Jawaharlal Nehru Technological University Hyderabad B.Pharmacy IV Year INSTRUMENTAL METHODS OF ANALYSIS Previous Question Paper (March-2021 - R17)

Beer-Lambert's law states that when a monochromatic light passes through a homogeneous absorbing medium, the rate of decrease in intensity of incident light with respect to the thickness of the medium is directly proportional to the intensity of the incident light and the concentration of the absorbing substance.

Mathematically, the law is represented as:

Where A is absorbance, I₀ is incident light intensity, It is transmitted light intensity, ε is molar absorptivity, b is the path length, and c is the concentration.

Derivation:

1. Consider a beam of monochromatic light of intensity I passing through a solution of thickness db. The decrease in intensity (-dI) is proportional to the intensity (I), the concentration (c), and the thickness (db): -dI ∝ Icdb or -dI = kIcdb.
2. Rearranging gives -dI / I = kcdb.
3. Integrating the expression from I₀ to It for intensity and 0 to b for thickness: ∫(-dI / I) = ∫kcdb.
4. This results in -ln(It / I₀) = kcb, or ln(I₀ / It) = kcb.
5. Converting to base 10 by dividing by 2.303, we get log(I₀ / It) = (k / 2.303)cb.
6. Substituting ε = k / 2.303, we obtain A = εbc.

UV-Visible spectroscopy is a versatile analytical technique used for both qualitative and quantitative analysis in pharmaceutical and chemical industries:

• Quantitative Analysis: It is widely used to determine the concentration of drugs in formulations by comparing the absorbance of the sample against a standard calibration curve following Beer-Lambert's law.
• Identification of Functional Groups: It helps in identifying the presence of chromophores (e.g., conjugated double bonds, carbonyl groups) in organic molecules based on characteristic absorption maxima (λmax).
• Purity Determination: It is used to detect impurities in substances; if an impurity absorbs at a specific wavelength, the absorbance value will deviate from the pure substance.
• Study of Chemical Kinetics: The rate of a reaction can be monitored by measuring the change in absorbance of a reactant or product over time.
• Molecular Weight Determination: By preparing derivatives of compounds with chromophores, the molecular weight can be estimated.
• Dissolution Studies: It is used to measure the rate at which a drug dissolves from a dosage form into a dissolution medium over time.