Tam K Y, Takács-Novák K
Sirius Analytical Instruments Ltd., East Sussex, United Kingdom.
Pharm Res. 1999 Mar;16(3):374-81. doi: 10.1023/a:1018817617432.
Acid dissociation constants (pKa values) denote the extent of ionization of drug molecules at different pH values, which is important in understanding their penetration through biological membranes and their interaction with the receptors. However, many drug molecules are sparingly soluble in water or contain ionization centres with overlapping pKa values, making precise pKa determination difficult using conventional spectrophotometric titration. In this work, we investigate a multiwavelength spectrophotometric titration (WApH) method for the determination of pKa values.
Spectral changes which arise during pH-metric titrations of substances with concentration of about 10(-5) M were captured by means of an optical system developed in this study. All experiments were carried out in 0.15 M KCI solution at 25 +/- 0.5 degrees C. Mathematical treatments based on the first derivative spectrophotometry procedure and the target factor analysis method were applied to calculate the pKa values from the multiwavelength absorption titration data.
pKa values were determined by the WApH technique for six ionizable substances, namely, benzoic acid, phenol, phthalic acid, nicotinic acid, p-aminosalicylic acid and phenolphthalein.
The pKa values measured using the WApH technique are in excellent agreement with those measured pH-metrically. We have demonstrated that the first derivative spectrophometry procedure provides a relatively simple way to visualize the pKa values which are consistent with those determined using the target factor analysis method. However, for ionization systems with insufficient spectral data obtained around the sought pKa values or with closely overlapping pKa values, the target factor analysis method outperforms the first derivative procedure in terms of obtaining the results. Using the target factor analysis method, it has been shown that the two-step ionization of phenolphthalein involves a colorless anion intermediate and a red colored di-anion.
酸解离常数（pKa值）表示药物分子在不同pH值下的电离程度，这对于理解它们透过生物膜的情况以及与受体的相互作用非常重要。然而，许多药物分子在水中溶解度很低，或者含有pKa值重叠的电离中心，这使得使用传统的分光光度滴定法精确测定pKa值变得困难。在这项工作中，我们研究了一种用于测定pKa值的多波长分光光度滴定法（WApH）。
通过本研究开发的光学系统捕获浓度约为10(-5) M的物质在pH滴定过程中出现的光谱变化。所有实验均在25±0.5℃的0.15 M KCl溶液中进行。基于一阶导数分光光度法和目标因子分析法的数学处理方法被用于从多波长吸收滴定数据中计算pKa值。
使用WApH技术测定了六种可电离物质的pKa值，即苯甲酸、苯酚、邻苯二甲酸、烟酸、对氨基水杨酸和酚酞。
使用WApH技术测得的pKa值与通过pH滴定法测得的值非常吻合。我们已经证明，一阶导数分光光度法提供了一种相对简单的方法来直观显示与使用目标因子分析法测定的pKa值一致的pKa值。然而，对于在所求pKa值附近获得的光谱数据不足或pKa值紧密重叠的电离系统，在获得结果方面，目标因子分析法优于一阶导数法。使用目标因子分析法已经表明，酚酞的两步电离涉及一个无色阴离子中间体和一个红色二价阴离子。
