Department of Chemistry, University of Guelph, Guelph, Ontario, Canada.
J Phys Chem B. 2011 Mar 31;115(12):3038-51. doi: 10.1021/jp110337j. Epub 2011 Mar 3.
Values of the ionization constant of acetic acid in H(2)O and D(2)O (K(HAc) and K(DAc)) and the deuterium isotope effect, ΔpK = pK(DAc) - pK(HAc), have been determined from T = 368 K to T = 548 K at p = 20 MPa, using a flow-through ac conductance cell built at the University of Delaware. Measurements were made on dilute (ionic strength ∼ 10(-4) mol·kg(-1)) solutions of acetic acid, sodium acetate, hydrochloric acid, and sodium chloride in H(2)O and D(2)O, injected in sequence at each temperature and pressure, so that systematic errors in the measured conductance of each solution would cancel. Experimental values for the molar conductivity, Λ, of the strong electrolytes were used to calculate the molar conductivity at infinite dilution, Λ°, using the Fuoss-Hsia-Fernández-Prini (FHFP) equation. These were used to calculate the molar conductivity at infinite dilution for acetic acid which was in turn used to calculate the degree of dissociation and finally the ionization constants of acetic acid. This same procedure was done for the pertinent deuterated solutes in D(2)O. Measured values of log K(HAc), log K(DAc), and ΔpK were obtained to a precision of ±0.008. The present results are in agreement with the only other accurate study at high temperatures and pressures (Mesmer, R. E.; Herting, D. L. J. Solution Chem.1978, 7, 901-913). The deuterium isotope effects, ΔpK, become independent of temperature above ∼420 K, at a value approximately 0.1 unit lower than that at 298 K. These values are ΔpK = 0.43 ± 0.01 and ΔpK = 0.51 ± 0.01, respectively. The temperature dependence of the Walden product ratio, (λ°η)(D(2)O)/(λ°η)(H(2)O), indicates a change in the relative hydration behavior of ions, whereby the effective Stokes radii of the sodium, chloride, and acetate ions in D(2)O relative to H(2)O reverse above ∼423 K. The results also suggest that the greater efficiency of the well-established proton-hopping transport mechanisms for OH(-) and H(3)O(+) at 298 K, relative to OD(-) and D(3)O(+), is significantly reduced as the temperature increases toward 548 K.
在 20 MPa 下，从 T = 368 K 到 T = 548 K，使用在特拉华大学建造的流通式交流电导池，测定了在 H(2)O 和 D(2)O 中的醋酸的电离常数（K(HAc) 和 K(DAc)）的值和氘同位素效应 ΔpK = pK(DAc) - pK(HAc)，其中醋酸、乙酸钠、盐酸和氯化钠在 H(2)O 和 D(2)O 中的稀溶液（离子强度约为 10(-4) mol·kg(-1)) 被依次注入，以在每个温度和压力下进行测量，从而消除了每种溶液的测量电导率中的系统误差。使用 Fuoss-Hsia-Fernández-Prini (FHFP) 方程，使用强电解质的摩尔电导率 Λ 的实验值计算出无限稀释时的摩尔电导率 Λ°。这些被用于计算在无限稀释时的醋酸的摩尔电导率，而这又被用于计算离解度，最终计算出醋酸的电离常数。对于 D(2)O 中的相关氘代溶质，进行了相同的处理。测量到的 log K(HAc)、log K(DAc) 和 ΔpK 值的精度达到 ±0.008。本研究结果与在高温高压下的唯一另一项精确研究（Mesmer，R.E.；Herting，D.L. J. Solution Chem.1978, 7, 901-913）一致。氘同位素效应 ΔpK 在约 420 K 以上变得与温度无关，其值比 298 K 时低约 0.1 个单位。这些值分别为 ΔpK = 0.43 ± 0.01 和 ΔpK = 0.51 ± 0.01。Walden 乘积比（(λ°η)(D(2)O)/(λ°η)(H(2)O)）的温度依赖性表明，离子的相对水合行为发生了变化，从而使 D(2)O 中的钠离子、氯离子和乙酸根离子的有效斯托克斯半径相对于 H(2)O 发生反转在约 423 K 以上。结果还表明，在 298 K 时，OH(-) 和 H(3)O(+) 的质子跳跃传输机制相对于 OD(-) 和 D(3)O(+)的效率更高，随着温度升高到 548 K，其效率显著降低。
