Szakács Zoltán, Hägele Gerhard
Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universität Düsseldorf, Universitätsstr. 1, D-40225 Düsseldorf, Germany.
Talanta. 2004 Mar 10;62(4):819-25. doi: 10.1016/j.talanta.2003.10.007.
The NMR titration methodology to determine acid dissociation constants in aqueous solutions is extended for pK(a) values between 0 and 2, where potentiometric titrations are no longer applicable. (1)H NMR spectra are acquired for single samples of constant acid concentration (e.g. 0.02M), controlled ionic strength (I=1M with HNO(3)/NaNO(3)) and varying pH. To avoid biased pH readings due to the acid error of the glass electrode, true, concentration-based pH values are deduced by combination of the charge balance equation with information from (1)H NMR chemical shifts of the investigated acid. The method has been tested on histidine (pK(1)=1.83+/-0.02) and yielded the dissociation constant of dichloroacetic acid (pK=1.06+/-0.01) for the first time with good accuracy and precision. Dichloroacetic acid is recommended as an NMR spectroscopical "indicator molecule" for in situ monitoring the pH in strong acidic solutions of other equilibrium systems.
用于测定水溶液中酸解离常数的核磁共振滴定方法被扩展到pK(a)值在0到2之间的范围，在此范围内电位滴定不再适用。对酸浓度恒定（如0.02M）、离子强度可控（用HNO(3)/NaNO(3)使I = 1M）且pH值变化的单个样品采集(1)H NMR谱。为避免因玻璃电极的酸误差导致pH读数有偏差，通过将电荷平衡方程与所研究酸的(1)H NMR化学位移信息相结合，推导出基于浓度的真实pH值。该方法已在组氨酸（pK(1)=1.83±0.02）上进行了测试，并首次以良好的准确度和精密度得出二氯乙酸的解离常数（pK = 1.06±0.01）。推荐二氯乙酸作为核磁共振光谱“指示分子”，用于原位监测其他平衡体系强酸性溶液中的pH值。
