Use of the twin-cell differential titration calorimeter for binding studies. I. EDTA and its calcium complex

The use of a twin-cell differential titration calorimeter is described which utilizes small volumes (1-3 ml) of modest concentrations of materials (0.001-0.01 M) and yields data of good precision. Operation is controlled by a microprocessor which regulates and controls the addition of reagents and collects and displays the data as time, temperature in volts, and the pH. Corrections for the titration of water are applied to the potentiometric data, and the thermal data are corrected for the initial temperature-time baseline, the changes in heat capacity, and the heat loss (or gain) to the external environment. Finally, the thermal signal is corrected for the heat derived from the formation of water due to the free hydrogen or hydroxyl ions present. The corrected data as pH, groups titrated and delta HT (kcal/mol) can then be used to obtain the parameters pK' and delta Hi involved with the equilibria by curve-fitting the observed data. The system has been applied to the ionization of EDTA and its calcium complex. The ionization constants, the heats of ionization, the stoichiometry of binding and the heat of binding have been determined and demonstrated to be in agreement with published values.