The pH in the solution was kept constant by adding drops of
either 0.5 M HCl or 0.5 M NaOH solutions, assuring that the total
liquid volume was not affected. The experiments were carried
out in duplicate (only Section 4.1 shows both results obtained,
thereafter all the results are the average values since the error
was less than 2% between the duplicates in all experiments) without
stirring and at fixed temperature (25 C).