Abstract
A method has been developed for the determination of acid-base equilibrium constants, which is applicable to compounds that absorb in the visible. The tristimulus (X, Y, Z) and the colorimetric coordinate values (<i>L*, a*, b*</i>) of the chromatic space CIELAB 1976 are calculated by means of the transmittances of the entire spectral range. The curves, in the <i>a,b</i> plane, of chromatic differences vs. pH intersect at a pH value agreeing with the p<i>K</i><sub><i>a</i></sub>; this configuration occurs because a chromatic equilibrium of both species takes place at this point, which corresponds to the chemical equilibrium. With this method, the acidity constants are found easily and quickly, and they yield very good results. The validity of the method has been tested with the acid-base indicator bromophenol blue, obtaining p<i>K</i><sub><i>a</i></sub>; = 3.943 at 25 °C; this result is compared with those obtained through the usual spectrophotometric methods. Also, thermodynamic magnitudes obtained from p<i>K</i><sub><i>a</i></sub> values at different temperatures from 20 to 70 °C have been determined, resulting in Δ<i>H</i><sub>0</sub> = -(13.93 ± 0.88) kJ mol<sup>-1</sup>, Δ<i>S</i><sup>0</sup> = -(122.6 ± 2.8) J K<sup>-1</sup> mol<sup>-1</sup>.
PDF Article
More Like This
Cited By
You do not have subscription access to this journal. Cited by links are available to subscribers only. You may subscribe either as an Optica member, or as an authorized user of your institution.
Contact your librarian or system administrator
or
Login to access Optica Member Subscription