The Beer-Lambert Law and Its Limitation Essay

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Introduction A complex consists of a central metal ion with a number of other molecules or ions surrounding it, which are known as ligands. Ligands donate at least a lone pair of electrons to the vacant d-orbitals of the metal ion to form coordinate covalent bond. The ligand field strength caused by different ligands can make the energy difference between the d-d orbitals splitting to vary. Therefore, different wavelength with equal energy with that energy difference is absorbed while the transmitted wavelengths in visible light can be observed as its colour. Hence, the formation of a complex in solution is usually being accompanied with the appearance of a colour. Optical density is also known as absorbance which is the measurement on a solution to illustrate the relative concentration of a complex ion formed in the solution. In ‘Jobs Method’, a series of solutions is prepared by mixing the metal ion and ligand solution in different compositions, however, the total concentrations of the metal ion and ligand in all the mixture remain constant. Initially, the measurement of optical density on the series of solutions should be made at a wavelength, λmax, where the maximum absorption by the complex formed occurs. Next, the measurement of optical density at this suitable wavelength, λmax, will show a maximum value for the solution in which the ratio of amount of ligand to metal is equal to that in a complex. In this solution, both metal ion and ligand in the solution become the limiting reagents in the complex formation reaction whereby the maximum amount of complex is produced compared to the solution with other compositions. This is because the solutions with other compositions contain a limiting reagent for one of the reactants and another exists in excess, thus, the amount of complex produced is restricted by that limiting reagent. The absorption law, also known

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