Spectrophotometric K Determinations

All methods of equilibrium constant determination using spectrophotometric measurements are based ultimately on the validity of Beer s Law. One of the reactants and/or products must have a characteristic spectrum from which the concentration of one of the species can be determined reliably. The concentrations of the remaining species in the equilibrated reaction mixture can then be deduced from the original solution composition and the reaction stoichiometry. Several methods will be described. 

Spectrophotometric Titration Method. This method is no more tedious or time consuming than the other two, and has the advantage of producing data of higher quality and abundance. It represents a close analogy to the potentiometric titration method, with measurement of absorbance at a large 

At pH values much higher and lower than the indicator pK the spectrum obtained for a solution of the indicator is  

When the pH of the solution is such that there are significant amounts of both In and HIn present, the absorbance is the sum of the contribution of the two. At any one wavelength 

The value of obtained at each of a series of wavelengths chosen so that 8jjj - 8j I is reasonably large [there may be one or more wavelengths where the 8jjj = 8j. These are called isosbestic points and are useful to measure C, total concentration]), is  

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