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 ( L, a, b) of the chromatic space CIELAB 1976 are calculated by means of the transmittances of the entire spectral range. The curves, in the a,b plane, of chromatic differences vs. pH intersect at a pH value agreeing with the p Ka; 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 Ka= 3.943 at 25 °C; this result is compared with those obtained through the usual spectrophotometric methods. Also, thermodynamic magnitudes obtained from p Ka values at different temperatures from 20 to 70 °C have been determined, resulting in Δ H0 = -(13.93 ± 0.88) kj mol−1 Δ S0 = -(122.6 ± 2.8) J K−1 mol−1.

The decolorizing effects of different quantities of activated carbon adsorbent on the color of sherry samples (Fine wine) were studied. The color change was measured through the use of chromatic parameters determined by the CIELAB 76 system, using transmittances taken at 1-nm intervals throughout the visible spectrum. The effect of the adsorbent on the color of the wine has been expressed by equations that relate the values of the chromatic parameters with the quality of the added adsorbent. These equations show that the discoloring process follows a Freundlich-type adsorption isotherm. Using these equations, it is possible to predict the amount of carbon adsorbent necessary to attain a desired color.

After a careful check of the response linearity of the spectrophotometer in absorbances and transmittances at wavelengths between 250 and 550 nm with potassium dichromate, the influence of the concentration of solutions on the tristimulus coordinates and the coordinates of the CIE 1931 (Commission Internationale de l'Éclairage) and CIELAB colorimetric systems was analyzed. How to use the value of only some chromatic coordinates to obtain a sample's concentration simultaneously is also shown.

Color characterization of olive oil may be of great importance to the industry. To determine the color of a solution, it is necessary to accurately measure a series of tristimulus coordinates for which several methods exist. This study analyzes the errors in the calculation of tristimulus values of olive oil color based on methods, by using several selected ordinates and an increasing number of weighted ordinates, and how these errors affect the values of the chromatic parameters defined in the various chromatic systems. The above analysis shows that the use of a large number of ordinates will lead to better results in the color definition of oils. For its determination, we have used the CIE 1931, CIELUV 1976 and CIELAB 1976 spaces; the latter yields the best results.