Our objective is to determine the strength of KMnO4 solution by titrating it against a standard solution of;
Titration is a common laboratory method of qualitative chemical analysis that can be used to determine the unknown concentration of a solution (analyte). The basis of this process is the reaction between the analyte and a solution of unknown concentration (standard solution). The analyte is taken in a conical flask using a pipette and the solution of known concentration is taken in a calibrated burette (titrant).
A solution whose concentration is known, is called a standard solution. The substance used to prepare a standard solution is called the primary standard. Oxalic acid and sodium carbonate are some examples.
Concentration of a solution is defined as the amount of a solute present in a definite volume of the solvent. Concentration of a solution can be expressed in different ways.
The endpoint of a titration is the point at which the reaction between the titrant and the analyte becomes complete. Generally the endpoint of a titration is determined using indicators. In some cases, either the reactant or the product can serve as the indicator. A best example is the redox titration using potassium permanganate.
Titrations can be classified as:
We will learn about Redox titrations.
The titration based on oxidation and reduction reaction between the titrant and analyte is called Redox titration. Oxidation is the process of the addition of oxygen or removal of hydrogen/electron and reduction involves the process of addition of hydrogen/electrons or removal of oxygen. Oxidizing agents are substances that gain one or more electrons and are reduced. Reducing agents are substances that lose one or more electrons and are oxidized. That is, oxidizing agents are electron acceptors, and reducing agents are electron donors.
In redox systems, the titration method can be adopted to determine the strength of a reductant/oxidant using a redox sensitive indicator. Redox titrations involving potassium permanganate are called permanganometric titrations. In these reactions, MnO4- ions acts as the self indicator.
The molecular mass of crystalline oxalic acid is, H2C2O4.2H2O = 126
Weight of oxalic acid crystals required to prepare 1000 ml of 1 M solution = 126 g
Therefore, weight of oxalic acid required to prepare 250 ml 0.1 M solution =
In this titration KMnO4 is the titrant and oxalic acid is the analyte. Here, potassium permanganate is the oxidizing agent and oxalic acid is the reducing agent. The reaction between potassium permanganate and oxalic acid is carried out in an acidic medium because permanganate ion in the acidic medium is a very strong oxidizing agent. Acidity is introduced by adding dil. H2SO4.
No other indicators are used to determine the endpoint, because KMnO4 acts as the indicator. Permanganate (MnO4-) ion has a dark purple colour. In an acidic medium, MnO4- is reduced to colourless manganous (Mn2+) ions. On reaching the end point, the addition of the last single drop of permanganate imparts a light pink colour to the solution. The chemical reaction that takes place during titration can be represented by the chemical equation.
From the balanced chemical equation, it is clear that 2 moles of KMnO4 reacts with 5 moles of oxalic acid.
According to the molarity equation,
If oxalic acid is to be titrated, add the required amount of dil. H2SO4 and heat the flask to 60°-70°C. The purpose of heating is to increase the rate of reaction, which otherwise is slow at room temperature.
The molecular mass of Mohr's salt is, FeSO4.(NH4)2SO4.6H2O= 392
Weight of Mohr's salt required to prepare 1000 ml of 1 M solution = 392 g
Therefore, weight of Mohr's salt required to prepare 250 ml 0.05 M Mohr's salt solution =
In this titration, potassium permanganate is the oxidizing agent and Mohr’s salt is the reducing agent. Mohr’s salt is a double salt of ferrous sulphate and ammonium sulphate and its composition is FeSO4.(NH4)2SO4.6H2O. It is a primary standard. Therefore, its standard solution can be prepared directly. Ferrous ions of Mohr’s salt undergo hydrolysis in aqueous solution. To prevent the hydrolysis, Conc. H2SO4 needs to be added to the Mohr’s salt crystals during the preparation of its standard solution.
In this titration, the MnO4- ion is reduced to Mn2+ in the presence of acid and Fe2+ ions of Mohr’s salt is oxidized to Fe3+
The chemical reaction that occurs in this titration can be represented by the following chemical equations.
In ionic form the reaction can be represented as,
From the overall balanced chemical equation, it is clear that 2 moles of potassium permanganate react with 10 moles of Mohr’s salt.