Evaluation of the sequestering ability of different complexones towards Ag+ ion

The interaction between Ag+ cation and different ligands was evaluated as a function of temperature and ionic strength by means of potentiometric techniques. Six aminopolycarboxylic (APCs) and aminopolyphosphonic (APPs) ligands were chosen, namely four APCs: nitrilotriacetic acid (NTA), ethylene-glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA), ethylenediamine-N,N,N',N'-tetraacetate (EDTA) and diethylenetriamine-N,N,N',N”,N”-pentaacetic acid (DTPA)] and two APPs: [(1-hydroxyethane-1,1-diyl)bis(phosphonic acid) (HEDP) and [[(Phosphonomethyl)imino]bis[2,1-ethanediylnitrilobis(methylene)]] tetrakis-phosphonic acid (DTPP). Different mononuclear species with general formula AgHiL and different number of protons were found. For example in the case of the DTPP, six species with i = 0 to 5 were determined. The ionic strength dependence of the formation constants was studied for all the six ligands, whereas the temperature dependence only for the EDTA, HEDP and DTPP. Among these ligands, DTPP shows the highest formation constant values for the AgL species. The stability trend is: DTPP > DTPA > EDTA ~ EGTA > HEDP > NTA. It was found that the formation constant values decrease with increasing temperature and ionic strength. All the complex formation reactions are endothermic and enthalpic driven. The sequestering ability was evaluated using a useful tool, named pL0.5, which uses a Boltzmann type equation to determine the percentage of metal bound to the ligand as a function of the ligand concentration and other chemico physical parameters (e.g. pH, temperature, ionic strength). Also in this case, the trend traces that of the formation constants, although the difference between DTPP and DTPA is lower.