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.     

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.     

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.     

Multiparametric curve fitting-XIII Reliability of formation constants determined by analysis of potentiometric titration data

The formation (protonation) constants log K_i, of the acid H_jL are determined by regression analysis of potentiometric titration data when common parameters (log K_i, i = 1,…, j) and group parameters (E⁰′, L₀, H_T) are refined. The influence of three kinds of error on the protonation constants has been investigated: error from the strategy of minimization, random error, and error from uncertain estimates of group parameters. An analysis of variance of the log K_i, matrix was made for 7 identical titrations and 8 computational strategies, or of 7 identical titrations and 8 different options of group parameters to be refined. The influence of the standard potential E⁰ of the glass-electrode cell on the systematic error in log K is greater than that of the acid concentration (L₀) or the concentration of titrant used (H_T). The ill-conditioned group parameters should be refined together with the common parameters (K_i), otherwise the estimates of log K_i, are not accurate enough. Two ways of calibrating the glass electrode cell were compared. Internal calibration (performed during titration) was more accurate than external calibration done separately. Of the programs tested ESAB and ACBA are the most powerful because they permit refinement of group parameters and internal calibration. Citric acid was chosen as model substance.     

I.M. Kolthoff,P.J. Elving and F.H. Stross (Editors), Treatise on analytical chemistry. Part III. Analytical chemistry in industry : Vol. 2, Wiley-Interscience,New York, 1971, xxiv+597 pp., price £ 12.00

The titration of some substituted azoles in non-aqueous solution has been studied and their relative basic strengths determined. Differentiating titration of mixtures of substituted imidazoles is only possible when one of the imidazoles is substituted with a powerful electronegative substituent, e.g. a nitro group. Azolium halides may be titrated with perchloric acid in anhydrous acetic acid after addition of mercury(II) acetate, or dissolved in acetic anhydride without addition of mercury(II) acetate; dissolved in methanol, they may be titrated as acids with tetrabutylammonium hydroxide in benzene/methanol. The acidic and basic properties of some substituted pyrazol-5-ones have been compared.     

Rapid determination of the equivalence volume in potentiometric acid-base titrations to a preset pH-I Theory and applications

A new approach to shorten the time needed for an acid-base titration has been made. The method developed is based on the equation for acid-base titrations derived by Ingman and Still. The equation is transformed into such a form that only one titration point is needed to calculate the equivalence volume when the titration is carried out to a preset pH which can be chosen according to the experimental conditions. The method is used for titration of acetic acid, log K(H)(HA) = 4.65, hydroxylammonium ion, log K(H)(HA) approximately 6.2, and boric acid, log K(H)(HA) approximately 9.1, with an error of 0.1-0.5%. In titration of hydrogen ascorbate ion, log K(H)(HA) approximately 11.3, the error obtained was about 0.3-2%.     

Process monitored spectrophotometric titration coupled with chemometrics for simultaneous determination of mixtures of weak acids

A new spectrophotometric titration method coupled with chemometrics for the simultaneous determination of mixtures of weak acids has been developed. In this method, the titrant is a mixture of sodium hydroxide and an acid-base indicator, and the indicator is used to monitor the titration process. In a process of titration, both the added volume of titrant and the solution acidity at each titration point can be obtained simultaneously from an absorption spectrum by least square algorithm, and then the concentration of each component in the mixture can be obtained from the titration curves by principal component regression. The method only needs the information of absorbance spectra to obtain the analytical results, and is free of volumetric measurements. The analyses are independent of titration end point and do not need the accurate values of dissociation constants of the indicator and the acids. The method has been applied to the simultaneous determination of the mixtures of benzoic acid and salicylic acid, and the mixtures of phenol, o-chlorophenol and p-chlorophenol with satisfactory results.     

Choice of chemical conditions in order to obtain linear titration curves in potentiometry

In the titration of an acid HA with a strong base, [HA] diminishes, on the whole, in proportion to the addition of titrant. However, [HA] = K_HA[H] [A], and if [A] is kept constant [HA] will be proportional to [H]. Therefore if [H] instead of [HA] is plotted against ml of titrant added, one obtains a linear titration curve. [A] is kept constant by addition of NaA before titration in such high concentration that the formation of A can be neglected during the titration. When an acid stronger than acetic acid is titrated, sodium acetate can be used as the added salt. Even mixtures of adds, mono- or polybasic, then yield'straight titration curves since one is in fact titrating acetic acid in the presence of excess of acetate. For weaker acids sodium sulphite is used. In the oxidation-reduction titration of, for example, ferrous iron with permanganate, the potential measured is dependent on the ratio [Fe²⁺]/[Fe³⁺]. If [Fe³⁺] is kept constant by the addition of ferric chloride before the titration, the potential win be dependent only on [Fe²⁺]. Since this diminishes in proportion to the added volume of titrant, straight titration curves can be obtained in this case also. No correction for dilution should be made.     

Linear titration plots for the analysis of mixtures of three weak acids or bases

Linearly related functions have been derived which enable mixtures of three weak acids or bases in solution to be analysed by means of pH-titration data. This theory has been tested with data from the titration of a variety of mixtures of acids. The data required for the functions are pH, volume of titrant, the equilibrium constants relevant to the mixture, and an independently determined equivalence volume. This may be the equivalence volume for one of the components of the mixture or the sum of the equivalence volumes for any two or all three of the components. It is immaterial how this equivalence volume is obtained, but it is usually possible to obtain the total equivalence volume from data in another part of the same titration curve (when a large excess of titrant has been added).     

GEOMETRIC REQUIREMENTS FOR THE INTRAMOLECULAR QUENCHING OF INDOLE FLUORESCENCE BY AMIDE AND CARBOXYLIC ACID GROUPS IN RIGID MOLECULES WITH THE [2,2,1]BICYCLOHEPTENYL SKELETON

Abstract— The geometric requirements for intramolecular fluorescence quenching of indole by carboxylic acid and amide functions have been determined in rigid norbornyl ([2.2,1]bicycloheptenyl) skeleton: trans and cis 3-(3'-indolyl) norbornene 2-dimethylcarboxamides or carboxylic acids. The reference compound was the 3-(3'-indolyl) N-dimethylpropionamide or -propanoic acid. The quenching occurs mainly in protic solvents. In the cis compounds, the quantum yields are strongly decreased when compared to the trans and open chain compounds. As determined by the pH dependence of fluorescence of the acids, it is in the acid form—COOH that the fluorescence is quenched; in the ionized species -CO⁻₂, only small quenching is observed. The K_s of Stern-Volmer equations are very small for the cis bicyclic acid and amide, the indole nucleus being shielded on one side only. The results support the hypothesis that small rearrangement of the peptide bond in protein conformation changes may be detected by fluorescence.     

Determination of sulphate by pH titration

A simple and inexpensive method has been developed for the determination of sulphate and other very weak bases and acids. It utilizes the partial protonation of the weak base or the partial dissociation of the weak acid, which has not been exploited for analytical purposes thus far. The procedure consists of three pH titrations: one with a test solution of known sulphate content, the second with the sulphate sample, and the third with a blank. This method can be used in the presence of several inorganic ions and organic matrices, including non-aqueous solvents. In aqueous medium sulphate contents above 10(-3)M can be determined. The use of solvent mixtures may increase the sensitivity of the method by two orders of magnitude.     

3-methyl-2-oxazolidone as a solvent for acid-base titrations

3-Methyl-2-oxazolidone has been evaluated as a solvent for the titration of various weak carboxylic acids and phenols. Its high dielectric constant and wide liquid range contribute to its outstanding solvent properties. Tetrabutylammonium hydroxide was the titrant. End-points were determined potentiometrically with a glass-calomel electrode system. FORTRAN computer programs were used to evaluate the results, and relative acid strengths were determined and related to the aqueous acidity. The accuracies obtained are comparable to those for other solvent media. An 800-mV potential span is available in this solvent, which may allow differentiating titrations to be performed. No titrations of acid mixtures were attempted, but it was found possible to distinguish both neutralization steps for salicylic acid and o-phthalic acid.     

Ultraviolet absorbance titration for determining stability constants of humic substances with Cu(II) and Hg(II)

We describe an ultraviolet (UV) absorbance titration method that can be used to determine complexing capacities (C_L) and conditional stability constants (log K) of humic substances (HSs) with metal ions such as Cu(II) and Hg(II). Two fulvic acids (FA) and one humic acid (HA) were used for this study. UV absorbance of HSs gradually increased with the addition of Cu(II) or Hg(II) after blank correction, and these increases followed the theoretical 1:1 (ligand:metal ion) binding model. The results from the absorbance titration calculation for HSs with Cu(II) and Hg(II) compared well with those from fluorescence quenching titration. The titration of the model compound l-tyrosine with Cu(II) proved the validity of this method, and the K and C_L were within 2.3% and 7.4% of the fluorescence quenching titration. The results suggest that the UV absorbance titration can be used to study the binding capacities of HSs and/or dissolved organic matter (DOM) with trace metals. The advantages and disadvantages of the absorbance titration method were also discussed.     

硅胶基质固定相离子对反相液相色谱法测定强离解化合物的正辛醇/水分配系数

反相液相色谱(RPLC)是测定正辛醇/水分配系数(log P)的有效方法,但由于缺少同类型模型化合物,RPLC在测定强离解化合物的log P时遇到挑战。该文在硅胶基质C18色谱柱上,采用离子抑制反相液相色谱(IS-RPLC)和离子对反相液相色谱(IP-RPLC)分别对中性化合物、酚酸、羧酸、磺酸及部分两性化合物的保留行为进行了系统研究。在IS-RPLC模式下,利用中性化合物、弱离解的酚酸和苯羧酸作为模型化合物,建立了表观正辛醇/水分配系数(log D)与纯水相保留因子对数值(log k_w)的定量结构-保留行为关系(QSRR)模型,测定了19种离解化合物的log D值,作为后续IP-RPLC的模型化合物及验证化合物。在IP-RPLC模式下,将中性、弱离解和强离解化合物作为混合模型组,以溶质静电荷n_e、氢键酸碱性参数A和B为桥梁,建立了线性良好的log D-log k_w-IP模型,采用3种不同类型的离解化合物进行了外部验证实验,预测值误差低于10%,证实了模型的可靠性。在此基础上,预测了8种强离解化合物的log D_7.0值(pH 7.0条件下的log D值)。研究表明,利用结构相关参数沟通不同类型的模型化合物,是实现IP-RPLC测定强离解化合物log D值的一种行之有效的方法。与聚乙烯醇基质色谱柱相比,通用型的硅胶基质色谱柱上尽管存在着更多的次级作用,但可以为强离解化合物log D的测定提供更灵活的选择。     

Single-point titrations: Part-I. The determination of bases

Equations are derived for the calculation of acid mixtures, which upon titration with base show a linear relation between pH and the amount of base. Three to five weak acids were used and a linearity of better than ±0.02 pH units was obtained. The use of such mixtures for analysis of the base content of samples by means of a single pH measurement is described. A procedure for obtaining conditional pK_a values of the components of the acid mixture is also described. The single-point titration method is advocated for use when better accuracy than that of direct potentiometry is desired but less than that of an ordinary titration can be accepted. It is not necessary to know the pK_b or the number of weak bases.     

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%.     

Acid-base indicators in acetonitrile: Their pK values and chromatic parameters

A series of acid—base indicators in acetonitrile has been studied. Their chromaticity coordinates, pK_a values, transition pH ranges, pH of maximum colour change, optimum concentration for titration, and quality of colour change in acetonitrile have been determined, together with the effect of ionic strength on their properties. Various bases and their mixtures have been titrated to test the practical usefulness of the indicators.     

The anilic acid number,an analytical index for testing dibasic acid anhydrides

For testing the purity of dibasic acid anhydrides and for analysing mixtures of these anhydrides with the corresponding free acids the determination of a new index number is recommended, indicated as the “anilic acid number”. The process is based on the fact that these acid anhydrides, when dissolved in acetone, under suitable experimental conditions can be added readily and quantitatively to aniline with resulting formation of the corresponding anilic acids.Immediately after addition, titration is effected with alcoholic potassium hydroxide on phenolphthalein. Pure dibasic acid anhydrides in this case yield the calculated anilic acid number. In the presence of free acids the values found are too high. The free acid content can be easily determined from the difference between the theoretical and the found anilic acid number.     

相对滴定法的研究Ⅱ．电位滴定法测定多种弱酸的混合溶液

本实验研究了两种弱酸混合溶液中的相对滴定法。利用该法可分别检测ｐＫａ值相差１的两种弱酸混合溶液中的两种弱酸。