滴定分析终点误差的通用高效计算策略

终点误差是滴定分析的核心内容之一,现行教材中众多导出公式致使终点误差的计算过于复杂。本文提出了一种通用高效的计算策略,统一了酸碱滴定、配位滴定以及氧化-还原滴定的终点误差计算;此外,还将准确滴定判别统一到终点误差的计算中。各种实例显示,这种策略不要求记忆任何导出公式即可完成计算,而且运算强度低。公式的统一性以及运算的便捷性极大地简化了终点误差的计算,在教学中能够显著减轻学生的记忆和运算负担,提升学习兴趣,从而能够将学习重点放在滴定体系的分析和处理上。本研究作为当前形势下分析化学教学改革的一个尝试,对传统课程内容进行优化,去冗存精,使其简明凝练,重点突出,定量分析的主旨更加明晰。     

氧化还原滴定方法理论新说

为了改造氧化还原滴定分析方法的标度,从而逐步建立统一的滴定分析标度计算式,提出了相对电子浓度Re-的概念,基于此发展了以p Re-为监测标度的氧化还原滴定方法新理论。该理论描述了氧化还原半反应的平衡,推导了氧化还原滴定标度p Re-的计算通式,探索了该标度在滴定过程中的一般变化规律,重新梳理了指示剂的指示原理。以Ce4+滴定Fe3+溶液为例,演绎了如何简单绘制滴定曲线、计算滴定突跃范围、判断可否直接准确滴定,讨论了氧化还原反应条件平衡常数Kτ?’的大小对滴定分析的影响。     

通过反函数或者隐函数快速准确地绘制滴定曲线

滴定曲线是定量化学分析的重点之一,包含了大量有用信息。本文提出通过反函数或者隐函数的方式绘制滴定曲线。与传统绘制方式相比,反函数或者隐函数不仅容易推导,而且易于编程实现,因此能够快速获得大量数据点,从而显著提高滴定曲线的准确性。这种绘制方法适用于各种滴定体系,尤其是传统方法难以解决的一些复杂体系。此外,借助一些高级数据处理软件,非计算机专业人员能够快速掌握相关编程。得益于高效的绘制方案和简洁的编程,高精度滴定曲线得以发挥其应有功能,包括指示剂选择、准确滴定判别、终点误差计算以及直观地研究滴定体系的各种影响因素。本研究作为计算机技术高度发展背景下分析化学教学改革的一个尝试,对传统课程内容进行优化,去冗存精,使其简明凝练,重点突出,定量分析的主旨更加明晰。     

高精度蠕动泵式称量滴定装置及称量滴定方法

滴定分析法是将一种已知准确浓度的试剂溶液,滴加到被测物质的溶液中,直到所加的试剂与被测物质按化学计量定量反应为止,根据试剂溶液的浓度和消耗的量,计算被测物质含量的方法。滴定分析法可分为容量滴定法和称量滴定法。目前实验室滴定分析普遍采用容量滴定法,因其所用设备简单、操作方便、迅速,滴定精度可达0.1%,被广泛用     

灰分分析法测定羧甲基壳聚糖羧甲基取代度

本文建立了表征羧甲基壳聚糖羧甲基取代度的化学分析方法－灰分分析法，并将该方法与元素分析和电位滴定方法进行了比较。统计分析的结果表明，灰分分析方法的精度和准确度要高于电位滴定，其准确度较接近元素分析，是一种准确的分析方法。     

活性水压裂液中氯化钾含量的检测方法

本发明公开了一种活性水压裂液中氯化钾含量的检测方法,该方法包括：在一定体积的活性水压裂液中加入一定体积且过量的四苯硼钠标准溶液,沉淀活性水压裂液中的钾离子;然后加入相应指示剂,用烃铵盐标准溶液滴定剩余的四苯硼钠标准溶液,滴定至相应的颜色;根据滴定所使用的烃铵盐标准溶液中的烃铵盐的质量,计算与其反应的四苯硼钠的质量,得到与活性水压裂液中的钾离子反应的四苯硼钠的质量,进而得到活性水压裂液中氯化钾的质量,该氯化钾的质量与所述一定体积的活性水压裂液质量的比值为该活性水压裂液中氯化钾的含量。本发明的技术方案结果准确,操作简便,不受仪器设备条件限制,能够在施工现场进行使用。     

从摩尔分数探讨多元酸分级滴定的条件

通过摩尔分数的计算探讨多元酸准确及分级滴定的条件，为多元酸和多元碱的准确滴定及分级滴定提供理论依据。     

锡阳极泥中铟对铋含量测定的影响研究

采用传统Na_2EDTA滴定方式测定锡阳极泥(当铟含量大于0.2mg时)试液会出现颜色无明显突跃和滴定终点难以判定的现象。当铟含量大于0.2mg时,Na_2EDTA滴定法测定锡阳极泥试样中铋含量,对铋量测定有影响。主要讨论了铟在三种滴定条件下的络合行为,优化了铋的滴定条件,论证了在10.00、20.00、30.00、40.00mg铋标准溶液中加入不同含量的铟标准溶液,并在实际锡阳极泥样品YTXN-7中加入不同含量的铟,探讨了两种计算方法。实验表明,通过调节试液pH=1.5～1.7,采用Na_2EDTA标准滴定溶液滴定至试液红色刚消失,加热试液至70～80℃,继续滴定,能够出现完成铋铟的络合滴定,当铟量在1.00～2.00mg时,采用方法1计算得到铋的加标回收率在97.8%～100%,方法2的加标回收率在99.8%～101%。实验数据表明方法2计算铋的方法更理想。     

聚酰胺-胺的酸碱电位滴定分析

聚酰胺-胺(PAMAM)是一种有着广泛用途的树形大分子,酸碱滴定分析可以表征合成的PAMAM中所含的伯胺基及叔胺基的数量,因此作为一种重要的辅助表征手段被PAMAM的合成及改性研究者所采用。文章对该方法原理进行了详细的论述,说明了滴定过程中产生的酰胺键水解不影响滴定及计算结果。通过实验表明在pH<2的条件下滴定可以保证叔胺的充分质子化。     

弱酸的滴定终点及其离解常数的微机直接计算法

在酸碱滴定中,确定滴定终点一般采用指示剂法、“S”形曲线法和直线图解法。前两种方法作为常用的方法,简便易行,但仅适用于具有pH突跃的各种酸碱滴定;而直线图解法虽然适用于极弱酸的滴定,但又需要经过比较复杂的计算和作图,才能求得结果,同样也不便于实际应用。我们采用了微机直接计算法,测算结果无论在准确、快速和简便等方面均取得了较为满意的效果。     

Kinetic detection of the end-point in titrations involving slow reactions Direct titration of polyhydroxy-compounds with periodate

A new titration technique is described in which the end-point is determined by measuring the relative reaction rate of the titration reaction. This technique is adequate for rather slow reactions where conventional direct titrations are not applicable. The titrations are done automatically under microcomputer control. The efficiency of this technique is demonstrated with direct titrations of certain polyhydroxy-compounds with standard periodate solution. Ethylene glycol and propylene glycol (0.05-0.3 mmole), glycerol (0.06-0.17 mmole) and mannitol (0.01-0.03 mmole) were determined with average relative errors of 0.1-0.3%.     

pH-stat techniques in titrimetric analysis: Part 3. The principles of pH-stat chelatometric titrations

Chelatometric titrations can be effectively monitored by means of the pH-stat technique. During the addition of the primary titrant (the chelating agent), the pH of the titrated solution is kept at a pre-selected value by adding a strong base as the auxiliary titrant. The equivalence point is characterised by a sudden change in the ratio between the added volumes of auxiliary and primary titrant solutions, i.e. in the slope of the linearly segmented titration plot representing the volume of the base against the volume of the chelating agent. The slope of the linear segments depends on the reactions involving proton exchange, which accompany the primary complexation reaction. The dependence of the shape of the plots on the imposed pH for typical titrations (magnesium(II) and copper(II)) is examined and the conditions for the feasibility of pH-stat chelatometric titrations are discussed.     

Logarithmic diagrams as an aid in complexometric titrations where 1:2 or 2:1 as well as 1:1 metal-ligand complexes are regarded as principal species

Logarithmic diagrams have earlier been used for theoretical treatment of complexometric titrations where only the 1:1 complex is formed in the main reaction between a metal ion M and a ligand L. This approach has now been extended to include titrations where 1:2 and 2:1 complexes are also formed. Four different types of diagram can be drawn. These diagrams can be used to determine pM (or pL) at the equivalence point, the titration ratio at different pM (or pL) values and the titration error. One of these diagrams is easy to construct and use. The method also has the advantage that it can directly indicate whether the titration is best performed with 1:1 or 2:1 (1:2) stoichiometry. Two typical examples, namely the titration of a manganese solution with triethylenetetraminehexa-acetic acid and of a copper solution with pyridine-2,6-dicarboxylic acid, are discussed. The two titrations proceed with the formation of the complexes Mn(2)L (2: 1) and CuL(2) (1:2) respectively, in addition to the 1:1 complex in each instance.     

Binding of Cu(II) to poly(methacrylic acid)

The binding of Cu(II) ions to partly neutralized poly(methacrylic acid) (PMA) has been investigated by potentiometric titration and dialysis to determine the stoichiometry the Cu–PMA complexes formed. Partly ionized PMA was titrated with solutions of the metal ion to enable a large range of metal ion/polymer ratios to be studied. Combination of the results from these two techniques at ionic strength 0.1 indicates that at very low Cu(II)/polymer ratios, a 4:1 complex exists, but at higher ratios the complex breaks down to give a mainly 2:1 coordination with some 1:1 binding. Conductance titrations support these results. Viscometric titrations show strong interactions between the metal and polymer, preventing the full extension of the polyion at high degrees of ionization, and spectrophotometric titrations support the existence of at least two types of complexes in the solution.     

Analysis of tellurides of lead and tin by automatic titrations

An analytical method based on automatic potentiometric EDTA and redox titrations was developed for the analysis of tellurides of lead and tin.First, the sum of lead and tin is titrated at pH 4.5 by adding a known excess of EDTA and back-titrating with a standard lead solution. After addition of ammonium fluoride to mask the tin, the EDTA released from the Sn-EDTA complex is titrated with standard lead solution. Alternatively, after the determination of total lead and tin, lead may be determined by back-titration with standard lead on a separate sample aliquot using tartaric acid to mask the tin. Tellurium is separated as tellurous acid, which is then dissolved in a dilute sulfuric acid solution and oxidized by permanganate at room temperature; the excess permanganate is back-titrated with iron(II) solution automatically. This titration may also be used to determine tellurium in the presence of lead and tin after treatment with perchloric acid. Accuracies of 0.1-0.2% can be obtained.     

Inclusion complexes of PBN-type nitrone spin traps and their superoxide spin adducts with cyclodextrin derivatives: parallel determination of the association constants by NMR titrations and 2D-EPR simulations

(1)H NMR and electron paramagnetic resonance (EPR) titrations were used to determine the association constants of the complexes of alpha-phenyl-N-tert-butylnitrone (PBN) analogues and their superoxide spin adducts, respectively, with methylated beta-cyclodextrins. A 1:1 stoichiometry for the nitrones with randomly methylated beta-cyclodextrin and 2,6-di-O-methyl-beta-cyclodextrin and 1:1 and 1:2 stoichiometries for the corresponding cyclodextrin-nitroxide complexes were observed. After the superoxide radical spin trapping reaction, EPR titrations afforded the association constants of the corresponding cyclodextrin-nitroxide complexes. Two-dimensional EPR simulations indicated a bimodal inclusion of the nitroxide free radical spin adducts into the cyclodextrins. For all the nitrone-cyclodextrin and nitroxide-cyclodextrin complexes, the association constants were always higher for the nitroxide complexes than for the nitrone complexes. A cooperative system concerning the complexation of the nitroxide spin adduct with a cyclodextrin was evidenced by EPR titrations. The efficiency of the cyclodextrin inclusion technique to trap superoxide and to resist bioreduction by sodium l-ascorbate was also investigated.     

Contrasting pKa of protonated bis(3-aminopropyl)-terminated polyethylene glycol "Jeffamine" and the associated thermodynamic parameters in solution and covalently attached to graphite surfaces

The pKa value of protonated Jeffamine (bis(3-aminopropyl) terminated polyethylene glycol) in solution and attached as a monolayer to graphite surfaces has been determined using potentiometric titration. The protonated Jeffamine was found to have a pKa value of 9.7 in solution at 25 degrees C, whereas this value decreases to 7.1 when it is attached to a graphite surface. Potentiometric titrations from 25 to 40 degrees C allowed us to determine the surface pKa of the protonated Jeffamine at each temperature studied and hence to determine the enthalpy, entropy and Gibbs energy changes associated with the deprotonation of the amino-terminated surface bound Jeffamine groups. It was found that the enthalpic contribution is negligibly small and the evaluation of these thermodynamic parameters controlling the shift in surface pKa value indicates that this process is controlled by entropic contribution arising from the ordering/disordering of solvent molecules at the carbon-water interface. This suggests that the long chain Jeffamine molecules are oriented on the carbon surface rather than existing in the bulk solution.     

A Simple Sensor for Potentiometric Titrations

Abstract

A sensor for potentiometric titrations was prepared by coating a spectroscopic graphite rod with a solution of poly(vinyl chloride) and dioctylphthalate in tetrahydrofuran. The reference was a Ag/AgCl single-junction electrode. The sensor was used in the following potentiometric titrations:

1. precipitation titrations 2. acid-base titrations, 3. compleximetric titrations, and 4. redox titrations.

A survey of its use in such titrations is presented.

Preparation of the coated-graphite sensor is simple and rapid. Moreover, it is quite inexpensive. A limitation is its applicability in aqueous media only, because organic solvents will dissolve the membrane.     

Inhibition of acidic corrosion of carbon steel by some mono and bis azo dyes based on 1,5 dihydroxynaphihalene

Inhibition of the corrosion of carbon steel in hydrochloric acid solution by some mono- and bis-azo dyes based on 1,5-dihydroxynaphthalene was studied in relation to the concentration of inhibitors using weight loss and potentiostatic polarization techniques. The percentage inhibition efficiency calculated from two methods is in a good agreement with each other. The inhibition mechanism of the additives was ascribed to the formation of complex compound adsorbed on the metal surface. The adsorption process follows Freundlich adsorption isotherm. The formation of the complex compound was studied by conductometric and potentiometric titrations. The stability constants of the Fe-complexes were determined using the latter technique and related to the inhibition efficiency.     

Diethylenetetra-ammonium sulphatocerate as volumetric reagent: Iodine monochloride method

The method of preparation of diethylenetetra-ammoniuin sulphatocerate is described. This substance has been used as an oxidant to determine potassium iodide, ferrous ammonium sulphate, arsenious oxide, stannous chloride, hydrazinc sulphate, thallous chloride. hydroquinone and potassium ferrocyanide by a volumetric method, using iodine inonochloride as catalyst and preoxidizer. During these titrations, normality of the solution with respect to hydrochloric acid has been kept at about 6N. Chloroform is used as an indicator. It is 'coloured pink owing to the liberation of iodine during the titration and becomes very pale yellow at the end-point because of the formation of iodine monochloride.