Lesson 3 Analytical Chemistry翻译.doc

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 in 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. 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 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 analyte. 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 concentration 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 original 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 Calculation 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 exact 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 often 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 changes. 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 titration 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 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 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 volume 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 complexation 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 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 potentiometric 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 from 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 of 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 sulfides. 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 equilibrium 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 any 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 & PhrasesamperometricAmpi: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 measurementn.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）。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