One of the important methods in Quantitative Analysis is Volumetric Analysis, a commonly used laboratory technique. It is used to determine the unknown concentration of a sample by measuring its volume. This process is also called titration. In a titration, a solution of unknown concentration is reacted with a solution of known concentration. The solution taken in the burette is called the titrant and the solution taken in the conical flask is called the analyte.
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.
A solution of known concentration is called the standard solution. A standard solution can be prepared by dissolving a known quantity of the substance in a definite volume of the solvent. The substance used to prepare the standard solution can be classified into two types.
A primary standard has the following features.
Oxalic acid, Mohr's salt, potassium dichromate are some examples of primary standards.
Substances whose standard solutions cannot be prepared directly are called secondary standards.
Some examples are sodium hydroxide and potassium permanganate.
The concentration of a solution can be expressed in the following ways.
Normality: It is defined as the number of gram equivalent of solute dissolved in one litre of the solution. It is denoted by the letter 'N'.
Molarity: It is defined as the number of gram moles of solute dissolved in one litre of the solution. It is denoted by the letter 'M'.
Molality: It is defined as the number of moles of the solute dissolved in 1Kg of the solvent. It is denoted by the letter 'm'.
Titration can be classified into various types depending upon the chemical reactions occurring during titration. One of the commonly known titrations is the Acid-Base titration. It is a method used to determine the strength of an acid or alkali and this type of titration is based on the neutralisation reaction. In this reaction, acids and bases react to form salt and water.
An indicator is a chemical substance that undergoes a colour change at the endpoint. The endpoint of an acid-base titration can be determined using acid-base indicators. Acid Base indicators are either weak organic acids or weak organic bases. The colour change of an indicator depends on the pH of the medium. The un-ionized form of an indicator has one colour, but its ionized form has a different colour.
For example, consider the indicator phenolphthalein, whose ionization can be written as,
|Phenolphthalein||8.0 - 10.0||Colourless||Pink|
|Methyl Orange||3.1 - 4.4||Red||Orange|
|Methyl Red||4.4 - 6.2||Red||Yellow|
|Phenol Red||6.4 - 8.0||Yellow||Red|
|Thymol Blue||1.2 - 2.8||Red||Yellow|
|Thymol Blue||8.0 - 9.6||Yellow||Blue|
|Methyl Yellow||2.9 - 4.0||Red||Yellow|
Acid-base titration can be classified into the following types.
Determination of the strength is based on the Law of Equivalents. According to this law, the number of equivalence of the substance to be titrated is equal to the number of equivalence of the titrant used.
Consider an acid-alkali titration. V1 cm3 is that of an acid solution of normality N1 required to neutralize V2 cm3 of a base of normality N2.
We know that 1000 cm3 of 1N acid solution contains acid = 1 gram equivalent
V1 cm3of 1N acid solution contains acid= V1/1000 gram equivalents
Thus number of gram equivalents of acid in V1 cm3 of N1 acid solution is;
Similarly, number of gram equivalents of base in V2 cm3 of N2 basic solution is;
By the law of equivalents, at the end point,
This is called the Normality Equation.
Similarly, Molarity equation can be written as,
Here we determine the strength of HCl by titrating it against a standard solution of sodium carbonate and they react to form NaCl, CO2 and water. The chemical reaction can be represented as;
Here 1 mole of sodium carbonate reacts with 2 moles of HCl. So according to Molarity equation,