Assessing ion-exchange properties and purity of lipophilic electrolytes by potentiometry and spectrophotometry

Abstract

Many ionic salts synthesized using metathesis are often found to contain significant amounts of impurities, despite careful control of the weighing of starting materials. In this work, a potentiometric method is devised to monitor ion-exchange properties (or 'purity') of an organic solvent containing a lipophilic electrolyte. Its permselective behaviour is monitored by treating the solvent as a liquid membrane and contacting it with two aqueous solutions with different electrolyte activities. This electrolyte mismatch results in a drastic potential change when excess lipophilic cation-exchanger is titrated with anion-exchanger, altering the membrane from being cation to anion responsive.Here, the cation-exchanger potassium tetrakis(4-chlorophenyl)borate (KTpClPB) dissolved in nitrobenzene was titrated with tetradodecylammonium chloride (TDDACl), in contact with Ag/AgCl electrodes placed in aqueous 1M and 10 -2 M KCl, respectively.The predicted potential change of -214mV was observed at the equivalence point, forming the inert lipophilic electrolyte ETH 500, in a very small concentration range of added anion-exchanger (0.8% for ±10mV), suggesting good precision. The approach was confirmed by monitoring absorbance and fluorescence intensity changes of the chromoionophore Nile Blue. This method may be applied for the synthesis of a range of highly lipophilic salts for which established metathesis protocols are not suitable. KeywordsETH 500, potentiometry, ionic liquid purity, inert lipophilic electrolyte purity, Nile Blue, spectrophotometry 2

Potentiometric determination of coextraction constants of potassium salts in ion-selective electrodes utilizing a nitrobenzene liquid membrane phase

“…Here, an anion exchanger is titrated into a membrane phase containing only a cation exchanger at a fixed concentration. The salts used as the cation and anion exchanger were chosen to be KTpClPB and TDDACl, respectively, for reasons described in our recent publication [6]. The organic phase is in contact with two aqueous phases, separated by a Celgard porous membrane, to prevent the mixing of the phases whilst allowing ion-transfer across the phase boundaries.…”

“…Based on our recent work [6], it is possible to predict the potential jump, ΔE, resulting from the titration of excess anion exchanger into a membrane phase containing a known concentration of cation exchanger (see equation (1)). The experiment involves treating the polar organic solvent, nitrobenzene, as a membrane phase, and contacting it with two aqueous phases of different ion activity.…”

“…Where the prime and double prime symbols denote activities/concentrations at the outer and inner phase boundaries of the liquid membrane phase, respectively. The potential can calculated at all concentrations of added anion exchanger by solving equation (5) for [I + ] org for each phase boundary and inserting the results into equation (6). While all other parameters are known (defined at the start of the experiment), the value of coextraction constant is varied until the best fit to experimental data is obtained.…”

“…In the examples where anions other than chloride are employed, a background of 10 -3 M KCl was added to maintain a constant potential at the Ag/AgCl elements. As a result, an additional parameter was added to equation (6) to account for the extra chloride concentration:…”

“…We recently introduced a potentiometric method at the ITIES to monitor the ionexchange properties of a nitrobenzene organic phase containing either excess cationexchanger or excess anion-exchanger, in contact with two aqueous phases containing potassium chloride with different ion activity [6]. The two salts used as the cation and anion exchanger in the organic phase were KTpClPB (potassium tetrakis(4chlorophenyl)borate) and TDDACl (tetradodecylammonium chloride), respectively.…”