Volumetric analysis
Volumetric analysis refers to the analytical method based on the measurement of the volume of a solution required to react completely with a definite volume of another solution. By comparing the volumes of the two solutions, the concentration of one substance can be calculated provided that the concentration of another solution is known. The volumes of the two solutions, their stoichiometric ratios in the chemical reaction, and
their concentrations are involved in the calculations. The volume required to complete a particular reaction is easily identified through colour changes of the resulting products or pH indicators used during titration.
Thus, titration is a chemical analysis in which the unknown quantity or concentration of a chemical species is determined by adding to a known quantity or concentration of another substance. In titration, the two
substances react in a definite known proportions. These proportions are known as stoichiometric coefficients.
The stoichiometric coefficients are the numbers which represent the actual ratios of the quantities of the reactants and products during the occurrence of a particular chemical reaction. They are represented by numbers appearing before each reactant and product species in a chemical equation. The stoichiometric coefficients are also called mole ratios. The solution with a known concentration is called a standard solution.
In titration, the exact volumes of solutions are measured by using apparatus such as burettes, pipettes, and volumetric flasks. The laboratory procedure involves measuring the volumes of solutions with known and unknown concentrations. The solution with known concentration is referred to as a titrant, and it is usually placed in the burette. The titrant is titrated against the solution with an unknown concentration called the titrand (analyte) which is kept in a conical flask. The completion of the reaction is called the end point. The end point is therefore the point where the analyte has completely reacted with the titrant. The completion of the reaction is indicated by colour change of an indicator used (See an example in Figure 5.1) or colour changes of the reacting species or products formed.
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Figure 5.1: Set-up of titration showing colour change at the end point
Chemical reactions of the involved substances in titration are represented by using balanced chemical equations. For example, a balanced chemical equation for a titration process which involves sulphuric acid and sodium hydroxide solutions is:
H2SO4(aq) + 2NaOH(aq)________Na2SO4 + 2H2O(l)
In this balanced chemical equation, the stoichiometric coefficients are 1, 2, 1, and 2 for H2SO4, NaOH, Na2SO4, and H2O, respectively. Thus, H2SO4 and NaOH react at known definite proportions of 1 to 2. These proportions will determine the reacting volumes of the acid and alkali in relation to their concentrations. If the concentration of the acid solution is known, then, by taking a fixed volume of the NaOH and adding H2SO4
from the burette dropwise, the end point can be identified by colour change of the pH indicator. Before the end point, the reaction mixture contains excess alkali. After the end point, the reaction mixture contains excess acid. In acid-base titration, a change in colour of the pH indicator is caused by the reaction between the pH indicator and an excess reagent when the other reagent has completely reacted.