Spectrophotometric titrations_Multicomponent Analysis.pptx

Spectroscopy

1. Spectrophotometric titration Instrumental Methods of Analysis Dr. Rajasekhar Reddy A M.Pharm., Ph.D

2. The process of determining the quantity of a sample by adding measured increments of a titrant until the end-point, at which essentially all of the sample has reacted, is reached. The titration is followed by measuring the absorbance of radiation in the range of ultraviolet to near-infrared by the sample. If at least one component in a titration process absorbs electromagnetic radiation, we can identify the end point by monitoring the titrand's absorbance at a carefully selected wavelength Spectrophotometric titration

3. For example, we can identify the end point for a titration of Cu 2+ with EDTA, in the presence of NH 3 by monitoring the titrand's absorbance at a wavelength of 745 nm, where the Cu(NH 3 ) 4 2+ complex absorbs strongly. At the beginning of the titration the absorbance is at a maximum. As we add EDTA the concentration of Cu(NH 3 ) 4 2+ , and thus the absorbance, decreases as EDTA displaces NH 3 . After the equivalence point the absorbance remains essentially unchanged. A corr  =  A  × ( V EDTA  +  V Cu )/ V Cu Note that the titration curve’s y-axis is not the actual absorbance, A, but a corrected absorbance, A corr Spectrophotometric titration

4. Correcting the absorbance for the titrand’s dilution ensures that the spectrophotometric titration curve consists of linear segments that we can extrapolate to find the end point. Spectrophotometric titration only the titrand absorbs; (b) only the titrant absorbs; (c) only the product of the titration reaction absorbs; (d) both the titrand and the titrant absorb; (e) both the titration reaction’s product and the titrant absorb; (f) only the indicator absorbs. 

5. The Spectrophotometric assay of drugs rarely involves the measurement of absorbance of samples containing only one absorbing component. The pharmaceutical analyst frequently encounters the situation where the concentration of one or more substances is required in samples known to contain other absorbing substances, which potentially interfere in the assay. If the formula of the samples is known, the identity and concentration of the interferents are known and the extent of interference in the assay may be determined. Multi-component Analysis:

6. Some of the commonly used Spectrophotometric methods are as follows : Simultaneous equation method (Vierdott’s method) Derivative Spectrophotometric method Absorbance ratio method ( Q-Absorbance method) Solvent extraction method Dual wavelength method Geometric correction method Orthogonal poly nominal method H-point standard addition method Least square approximation method

7. Th e b a sis o f all the Spe c t r oph o t o m e tric t echniq u e s f or multicomponent samples is the property that at all wavelengths: The absorbance of a solution is the sum of absorbance of the individual components or The measured absorbance is the difference between the total absorbance of the solution in the sample cell and that of the solution in the reference cell. And most importantly the excipients present in the formulation are not absorbing at the wavelength of experiment. If all o f th e se condi t i ons a r e s a ti s f i e d th e n w e c a n a p ply t h e s e methods satisfactorily.

8. 1. Simultaneous equation method : If a sample contain two absorbing drugs (X & Y) each of this absorbs at the λ max of each other i.e. λ 1 and λ 2 (figure ), it may be possible to determine both the drugs by the technique of simultaneous equation method provided that certain criteria apply.

9. The information required is: The absorptivity of X at λ 1 and λ 2 is a x1 and a x2 respectively. The absorptivity of Y at λ 1 and λ 2 is a y1 and a y2 respectively. The absorbance of the diluted sample at λ 1 and λ 2, is A 1 and A 2 respectively. Let C x & C y be the concentration of X & Y respectively in the diluted sample. Two equations are constructed based upon the fact that at λ 1 and λ 2 the absorbance of the mixture is the sum of the individual absorbance of X & Y. At λ 1 A 1 = a x1 bc x + a y1 bc y ………(1) At λ 2 A 2 = a x2 bc x + a y2 bc y ………(2)

10. Substituting for c y in eq. (1). And rearranging gives c x = A 1 – a y1 c y /a x1 c x = A 1 a y 2 – A 2 a y 1 / a x 1 a y 2 – a x 2 a y 1 And c y = A 2 a x 1 – A 1 a x 2 / a x 1 a y 2 – a x 2 a y 1 These criteria are satisfied only when the λ max of the two components are reasonably dissimilar. An additional criterion is that the two components must not interact chemically, thereby negating the initial assumption that the total absorbance is the sum of individual absorbance. A 2 = a x2 bc x + a y2 bc y ………(2) A 1 = a x1 bc x + a y1 bc y ………(1) Rearrange eq. (2). c y = A 2 - a x2 c x / a y2

11. It depends on the property that for a substance which obeys Beer’s law at all wavelengths, the ratio of absorbance at any two wavelengths is a constant value independent of concentration or pathlength Q value (Ratio of absorbance at two wavelengths = A1/A2