Potentiometric Determination of Coextraction Constants

1 A theoretical treatment of potentiometric data is applied to calculate coextraction 2 constants (K IA) for three potassium salts from water into a liquid nitrobenzene phase. 3 The experiment involves treating nitrobenzene as a membrane and contacting it with 4 two aqueous solutions of different ion activities. In the presence of either a cation or 5 anion exchanger, the ratio of activities of ions in the two aqueous phases gives rise to 6 a potential difference across the membrane that depends upon the nature and charge 7 of the counter ion of the ion-exchanger in excess. Here, the cation exchanger was 8 chosen to be potassium tetrakis(4-chlorophenyl)borate (KTpClPB) and the anion 9 exchanger was tetradodecylammonium chloride (TDDACl). TDDACl was 10 incrementally added to the nitrobenzene phase containing a fixed concentration of 11 KTpClPB, and the corresponding emf was recorded as a function of concentration of 12 TDDACl. The membrane changes from one with cation exchanger properties (excess 13 KTpClPB) to one with anion exchanger properties (excess TDDACl). The potential 14 difference and shape of the titration curve can be predicted by theory based on the 15 phase boundary potential model. Log(K IA) values calculated for KCl, KNO 3 and 16 KClO 4 in nitrobenzene were found as:-10.53 (± 0.09),-8.16 (± 0.05) and-5.63 (± 17 0.03) respectively, in accordance with the Hofmeister series of lipophilicity, and 18 similar to those observed in PVC membranes containing other plasticizers. The 19 method presented here offers the advantage over other methods to calculate K IA , in 20 that it is relatively experimentally simple without compromising the accuracy of the 21 calculated coextraction constants. The ability to titrate directly into the liquid 22 membrane phase affords a higher precision compared to the preparation of a series of 23 PVC/plasticizer membranes with different compositions.