Measuring an Equilibrium Constant The Scatchard Plot

To measure an equilibrium constant, we measure concentrations (actually activities) of species at equilibrium. This section shows how spectrophotometry can be used for this purpose. 

Let s examine the equilibrium in which the species P and X react to form PX. 

Consider a series of solutions in which increments of X are added to a constant amount of P. Letting P(t be the total concentration of P (in the forms P and PX), we can write 

Absorbance is proportional to concentration (not activity), so concentrations must be converted into activities to get true equilibrium constants. 

Clearing the cobwebs from your brain, you realize that Equation 19-12 is a mass balance. 

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