运用pH滴定法测定弱酸的电离常数

运用pH传感器和滴数传感器,采取pH滴定法测定了乙酸的电离常数和草酸的二级电离常数,分析了实验误差以及不宜选用此法测定中强酸的电离常数的原因。     

关于多元弱酸逐级电离常数递减原因的探讨

多元弱酸逐级电离常数（Ka,n）锐减的真正原因：一是电离过程,即从带负电荷越多的（含氢）酸根负离子中电离出H^＋,受到的静电引力越大,越难电离;二是酸根离子得到质子过程,即H^＋与负电荷越多的负离子间的库仑引力越强,致使平衡左移.故有Ka,1〉〉Ka,2〉〉Ka,3.前一级电离出的H＋引起的同离子效应并不是多元弱酸逐级电离常数锐减的原因,也违背多元弱酸逐级电离常数的热力学定义.     

乙酸和一氯乙酸电离常数的高效毛细管区带电泳法测定

首次建立了测定一氯乙酸和乙酸的电离常数的高效毛细管区带电泳新方法.该方法利用中性标记物和电流突跃两种方法来标记电渗流,通过测定乙酸和一氯乙酸在一定pH的缓冲溶液中的电泳淌度,结合数据回归分析拟合,求得乙酸和一氯乙酸的电离常数;所得数据和文献报道值较为接近.总体而言,毛细管区带电泳法可简单、快速、可靠地用于测定待测化合物的电离常数.     

用紫外分光光度法和电位滴定法测定几种植物激素的电离常数

分别采用紫外分光光度法和电位滴定法测定了脱落酸、3-吲哚乙酸、赤霉素、水杨酸等几种植物激素的电离常数,对比分析了两种测试方法的原理、测试步骤和数据处理方法.结果表明,采用两种方法测定的四种样品的电离常数与文献报道值一致,且测量结果的重现性较好;这表明紫外分光光度法和电位滴定法可方便地用于测定有机化合物的电离常数.     

二-(2-乙基已基)膦酸(P229)的基本常数测定

有关P229萃取金属离子的研究,目前文献上还很少报道,为研究P229萃取金属离子的机理,需要知道它的一系列物理化学常数,特别是关于两相体系的基本常数,本文报道了P229在水中的溶解度S、电离常数K_a、在正辛醇-水两相中的两相电离常数K_ac及分配常数K_d的测定及计算。     

运用手持技术测定甲基橙的电离常数Ka

利用吸光光度法,基于朗伯比尔定律,联合使用手持技术仪器中的pH传感器和色度计,测定了甲基橙的电离常数Ka.实验装置简单、操作方便、所得结果准确,有利于学生理解电离平衡的实质和电离常数的意义.     

基于“宏观辨识与微观探析”核心素养培养的“电解质的电离”教、学、评一体化实践

“电解质的电离”作为高中化学的重要学科内容,受到教师广泛重视,但是在实践中存在着教学效果不理想的情况,主要表现是学生未建立微观认识物质的视角,这与教师在教学过程中过于关注电解质概念,进行具体性知识教学而忽视具有核心素养培养功能的电离概念的教学有直接关系。从素养发展来看,“电解质的电离”是培养学生“宏观辨识与微观探析”素养方面时重要的内容载体,而教、学、评一体化的落实情况决定“电解质的电离”这一核心概念的教学效果。通过凸显水的作用、导电性实验的功能化处理、加强电离概念的功能化教学等区别于传统教学的尝试,在教、学、评一体化的实践中取得了较好的教学效果。     

对、间位卤代苯甲酸在水-乙醇混合溶剂中电离过程的焓、熵贡献

用LKB-2277Bioactivity Monitor测定了25℃时间氯、对氯和对氟苯甲酸在水-乙醇混合溶剂中的标准电离焓, 计算了相应体系的标准电离熵。利用Hammett方程对溶剂中酸的取代基常数σ进行了计算, 求取了对应的焓、熵取代基常数σH和σS值及熵反应常数ρS, 利用内部-环境模型和上述常数对溶剂效应和取代基效应进行了解释。     

氧化还原反应中2个重要关系式用于求算非氧化还原反应平衡常数的方法讨论

电化学中的KƟ-EƟ关系式和φ-φƟ关系式是分别求算原电池氧化还原反应平衡常数和氧化还原电对电极电势的重要公式。实际上,这2个公式也可以用来求解非氧化还原反应的平衡常数。以实例的形式讨论了这一思想的根据、方法,及其在求算水电离常数、弱酸或弱碱电离常数、难溶性电解质溶度积常数和配离子形成常数等方面的应用。     

弱酸电离常数和电离度测定方法的改进

在很多无机化学和基础化学实验讲义中,弱酸的电离度和电离常数的测定,都是使用pH计,测定几个不同浓度的弱酸溶液的pH,以酸碱滴定法测定弱酸的初始浓度,根据α=[H~+]/c_HA和K_α=cα~2/(1-α)公式求各浓度下的电离度和电离常数,操作麻烦,误差较大,且受弱酸强度的限制.有人提出通过测定弱酸及其盐构成的缓冲比为1的缓冲溶液的pH求PK_a,但配制1:1的缓冲溶液较难.有人提出作滴定曲线法求pK_a,方法较准确,但实验点太多,操作麻烦.综合以上方法,我们提出了一种不必知道弱酸及所用强碱的准确浓度,只要测定五次pH就可较准确地求出弱酸的电离常数和电离度的方法,仪器简单只需酸度计、滴定管、烧杯等,操作方便,且不受弱酸强度的限制.     

Microscopic acid dissociation constants of 3,4-dihydroxyphenylpropionic acid and related compounds, and 3,4-dihydroxyphenylalanine (DOPA)

The dissociation constants of 3,4-dihydroxyphenylpropionic acid and related compounds and of DOPA were determined by potentiometric titration and complementary tristimulus colorimetry at 25 degrees and mu = 0.1 (NaClO(4)) in aqueous solution. The thermodynamic parameters were calculated from the values of the dissociation constants at various temperatures. The dissociation constants and corresponding thermodynamic parameters for the first phenol group of the catechols showed almost the same values as those of the phenol derivatives. In the dissociation of the second phenol group of the catechols, formation of an intramolecular hydrogen bond was indicated. The microscopic acid dissociation constants of 3,4-dihydroxyphenylacetic acid and 3,4-dihydroxyphenylpropionic acid were calculated by two different methods. In the physiological pH-region (pH 7.2-7.4), 3,4-hydroxyphenylpropionic acid is present in the carboxylate form, and the two possible monophenolate anions are present to the extent of about 45% and 40%, respectively, at pH 11.0. The twelve micro-constants for the eight chemical species from DOPA were similarly evaluated.     

Studies on the riboflavin-resorcinol interaction in an aqueous medium and its pH dependence

The equilibrium constants, K', for the formation of the molecular association complex between riboflavin and resorcinol in the pH range 6-8 were found to be in the range 5-25 mol(-1)dm(3) by difference absorption spectroscopy. The equilibrium constants from emission spectroscopy were estimated to be in the range 38-55 mol(-1)dm(3) over the same pH range. Both sets of K' show a remarkable dependence on pH. The equilibrium constants K(A) and K(B) describing the interaction of riboflavin with neutral and ionic forms of resorcinol, respectively, obtained by resolving K', shows that K(B) is much greater than K(A). Molecular modeling studies suggest a stacked conformation of the two components in the complexed form. The interaction energies obtained from modeling studies also suggest a stronger interaction between the ionic form of resorcinol and riboflavin.     

Determination of the kinetic constants of the reversible opening of a triazolo‐1,4‐thienodiazepine in water at different pH values: A striking example of the determination of intimately intricate kinetic and equilibrium constants

Apparent kinetic constants of the reversible opening of a triazolo‐1,4‐thienodiazepine were determined by UV‐spectrophotometry and by polarography in water at several pH values assuming as a hypothesis a stationary state for the carbinolamine intermediate. Both the apparent kinetic constants, k_f and k_r, exhibited a maximum for the values H₀ = −0.25 and pH = 6.07. A possible determination of the elementary kinetic constants of the several acido‐basic species which may be involved in the opening and closing process according to the pH and pKa values was studied and is discussed. The results are consistent with the hypothesis that both the opening of the diazepine cycle and the closing of the opened form proceed through a mechanism suggesting that the protonated form of the carbinolamine function of the intermediate is involved. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 826–834, 1999     

Electronic absorption study on acid-base equilibria for some keto and thioketo pyrimidine derivatives. Experimental and theoretical evidence of enolization and solute-solvent interactions

The UV-vis spectra of recently synthesized 1-amino-5-benzoyl-4-phenyl-1H-pyrimidine-2-one, (I), and 1-amino-5-benzoyl-4-phenyl-1H-pyrimidine-2-thione (II), were studied in aqueous methanol (5%, v/v, methanol) and pure methanol. The nature of the electronic transitions and the role of carbonyl oxygen of I and thiocarbonyl sulfur of II in the behavior of the observed UV-vis spectra were discussed. The carbonyl group at position 2 of I and the thiocarbonyl group of II were found to be enolized instead of protonation. Quantum chemical calculations showed agreement with the experimental evidence. However, the carbonyl group of the benzoyl moiety at position 5 of both compounds underwent neither enolization nor protonation. Acid-base equilibria of the compounds against varying pH have been examined in detail. The pKa values of all related equilibria were determined at room temperature and an ionic strength of 0.10 M from the pH-dependence of the absorbance values using the Henderson-Haselbalch equation and graphical logarithmic analysis. The mean acidity constants for the protonated forms of the compounds were determined as pKa1=4.214 and pKa2=6.678 for I and pKa1=3.739 and pKa2=6.258 for II. The mean acidity constants (pKa3) for the enol form of I and the thioenol form of II were determined as 11.278 and 11.063, respectively. The preferred dissociation mechanisms were discussed based on the data of UV-vis spectroscopy and a mechanism was proposed for each compound. The formation of intramolecular and intermolecular hydrogen bonding were found with I but not with II. The intramolecular bonding stabilizing the enol form was favoured at pH values corresponding to pKa1 and above. On the other hand, the intermolecular hydrogen bonding stabilizing the free form of the carbonyl group was favoured at all pH values.     

Effect of pH on the reactivity of dipeptides and α-amino acids in the N-acylation

The reaction kinetics of Gly, L-??-Ala, Gly-Gly, L-??-Ala-L-??-Ala and ??-Ala-??-Ala with picryl benzoate in water (40 wt %)-2-propanol was investigated. At pH = 4?C8 the rate constants of N-acylation of the anionic form of dipeptides are less than those of the corresponding amino acid anions, in agreement with their basicity, whereas the relative effective rate constants of reactions depend on pH: in acidic, neutral and slightly alkaline media the k _ef values are higher for the dipeptides, and in a strongly alkaline medium, for the amino acids. These differences are due to the changes in the concentrations of reactive forms of amino acids and dipeptides in the system at varying the medium pH.     

Adjusting the Structure of β-Cyclodextrin to Improve Complexation of Anthraquinone-Derived Drugs

β-Cyclodextrin (CD) derivatives containing an aromatic triazole ring were studied as potential carriers of the following drugs containing an anthraquinone moiety: anthraquinone-2-sulfonic acid (AQ2S); anthraquinone-2-carboxylic acid (AQ2CA); and a common anthracycline, daunorubicin (DNR). UV-Vis and voltammetry measurements were carried out to determine the solubilities and association constants of the complexes formed, and the results revealed the unique properties of the chosen CDs as effective pH-dependent drug complexing agents. The association constants of the drug complexes with the CDs containing a triazole and lipoic acid (βCDLip) or galactosamine (βCDGAL), were significantly larger than that of the native βCD. The AQ2CA and AQ2S drugs were poorly soluble, and their solubilities increased as a result of complex formation with βCDLip and βCDGAL ligands. AQ2CA and AQ2S are negatively charged at pH 7.4. Therefore, they were less prone to form an inclusion complex with the hydrophobic CD cavity than at pH 3 (characteristic of gastric juices) when protonated. The βCDTriazole and βCDGAL ligands were found to form weaker inclusion complexes with the positively charged drug DNR at an acidic pH (pH 5.5) than in a neutral medium (pH 7.4) in which the drug dissociates to its neutral, uncharged form. This pH dependence is favorable for antitumor applications.     

The Reversible Proxibarbal-Valofan Isomerisation. Part II. Kinetic studies in a biphasic octanol/water system

The tautomerisation between proxibarbal (I) and the two diasterloisomers of valofan ( IIX and IIY ) was investigated at pH 7.4 in a biphasic octanol/H₂O system. The rate constants of isomerisation and the equilibrium constants of (partition coefficients) were calculated by compartmental analysis. The rate constants of isomerisation were comparable with those determined in monophasic aqueous solutions, whereas at pH 7.4 and 37° the duration necessary for a global equilibrium to be reached was 4–6 times longer in the biphasic system. These reduced rates and the higher lipophilicity of IIX and IIY as compared to I may be of pharmacokinetic and pharmacodynamic significance. They may also have relevance for a number of drugs known or suspected to form lactonic or lactamic metabolites.     

Spectroscopic determination of acid dissociation constants of N-substituted-6-acylbenzothiazolone derivatives

The acid dissociation constants of twelve novel drug precursor N-substituted-6-acylbenzothiazolone derivatives were determined by using the UV-vis spectroscopic technique. The protonation and deprotonation behaviors of the investigated molecules were researched from the super basic to super acid regions (i.e., 8 mol·L(-1) KOH to 98% H(2)SO(4)) including the pH region. It is observed that all of the molecules are protonated in the super acidic region. The calculated relative stability values of possible tautomer structures indicate that the keto form of investigated molecules is favored over the enol form. It was predicted that protonation occurs at the amide (oxo) group found in the keto form.     

配位化学中的直线自由能关系XXI: 铜(II)-13-取代苄基-1, 4, 8, 11-四氮杂环十四烷-12, 14-二酮配合物的生成及酸分解行为研究

采用pH法, 在25.0±0.1℃, I=0.1 mol.dm^-^3 (KNO3)条件下, 测定了13-取代苄基-1, 4, 8, 11-四氮杂环十四烷-12,14-二酮的质子化常数及其与Cu(II)配位的平衡常数。讨论了配体与金属离子的配位方式。在25.0±0.1℃, 离子强度为0.1mol.dm^-^3 (KNO3)下, 采用分光光度法, 研究了这些配体铜(II)配合物的酸分解动力学行为。探讨了配合物酸分解机理,得到了速控步的速率常数。发现配位反应平衡常数与配体的质子化常数及配合物酸分解反应速率常数之间存在较好的Hammett型和Bronsted型直线自由能关系。同时探讨了取代基对配合物生成及酸分解的影响情况。     

The TcO4-binding to human serum albumin --the stoichiometry and characteristics of the binding equilibrium.

The stoichiometry and the characteristics of the TcO4-binding equilibrium to human serum albumin were investigated by the use of 99TcO4-. Based on the Scatchard plots, the number of binding sites and the association constants were obtained at pH 7.38,6.04, 5.10, 4.18, 2.80 and 0.65, respectively. From the parameters at pH 7.38, it was estimated that 64% of TcO4- added was bounded to human serum albumin under physiological condition. Variance in the values at pH 7.38, 6.04 and 5.10 shows that these bindings are stabilized by electrostatic forces. Below pH 4.18, the number of binding sites increased and the association constants constants diminished. These phenomena may be attributed to the conformation change of human serum albumin.