基于思维发展的化学实验仪器教学——以滴定管的教学为例

以滴定管的教学为例,分析在实验仪器教学过程中,如何提高学生的学习效果,促进学生的思维能力发展。在实验仪器教学过程中,应该引入合适的教学情境,让学生结合自己的直接经验和科学发展过程,正确认识和使用实验仪器,实现思维能力的发展。     

Carboxyl groups in pre-treated regenerated cellulose fibres

The influence of peroxide bleaching and slack-mercerization on the amount of acidic groups in regenerated fibres (viscose, modal and lyocell) were studied. Conductometric titration was used to determine the total content of acidic carboxylic groups. Polyelectrolyte titration was used for surface and total charge determination, and to obtain information about the charge distribution and accessibilities of charged groups. Changes in fibre crystallinity to pre-treatment processes were characterized using iodine sorption (Schwertassek method) and correlated to treatments and the amount of carboxylic groups. For all three types of fibres the amount of accessible carboxyl groups was lowered by an increase in the degree of crystallinity. Bleaching with hydrogen peroxide causes some oxidative cellulose damage and, therefore, a larger amount of carboxyl groups (presumably formed at the end of cellulose chains). Slack-mercerization did not significantly change the total amount of acidic groups in the fibres, but their accessibility to cationic polyelectrolytes, in particular to polymers with high molecular weight was substantially lowered. Lidija Fras Zemljič, Zdenka Peršin, and Karin Stana Kleinschek are the members of the European Polysaccharide Network of Excellence (EPNOE).     

Topochemical modification of cotton fibres with carboxymethyl cellulose

Adsorption of carboxymethyl cellulose (CMC) as a method to introduce charged (ionizable) groups onto cellulose cotton fibre surfaces was investigated. The method was based on application of a previously published method used for wood fibres. The amount of adsorbed ionizable groups was determined indirectly by analysis of CMC in solution by the phenol–sulphuric acid method and directly by conductometric titration of the fibres. Results from the two methods correlated well. The molecular weight and purity of the CMC had an influence on its adsorption onto cotton; high molecular weight CMC was preferentially adsorbed. The adsorbed charge correlated linearly with the amount of CMC adsorbed. The total charge of the cotton fibres could be increased by more than 50% by adsorption of CMC. It is expected that this modification procedure can be used in a wide spectrum of practical applications. Lidija Fras Zemljič and Karin Stana-Kleinschek are the members of the European Polysaccharide Network of Excellence (EPNOE).     

Development of algorithms for automated spectrophotometric titrations

A versatile method for evaluation of equilibrium constants from automated spectrophotometric titrations is described. General algorithms were derived for normalized titration curves of reactions having various stoichiometric ratios. Normalization of data involve converting it from absorption versus time to product formed (0 to 1) versus equivalents of titrant added. Relevance of derived algorithms was assessed by the quality of their fit to real data. The demonstration system chosen for this investigation was the 1:3 complexation reaction of Fe(II) and 1,10-phenantroline (o-Phen). Titrations were performed at three distinct infusion rates under controlled pH of 5.0 and at 25.0 °C, with sets produced using both Fe(II) and o-Phen as the titrant. Direct fitting of titration curves yielded values for overall equilibrium constants. Furthermore, resulting curves revealed reproducible initial deviation, consistent with step-wise reaction, when titrating Fe(II) with o-Phen. No unusual deviations were evident anywhere in the titration curves when Fe(II) was the titrant. Overall, our approach facilitates adoption of automated titrimetry and data analysis methodologies for the determination of stoichiometries and equilibrium constants of many UV/Vis active reactions.     

Direct analysis of bottom sediments by a microcoulometric titration method for determination of total organic halide pollutants

Up to now the content of the organic halides in sediments was measured as extractable organic halides compounds by different methods including microcoulometric titration one. We describe a procedure for microcoulometric determination of total organic halide pollutants by using a direct combustion of the sediments into the microcoulometric system furnace. An accelerated removal of the inorganic chlorides by reaction with potassium nitrate using ultrasonic radiation was attained. The procedure was validated for analysis of bottom sediments from natural and anthropogenic sources. The repeatability RSD = 8.5%, the expanded relative uncertainty U (n = 7, P = 95%, k = 2) = 6.4%, the reproducibility RSD was within the range 10.0–7.3%, the average recovery R = 97.6% and method LOD = 11 mg kg⁻¹ Cl. Correspondence: Zara V. Aneva, Analytical Department, University “Prof. Dr. Assen Zlatarov”, Nr. 1 Prof. Yakimov street, 8010 Bourgas, Bulgaria     

A New Approach for Titration Calorimetric Data Analysis on the Binding of Magnesium Ion with Myelin Basic Protein

The interaction of the myelin basic protein (MBP) from the bovine central nervous system with divalent magnesium ion was studied by isothermal titration calorimetry at 27 °C in aqueous solution. A simple rapid method for determination of the dissociation binding constants for Mg²⁺-MBP interaction was introduced using the isothermal titration calometric data. The binding isotherm for Mg²⁺-MBP interaction is easily obtained by carrying out a titration calorimetric experiment using only one set of concentrations of MBP. There are two identical independent intrinsic association constants equal to 0.021 μmol⋅L⁻¹ in the first- and second-binding sites, respectively.     

The structure of a valinomycin–hexaaquamagnesium trifluoromethanesulfonate compound

Valinomycin is a naturally occurring cyclic dodecadepsipeptide with the formula cyclo‐[d ‐HiVA→l ‐Val →l ‐LA→l ‐Val]₃ (d ‐HiVA is d ‐α‐hydroxyisovaleic acid, Val is valine and LA is lactic acid), which binds a K⁺ ion with high selectively. In the past, several cation‐binding modes have been revealed by X‐ray crystallography. In the K⁺, Rb⁺ and Cs⁺ complexes, the ester O atoms coordinate the cation with a trigonal antiprismatic geometry, while the six amide groups form intramolecular hydrogen bonds and the network that is formed has a bracelet‐like conformation (Type 1 binding). Type 2 binding is seen with the Na⁺ cation, in which the valinomycin molecule retains the bracelet conformation but the cations are coordinated by only three ester carbonyl groups and are not centrally located. In addition, a picrate counter‐ion and a water molecule is found at the center of the valinomycin bracelet. Type 3 binding is observed with divalent Ba²⁺, in which two cations are incorporated, bridged by two anions, and coordinated by amide carbonyl groups, and there are no intramolecular amide hydrogen bonds. In this paper, we present a new Type 4 cation‐binding mode, observed in valinomycin hexaaquamagnesium bis(trifluoromethanesulfonate) trihydrate, C₅₄H₉₀N₆O₁₈·[Mg(H₂O)₆](CF₃SO₃)₂·3H₂O, in which the valinomycin molecule incorporates a whole hexaaquamagnesium ion, [Mg(H₂O)₆]²⁺, via hydrogen bonding between the amide carbonyl groups and the hydrate water H atoms. In this complex, valinomycin retains the threefold symmetry observed in Type 1 binding, but the amide hydrogen‐bond network is lost; the hexaaquamagnesium cation is hydrogen bonded by six amide carbonyl groups. ¹H NMR titration data is consistent with the 1:1 binding stoichiometry in acetonitrile solution. This new cation‐binding mode of binding a whole hexaaquamagnesium ion by a cyclic polypeptide is likely to have important implications for the study of metal binding with biological models under physiological conditions.     

Advantages of organic halogen bonding for halide recognition

The synthesis of a bidentate halogen bonding receptor and a nearly isostructural hydrogen bonding analogue is described. Crystal structures reveal the interactions of each receptor with anions in the solid state, while NMR titrations elucidate bidentate binding and association constants in solution. Of the two, the halogen bonding receptor demonstrates stronger, water resistant halide binding in competitive solvents.     

一元强碱滴定二元弱酸的林邦滴定曲线方程及pH_(sp)计算

用质子条件式PBE、物料平衡式MBE、副反应系数和条件稳定常数等知识,推导出一元强碱滴定二元弱酸溶液的林邦滴定曲线方程,并用其推导出第一、第二化学计量点pH_(sp1)和pH_(sp2)的计算公式。     

分析化学中四种容量分析法的归纳与比较

对分析化学中的四种容量分析方法进行了归纳和比较,从滴定的可行性、终点误差的计算公式、指示剂的作用原理、突跃范围及影响因素等方面比较其异同点,推导了各种滴定方法突跃范围的计算公式.