:
By these principles, the titration of Fe (II) solutions with different redox reagents is easy. As long as the chemical conditions are optimized then potassium permanganate which is an oxidation reagent is able to meet the above principles.
The reaction equation
The overall reaction requires that the oxidation be taken five times for each reduction
MnO4-(aq) +8H3O+(aq) +5Fe2+(aq) 5Fe3+(aq) + Mn2+(aq) + 12H2O(l)
The half reaction is:
Fe2+ reducing agent: Fe2+ Fe3+ + 1e-
MnO4- oxidizing agent: MnO4- + 8H3O+ +5e- Mn2+ + 12H2O
Excess of acid (H2SO4) and titration at higher temperature can be done to speed up the reaction because the rate of at which equilibrium is achieved is slow.
The oxidizing agent gives its adequate indicator because the reagent MnO4- is extremely purple while the product Mn2+ is colourless. Therefore the end point must be read quickly because the MnO4- end point in an acidic solution disappears slowly due to the reaction of MnO4- with the Mn (II) that is formed during the titration. The colourless Fe2+ does not react but the orange hydrated Fe3+ can be unclear at the end point. Side reactions can be avoided in the titration by chemical procedures. If the reaction is not acidic, then in neutral solutions the reaction will produce MnO2 while in an alkaline solution MnO42- will be produced at a low rate.
Standardization of Permanganate...