In a complexometric titration of metal ion with ligand $M ($ Metal ion $)+ L ($ Ligand $) \rightarrow C ($ Complex $)$ end point is estimated spectrophotometrically (through light absorption). If 'M' and 'C' do not absorb light and only 'L' absorbs, then the titration plot between absorbed light
( $A$ ) versus volume of ligand 'L' $(V)$ would look like
To solve this problem, we need to understand how the concentration of the ligand 'L' during the titration affects the absorption of light. Let's break down the process:
The reaction outlined is M + L \rightarrow C where M is the metal ion, L is the ligand, and C is the complex.
Given that M and C do not absorb light, and only L absorbs, the absorption of light (A) measured during the titration is directly proportional to the concentration of L, i.e., A \propto [L].
Initially, as we add ligand L, the solution starts absorbing more light due to the increase in free L concentration. Thus, the absorption (A) increases.
As L reacts with M to form complex C, the concentration of free L in the solution will decrease, causing a decrease in the absorption after a certain volume of L has been added. This is because once the complex forms, L is consumed and hence its free concentration decreases.
The endpoint of the titration occurs when all of M has reacted with L to form C. Beyond this point, any additional ligand would result in an increase in absorption, as the added L is no longer reacting and stays free in the solution.
Based on this reasoning, the absorption vs. volume of ligand 'L' plot will first increase, reach a peak (at the endpoint), and then start increasing again as more free L is added beyond the endpoint. The correct option illustrating this plot is the second one:
This plot shows an initial increase in absorption, reaching a maximum point (corresponding to the endpoint of the titration), and then a further increase as additional L contributes to more absorption.
