Determination of the Dissociation Constants of Urocanic Acid Isomers in Aqueous Solutions

(E)- and (Z)-Urocanic acids are endogenous chemicals in the normal mammalian skin. The first and the second thermodynamic dissociation constants (pK _a1 and pK _a2) of urocanic acid isomers were determined using UV spectrophotometry in aqueous solutions. The values with standard deviation (pK _a1 = 3.43 ± 0.12 and pK _a2 = 5.80 ± 0.04) and (pK _a1 = 2.7 ± 0.3 and pK _a2 = 6.65 ± 0.04) were obtained to (E)- and (Z)-urocanic acids, respectively. The second dissociations were studied also by potentiometric titration in aqueous sodium chloride solutions up to the isotonic salt concentration (0.154 mol dm⁻³), and the second thermodynamic dissociation constants as well as activity parameters for both isomers were determined at temperature 25°C and for (E)-urocanic acid also at 37°C. The obtained pK _a2 values were close to those found by UV spectrophotometry. The equations for the calculation of the second stoichiometric dissociation constants of urocanic acid isomers on molality and molarity scale in aqueous sodium chloride solutions were derived. The obtained pK _a1 and pK _a2 values for (Z)-urocanic acid appear to be essentially lower than some previously reported values in literature.     

聚天冬氨酸酸解离常数的测定

采用pH电位滴定法，利用先进的BEST程序，针对聚羧酸在水溶液中解离及平衡常数测定的复杂性，构建了聚天冬氨酸（PASP）在水溶液中解离的H4L和H2L两种模型，分别测得了相应的各级酸解离常数，并得到了PASP在不同pH条件下的物种分布曲线。在35℃，I=0．1时，对于H4L模型，各级酸解离常数的pK,值为：2．27、3．66、4．22、5．40；对于H2L模型，相应的pK,值为2．92、4．84。研究结果表明，两种模型计算结果均能与实验结果很好吻合，但H4L模型误差更小，因此是一种更为合理的模型。     

甘氨酸在乙醇—水混合溶剂中离解常数的测定

本文利用电位滴定法测定了２５℃，ＫＣｌ离子强度Ｉ＝０．１ｍｏｌ．Ｌ＾－１时，甘氨酸在纯水及８个乙醇－水混合溶剂中的二级离解常数ｐＫａ，对应的碱离解常数ｐＫｂ和混合溶剂的质子自递常数ｐＫ。实验结果表明：在所研究的浓度范围内，随着混合溶剂中乙醇含量的增加，甘氨酸的酸离解常数及溶剂的质子自递常数逐渐减小，碱离解常数则逐渐增大。     

UV-Vis Spectrophotometric Determination of the Dissociation Constants for Monochlorophenols in Aqueous Solution at Elevated Temperatures

The dissociation constants of monochlorophenols (2-, 3-, 4-chlorophenols) were examined using direct UV-vis spectroscopy at temperatures from 25 to 175^°C and at saturated vapor pressures in a high-temperature, high-pressure cell. The dissociation constant, K _a increased under experimental temperatures in the order: 2-chlorophenol, 3-chlorophenol, and 4-chlorophenol. The dissociation constant of 4-chlorophenol increased with increasing temperature under experimental conditions, while those of 2- and 3-chlorophenol reached maximum values at around 125^°C, and then decreased with further increases in temperature. The slope of (log K)/ (1/T) was nonconstant and positive, that is, endothermic, below 150^°C. The data on dissociation constants were analyzed by simultaneous regression to yield a five-term equation that described the Van't Hoff isobar. The magnitude of enthalpy H increased at 25^°C in the order: 3-chlorophenol, 4-chlorophenol, and 2-chlorophenol. The decrease in enthalpy at the absolute temperature was larger for 3-chlorophenol than for either 2- or 4-chlorophenol. Considering the equilibrium constant K _b for the isocoulombic reaction of monochlorophenol with OH^–, the nearly linear relationship of log K _b vs. 1/T for temperatures between 25 and 175^°C indicates that the C_p values for this isocoulombic reaction are low.     

微量光度滴定法测定咔咯化合物的质子化和去质子化常数

采用微量光度滴定法测定了两种具有不同取代基的新型咔咯化合物,三(4-氯苯基)咔咯(化合物1)和三(2,4-二氯苯基)咔咯(化合物2)在非水溶剂中的质子化和去质子化常数。结果表明:化合物1和化合物2在二氯甲烷中均可以与三氟乙酸反应得到一个质子生成正一价阳离子,其质子化常数(lgKb)分别为4.2和4.0。在甲醇溶液中,化合物1和化合物2与氢氧化钠反应时可以失去一个质子生成负一价阴离子,其去质子化常数(lgKa)分别为3.4和3.5。而在二氯甲烷中与碱反应时,化合物1和化合物2均能够一步失去两个质子生成负二价阴离子,其累积去质子化常数(lgβ2)分别为7.9和11.0。     

采用电位滴定法在DMSO/H2O混合溶剂体系中测定磺酰脲(胺)类除草剂的酸离解常数

采用电位滴定法在DMSO/H₂O(质量比80:20)混合体系中测定了磺酰脲和三唑并嘧啶磺酰胺类除草剂的酸离解常数,进而采用回归分析方法探讨了测得的pK_a值与化合物的前沿轨道能级以及取代基效应之间的相关性,结果较为满意,从而证明从该体系测得的pK_a值用于构效关系分析是完全可行的.     

Multiwavelength Spectrophotometric Determination of Acid Dissociation Constants. Deconvolution of Binary Mixtures of Ionizable Compounds

ABSTRACT

A spectral deconvolution method based on target factor analysis has been developed to determine pK_a values of binary mixtures of monoprotic and/or diprotic ionizable compounds. The technique makes use of the approach of Frans and Harris, which has been implemented previously for mixtures of monoprotic compounds (Anal. Chem. 1985, 57, 1718-1721), to extract the unknown pK_a values. The method has been illustrated by using the multiwavelength spectrophotometrictitation data of binary mixtures of: I. benzoic acid and phenol, 2. benzoic acid and nicotinic acid, 3. p-aminosalicylic acid and phthalic acid. It was demonstrated that the pK_a values as determined in this study are in good agreement with the literature.     

电位滴定法测定弱酸电离常数与混合酸组分浓度的一种通用处理方法

本文导出了由电位滴定数据求算弱酸电离常数与混合酸组分浓度的通用算式,按Rosenbrock算法设计程序并处理了某些实验数据,得到良好的结果。与处理同类问题的其它方法相比,本法是方便并可靠的。     

仲辛基苯氧基乙酸基本常数的测定

在 2 5℃ ,利用两相滴定法测定了仲辛基苯氧基乙酸 (CA 1 2 )在水中的溶解度S ,离解常数Ka,庚烷中的二聚常数K2 和水 庚烷两相间的分配常数Kd 。实验测得S =1 .40× 1 0 - 4mol L ,Ka=3 .0 2×1 0 - 4,K2 =3 .5 6× 1 0 2 ,Kd=4.0 6× 1 0 2 。     

The Determination of the Protonation Constants of A New Macrocyclic Dinucleating Ligand, BDBPH

One of our current interests focuses on the design and synthesis of polyaza macrocyclic ligands, the determination of their protonation constants, and the stabilities of corresponding metal complexes. Two polyazadiphenol macrocycles, R(babp)2(dfc)21 and [24]RBPyBC2, one octaazamacrocycle, BPBD3,4, and several hexaaza macrocyclic ligands, BFBD5, BMXD6, and OBISDIEN7, have been reported recently. We have also reported the synthesis of a new dinucleating 24-membered hexaazadiphenol macr…     

Gas-Chromatographic Determination of Acrylic Acid in Aqueous Solutions

A procedure was developed for determining acrylic acid in aqueous solutions at its concentration within 0.5-20 MPC.     

Isothermal titration calorimetric studies of the acid-base properties of poly(N-isopropylacrylamide-co-4-vinylpyridine) cationic polyelectrolyte colloidal microgels

Isothermal titration calorimetry (ITC) and potentiometric titration were used to study the protonation properties of the 4-vinylpyridine (4-VP) moiety in cationic poly(N-isopropylacrylamide-co-4-vinylpyridine) colloidal microgels [poly(NIPAM-co-4-VP)]. Calorimetric pH titrations were performed using microgels of different 4-VP content and the influence of ionic strength and counter ions have been examined. The calorimetric titration output consists of several thermal contributions reflecting the complex nature of the interactions in the aqueous microgel dispersions. In contrast to the potentiometric results, the calorimetric titration data could not be completely described by a theoretical model solely taking into account protonation equilibria. Deviations from the proposed model correlate with swelling or shrinking of the gel particles. The calorimetric results also reveal a pronounced counter-ion effect of perchlorate compared to chloride ions. In the presence of perchlorate ions, small secondary thermal effects accompany protonation of the 4-VP moiety due, in part, to kinetically limited conformational changes in the co-polymer microgel.     

Pressure Dependence of the Acid/Base Equilibria of Methyl Orange in Aqueous Solutions to 1000 bar at 20°C

The pressure dependence on the acid/base equilibria of methyl orange in aqueous solution was measured at 20 °C in the 1–1000 bar range with a newly designed flow-through spectrophotometric cell. Combined chemometric and thermodynamic analyses of the UV-Vis spectrophotometric data were used to extract the dissociation constants as well as the changes in molar volume and isothermal compressibility of methyl orange as a function of pressure. The results show that increasing the pressure promotes the deprotonation of methyl orange, with pK values ranging from 3.505 at 1 bar to 3.445 ± 0.002 at 1000 bar. Increasing the pressure also yields small negative changes in the molar volume ranging from –6.9 cm³·mol⁻¹ at 1 bar to −1.7 cm³·mol⁻¹ at 1000 bar. The isothermal compressibility of methyl orange in this pressure range was estimated using the second derivative of second and third order polynomial fits to the constants, and resulted in a constant value of –48.4 × 10⁻⁴ cm³·mol⁻¹·bar⁻¹ in the former case, but increasing values from –107 × 10⁻⁴ cm³·mol⁻¹·bar⁻¹ at 1 bar to 3.43 × 10⁻⁴ cm³·mol⁻¹·bar⁻¹ at 1000 bar in the latter case. Molar absorption coefficients for the protonated and deprotonated species were also shown to be only slightly effected by pressure changes and can be used to accurately predict the absorption spectra of methyl orange as a function of pressure.     

苯氧乙酸类化合物的酸离解常数测定及其量子化学计算

苯氧乙酸;酸离解常数;键电荷密度;量子化学计算;苯氧乙酸类化合物的酸离解常数测定及其量子化学计算     

强酸性环境中氧氟沙星的质子化模型研究

利用核磁共振波谱技术研究了不同浓度硫酸溶液中氧氟沙星(OFL)的1H,19F和13C核磁共振谱,对不同硫酸浓度引起的δH,δF,δC和JFC耦合常数的变化进行比较分析,由此推测其结构状态.综合1H,19F和13C核磁共振谱特点及其变化,提出OFL分子在强酸性环境中N1'被进一步质子化的结构模型.在浓硫酸溶液中,N1'被进一步质子化,并与F9形成氢键(N1'—H+┈F9),该结构使分子的共轭程度大幅降低,导致其荧光发射波长、荧光激发波长及紫外吸收波长均发生蓝移.硫酸溶液中氧氟酸和甲基氧氟沙星的荧光光谱行为进一步证明了OFL分子在浓硫酸溶液中质子化模型的合理性.     

异丙基膦酸单(1-已基-4-乙基)辛酯的某些基本常数的测定

用两相滴定法测定异丙基膦酸单（１－己基－４－乙基）辛酯（ＰＴ－２，ＨＬ）在水中的溶解度Ｓ，在水中的解离常数Ｋ在水－正庚烷中的分配常数Ｋｄ及二聚常数Ｋ２，利用ＳＯＬＷＲ计算程序，简单快速地处理两相滴定数据，得到结果为：Ｓ＝３．６８ × １０－５ｍｏｌ／Ｌ，ｐＫａ＝５．４９，ｌｏｇ Ｋ２＝４． ６７，ＩｏｇＫｄ＝ ２． ６７（２５±０． ５℃）．     

Determination of Acid Dissociation Constants of Neamine by Potentiometric and Electrospray Mass Spectral Techniques

The acid dissociation constants of neamine have been measured using potentiometric titrations. The pK_a1, pK_a2, and pK_a3 values of neamine are 6.35 ± 0.2, 7.73 ± 0.15, and 8.62 ± 0.08, respectively. Neamine is readily characterized using positive-ion electrospray ionization–mass spectrometry (ESI–MS). Various protonated species and their solvated ions are mainly observed. Nevertheless, the abundances of the observed speciation over the pH range 5.0–9.8 do not reflect the variation in the bulk solution. Hence, reaction quotients determined from the ESI–MS investigations vary significantly from the pK_a's determined from potentiometric titrations, thereby illustrating that care must be taken in determining thermodynamic properties using the former technique.