Abstract:

Analytical chemistry of solutions deals with reactions and equilibrium state of solutes in order to find appropriate conditions to characterize or quantify analytes.
Acid-base indicators are used in many procedures of classical analysis and the color change is the result of electronic changes of the indicator molecules or of its ionized forms, caused by the exchange of protons . The exchange of protons takes place at pH values in the vicinity of pKa of the molecule of indicator.

The teaching of how acid-base indicators work may be purely phenomenological, based only on pKa values and the Henderson-Hasselbalch relation, or it can be visualized through structural changes and its implications on the electronic absorption spectra in the visible region. Most frequently used acid-base indicators are discussed using molecular modeling programs like Odyssey and Spartan. This is a valuable help for students not only to enhance learning of the concept of pH but also to correlate the change of color with the change of the structure of the indicators.
Similarly complex formation between metallic ions in aqueous solutions and polydentate ligands like aminopolycarboxylic acids may be simulated using molecular modeling programs. The process of complex formation between EDTA and a series of metallic cations allows discussion of the effect pH and of ionic radius of metallic cations and thermodynamic stability of formed complexes.
With the aid of 3D structures the understanding of the process is easier for students.

Keywords:

Analytical chemistry, molecular modeling, pH, acid-base indicators.