Chemical Speciation: A Guide to Understand Titrimetric Analysis

Chemical speciation modeling (equilibrium simulation program, MINTEQA2) was used to understand the chemistry behind titrimetric analysis and to monitor changes in experimental conditions in chemical titrations quickly and easily. Determination of magnesium and both magnesium and calcium in a mixture by EDTA titrations can be done at pH 10 in the presence of Eriochrom black T indicator. The end point is detected by the color change from red to blue. Simulation results of these titrations highlighted the fact that over the pH range 7-11, complexation of magnesium and calcium by EDTA is independent of pH while indicator speciation is highly dependent on pH. Thus the quantitative determination of magnesium and total magnesium and calcium by EDTA titrations using Eriochrom black T should be done at pH values of 9-10. By using computer simulation programs it is easy to calculate the chemical speciation at each step in a titration and to gain a better understanding of why a particular procedure does or does not work under particular experimental conditions.