Spectrophotometric Titrations

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 Cu2+ with EDTA, in the presence of NH3 by monitoring the titrand’s absorbance at a wavelength of 745 nm, where the Cu(NH3)42+ complex absorbs strongly. At the beginning of the titration the absorbance is at a maximum. As we add EDTA the concentration of Cu(NH3)42+, and thus the absorbance, decreases as EDTA displaces NH3. After the equivalence point the absorbance remains essentially unchanged. 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, Acorr

Acorr = A × (VEDTA + VCu)/VCu

where VEDTA and VCVCuu 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.

Figure9.31

Examples of spectrophotometric titration curves: (a) 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. The red arrows indicate the end points for each titration curve.

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