Lesson3AnalyticalChemistry翻译.doc

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1、Lesson 3 Analytical ChemistryAnalytical chemistry is the science of making quantitative measurements. In practice, quantifying analytes in a complex sample becomes an exercise in problem solving. To be effective and efficient, analyzing samples requires expertise in: 1 the chemistry that can occur i

2、n a sample 2 analysis and sample handling methods for a wide variety of problems (the tools-of-the-trade) 3 proper data analysis and record keeping To meet these needs, Analytical Chemistry courses usually emphasize equilibrium, spectroscopic and electrochemical analysis, separations, and statistics

3、. Analytical chemistry requires a broad background knowledge of chemical and physical concepts. With a fundamental understanding of analytical methods, a scientist faced with a difficult analytical problem can apply the most appropriate technique(s). A fundamental understanding also makes it easier

4、to identify when a particular problem cannot be solved by traditional methods, and gives an analyst the knowledge that is needed to develop creative approaches or new analytical methods. 1 GravimetryGravimetry is the quantitative measurement of an analyte by weighing a pure, solid form of the analyt

5、e. Obtaining pure solids from solutions containing an unknown amount of a metal ion is done by precipitation. Since gravimetric analysis is an absolute measurement, it is a principal method for analyzing and preparing primary standards. A typical experimental procedure to determine an unknown concen

6、tration of an analyte in solution is as follows: quantitatively precipitate the analyte from solution collect the precipitate by filtering and wash it to remove impurities dry the solid in an oven to remove solvent weigh the solid on an analytical balance calculate the analyte concentration in the o

7、riginal solution based on the weight of the precipitate Gravimetric Determination of Iron: Determine constant weight of the crucibles Oxidation of iron sample Precipitation of iron hydroxide Ignition of iron hydroxide to iron oxide Determine constant weight of the crucibles plus iron oxide Calculati

8、on of iron in the sample2 TitrationTitration is the quantitative measurement of an analyte in solution by completely reacting it with a reagent solution. The reagent is called the titrant and must either be prepared from a primary standard or be standardized versus a primary standard to know its exa

9、ct concentration.The point at which all of the analyte is consumed is the equivalence point. The number of moles of analyte is calculated from the volume of reagent that is required to react with all of the analyte, the titrant concentration, and the reaction stoichiometry.The equivalence point is o

10、ften determined by visual indicators are available for titrations based on acid-base neutralization, complexation, and redox reactions, and is determined by some type of indicator that is also present in the solution. For acid-base titrations, indicators are available that change color when the pH c

11、hanges. When all of the analyte is neutralized, further addition of the titrant causes the pH of the solution to change causing the color of the indicator to change.If the pH of an acid solution is plotted against the amount of base added during a titration, the shape of the graph is called a titrat

12、ion curve. All acid titration curves follow the same basic shapes.Strong Acid Titration CurveAt the beginning, the solution has a low pH and climbs as the strong base is added. As the solution nears the point where all of the H+ are neutralized, the pH rises sharply and then levels out again as the

13、solution becomes more basic as more OH- ions are added.Manual titration is done with a buret, which is a long graduated tube to accurately deliver amounts of titrant. The amount of titrant used in the titration is found by reading the volume of titrant in the buret before beginning the titration and

14、 after reaching the endpoint. The difference in these readings is the volume of titrant to reach the endpoint. The most important factor for making accurate titrations is to read the buret volumes reproducibly. The figure shows how to do so by using the bottom of the meniscus to read the reagent vol

15、ume in the buret. The end point can be determined by an indicator as described above or by an instrumental method. The most common instrumental detection method is potentiometric detection. The equivalence point of an acid-base titration can be detected with a pH electrode. Titrations, such as compl

16、exation or precipitation, involving other ions can use an ion-selective electrode (ISE). UV-vis absorption spectroscopy is also common, especially for complexometric titrations where a subtle color change occurs.For repetitive titrations, autotitrators with microprocessors are available that deliver

17、 the titrant, stop at the endpoint, and calculate the concentration of the analyte. The endpoint is usually detected by some type of electrochemical measurement. Some examples of titrations for which autotitrators are available include: Acid or base determination by pH measurement with potentiometri

18、c detection. Determination of water by Karl Fischer reagent (I2 and SO2 in methyl alcohol and pyridine) with coulometric detection. Determination of Cl in aqueous solution with phenylarsene oxide using amperometric detection. 3 ExtractionExtractions use two immiscible phases to separate a solute fro

19、m one phase into the other. The distribution of a solute between two phases is an equilibrium condition described by partition theory. Boiling tea leaves in water extracts the tannins, theobromine, and caffeine (the good stuff) out of the leaves and into the water. More typical lab extractions are o

20、f organic compounds out of an aqueous phase and into an organic phase.Analytical Extractions4 Precipitation (Insoluble Salts)Many metal ions form compounds that are insoluble in water. We call them insoluble salts or precipitates. Common precipitates are carbonates, hydroxides, sulfates, and sulfide

21、s. Ions that we consider spectator ions when discussing acid-base equilibria will form insoluble salts.An insoluble salt in contact with water maintains an equilibrium with the ions. In simple cases where there are no common ions or competing equilibria, the ion concentrations depend only on the equ

22、ilibrium constant for the particular precipitate. When we talk about solubility equilibria we always write the equilibrium with the solid on the left. For example:Ba(IO3)2 (s) Ba2+(aq) + 2 IO3-(aq)The equilibrium constant expression for an insoluble salt is written following the same rules as for an

23、y other equilibrium. The equilibrium constant is called the solubility product, Ksp. The Ksp expression for the above equilibrium is:Ksp = Ba2+IO3-2Ksp Values for Some PrecipitatesFormulaNameKspAgClsilver chloride1.810-10Al(OH)3aluminum hydroxide210-32BaCO3barium carbonate510-9Words & Phrasesamperom

24、etricAmpi:rE5metrikadj.测量电流的analyte5AnElaitn. （被）分析物buretbjuE5retn. 滴定管；量筒carbonate5kB:bEneitn.碳酸盐complexometric7kCmpleks5Cmitrin.络合滴定（法）coulometric5ku:lC5metrikn.库仑滴定crucible5kru:sibln.坩埚endpoint5endpCintn.端点equilibrium7i:kwi5libriEmn.平衡（复数形式：equilibria）filtering5filtEriNn.过滤gravimetryrE5vimitrin.重

25、量测定法hydroxidehai5drCksaidn.氢氧化物impurityim5pjuEritin.不纯，杂质insolublein5sCljubladj.不能溶解的，不能解决的neutralization7nju:trElai5zeiFEnn.中和（作用）reagentri(:)5eidVEntn.反应物, 试剂solute5sClju:tn.溶解物，溶质solvent5sClvEntn.溶剂spectroscopic9spektrEskCpikadj.分光镜的，借助分光镜的sulfate5sQlfeitn.硫酸盐sulfide5sQlfaidn.硫化物tannin5tAninn.单宁酸

26、theobromin7Wi:E5brEuminn.可可碱titrant5taitrEntn.滴定剂（滴定标准液）analytical balancen.分析天平aqueous phasen.水相equivalence pointn.等量点graduated tuben.刻度管immiscible phasen.不混溶相ion-selective electroden.选择性离子电极organic phasen.有机相partition theoryn.分配理论potentiometricadj.电势测定的precipitationn.沉淀(作用)quantitative measurement

27、n.定量测量solubility solution productn.溶度积stoichiometryn.化学计量法，化学计量学 第3课 分析化学 分析化学是定量测量的科学。在实践中，混合物样品定量分析成为了解决问题的应用题。为了有效和高效率，样品分析需要专业知识： （1）.样品中可能发生化学反应 （2）.析和处理样品对各种各样的问题（职业的工具）的方法 （3）.正确的数据分析和记录保存 为了满足这些需求，分析化学课程通常强调平衡，光谱及电化学分析，分离和统计数据。 分析化学需要大量的化学和物理概念等知识做铺垫。随着对分析方法的基本了解，科学家面临一个困难的分析问题就是应用最合适的方法（s）。

28、当一个特定的问题不能用传统的方法解决的时候，基本的了解能够使它变的更容易，并且给一个分析师的是创造性的方法或开发新的分析方法的知识。 1重力 重力是通过称量纯净的固体分析物来定量测量分析物的方法。从一个含有未知量的金属离子的溶液中活的纯净物的方法是沉淀法。由于重量分析是绝对测量，它是分析和制定基本标准的主要方法。一个典型的确定一个未知的溶液中分析物浓度的实验过程如下： 溶液中沉淀的定量分析 通过过滤洗涤除去杂质，收集沉淀。 在烘箱中干燥固体，去除溶剂。 用分析天平称量固体 以沉淀的重量为基础，计算所分析的溶液的浓度。重量法测定铁： 确定恒重的坩埚 样品铁的氧化 氢氧化铁沉淀 氢氧化铁燃烧生成氧

29、化铁 确定加氧化铁的坩埚恒重 计算样品中的铁 2滴定 滴定法是一个完全由两种溶液发生化学反应来对分析物进行定量测量的方法。该试剂被称为滴定剂并且它必须是标准试剂或者是被标准试剂标定后的，知道它的确切含量。 所有分析物所消耗的点是等价点。分析物的摩尔数是通过发生反应的试剂的体积、浓度和化学计量比等计算出来的。 终点通常由视觉来判断，并以酸碱中和，络合，氧化还原滴定反应为基础，由某种类型的指示剂来判断，并在溶液中能够呈现出来的。对于酸碱滴定，当pH值改变时，指示剂的颜色会发生变化。当分析物全部分解，进一步增加了滴定剂使溶液的pH值改变从而引起指示剂颜色的改变。 在滴定过程中，如果酸溶液的pH值是通

30、过碱的加入量相反绘制出来的，这个图形被叫做滴定曲线。所有的滴定曲线都和基本形状相同。强酸滴定曲线 在开始的时候，溶液具有较低的pH值 ，随着强碱的加入，曲线上升一个很大的坡度。由于在H离子被中和的时候，溶液接近终点，pH值急剧上升，然后再次水平，因为溶液中OH根离子的增加时碱性增强。手动滴定法，是用滴定管，这是一个长的有刻度的管，用来提供有准确量的滴定剂。在滴定中的滴定剂的用量是通过读滴定开始之前滴定管中滴定剂的体积 和到达终点之后滴定剂的体积来确定的。在这些读数的差值就是达到滴定终点是滴定剂的体积。做滴定计算中最重要的因素是能再现滴定管的体积。该图显示了如何通过半月型底部来读出滴定光中试剂的

31、体积。终点可通过上述的指示剂来确定或由仪器法来确定。最常见的仪器检测方法是电位检测。一个酸碱滴定的当量点可检测到pH电极。滴定法，比如络合滴定法或沉淀滴定法等，涉及其他离子可以使用离子选择性电极（ISE）。紫外可见吸收光谱也很普遍，特别是在发生了微妙的颜色变化的络合滴定中。 对于重复滴定，可用微处理器提供滴定剂，停在终点，并计算分析物的浓度。终点通常是通过一些类型的电化学测量仪器测得的。Autotitrators是可用的滴定方法包括一些例子： 通过用电位检测酸或碱的pH来确定。 用卡尔费休试剂（甲醇和吡啶中加入碘和二氧化硫）和库伦检测方法检测水。 用安培检测法测定水溶液中CL含量。 3提取 提

32、取物可用两个阶段把溶质从一个到另一个阶段中分开。两阶段的溶质分布是分配理论所描述的均衡的情况。沸腾茶叶提取从叶子中华出来的单宁酸，可可碱，咖啡因（好东西）并进入水中。更典型的实验室提取是有机物的水相和成一个有机的阶段。 分析提取物 4沉淀物（不溶盐） 很多金属离子形成的化合物不溶于水。我们称他们为不溶性盐类或沉淀。常见的沉淀有碳酸盐，氢氧化物，硫酸盐和硫化物。在讨论酸碱平衡我们认为其它离子会形成不溶性盐。 在与水中的不溶性盐离子保持平衡。在没有共同的竞争平衡离子或简单的情况下，离子的浓度仅依赖于特定的沉淀平衡的常数。当我们谈论我们关于溶解度平衡时我们总是把盐写在方程式左边。例如： Ba(IO3)2 (s) Ba2+(aq) + 2 IO3-(aq) 不溶性的盐平衡常数表达式以遵循任何其他平衡相同的规则被书写。所谓的平衡常数是产品的溶解度，溶度积。为上述平衡溶度积表达式是： 溶度积= Ba2+IO3-2 8