Spectrophotometric titrations

INSTRUMENTAL METHODS OF ANALYSIS, B.PHARM 7TH SEM. AND FOR BSC,MSC CHEMISTRY. This is Geeta prasad kashyap (Asst. Professor), SVITS, Bilaspur (C.G) 495001

1. Spectrophotometric Titration s If at least one species in a complexation titration absorbs electromagnetic radiation, we can identify the end point by monitoring the titrand’s absorbance at a carefully selected wavelength. 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

2. A corr  =  A  × ( V EDTA  +   V Cu )/ V Cu where  V EDTA  and  V C V Cu u are, respectively, the volumes of EDTA and Cu. 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. Other common spectrophotometric titration curves are shown in panels b-f. The resulting spectrophotometric titration is shown below in panel (a). Note that the titration curve’s  y -axis is not the actual absorbance,  A , but a corrected absorbance,  A corr

3. 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