Potentiometric titrations with ion-selective electrodes of aqueous carbonate systems and study of a novel carbonate species

Abstract

Mathematical consideration of the course of neutralization of a solution containing a strong base (e. g. NaOH) and a salt of a weak dibasic acid (e. g. Na 2 CO 3) with a strong acid (e. g. HCI) shows that first derivative of the potentiometric titration curve should contain three maxima. The first corresponds to neutralization of most of the hydroxide, the second and the third peak correspond to conversion of CO 2- 333to2 32H2C O- and HCO to H CO (CO +H 0) respectively. Plots of (dE/dV) against Vol. of HCI (V) for sodium hydroxide containing different concentrations of Na 2 CO 3 using glass electrodes show an additional, unexpected maximum in the titration curves. This is not detected when ethanol (mole fraction > 0.2) or t-butanol (mole fraction> 0.1) are added, when the temperature is raised above 55 0 C, when hydrogen gas is bubbled through the solution or when the hydrogen gas electrode is used instead of the glass electrode. Change of solvent from H20 to D20 and change of cation from Na + to K+ had no effect on the additional maximum, and also when back titrations of a solution of hydrochloric acid containing different concentrations of dissolved CO 2 with NaOH were carried out. The rate of addition of hydrochloric acid or, in other words, the rate of introducing CO 2 into the solution during the course of titration has an effect on the position of this unexpected maximum. This suggests that a kinetic stage is involved in formation of the maximum. The extra feature was not recorded when a trace of carbonic anhydrase was introduced into the titration solution. Computer simulation was carried out considering the effect of slow kinetics of hydration of CO 2 on the form of the titration curve, but the only effect was on the height of the maximum for the conversion of H2 CO 3 to HCO 3 There is strong evidence that the extra maximum is a result of formation of a complex of CO 2 and HCO 3' in the presence of H20 (H 3C206)' Assuming values for the equilibrium constant for this process, the appropriate equations were solved on the computer to obtain titration curves identical to the experimental curves. Also the molecular structure of this new species was plotted by using an IBM/360/370 computer. Along with analysis of titration curves attempts were made to use liquid membrane electrodes, PVC membrane electrodes, Ag/Ag2CO3 electrodes, In direct potentiometric measurements of carbonate ions. Also isopiestic technique was applied to measure osmotic coefficients of K2 CO 3 and NaHCO 3 solutions.