Spectroscopic, Chromatographic and Visual Investigation of Organic Dyes

An introductory level laboratory experiment is presented in which students learn about color using spectroscopy and chromatography. The pedagogical approach is discovery-based; students are given only enough background information to enable them to take good data. Commercially available dyes are dissolved in water to make concentrated stock solutions, which students then dilute to prepare solutions of primary, secondary and tertiary colors. The class works as a team to study colored solutions representing a range of concentrations and combinations of the three primary colors: cyan, yellow, and magenta. Students record transmission and absorption spectra and compare the results with human perception. They show the number of components in each solution by paper chromatography. They explore the filter nature of dyes, the effects of concentration, and the cumulative effects of mixing dyes. From this information they deduce the principles of color printing.The experiment is best performed with a photodiode array spectrophotometer; alternative approaches include spectrometers and simple spectroscopes which the students make from cereal boxes. The experiment can be performed in either a single three-hour laboratory period, or alternatively, three one-hour sessions.     

Freezing on heating of liquid solutions

We report a reversible liquid-solid transition upon heating of a simple solution composed of a-cyclodextrine (alpha CD), water, and 4-methylpyridine. These solutions are homogeneous and transparent at ambient temperature and solidify when heated to temperatures between 45 degrees and 75 degrees. Quasielastic and elastic neutron scattering show that molecular motions are slowed down in the solid and that crystalline order is established. The solution "freezes on heating." This process is fully reversible, on cooling the solid melts. A rearrangement of hydrogen bonds is postulated to be responsible for the observed phenomenon.     

翻转课堂模式在光度法测铁实验教学中的应用

翻转课堂模式有助于使课堂教学从以教师为中心向以学生为中心转变,因而受到教育界广泛关注。以邻二氮菲分光光度法测定水中微量铁为例,探索了翻转课堂教学模式在非化学化工类大学化学实验教学中的应用。依据翻转课堂教学流程研究了课前视频、课前在线检测题及课上活动的设计与实施方案,探索了将科技文献用于课前视频的教学方法。利用课上活动给学生提供自我展示的机会;利用课前视频和课前在线检测使学生的课前预习落到实处、拓宽学生视野、激发学生对化学实验的兴趣。     

Thermodynamic and kinetic models for the extraction of essential oil from savory and polycyclic aromatic hydrocarbons from soil with hot (subcritical) water and supercritical CO2

Mechanisms that control the extraction rates of essential oil from savory (Satureja hortensis) and polycyclic aromatic hydrocarbons (PAHs) from historically-contaminated soil with hot water and supercritical carbon dioxide were studied. The extraction curves at different solvent flow-rates were used to determine whether the extractions were limited primarily by the near equilibrium partitioning of the analyte between the matrix and solvent (i.e. partitioning thermodynamics, or the "elution" step) or by the rate of analyte desorption from the matrix (i.e. kinetics, or the "initial desorption" step). Two simple models were applied to describe the extraction profiles obtained with hot water and with supercritical CO2: (1) a model based solely on the thermodynamic distribution coefficient KD, which assumes that analyte desorption from the matrix is rapid compared to elution. and (2) a two-site kinetic model which assumes that the extraction rate is limited by the analyte desorption rate from the matrix, and is not limited by the thermodynamic (KD) partitioning that occurs during elution. For hot water extraction, the thermodynamic elution of analytes from the matrix was the prevailing mechanism as evidenced by the fact that extraction rates increased proportionally with the hot water flow-rate. This was also confirmed by the fact that simple removal calculations based on a single KD (for each essential oil compound) gave good fits to experimental data for flow-rates from 0.25 to 4 ml/min. In contrast, supercritical CO2 extraction showed only minimal dependence on flow-rate, and the simple KD model could only describe the initial 20-50% of the extraction. However, a simple two-site kinetic model gave a good fit for all CO2 flow-rates tested. The results of these investigations demonstrated that very simple models can be used to determine and describe extractions which are limited primarily by partitioning thermodynamics, or primarily by desorption kinetics. Furthermore, these results show that the time required for the recovery of essential oil from savory with hot water can be minimized by increasing flow-rate, with little change in the total volume of water required. In contrast, raising the flow-rate of supercritical CO2 has little effect on the mass of essential oils recovered per unit of time, indicating that optimal recovery of these compounds with supercritical CO2 (amount recovered for the lowest amount of CO2) requires longer extraction times rather than faster flow-rates.     

Statistical Comparison of Multiple Methods for the Determination of Dissolved Oxygen Levels in Natural Waters

The following experiment reinforces students working knowledge of statistics by utilizing the t test to compare the results of two independent methods for the determination of dissolved oxygen (DO). In this experiment students utilize a dissolved oxygen probe to determine the levels of DO in natural waters at two sampling locations while obtaining samples of water from the laboratory for analysis using the classic Winkler titration. The importance of using proper sampling methods and techniques to obtain representative samkples is a large focus of the prelaboratory discussion and is continually stressed during fieldwork. After analyzing the water samples by the DO mete and the Winkler titration, students pool the class data and are asked to determine if the two methods for dissolved oxygen agree at each sampling location. The students are then asked to determine if the DO levels at the different sampling locations are statistically different or not. The students are asked to consider why their results agree or differ from the theoretical value they calculate using Henrys law.     

Effects of different steeping methods and storage on caffeine, catechins and gallic acid in bag tea infusions

Bag teas, packed 3g of ground black, green, oolong, paochoung and pu-erh tea leaves (the particle size used was 1-2mm), were steeped in 150 mL of 70, 85 or 100 degrees C hot water to study the effects of the number of steeping (the same bag tea was steeped repeatedly eight times, 30s each time, as done in China for making ceremonial tea) and varied steeping durations (0.5-4 min) on caffeine, catechins and gallic acid in tea infusions. The changes in tea infusions during storage at 4 or 25 degrees C for 0-48 h and the variations in these compounds of bag tea infused with 150 mL of 4 or 25 degrees C cold water for 0.5-16 h were also investigated. A HPLC method with a C18 column and a step gradient solvent system consisting of acetonitrile and 0.9% acetic acid in deionized water was used for analysis. Results for all kinds of tea samples showed that the second tea infusion contained the highest contents of caffeine, catechins and gallic acid when bag teas were steeped in 70 degrees C water. It was different from that steeped at 85 and 100 degrees C, the highest contents existed in the first infusion. These compounds decreased gradually in later infusions. Higher amounts of caffeine, catechins and gallic acid could be released from bag teas as hotter water was used. As steeping duration prolonged, these ingredients increased progressively, however, their levels were lower than that cumulated from the infusions with the identical bag tea prepared recurrently at the same temperature and time points. (-)-Gallocatechin gallate and (+)-catechin existed in these tea infusions rarely and could not be detected until a certain amount of them infusing. Except gallic acid that showed a significant increase and caffeine that exhibited no significant change, all kinds of catechins decreased appreciably after tea infusions were stored at 25 degrees C for 36 h; nevertheless, all of them showed no evident changes at 4 degrees C storage. The caffeine, catechins and gallic acid in tea infused with cold water also increased with increasing duration. Their contents in 25 degrees C steeped tea were higher than that made at 4 degrees C; moreover, their infusion rates from bag teas to cold water were markedly lower than that steeped in hot water. Infusing efficiencies of non-gallated catechins were higher than gallated catechins under cold water steeping.     

Temperature-actuated changes in wettability at elastomer/water interfaces

Surface modification of 1,4-polybutadiene and cis-1,4-polyisoprene to introduce polar functional groups provided surfaces that reconstructed reversibly against water as a function of temperature. These surfaces became hydrophobic in contact with hot water, but their original hydrophilicity returned upon equilibration against cold water. Repeated cycling between hot and cold water, however, led to a damping of this reversibility. A series of parallel experiments on both the interfacial and bulk behavior of these elastomers strongly indicated that this damping was due to the alignment of extended interfacial chains during temperature cycling and to a decay of the restoring force on the interfacial chains under extension. These studies thus demonstrate that the interfacial behavior of elastomers can display close analogies to the bulk viscoelastic properties of the solid.     

Isotope effects in liquid water by infrared spectroscopy. II. Factor analysis of the temperature effect on H2O and D2O

Some 500 infrared (IR) spectra of light and heavy waters were obtained between 29 and 93 degrees C in order to identify the species present in liquid water. Factor analysis of these gives two species for each type of water with their IR spectra and abundance curves. Using an orthogonalization procedure, we obtained the temperature factor limits of -22 and +118 degrees C (+/-5 degrees C) that we coined cold and hot factors, respectively. Within experimental error, these limits are the same for light and heavy waters. The spectra of the orthogonalized factors presented show a decrease of the OH (OD) stretch band integrated intensities of almost 36% from the cold to the hot factors. No "free" OH (OD) group is present or formed in the temperature ramp. This indicates that all water molecules in the bulk are made of an oxygen atom surrounded with four hydrogen atoms, two covalently bonded, and two hydrogen bonded. This is consistent with the previous study of mixtures of H(2)O and D(2)O [part I: J.-J. Max and C. Chapados, J. Chem. Phys. 116, 4626 (2002)]. To maintain the ordinary liquid within the limits of 0 and 100 degrees C at atmospheric pressure, a fraction of the cold and hot factors are necessary. With the spectra of the cold and hot factors and the abundance curves, one can generate the spectrum at any temperature between -22 and +118 degrees C of light and heavy liquid waters.     

槐米中芦丁提取、精制与鉴定的创新实验教学

In order to solve the frequently encountered problems on the experiment of extraction, refining and identification of Rutin in the laboratory class, new methods of teaching design and experiment process were adopted to suit different experimental procedure. With the improvement, both the success rate and the students' confidence were improved. The innovation experiment also illustrated the importance of process improvement to improve the quality of the products. Through the mutual improvement of teaching and learning in this experiment, teachers and students achieved the goal of learning together.     

婴儿湿巾缓冲能力的测试——一个贴近生活的实验

为了有效提升学生在缓冲溶液学习过程中的学习兴趣及积极性,在此推荐一个将现实中的生活用品作为实验对象引入实验课教学过程中的案例。该实验设计将性质实验扩充到婴儿湿巾,将实验内容变为对婴儿湿巾工作原理的验证,而不再是过去简单的酸碱体系的配制及相关验证实验。通过该实验的实施,学生普遍反映能够有效激发其在学习缓冲溶液相关课程中的学习积极性与兴趣,并最终促使学生认知缓冲体系内pH变化的特点及缓冲机理。     

Assembly of a Thin-Falling-Film Exchanger for Laboratory Demonstrations: Calculation of the Individual Heat-Transfer Coefficient

Thin-film exchangers have a large heat-transfer area per unit liquid volume that make them very efficient for industrial use. Unfortunately, they are usually less applied for education purposes. In this work, a simple experimental device with a fluid flowing as a falling film has been assembled. In addition, an experimental guide allows students to determine the individual heat-transfer coefficient in the exchanger.The experimental setup consists of two pipes bundled as a shell-and-tube heat exchanger. The cold liquid flows in the form of a thin film that runs down inside the inner tube; the hot liquid flows down concurrently in the external one.In order to test the operation of the device, experimental results are given for a simple test with water. Heat-transfer coefficients are calculated and a theoretical correlation obtained from the literature is compared with the experimental results. In addition, a fit of the data allows for the formulation of a new empirical equation for the transient regime.     

Quantitative Determination of Copper: Combining Project-Based Laboratories with Single-Analyte Detection

A multiweek experiment is presented for use in undergraduate instrumental analysis courses. The experiment combines project-based laboratories and single-analyte detection to provide students with experience in method development and validation, and to give them a more realistic experience in the analytical laboratory. Working together as a team, students develop methods for the detection of an analyte (i.e., copper) in water samples using at least two spectroscopic instruments (e.g., ICP-AES, AA, UV-vis, fluorescence). Student teams are given only topical information about their projects, and must research and plan the analyses, learn the instrumental methods to be used, obtain figures of merit (e.g., detection limits) from Beers law plots, analyze commercial water samples, and produce a standard operating protocol for one of their methods, which will be validated by another team during a subsequent laboratory. Goals of this approach include promoting teamwork and building student confidence in approaching and operating unfamiliar instrumentation. Even more importantly, students are placed in the position of being scientists and having to make decisions and recommendations. Each step of the analytical process must be carefully considered, and its significance assessed as there are no recipes to follow as they develop their methods and make comparisons between different techniques for the determination of a single analyte.     

Laser-induced fluorescence determination of thallium in sediments

A simple dissolution procedure is decribed for sediments to be analyzed for thallium by Laser-Excited Atomic Fluorescence Spectrometry (LEAFS). It simply uses a nitric – hydrofluoric acid mixture at room temperature (a “cold dissolution” procedure as opposed to the hot acid digestion) followed by a dilution with water (as opposed to the tedious steps of separation and preconcentration). Excellent accuracy (91–106% recoveries) and precision (4–10% relative standard deviation) were demonstrated by the use of five sediment reference materials of diverse origins. The detection limit was estimated to be 0.5 ng/g of thallium. Additionally, a hot plate digestion procedure, using an in-house designed semi-enclosed Teflon beaker, was also investigated; its analytical results agreed with certified values and confirmed the adequacy of the cold dissolution technique. The method is being applied to study the sediment – water interactions in lake environments. Received: 30 December 1996 / Revised: 20 March 1997 / Accepted: 30 March 1997     

The Making of a Solution: A Simple but Poorly Understood Concept in General Chemistry

A primary goal in many general chemistry laboratories is to teach students to properly perform dilutions and make solutions. This article presents a simple exercise to test if your students have acquired this ability from their introductory laboratory exercises. Our results indicate that approximately 50% of the students cannot perform this task on their first attempt, but with guidance and additional attempts their success rate improves. Approximately 30% of the students still fail at this task after additional attempts, but this exercise does improve the laboratory technique of all students. Students used a variety of dilution strategies to achieve the same final concentration; but the most common strategy used by the students was the one deemed most logical by the professors.     

“In situ” extraction of essential oils by use of Dean–Stark glassware and a Vigreux column inside a microwave oven: a procedure for teaching green analytical chemistry

One of the principal objectives of sustainable and green processing development remains the dissemination and teaching of green chemistry in colleges, high schools, and academic laboratories. This paper describes simple glassware that illustrates the phenomenon of extraction in a conventional microwave oven as energy source and a process for green analytical chemistry. Simple glassware comprising a Dean-Stark apparatus (for extraction of aromatic plant material and recovery of essential oils and distilled water) and a Vigreux column (as an air-cooled condenser inside the microwave oven) was designed as an in-situ extraction vessel inside a microwave oven. The efficiency of this experiment was validated for extraction of essential oils from 30 g fresh orange peel, a by-product in the production of orange juice. Every laboratory throughout the world can use this equipment. The microwave power is 100 W and the irradiation time 15 min. The method is performed at atmospheric pressure without added solvent or water and furnishes essential oils similar to those obtained by conventional hydro or steam distillation. By use of GC-MS, 22 compounds in orange peel were separated and identified; the main compounds were limonene (72.1%), β-pinene (8.4%), and γ-terpinene (6.9%). This procedure is appropriate for the teaching laboratory, does not require any special microwave equipment, and enables the students to learn the skills of extraction, and chromatographic and spectroscopic analysis. They are also exposed to a dramatic visual example of rapid, sustainable, and green extraction of an essential oil, and are introduced to successful sustainable and green analytical chemistry.     

An improved thermodynamic perturbation theory for Mercedes-Benz water

We previously applied Wertheim's thermodynamic perturbation theory for associative fluids to the simple Mercedes-Benz model of water. We found that the theory reproduced well the physical properties of hot water, but was less successful in capturing the more structured hydrogen bonding that occurs in cold water. Here, we propose an improved version of the thermodynamic perturbation theory in which the effective density of the reference system is calculated self-consistently. The new theory is a significant improvement, giving good agreement with Monte Carlo simulations of the model, and predicting key anomalies of cold water, such as minima in the molar volume and large heat capacity, in addition to giving good agreement with the isothermal compressibility and thermal expansion coefficient.     

In-flight particle diagnostics in induction plasma processing

A laser Doppler anemometer combined with a particle-emission spectrometer, are used for the study of the induction plasma spraying process. For this, the effects of chamber pressure, spray distance and torch nozzle design on the particle surface temperature and velocity as well as the fraction of hot particles included in the stream of processed material, were investigated. A comparison between the velocity measurements by laser Doppler anemometry (LDA) and by the particle time-of-flight technique is presented in order to emphasize the deference between the velocity of the hot particles, and that of the total particle population, cold and hot. The influence of the individual particle mass on particle entrainment in the plasma jet from the ambient atmosphere in the vacuum chamber is discussed.     

一种新型有机质子响应荧光分子的合成在实验教学中的设计与探索

本实验将最新的科研热点——有机发光材料引入实验教学,开发了基于一个荧光分子二甲氨基苯乙烯基苯并噁唑的合成实验,并考查了其质子响应性质。本实验反应温和易控,操作简单,现象明显,绿色环保,并且可以用于模块化教学,拆分成多个环节实验,适合不同学时的实验教学要求,既可培养学生的基本操作,又能激发学生的实验兴趣,锻炼学生的综合能力。     

对“Co2+鉴定”实验的再认识——批判性思维教育的最好案例之一

Taking the experiment of the "identification of Co²⁺" as an example, this article elaborates the flexible application of "flipped classroom" model in the chemical laboratory teaching. Specifically, students do the experiment first and have questions during the experimental process, then the teacher discusses and demonstrates with students base on their questions, leading students to reunderstand the classic experiment of the "identification of Co²⁺". Thus this article reveals about how to educate students to think and criticize in the teaching process.     

将科研软件融入化学实验与理论教学

The imbalance between teaching and learning, because of individual difference and the simplicity of teaching, can lead to interest losing on study, which will hinder the further learning. Here, based on the course content, certain scientific software are introduced in chemical laboratory lesson before and after class, thus, students can understand the experimental phenomena with the assistant of this teaching approach. Then laboratory lesson is connected with theory lessons to demonstrate the compounds they synthesized during experiment through a scientific software. The design helps the students understand the role of the software in judging and solving scientific issues. At last, the teaching practice is summarized and reconsidered to point out the essentials in the designing process.