关于一元酸准确滴定判据的商榷

化学分析教科书中给出的弱酸能被准确滴定的条件是cKa≥1.0×10-8。本文根据这一判据的来源(滴定突跃≥0.3pH单位),对一元酸体系的滴定情况进行了严格数学处理。计算结果表明,只有同时满足弱酸的浓度c1.0×10-3mol/L和cKa≥1.0×10-8条件下,弱酸才能被准确滴定。对于浓度极稀的酸溶液(c≤1.4×10-4mol/L),无论强酸还是弱酸都不能被准确滴定。对教材很少涉及的不同浓度酸碱互相滴定的情况也给出了有价值的结论。     

计算机与化学Origin在精确绘制酸碱滴定曲线中的应用

根据林邦副反应思想,分别得到一元强碱滴定一元强酸和一元强碱滴定一元弱酸的精确滴定曲线为一元二次和一元三次方程.利用Origin软件解析酸碱滴定曲线方程,并以滴定分数为横坐标,pH为纵坐标绘制滴定曲线.     

强酸和弱酸混合溶液直接滴定的条件

讨论了强酸和弱酸混合溶液直接滴定的条件,并绘制了混合溶液滴定曲线,为强酸和弱酸混合溶液的滴定提供了理论根据。     

基于滴定突跃的计算探讨多元弱酸分步滴定的准确条件

多元弱酸分步滴定的准确条件一直是酸碱滴定中的一个教学难点。基于分布分数的分析指出了当前分析化学教材中有关探讨磷酸分步滴定的某些错误结论,并在此基础上,通过滴定突跃大小的计算,对多元弱酸分步滴定的准确条件进行了探讨分析。结果表明：若人眼判断滴定终点时有±0.2pH的不确定性时,滴定误差控制在0.5%,强碱滴定多元弱酸时,分步滴定的准确条件是：Kai/Ka(i+1)≥105.0;cKai≥10-8.7。     

一元弱酸(弱碱)准确滴定界限的简易推导

１　引言　　在酸碱滴定分析中,一般常用指示剂颜色的变化来目测终点,判断强碱（或强酸）能否准确滴定一元弱酸（或弱碱）的界限为［１,２］：ｃａｋａ≥１０－８或ｃｂｋｂ≥１０－８。对于这个滴定界限的提出,少数教材及教学参考书进行了简要的推导［３,４］。王毓芳等［５］采用多步近似法对滴定界限作了理论上的求证,但推导过程太繁琐,且近似处理欠严密。本文提出另一种方法,从缓冲容量的定义出发,通过引入滴定分数和敏锐指数,导出了弱酸（弱碱）能否被直接滴定的界限。２　滴定界限的推证　　首先引入缓冲容量β,教材［１］…     

关于一元弱酸盐及弱酸溶液几个问题的系统论证

系统论证了等物质的量浓度或等pH的一元弱酸盐或一元弱酸溶液, 在加水稀释或加强酸过程中溶液pH变化情况与相应K之间的关系, 最终得出一个简单而统一的结论。     

强酸与弱酸分步滴定的误差公式及可行性判断

一般分析化学教科书［１～３］在论述多元酸或混合酸分步滴定的可能性时,以Ｋａ（ＨＡ）／Ｋａ（ＨＢ）＞１０５（ｃ（ＨＡ）＝ｃ（ＨＢ））作为判断能否分步滴定的判别式,然而这一判别式却不能适用于强酸和弱酸混合物的情况。例如ＨＣｌ＋ＨＡｃ混合酸是不能分步准确滴定的,虽然它们的Ｋａ值之比Ｋａ（ＨＣｌ）／Ｋａ（ＨＡｃ）远大于１０５。Ａｃ－也是不可能藉加入过量的ＨＣｌ再用ＮａＯＨ溶液进行返滴定的。　　对于这一问题,可从讨论终点误差入手。返滴定至终点时,溶液的质子平衡式为［Ｈ＋］终＋ｃ（ＮａＯＨ）＝ｃ（ＨＣｌ）＋…     

强碱滴定一元弱酸突跃范围的计算

针对现有教材及文献对弱酸滴定计量点附近某些不甚合理的简化处理过程以及某些值得商榷的结论,根据滴定弱酸计量点附近溶液的实际组成,推导了合理的、适用任何情况的强碱滴定一元弱酸的突跃范围计算公式,从而给出了一元弱酸被准确滴定的判据。     

近似计算的普适性：一元弱酸的滴定突跃范围

突跃范围是选择指示剂的主要依据,也是决定能否准确滴定的重要条件。滴定一元弱酸HB溶液时,计量点前(滴定分数a=0.999),许多溶液中c_(HB)与[OH~-]或c_(HB)与[H~+]具有相同数量级,因此通用教科书介绍的近似式■存在较大局限性。本文新推导出了满足误差要求(在±0.02pH以内)的适用滴定各种浓度一元弱酸HB的pH的近似公式。计量点前,溶液酸性情形(pK_a≤4.0),溶液■;溶液碱性情形(pK_a4.0),溶液■。计量点后(a=1.001),考虑弱碱B~-对溶液pH影响,则有溶液■。     

多元酸碱分步滴定的可行性研究

分析化学教科书中,对多元酸能否分步滴定都给出了判据。以二元酸的分步滴定为例,当△pK_a≥5,c_aK_(a_1)≥10~8,△pH=±0.3时,滴定误差|TE|≤0.5%。基于A.Ringbom误差公式导出的这一判据被广泛的采用。在我们的教学中,曾有人对此提出了疑向:(1)在c_aK_a≥10~(-8)的前提下,分步滴定的准确度是否仅决定于△pK_a,而与被滴酸的浓度大小无关?(2)c_aK_(a_1)≥10~(-8),是一元酸被准确滴定的条件,对一元酸准确滴定的误差要求是0.1%,而多元酸分步滴定的误差要求是0.5%。所以分步滴定中仍要求c_aK_(a_1)≥10~(-8)是否合理?本文拟从滴定过程的质子条件出发,导出计算分步滴定误差的精确算式,借助计算机进行计算,得到分步滴定的确切条件,同时引用浓度对数图对此条件作出直观形象的说明。     

The accuracy and precision of the classical technique for locating the point of maximum slope on a potentiometric titration curve

When it is applied to data simulating those that might be obtained in the titration of either a strong acid or a monobasic weak acid with a strong base at the same concentration, the classical-technique for locating the end-point of a potentiometric titration has a bias that is statistically indistinguishable from zero. Its precision depends on the standard error of measurement of the volume of reagent but exhibits no detectable dependence on the standard error of measurement of the pH. If the acid is strong, the precision is improved by decreasing its concentration; if the acid a weak, the precision is improved by inccreasing its pK_a value or by decreasing its concentration.     

Simultaneous determination of equivalence volumes and acid dissociation constants from potentiometric titration data

New iterative methods for analysis of potentiometric titration data of (a) mixtures of weak monoprotic acids with their conjugate bases, (b) solutions of polyprotic (di- and triprotic) acids, and (c) mixtures of two diprotic acids are presented. These methods, using data exclusively resulting from the acidic region of the titration curve permits the accurate determination of the analytical concentration of one or more acids even if the titration is stopped well before the end point of the titration. For the titration of a solution containing a conjugate acid/base pair, the proposed procedure enables the extraction of the initial composition of the mixture, as well as the dissociation constant of the concerned acid. Thus, it is possible by this type of analysis to distinguish whether a weak acid has been contaminated by a strong base and define the extent of the contamination. On the other hand, for the titration of polyprotic acids, the proposed approach enables the extraction of the accurate values of the equivalence volume and the dissociation constants K(i) even when the ionization stages overlap. Finally, for the titration of a mixture of two diprotic acids the proposed procedure enables the determination of the composition of the mixture even if the sum of the concentrations of the acids is not known. This method can be used in the analysis of solutions containing two diastereoisomeric forms of a weak diprotic acid. The test of the proposed procedures by means of ideal and Monte Carlo simulated data revealed that these methods are fairly applicable even when the titration data are considerably obscured by 'noise' or contain an important systematic error. The proposed procedures were also successfully applied to experimental titration data.     

Improved linear titration plots for weak-acid titrations

Linear titration plot functions are derived for the following types of titration: weak monobasic acid-strong base, weak dibasic acid-strong base weak acid-weak base. These functions are compared with the corresponding Gran functions, where possible, and the effects of the following sources of error are discussed: the neglect of activity coefficients, errors in equilibrium constants and the presence of a hydrolysable salt.     

无需标准溶液的酸碱滴定计算分析研究

无需标准溶液的滴定计算分析能同时求出滴定剂与被测物质浓度.本文用非线性规划问题的单纯形加速法对无需标准溶液的酸碱滴定进行了研究.单元酸、多元酸及混合酸的测定得到了比较满意的结果.     

Approximation of Potentiometric Titration Curves by Logarithmic Functions: Titration of a Mixture of Two Monobasic Acids

The approximation of titration curves by logarithmic functions was described for the titration of a mixture of two monobasic acids. It was demonstrated with the titration of a mixture of CHCl₂COOH, CH₂ClCOOH, C₆H₅COOH, and H₃BO₃ that this method can be used for the precise determination of equivalence points. The relative error in the determination of titration end-points depends on the values of the corresponding jumps and, to some extent, on the choice of a weight function and lies in the range from 0.1 to 1%.     

Determination of component concentrations in mixtures of weak and strong acids and bases by linear algebraic methods

A general expression for transforming potentiometric titration curves of mixtures of weak acids into a system of linear equations is derived. The solution of the linear equations gives directly the concentrations of the components. This linear transformation method is illustrated by the analysis of mixtures of weak acids with overlapping dissociation equilibria. The possible presence of a strong acid or strong base in the mixture can also be detected and its concentration simultaneously determined. The method can also be used for analysis of an ampholyte and solutions containing a weak acid and its conjugate base. For example a mixture of hydroxyacetic acid (pK approximately 3.6), acetic acid (pK approximately 4.6) and hydroxylamine hydrochloride (pK approximately 6) was analysed in the presence of strong acid with an average relative error of approximately 2%.     

简便快速计算线性作图法测定稀的极弱碱

提出了HCl滴定碱的线性方程简化式,应用于测定稀的极弱碱、多元碱、强弱碱混合液及抗酸药碳酸氢钠片的含量,不需要极弱碱的稳定常数KS,即可简便快速计算V-pH数据,作直线确定反应终点,其结果的相对标准偏差和绝对误差分别为0.16%～0.66%和0.090%～0.91%,符合一般化学分析要求     

Distinguishing polyfunctional from monofunctional acids and bases by acid-base titrimetry, multiparametric curve-fitting, and deviation-pattern recognition

A new technique for distinguishing diacidic from monoacidic weak bases (or dibasic from monobasic weak acids) is based on fitting the data obtained in a potentiometric acid-base titration to theoretical equations for the titration of a monoacidic base (or monobasic acid). If the substance titrated is not monofunctional the best fit to these equations will involve systematic deviations that, when plotted against the volume of reagent, yield a "deviation pattern" with a shape characteristic of polyfunctional behaviour. Ancillary criteria based on the values of the parameters obtained from the fit are also described. There is a range of uncertainty associated with each of these criteria in which the ratios of successive dissociation constants are so close to the statistical values that it is impossible in the face of the errors of measurement to decide whether the substance is monofunctional or polyfunctional. If the data from one titration prove to lie within that range, the decision may be based on the results of a second titration performed at a different ionic strength. Further fitting to the equations describing more complex behaviour provides a basis for distinguishing non-statistical difunctional substances from trifunctional ones, trifunctional ones from tetrafunctional ones, and so on.     

An online computer method for the potentiometric titration of mixtures of a strong and a weak acid

A PDP-11 online computer method for the titration of mixtures or a strong and a weak acid is described.The method is based on multiparametric curve-fitting. One or the parameters found from the calculations is the dissociation constant of the weak acid, hence the method can be applied even when this constant is unknown. Accurate results (relative error ±1%) were obtained for weak acids with pK_a values of 0.2–10. A complete titration and calculation takes about 20 min.     

Graphic determination of equivalence volumes in potentiometric titrations of mixtures of weak acids-I Two monobasic acids

A titration method for analysis of binary mixtures of weak acids is given. The equivalence volumes of the individual acids can be determined graphically from only two titration points. However, use of more than two points gives a more accurate result. Mixtures of acetic acid with ascorbic acid, hydroxyacetic and monochloroacetic (Delta log K(H)(HA), = 0.5-1.9) are titrated with an error of approximately, 0.5-2.5% in all three cases.