Table nbsp Transport number of counter
The thermolytic solution of NH4HCO3 creates a greater junction potential across the AEM (AMV membrane) than that of (NH4)2CO3 by 21–65% (Fig. 4A). Note that the pH in NH4HCO3 solutions was between 7.9 and 8.1 (Fig. 1B), while it was between 9.1 and 9.2 for (NH4)2CO3 solutions. This relatively high pH can explain the smaller junction potential across the AEM with (NH4)2CO3 (Fig. 4A). For pH between 9.1 and 9.2, the concentration of hydroxide ion (OH-) ranges between 1.3 × 10− 5 and 1.6 × 10− 5 M, and since this OH− concentration is almost identical on both sides of the AEM, then the OH− contribution to the junction potential should be negligible. Since the total amount of carbonate species in the LC solution was as small as 1 × 10− 3 M (CR = 500), the amount of hydroxide ions was up to 1.3–1.6% of the total anions in the LC solution. While this fraction is still small, the GNE-7915 coefficient (and, thus, the ionic mobility) of OH- at infinite dilution (5.273 × 10− 9 m2/s) is about 4.4 times that of HCO3− (1.185 × 10− 9 m2/s), increasing the transport number of OH−. Furthermore, AEMs are known to favor hydroxide ions over carbonate ions, so for example, with a selectivity coefficient of 3.0 for OH− over HCO3−, the fraction of hydroxide ions in the membrane would be significantly greater . As a result, the transport number of hydroxide ions increases in proportion to the fraction, making the transport number of carbonate ions small. Since the concentration ratio (or activity ratio) of hydroxide ions across the AEM is close to unity, the junction potential decreases with the increasing transport number of hydroxide ions (Eq. (2)). Consequently, the junction potential with (NH4)2CO3 is smaller than that with NH4HCO3 (Fig. 4A). Note that the transport number of the hydroxide ion in NH4HCO3 is smaller than that in (NH4)2CO3 by an order of magnitude because the hydroxide ion concentration is smaller by an order of magnitude.