Methodological aspects in the study of alkali metal ion weak complexes using different ISEs electrodes

The detection of alkali metal complex formation with polycarboxylate, hydroxo-polycarboxylate and amino-polycarboxylate ligands has been investigated employing ISE-Na+ potentiometry. To minimize the effects due to the variation of activity coefficients, a set of measurements were made at constant ionic strength. The formation of alkali metal complexes in solution has been evidenced unequivocally comparing the experimental values of free Na+ ions with those of total sodium added for each point of titration curve. There are no doubts about the formation of alkali metal complexes in solution. After detection, the formation constants were determined and their values, compared with those evaluated by pH-metric technique (previously) or by ISE-Na+ potentiometry at variable ionic strengths (this work), show a good agreement, suggesting that also potentiometric techniques at variable ionic strengths can be used in the study of this topic with a good accuracy. Moreover, our results confirm the low stability of alkali metal complexes (ion pairs) in aqueous solution, which is founded on coulomb interactions. In general, the values of stability constants depend not only on the number of charged oxygen donor groups, but also on the presence of amino donor(s) (NTA, EDTA and EDDS) and hydroxyl group (citrate).