Modern Analytical Chemistry Ebook

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1400-Fm 9/9/99 7:37 AM Page i C Ch he em mi is st tr ry y Modern Analytical Chemistry David Harvey DePauw University Boston Burr Ridge, IL Dubuque, IA Madison, WI New York San Francisco St. Louis Bangkok Bogotá Caracas Lisbon London Madrid Mexico City Milan New Delhi Seoul Singapore Sydney Taipei Toronto1400-Fm 9/9/99 7:37 AM Page ii McGraw-Hill Higher Education A Division of The McGraw-Hill Companies MODERN ANALYTICAL CHEMISTRY Copyright © 2000 by The McGraw-Hill Companies, Inc. All rights reserved. Printed in the United States of America. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a data base or retrieval system, without the prior written permission of the publisher. This book is printed on acid-free paper. 1 2 3 4 5 6 7 8 9 0 KGP/KGP 0 9 8 7 6 5 4 3 2 1 0 ISBN 0–07–237547–7 Vice president and editorial director: Kevin T. Kane Publisher: James M. Smith Sponsoring editor: Kent A. Peterson Editorial assistant: Jennifer L. Bensink Developmental editor: Shirley R. Oberbroeckling Senior marketing manager: Martin J. Lange Senior project manager: Jayne Klein Production supervisor: Laura Fuller Coordinator of freelance design: Michelle D. Whitaker Senior photo research coordinator: Lori Hancock Senior supplement coordinator: Audrey A. Reiter Compositor: Shepherd, Inc. Typeface: 10/12 Minion Printer: Quebecor Printing Book Group/Kingsport Freelance cover/interior designer: Elise Lansdon Cover image: © George Diebold/The Stock Market Photo research: Roberta Spieckerman Associates Colorplates: Colorplates 1–6, 8, 10: © David Harvey/Marilyn E. Culler, photographer; Colorplate 7: Richard Megna/Fundamental Photographs; Colorplate 9: © Alfred Pasieka/Science Photo Library/Photo Researchers, Inc.; Colorplate 11: From H. Black, Environ. Sci. Technol., 1996, 30, 124A. Photos courtesy D. Pesiri and W. Tumas, Los Alamos National Laboratory; Colorplate 12: Courtesy of Hewlett-Packard Company; Colorplate 13: © David Harvey. Library of Congress Cataloging-in-Publication Data Harvey, David, 1956– Modern analytical chemistry / David Harvey. — 1st ed. p. cm. Includes bibliographical references and index. ISBN 0–07–237547–7 1. Chemistry, Analytic. I. Title. QD75.2.H374 2000 543—dc21 99–15120 CIP INTERNATIONAL EDITION ISBN 0–07–116953–9 Copyright © 2000. Exclusive rights by The McGraw-Hill Companies, Inc. for manufacture and export. This book cannot be re-exported from the country to which it is consigned by McGraw-Hill. The International Edition is not available in North America. www.mhhe.com1400-Fm 9/9/99 7:37 AM Page iii Contents Contents Preface xii 2C.5 Conservation of Electrons 23 2C.6 Using Conservation Principles in Stoichiometry Problems 23 Chapter 1 2D Basic Equipment and Instrumentation 25 Introduction 1 2D.1 Instrumentation for Measuring Mass 25 1A What is Analytical Chemistry? 2 2D.2 Equipment for Measuring Volume 26 1B The Analytical Perspective 5 2D.3 Equipment for Drying Samples 29 1C Common Analytical Problems 8 2E Preparing Solutions 30 1D Key Terms 9 2E.1 Preparing Stock Solutions 30 1E Summary 9 2E.2 Preparing Solutions by Dilution 31 1F Problems 9 2F The Laboratory Notebook 32 1G Suggested Readings 10 2G Key Terms 32 1H References 10 2H Summary 33 2I Problems 33 2J Suggested Readings 34 Chapter 2 2K References 34 Basic Tools of Analytical Chemistry 11 3 2A Numbers in Analytical Chemistry 12 Chapter 2A.1 Fundamental Units of Measure 12 The Language of Analytical Chemistry 35 2A.2 Significant Figures 13 2B Units for Expressing Concentration 15 3A Analysis, Determination, and Measurement 36 2B.1 Molarity and Formality 15 3B Techniques, Methods, Procedures, and Protocols 36 2B.2 Normality 16 3C Classifying Analytical Techniques 37 2B.3 Molality 18 3D Selecting an Analytical Method 38 2B.4 Weight, Volume, and Weight-to-Volume Ratios 18 3D.1 Accuracy 38 2B.5 Converting Between Concentration Units 18 3D.2 Precision 39 2B.6 p-Functions 19 3D.3 Sensitivity 39 2C Stoichiometric Calculations 20 3D.4 Selectivity 40 2C.1 Conservation of Mass 22 3D.5 Robustness and Ruggedness 42 2C.2 Conservation of Charge 22 3D.6 Scale of Operation 42 2C.3 Conservation of Protons 22 3D.7 Equipment, Time, and Cost 44 2C.4 Conservation of Electron Pairs 23 3D.8 Making the Final Choice 44 iii1400-Fm 9/9/99 7:37 AM Page iv iv Modern Analytical Chemistry 3E Developing the Procedure 45 4E.4 Errors in Significance Testing 84 3E.1 Compensating for Interferences 45 4F Statistical Methods for Normal Distributions 85 – 3E.2 Calibration and Standardization 47 4F.1 Comparing X to m 85 2 2 3E.3 Sampling 47 4F.2 Comparing s to s 87 3E.4 Validation 47 4F.3 Comparing Two Sample Variances 88 3F Protocols 48 4F.4 Comparing Two Sample Means 88 3G The Importance of Analytical Methodology 48 4F.5 Outliers 93 3H Key Terms 50 4G Detection Limits 95 3I Summary 50 4H Key Terms 96 3J Problems 51 4I Summary 96 3K Suggested Readings 52 4J Suggested Experiments 97 3L References 52 4K Problems 98 4L Suggested Readings 102 4 4M References 102 Chapter Evaluating Analytical Data 53 5 Chapter 4A Characterizing Measurements and Results 54 Calibrations, Standardizations, 4A.1 Measures of Central Tendency 54 and Blank Corrections 104 4A.2 Measures of Spread 55 4B Characterizing Experimental Errors 57 5A Calibrating Signals 105 4B.1 Accuracy 57 5B Standardizing Methods 106 4B.2 Precision 62 5B.1 Reagents Used as Standards 106 4B.3 Error and Uncertainty 64 5B.2 Single-Point versus Multiple-Point 4C Propagation of Uncertainty 64 Standardizations 108 4C.1 A Few Symbols 65 5B.3 External Standards 109 4C.2 Uncertainty When Adding or Subtracting 65 5B.4 Standard Additions 110 4C.3 Uncertainty When Multiplying or 5B.5 Internal Standards 115 Dividing 66 5C Linear Regression and Calibration Curves 117 4C.4 Uncertainty for Mixed Operations 66 5C.1 Linear Regression of Straight-Line Calibration 4C.5 Uncertainty for Other Mathematical Curves 118 Functions 67 5C.2 Unweighted Linear Regression with Errors 4C.6 Is Calculating Uncertainty Actually Useful? 68 in y 119 4D The Distribution of Measurements and 5C.3 Weighted Linear Regression with Errors Results 70 in y 124 4D.1 Populations and Samples 71 5C.4 Weighted Linear Regression with Errors 4D.2 Probability Distributions for Populations 71 in Both x and y 127 4D.3 Confidence Intervals for Populations 75 5C.5 Curvilinear and Multivariate Regression 127 4D.4 Probability Distributions for Samples 77 5D Blank Corrections 128 4D.5 Confidence Intervals for Samples 80 5E Key Terms 130 4D.6 A Cautionary Statement 81 5F Summary 130 4E Statistical Analysis of Data 82 5G Suggested Experiments 130 4E.1 Significance Testing 82 5H Problems 131 4E.2 Constructing a Significance Test 83 5I Suggested Readings 133 4E.3 One-Tailed and Two-Tailed Significance Tests 84 5J References 1341400-Fm 9/9/99 7:38 AM Page v v Contents 7 Chapter Chapter 6 Obtaining and Preparing Samples Equilibrium Chemistry 135 for Analysis 179 6A Reversible Reactions and Chemical 7A The Importance of Sampling 180 Equilibria 136 7B Designing a Sampling Plan 182 6B Thermodynamics and Equilibrium Chemistry 136 7B.1 Where to Sample the Target Population 182 6C Manipulating Equilibrium Constants 138 7B.2 What Type of Sample to Collect 185 6D Equilibrium Constants for Chemical Reactions 139 7B.3 How Much Sample to Collect 187 6D.1 Precipitation Reactions 139 7B.4 How Many Samples to Collect 191 6D.2 Acid–Base Reactions 140 7B.5 Minimizing the Overall Variance 192 6D.3 Complexation Reactions 144 7C Implementing the Sampling Plan 193 6D.4 Oxidation–Reduction Reactions 145 7C.1 Solutions 193 6E Le Châtelier’s Principle 148 7C.2 Gases 195 6F Ladder Diagrams 150 7C.3 Solids 196 6F.1 Ladder Diagrams for Acid–Base Equilibria 150 7D Separating the Analyte from Interferents 201 6F.2 Ladder Diagrams for Complexation Equilibria 153 7E General Theory of Separation Efficiency 202 6F.3 Ladder Diagrams for Oxidation–Reduction Equilibria 155 7F Classifying Separation Techniques 205 6G Solving Equilibrium Problems 156 7F.1 Separations Based on Size 205 6G.1 A Simple Problem: Solubility of Pb(IO ) in 3 2 7F.2 Separations Based on Mass or Density 206 Water 156 7F.3 Separations Based on Complexation 6G.2 A More Complex Problem: The Common Ion Reactions (Masking) 207 Effect 157 7F.4 Separations Based on a Change 6G.3 Systematic Approach to Solving Equilibrium of State 209 Problems 159 7F.5 Separations Based on a Partitioning Between 6G.4 pH of a Monoprotic Weak Acid 160 Phases 211 6G.5 pH of a Polyprotic Acid or Base 163 7G Liquid–Liquid Extractions 215 6G.6 Effect of Complexation on Solubility 165 7G.1 Partition Coefficients and Distribution Ratios 216 6H Buffer Solutions 167 7G.2 Liquid–Liquid Extraction with No Secondary 6H.1 Systematic Solution to Buffer Reactions 216 Problems 168 7G.3 Liquid–Liquid Extractions Involving 6H.2 Representing Buffer Solutions with Acid–Base Equilibria 219 Ladder Diagrams 170 7G.4 Liquid–Liquid Extractions Involving Metal 6I Activity Effects 171 Chelators 221 6J Two Final Thoughts About Equilibrium 7H Separation versus Preconcentration 223 Chemistry 175 7I Key Terms 224 6K Key Terms 175 7J Summary 224 6L Summary 175 7K Suggested Experiments 225 6M Suggested Experiments 176 7L Problems 226 6N Problems 176 7M Suggested Readings 230 6O Suggested Readings 178 7N References 231 6P References 1781400-Fm 9/9/99 7:38 AM Page vi vi Modern Analytical Chemistry 9B.7 Characterization Applications 309 Chapter 8 9B.8 Evaluation of Acid–Base Titrimetry 311 Gravimetric Methods of Analysis 232 9C Titrations Based on Complexation Reactions 314 9C.1 Chemistry and Properties of EDTA 315 8A Overview of Gravimetry 233 9C.2 Complexometric EDTA Titration Curves 317 8A.1 Using Mass as a Signal 233 9C.3 Selecting and Evaluating the End Point 322 8A.2 Types of Gravimetric Methods 234 9C.4 Representative Method 324 8A.3 Conservation of Mass 234 9C.5 Quantitative Applications 327 8A.4 Why Gravimetry Is Important 235 9C.6 Evaluation of Complexation Titrimetry 331 8B Precipitation Gravimetry 235 9D Titrations Based on Redox Reactions 331 8B.1 Theory and Practice 235 9D.1 Redox Titration Curves 332 8B.2 Quantitative Applications 247 9D.2 Selecting and Evaluating the End Point 337 8B.3 Qualitative Applications 254 9D.3 Representative Method 340 8B.4 Evaluating Precipitation Gravimetry 254 9D.4 Quantitative Applications 341 8C Volatilization Gravimetry 255 9D.5 Evaluation of Redox Titrimetry 350 8C.1 Theory and Practice 255 9E Precipitation Titrations 350 8C.2 Quantitative Applications 259 9E.1 Titration Curves 350 8C.3 Evaluating Volatilization Gravimetry 262 9E.2 Selecting and Evaluating the End Point 354 8D Particulate Gravimetry 262 9E.3 Quantitative Applications 354 8D.1 Theory and Practice 263 9E.4 Evaluation of Precipitation Titrimetry 357 8D.2 Quantitative Applications 264 9F Key Terms 357 8D.3 Evaluating Precipitation Gravimetry 265 9G Summary 357 8E Key Terms 265 9H Suggested Experiments 358 8F Summary 266 9I Problems 360 8G Suggested Experiments 266 9J Suggested Readings 366 8H Problems 267 9K References 367 8I Suggested Readings 271 8J References 272 Chapter 10 9 Spectroscopic Methods Chapter of Analysis 368 Titrimetric Methods of Analysis 273 10A Overview of Spectroscopy 369 9A Overview of Titrimetry 274 10A.1 What Is Electromagnetic Radiation 369 9A.1 Equivalence Points and End Points 274 10A.2 Measuring Photons as a Signal 372 9A.2 Volume as a Signal 274 10B Basic Components of Spectroscopic 9A.3 Titration Curves 275 Instrumentation 374 9A.4 The Buret 277 10B.1 Sources of Energy 375 9B Titrations Based on Acid–Base Reactions 278 10B.2 Wavelength Selection 376 9B.1 Acid–Base Titration Curves 279 10B.3 Detectors 379 9B.2 Selecting and Evaluating the 10B.4 Signal Processors 380 End Point 287 10C Spectroscopy Based on Absorption 380 9B.3 Titrations in Nonaqueous Solvents 295 10C.1 Absorbance of Electromagnetic Radiation 380 9B.4 Representative Method 296 10C.2 Transmittance and Absorbance 384 9B.5 Quantitative Applications 298 10C.3 Absorbance and Concentration: Beer’s 9B.6 Qualitative Applications 308 Law 3851400-Fm 9/9/99 7:38 AM Page vii vii Contents 10C.4 Beer’s Law and Multicomponent 11B Potentiometric Methods of Analysis 465 Samples 386 11B.1 Potentiometric Measurements 466 10C.5 Limitations to Beer’s Law 386 11B.2 Reference Electrodes 471 10D Ultraviolet-Visible and Infrared 11B.3 Metallic Indicator Electrodes 473 Spectrophotometry 388 11B.4 Membrane Electrodes 475 10D.1 Instrumentation 388 11B.5 Quantitative Applications 485 10D.2 Quantitative Applications 394 11B.6 Evaluation 494 10D.3 Qualitative Applications 402 11C Coulometric Methods of Analysis 496 10D.4 Characterization Applications 403 11C.1 Controlled-Potential Coulometry 497 10D.5 Evaluation 409 11C.2 Controlled-Current Coulometry 499 10E Atomic Absorption Spectroscopy 412 11C.3 Quantitative Applications 501 10E.1 Instrumentation 412 11C.4 Characterization Applications 506 10E.2 Quantitative Applications 415 11C.5 Evaluation 507 10E.3 Evaluation 422 11D Voltammetric Methods of Analysis 508 10F Spectroscopy Based on Emission 423 11D.1 Voltammetric Measurements 509 10G Molecular Photoluminescence 11D.2 Current in Voltammetry 510 Spectroscopy 423 11D.3 Shape of Voltammograms 513 10G.1 Molecular Fluorescence and 11D.4 Quantitative and Qualitative Aspects Phosphorescence Spectra 424 of Voltammetry 514 10G.2 Instrumentation 427 11D.5 Voltammetric Techniques 515 10G.3 Quantitative Applications Using Molecular 11D.6 Quantitative Applications 520 Luminescence 429 11D.7 Characterization Applications 527 10G.4 Evaluation 432 11D.8 Evaluation 531 10H Atomic Emission Spectroscopy 434 11E Key Terms 532 10H.1 Atomic Emission Spectra 434 11F Summary 532 10H.2 Equipment 435 11G Suggested Experiments 533 10H.3 Quantitative Applications 437 11H Problems 535 10H.4 Evaluation 440 11I Suggested Readings 540 10I Spectroscopy Based on Scattering 441 11J References 541 10I.1 Origin of Scattering 441 10I.2 Turbidimetry and Nephelometry 441 Chapter 12 10J Key Terms 446 Chromatographic and Electrophoretic 10K Summary 446 Methods 543 10L Suggested Experiments 447 10M Problems 450 12A Overview of Analytical Separations 544 10N Suggested Readings 458 12A.1 The Problem with Simple 10O References 459 Separations 544 12A.2 A Better Way to Separate Mixtures 544 12A.3 Classifying Analytical Separations 546 Chapter 11 12B General Theory of Column Electrochemical Methods of Analysis 461 Chromatography 547 12B.1 Chromatographic Resolution 549 11A Classification of Electrochemical Methods 462 12B.2 Capacity Factor 550 11A.1 Interfacial Electrochemical Methods 462 12B.3 Column Selectivity 552 11A.2 Controlling and Measuring Current and 12B.4 Column Efficiency 552 Potential 4621400-Fm 9/9/99 7:38 AM Page viii viii Modern Analytical Chemistry 12B.5 Peak Capacity 554 12O Suggested Readings 620 12B.6 Nonideal Behavior 555 12P References 620 12C Optimizing Chromatographic Separations 556 3 12C.1 Using the Capacity Factor to Optimize Chapter 1 Resolution 556 Kinetic Methods of Analysis 622 12C.2 Using Column Selectivity to Optimize Resolution 558 13A Methods Based on Chemical Kinetics 623 12C.3 Using Column Efficiency to Optimize 13A.1 Theory and Practice 624 Resolution 559 13A.2 Instrumentation 634 12D Gas Chromatography 563 13A.3 Quantitative Applications 636 12D.1 Mobile Phase 563 13A.4 Characterization Applications 638 12D.2 Chromatographic Columns 564 13A.5 Evaluation of Chemical Kinetic 12D.3 Stationary Phases 565 Methods 639 12D.4 Sample Introduction 567 13B Radiochemical Methods of Analysis 642 12D.5 Temperature Control 568 13B.1 Theory and Practice 643 12D.6 Detectors for Gas Chromatography 569 13B.2 Instrumentation 643 12D.7 Quantitative Applications 571 13B.3 Quantitative Applications 644 12D.8 Qualitative Applications 575 13B.4 Characterization Applications 647 12D.9 Representative Method 576 13B.5 Evaluation 648 12D.10 Evaluation 577 13C Flow Injection Analysis 649 12E High-Performance Liquid 13C.1 Theory and Practice 649 Chromatography 578 13C.2 Instrumentation 651 12E.1 HPLC Columns 578 13C.3 Quantitative Applications 655 12E.2 Stationary Phases 579 13C.4 Evaluation 658 12E.3 Mobile Phases 580 13D Key Terms 658 12E.4 HPLC Plumbing 583 13E Summary 659 12E.5 Sample Introduction 584 13F Suggested Experiments 659 12E.6 Detectors for HPLC 584 13G Problems 661 12E.7 Quantitative Applications 586 13H Suggested Readings 664 12E.8 Representative Method 588 13I References 665 12E.9 Evaluation 589 12F Liquid–Solid Adsorption Chromatography 590 4 Chapter 1 12G Ion-Exchange Chromatography 590 Developing a Standard Method 666 12H Size-Exclusion Chromatography 593 12I Supercritical Fluid Chromatography 596 14A Optimizing the Experimental Procedure 667 12J Electrophoresis 597 14A.1 Response Surfaces 667 12J.1 Theory of Capillary Electrophoresis 598 14A.2 Searching Algorithms for Response 12J.2 Instrumentation 601 Surfaces 668 12J.3 Capillary Electrophoresis Methods 604 14A.3 Mathematical Models of Response 12J.4 Representative Method 607 Surfaces 674 12J.5 Evaluation 609 14B Verifying the Method 683 12K Key Terms 609 14B.1 Single-Operator Characteristics 683 12L Summary 610 14B.2 Blind Analysis of Standard Samples 683 12M Suggested Experiments 610 14B.3 Ruggedness Testing 684 12N Problems 615 14B.4 Equivalency Testing 6871400-Fm 9/9/99 7:38 AM Page ix ix Contents 14C Validating the Method as a Standard 15D Key Terms 721 Method 687 15E Summary 722 14C.1 Two-Sample Collaborative Testing 688 15F Suggested Experiments 722 14C.2 Collaborative Testing and Analysis of 15G Problems 722 Variance 693 15H Suggested Readings 724 14C.3 What Is a Reasonable Result for a 15I References 724 Collaborative Study? 698 14D Key Terms 699 Appendixes 14E Summary 699 Appendix 1A Single-Sided Normal Distribution 725 14F Suggested Experiments 699 Appendix 1B t-Table 726 14G Problems 700 Appendix 1C F-Table 727 14H Suggested Readings 704 Appendix 1D Critical Values for Q-Test 728 14I References 704 Appendix 1E Random Number Table 728 Appendix 2 Recommended Reagents for Preparing Primary Standards 729 5 Chapter 1 Appendix 3A Solubility Products 731 Quality Assurance 705 Appendix 3B Acid Dissociation Constants 732 Appendix 3C Metal–Ligand Formation Constants 739 15A Quality Control 706 Appendix 3D Standard Reduction Potentials 743 15B Quality Assessment 708 Appendix 3E Selected Polarographic Half-Wave Potentials 747 Appendix 4 Balancing Redox Reactions 748 15B.1 Internal Methods of Quality Appendix 5 Review of Chemical Kinetics 750 Assessment 708 Appendix 6 Countercurrent Separations 755 15B.2 External Methods of Quality Appendix 7 Answers to Selected Problems 762 Assessment 711 15C Evaluating Quality Assurance Data 712 Glossary 769 15C.1 Prescriptive Approach 712 Index 781 15C.2 Performance-Based Approach 7141400-Fm 9/9/99 7:38 AM Page x x Modern Analytical Chemistry A Guide to Using This Text . . . in Chapter Representative Methods 246 Modern Analytical Chemistry Annotated methods of typical An additional problem is encountered when the isolated solid is non- analytical procedures link theory with 2+ stoichiometric. For example, precipitating Mn as Mn(OH)2, followed by heating to produce the oxide, frequently produces a solid with a stoichiometry of MnOx, practice. The format encourages where x varies between 1 and 2. In this case the nonstoichiometric product results from the formation of a mixture of several oxides that differ in the oxidation state students to think about the design of of manganese. Other nonstoichiometric compounds form as a result of lattice de- 6 fects in the crystal structure. the procedure and why it works. Representative Method The best way to appreciate the importance of the theoreti- cal and practical details discussed in the previous section is to carefully examine the procedure for a typical precipitation gravimetric method. Although each method 2+ has its own unique considerations, the determination of Mg in water and waste- water by precipitating MgNH4PO4 6H2O and isolating Mg2P2O7 provides an in- structive example of a typical procedure. Margin Notes 2+ 7 Method 8.1 Determination of Mg in Water and Wastewater Margin notes direct students Description of Method. Magnesium is precipitated as MgNH4PO4 6H2O using to colorplates located toward (NH4)2HPO4 as the precipitant. The precipitate’s solubility in neutral solutions (0.0065 g/100 mL in pure water at 10 °C) is relatively high, but it is much less soluble in the presence of dilute ammonia (0.0003 g/100 mL in 0.6 M NH ). The precipitant is 3 the middle of the book 2+ not very selective, so a preliminary separation of Mg from potential interferents is necessary. Calcium, which is the most significant interferent, is usually removed by its prior precipitation as the oxalate. The presence of excess ammonium salts from the precipitant or the addition of too much ammonia can lead to the formation of Mg(NH4)4(PO4)2, which is subsequently isolated as Mg(PO3)2 after drying. The precipitate is isolated by filtration using a rinse solution of dilute ammonia. After 110 Modern Analytical Chemistry filtering, the precipitate is converted to Mg P O and weighed. 2 2 7 2+ either case, the calibration curve provides a means for relating S to the ana- Procedure. Transfer a sample containing no more than 60 mg of Mg into a samp 600-mL beaker. Add 2–3 drops of methyl red indicator, and, if necessary, adjust the lyte’s concentration. volume to 150 mL. Acidify the solution with 6 M HCl, and add 10 mL of 30% w/v (NH ) HPO . After cooling, add concentrated NH dropwise, and while constantly 4 2 4 3 5 3 EXAMPLE . stirring, until the methyl red indicator turns yellow (pH 6.3). After stirring for 5 min, add 5 mL of concentrated NH3, and continue stirring for an additional 10 min. Color plate 1 shows an example of a set of A second spectrophotometric method for the quantitative determination of Allow the resulting solution and precipitate to stand overnight. Isolate the external standards and their corresponding 2+ Pb levels in blood gives a linear normal calibration curve for which precipitate by filtration, rinsing with 5% v/v NH . Dissolve the precipitate in 50 mL 3 normal calibration curve. of 10% v/v HCl, and precipitate a second time following the same procedure. After –1 S = (0.296 ppb ) · C + 0.003 stand S filtering, carefully remove the filter paper by charring. Heat the precipitate at 500 °C until the residue is white, and then bring the precipitate to constant weight at 2+ What is the Pb level (in ppb) in a sample of blood if S is 0.397? samp 1100 °C. SOLUTION Questions 2+ To determine the concentration of Pb in the sample of blood, we replace 1. Why does the procedure call for a sample containing no more than 60 mg of S in the calibration equation with S and solve for C stand samp A S –. 0 003 samp 0.– 397 0.003 C== = 13 . 3 ppb A –1 –1 0.296 ppb 0.296 ppb It is worth noting that the calibration equation in this problem includes an extra term that is not in equation 5.3. Ideally, we expect the calibration curve to qy give a signal of zero when C is zero. This is the purpose of using a reagent There is a serious limitation, however, to an external standardization. The S blank to correct the measured signal. The extra term of +0.003 in our relationship between Sstand and CS in equation 5.3 is determined when the ana- calibration equation results from uncertainty in measuring the signal for the lyte is present in the external standard’s matrix. In using an external standardiza- reagent blank and the standards. tion, we assume that any difference between the matrix of the standards and the sample’s matrix has no effect on the value of k. A proportional determinate error is introduced when differences between the two matrices cannot be ignored. This An external standardization allows a related series of samples to be analyzed is shown in Figure 5.4, where the relationship between the signal and the amount using a single calibration curve. This is an important advantage in laboratories of analyte is shown for both the sample’s matrix and the standard’s matrix. In where many samples are to be analyzed or when the need for a rapid throughput of this example, using a normal calibration curve results in a negative determinate l i iti l t i i l f th t l t d error. When matrix problems are expected, an effort is made to match the matrix of the standards to that of the sample. This is known as matrix matching. When the sample’s matrix is unknown, the matrix effect must be shown to be negligi- Examples of Typical Problems ble, or an alternative method of standardization must be used. Both approaches are discussed in the following sections. matrix matching Each example problem includes a Adjusting the matrix of an external standard so that it is the same as the 5 4 B. Standard Additions detailed solution that helps students in matrix of the samples to be analyzed. The complication of matching the matrix of the standards to that of the sample can be avoided by conducting the standardization in the sample. This is known applying the chapter’s material to method of standard additions as the method of standard additions. The simplest version of a standard addi- A standardization in which aliquots of a standard solution are added to the tion is shown in Figure 5.5. A volume, V , of sample is diluted to a final volume, o practical problems. sample. V , and the signal, S is measured. A second identical aliquot of sample is f samp Bold-faced Key Terms with Margin Definitions Key words appear in boldface when they are introduced within the text. The term and its definition appear in the margin for quick review by the student. All key words are also defined in the glossary. x Representative Methods1400-Fm 9/9/99 7:38 AM Page xi . . . End of Chapter yy List of Key Terms 5 E KEY TERMS The key terms introduced within the chapter are aliquot (p. 111) multiple-point standardization (p. 109) secondary reagent (p. 107) external standard (p. 109) normal calibration curve (p. 109) single-point standardization (p. 108) listed at the end of each chapter. Page references internal standard (p. 116) primary reagent (p. 106) standard deviation about the regression (p. 121) direct the student to the definitions in the text. linear regression (p. 118) reagent grade (p. 107) total Youden blank (p. 129) matrix matching (p. 110) residual error (p. 118) method of standard additions (p. 110) Summary 5 F SUMMARY The summary provides the student with a brief In a quantitative analysis, we measure a signal and calculate the and the use of an internal standard. The most desirable standard- amount of analyte using one of the following equations. ization strategy is an external standardization. The method of review of the important concepts within the chapter. standard additions, in which known amounts of analyte are added S = kn + S meas A reag to the sample, is used when the sample’s matrix complicates the analysis. An internal standard, which is a species (not analyte) S = kC + S meas A reag added to all samples and standards, is used when the procedure does not allow for the reproducible handling of samples and Suggested Experiments To obtain accurate results we must eliminate determinate errors standards. affecting the measured signal, S , the method’s sensitivity, k, meas Standardizations using a single standard are common, but also and any signal due to the reagents, S . reag An annotated list of representative experiments is are subject to greater uncertainty. Whenever possible, a multiple- To ensure that S is determined accurately, we calibrate meas point standardization is preferred. The results of a multiple-point the equipment or instrument used to obtain the signal. Balances provided from the Journal of Chemical Education. standardization are graphed as a calibration curve. A linear regres- are calibrated using standard weights. When necessary, we can sion analysis can provide an equation for the standardization. also correct for the buoyancy of air. Volumetric glassware can A reagent blank corrects the measured signal for signals due to be calibrated by measuring the mass of water contained or de- reagents other than the sample that are used in an analysis. The livered and using the density of water to calculate the true vol- most common reagent blank is prepared by omitting the sample. ume. Most instruments have calibration standards suggested by When a simple reagent blank does not compensate for all constant the manufacturer. sources of determinate error, other types of blanks, such as the An analytical method is standardized by determining its sensi- total Youden blank, can be used. tivity. There are several approaches to standardization, including the use of external standards, the method of standard addition, Suggested Readings 5 G Suggested EXPERIMENTS Suggested readings give the student The following exercises and experiments help connect the material in this chapter to the analytical laboratory. access to more comprehensive Calibration—Volumetric glassware (burets, pipets, and Standardization—External standards, standard additions, discussion of the topics introduced volumetric flasks) can be calibrated in the manner described and internal standards are a common feature of many in Example 5.1. Most instruments have a calibration sample quantitative analyses. Suggested experiments using these within the chapter. that can be prepared to verify the instrument’s accuracy and standardization methods are found in later chapters. A good precision. For example, as described in this chapter, a project experiment for introducing external standardization, solution of 60.06 ppm K Cr O in 0.0050 M H SO should standard additions, and the importance of the sample’s 2 2 7 2 4 yy give an absorbance of 0.640 ± 0.010 at a wavelength of matrix is to explore the effect of pH on the quantitative 350.0 nm when using 0.0050 M H SO as a reagent analysis of an acid–base indicator. Using bromothymol blue 2 4 blank. These exercises also provide practice with using as an example, external standards can be prepared in a pH 9 1G SUGGESTED READINGS volumetric glassware, weighing samples, and preparing buffer and used to analyze samples buffered to different pHs solutions. in the range of 6–10. Results can be compared with those The role of analytical chemistry within the broader discipline of Laitinen, H. A. “Analytical Chemistry in a Changing World,” obtained using a standard addition. chemistry has been discussed by many prominent analytical Anal. Chem. 1980, 52, 605A–609A. chemists. Several notable examples follow. Laitinen, H. A. “History of Analytical Chemistry in the U.S.A.,” Baiulescu, G. E.; Patroescu, C.; Chalmers, R. A. Education and Talanta 1989, 36, 1–9. Teaching in Analytical Chemistry. Ellis Horwood: Chichester, Laitinen, H. A.; Ewing, G. (eds). A History of Analytical 1982. Chemistry. The Division of Analytical Chemistry of Hieftje, G. M. “The Two Sides of Analytical Chemistry,” Anal. the American Chemical Society: Washington, D.C., References Chem. 1985, 57, 256A–267A. 1972. Kissinger, P. T. “Analytical Chemistry—What is It? Who Needs It? McLafferty, F. W. “Analytical Chemistry: Historic and Modern,” The references cited in the Why Teach It?” Trends Anal. Chem. 1992, 11, 54–57. Acc. Chem. Res. 1990, 23, 63–64. chapter are provided so the 1H REFERENCES student can access them for 1. Ravey, M. Spectroscopy 1990, 5(7), 11. 113–119; (c) Atkinson, G. F. J. Chem. Educ. 1982, 59, 201–202; further information. (d) Pardue, H. L.; Woo, J. J. Chem. Educ. 1984, 61, 409–412; 2. de Haseth, J. Spectroscopy 1990, 5(7), 11. (e) Guarnieri, M. J. Chem. Educ. 1988, 65, 201–203; (f) de Haseth, J. 3. Fresenius, C. R. A System of Instruction in Quantitative Chemical Spectroscopy 1990, 5, 20–21; (g) Strobel, H. A. Am. Lab. 1990, Analysis. John Wiley and Sons: New York, 1881. October, 17–24. 4. Hillebrand, W. F.; Lundell, G. E. F. Applied Inorganic Analysis, John 8. Hieftje, G. M. Am. Lab. 1993, October, 53–61. Wiley and Sons: New York, 1953. 9. See, for example, the following laboratory texts: (a) Sorum, C. H.; 5. Van Loon, J. C. Analytical Atomic Absorption Spectroscopy. Academic Lagowski, J. J. Introduction to Semimicro Qualitative Analysis, 5th ed. Press: New York, 1980. Prentice-Hall: Englewood Cliffs, NJ, 1977.; (b) Shriner, R. L.; Fuson, 6. Murray, R. W. Anal. Chem. 1991, 63, 271A. R. C.; Curtin, D. Y. The Systematic Identification of Organic 3 J PROBLEMS 7. For several different viewpoints see (a) Beilby, A. L. J. Chem. Educ. Compounds, 5th ed. John Wiley and Sons: New York, 1964. 1970, 47, 237–238; (b) Lucchesi, C. A. Am. Lab. 1980, October, 1. When working with a solid sample, it often is necessary to 4. A sample was analyzed to determine the concentration of an bring the analyte into solution by dissolving the sample in a analyte. Under the conditions of the analysis, the sensitivity is –1 suitable solvent. Any solid impurities that remain are 17.2 ppm . What is the analyte’s concentration if Smeas is 35.2 removed by filtration before continuing with the analysis. and S is 0.6? reag In a typical total analysis method, the procedure might 2+ Problems 5. A method for the analysis of Ca in water suffers from an read 2+ interference in the presence of Zn . When the concentration 2+ 2+ of Ca is 50 times greater than that of Zn , an analysis for After dissolving the sample in a beaker, remove any A variety of problems, many based 2+ Ca gives a relative error of –2.0%. What is the value of the solid impurities by passing the solution containing selectivity coefficient for this method? the analyte through filter paper, collecting the on data from the analytical literature, solution in a clean Erlenmeyer flask. Rinse the beaker 6. The quantitative analysis for reduced glutathione in blood is with several small portions of solvent, passing these complicated by the presence of many potential interferents. provide the student with practical rinsings through the filter paper, and collecting them In one study, when analyzing a solution of 10-ppb in the same Erlenmeyer flask. Finally, rinse the filter glutathione and 1.5-ppb ascorbic acid, the signal was 5.43 examples of current research. paper with several portions of solvent, collecting the times greater than that obtained for the analysis of 10-ppb 12 rinsings in the same Erlenmeyer flask. glutathione. What is the selectivity coefficient for this analysis? The same study found that when analyzing a For a typical concentration method, however, the procedure solution of 350-ppb methionine and 10-ppb glutathione the might state signal was 0 906 times less than that obtained for the analysis xi Experiments1400-Fm 9/9/99 7:38 AM Page xii Preface Preface As currently taught, the introductory course in analytical chemistry emphasizes quantitative (and sometimes qualitative) methods of analysis coupled with a heavy dose of equilibrium chemistry. Analytical chemistry, however, is more than equilib- rium chemistry and a collection of analytical methods; it is an approach to solving chemical problems. Although discussing different methods is important, that dis- cussion should not come at the expense of other equally important topics. The intro- ductory analytical course is the ideal place in the chemistry curriculum to explore topics such as experimental design, sampling, calibration strategies, standardization, optimization, statistics, and the validation of experimental results. These topics are important in developing good experimental protocols, and in interpreting experi- mental results. If chemistry is truly an experimental science, then it is essential that all chemistry students understand how these topics relate to the experiments they conduct in other chemistry courses. Currently available textbooks do a good job of covering the diverse range of wet and instrumental analysis techniques available to chemists. Although there is some disagreement about the proper balance between wet analytical techniques, such as gravimetry and titrimetry, and instrumental analysis techniques, such as spec- trophotometry, all currently available textbooks cover a reasonable variety of tech- niques. These textbooks, however, neglect, or give only brief consideration to, obtaining representative samples, handling interferents, optimizing methods, ana- lyzing data, validating data, and ensuring that data are collected under a state of sta- tistical control. In preparing this textbook, I have tried to find a more appropriate balance between theory and practice, between “classical” and “modern” methods of analysis, between analyzing samples and collecting and preparing samples for analysis, and between analytical methods and data analysis. Clearly, the amount of material in this textbook exceeds what can be covered in a single semester; it’s my hope, however, that the diversity of topics will meet the needs of different instructors, while, per- haps, suggesting some new topics to cover. The anticipated audience for this textbook includes students majoring in chem- istry, and students majoring in other science disciplines (biology, biochemistry, environmental science, engineering, and geology, to name a few), interested in obtaining a stronger background in chemical analysis. It is particularly appropriate for chemistry majors who are not planning to attend graduate school, and who often do not enroll in those advanced courses in analytical chemistry that require physical chemistry as a pre-requisite. Prior coursework of a year of general chemistry is assumed. Competence in algebra is essential; calculus is used on occasion, however, its presence is not essential to the material’s treatment. xii1400-Fm 9/9/99 7:38 AM Page xiii xiii Preface Key Features of This Textbook Key features set this textbook apart from others currently available. • A stronger emphasis on the evaluation of data. Methods for characterizing chemical measurements, results, and errors (including the propagation of errors) are included. Both the binomial distribution and normal distribution are presented, and the idea of a confidence interval is developed. Statistical methods for evaluating data include the t-test (both for paired and unpaired data), the F-test, and the treatment of outliers. Detection limits also are discussed from a statistical perspective. Other statistical methods, such as ANOVA and ruggedness testing, are presented in later chapters. • Standardizations and calibrations are treated in a single chapter. Selecting the most appropriate calibration method is important and, for this reason, the methods of external standards, standard additions, and internal standards are gathered together in a single chapter. A discussion of curve-fitting, including the statistical basis for linear regression (with and without weighting) also is included in this chapter. • More attention to selecting and obtaining a representative sample. The design of a statistically based sampling plan and its implementation are discussed earlier, and in more detail than in other textbooks. Topics that are covered include how to obtain a representative sample, how much sample to collect, how many samples to collect, how to minimize the overall variance for an analytical method, tools for collecting samples, and sample preservation. • The importance of minimizing interferents is emphasized. Commonly used methods for separating interferents from analytes, such as distillation, masking, and solvent extraction, are gathered together in a single chapter. • Balanced coverage of analytical techniques. The six areas of analytical techniques—gravimetry, titrimetry, spectroscopy, electrochemistry, chromatography, and kinetics—receive roughly equivalent coverage, meeting the needs of instructors wishing to emphasize wet methods and those emphasizing instrumental methods. Related methods are gathered together in a single chapter encouraging students to see the similarities between methods, rather than focusing on their differences. • An emphasis on practical applications. Throughout the text applications from organic chemistry, inorganic chemistry, environmental chemistry, clinical chemistry, and biochemistry are used in worked examples, representative methods, and end-of-chapter problems. • Representative methods link theory with practice. An important feature of this text is the presentation of representative methods. These boxed features present typical analytical procedures in a format that encourages students to think about why the procedure is designed as it is. • Separate chapters on developing a standard method and quality assurance. Two chapters provide coverage of methods used in developing a standard method of analysis, and quality assurance. The chapter on developing a standard method includes topics such as optimizing experimental conditions using response surfaces, verifying the method through the blind analysis of standard samples and ruggedness testing, and collaborative testing using Youden’s two-sample approach and ANOVA. The chapter on quality assurance covers quality control and internal and external techniques for quality assessment, including the use of duplicate samples, blanks, spike recoveries, and control charts.1400-Fm 9/9/99 7:38 AM Page xiv xiv Preface • Problems adapted from the literature. Many of the in-chapter examples and end- of-chapter problems are based on data from the analytical literature, providing students with practical examples of current research in analytical chemistry. • An emphasis on critical thinking. Critical thinking is encouraged through problems in which students are asked to explain why certain steps in an analytical procedure are included, or to determine the effect of an experimental error on the results of an analysis. • Suggested experiments from the Journal of Chemical Education. Rather than including a short collection of experiments emphasizing the analysis of standard unknowns, an annotated list of representative experiments from the Journal of Chemical Education is included at the conclusion of most chapters. These experiments may serve as stand alone experiments, or as starting points for individual or group projects. The Role of Equilibrium Chemistry in Analytical Chemistry Equilibrium chemistry often receives a significant emphasis in the introductory ana- lytical chemistry course. While an important topic, its overemphasis can cause stu- dents to confuse analytical chemistry with equilibrium chemistry. Although atten- tion to solving equilibrium problems is important, it is equally important for stu- dents to recognize when such calculations are impractical, or when a simpler, more qualitative approach is all that is needed. For example, in discussing the gravimetric + analysis of Ag as AgCl, there is little point in calculating the equilibrium solubility – of AgCl since the concentration of Cl at equilibrium is rarely known. It is impor- – tant, however, to qualitatively understand that a large excess of Cl increases the sol- ubility of AgCl due to the formation of soluble silver-chloro complexes. Balancing the presentation of a rigorous approach to solving equilibrium problems, this text also introduces the use of ladder diagrams as a means for providing a qualitative pic- ture of a system at equilibrium. Students are encouraged to use the approach best suited to the problem at hand. Computer Software Many of the topics covered in analytical chemistry benefit from the availability of appropriate computer software. In preparing this text, however, I made a conscious decision to avoid a presentation tied to a single computer platform or software pack- age. Students and faculty are increasingly experienced in the use of computers, spreadsheets, and data analysis software; their use is, I think, best left to the person- al choice of each student and instructor. Organization The textbook’s organization can be divided into four parts. Chapters 1–3 serve as an introduction, providing an overview of analytical chemistry (Chapter 1); a review of the basic tools of analytical chemistry, including significant figures, units, and stoi- chiometry (Chapter 2); and an introduction to the terminology used by analytical chemists (Chapter 3). Familiarity with the material in these chapters is assumed throughout the remainder of the text. Chapters 4–7 cover a number of topics that are important in understanding how a particular analytical method works. Later chapters are mostly independent of the material in these chapters. Instructors may pick and choose from among the topics1400-Fm 9/9/99 7:38 AM Page xv xv Preface of these chapters, as needed, to support individual course goals. The statistical analy- sis of data is covered in Chapter 4 at a level that is more complete than that found in other introductory analytical textbooks. Methods for calibrating equipment, stan- dardizing methods, and linear regression are gathered together in Chapter 5. Chapter 6 provides an introduction to equilibrium chemistry, stressing both the rigorous solution to equilibrium problems, and the use of semi-quantitative approaches, such as ladder diagrams. The importance of collecting the right sample, and methods for separating analytes and interferents are covered in Chapter 7. Chapters 8–13 cover the major areas of analysis, including gravimetry (Chapter 8), titrimetry (Chapter 9), spectroscopy (Chapter 10), electrochemistry (Chapter 11), chromatography and electrophoresis (Chapter 12), and kinetic meth- ods (Chapter 13). Related techniques, such as acid–base titrimetry and redox titrimetry, or potentiometry and voltammetry, are gathered together in single chap- ters. Combining related techniques together encourages students to see the similar- ities between methods, rather than focusing on their differences. The first technique presented in each chapter is generally that which is most commonly covered in the introductory course. Finally, the textbook concludes with two chapters discussing the design and maintenance of analytical methods, two topics of importance to analytical chemists. Chapter 14 considers the development of an analytical method, including its opti- mization, verification, and validation. Quality control and quality assessment are discussed in Chapter 15. Acknowledgments Before beginning an academic career I was, of course, a student. My interest in chemistry and teaching was nurtured by many fine teachers at Westtown Friends School, Knox College, and the University of North Carolina at Chapel Hill; their col- lective influence continues to bear fruit. In particular, I wish to recognize David MacInnes, Alan Hiebert, Robert Kooser, and Richard Linton. I have been fortunate to work with many fine colleagues during my nearly 17 years of teaching undergraduate chemistry at Stockton State College and DePauw University. I am particularly grateful for the friendship and guidance provided by Jon Griffiths and Ed Paul during my four years at Stockton State College. At DePauw University, Jim George and Bryan Hanson have willingly shared their ideas about teaching, while patiently listening to mine. Approximately 300 students have joined me in thinking and learning about ana- lytical chemistry; their questions and comments helped guide the development of this textbook. I realize that working without a formal textbook has been frustrating and awkward; all the more reason why I appreciate their effort and hard work. The following individuals reviewed portions of this textbook at various stages during its development. Wendy Clevenger David Ballantine University of Tennessee–Chattanooga Northern Illinois University Cathy Cobb John E. Bauer Augusta State University Illinois State University Paul Flowers Ali Bazzi University of North Carolina–Pembroke University of Michigan–Dearborn Nancy Gordon Steven D. Brown University of Southern Maine University of Delaware1400-Fm 9/9/99 7:38 AM Page xvi xvi Preface Virginia M. Indivero Vincent Remcho Swarthmore College West Virginia University Michael Janusa Jeanette K. Rice Nicholls State University Georgia Southern University J. David Jenkins Martin W. Rowe Georgia Southern University Texas A&M University Richard S. Mitchell Alexander Scheeline Arkansas State University University of Illinois George A. Pearse, Jr. James D. Stuart Le Moyne College University of Connecticut Gary Rayson Thomas J. Wenzel New Mexico State University Bates College David Redfield David Zax NW Nazarene University Cornell University I am particularly grateful for their detailed written comments and suggestions for improving the manuscript. Much of what is good in the final manuscript is the result of their interest and ideas. George Foy (York College of Pennsylvania), John McBride (Hofstra University), and David Karpovich (Saginaw Valley State University) checked the accuracy of problems in the textbook. Gary Kinsel (University of Texas at Arlington) reviewed the page proofs and provided additional suggestions. This project began in the summer of 1992 with the support of a course develop- ment grant from DePauw University’s Faculty Development Fund. Additional finan- cial support from DePauw University’s Presidential Discretionary Fund also is acknowledged. Portions of the first draft were written during a sabbatical leave in the Fall semester of the 1993/94 academic year. A Fisher Fellowship provided release time during the Fall 1995 semester to complete the manuscript’s second draft. Alltech and Associates (Deerfield, IL) graciously provided permission to use the chromatograms in Chapter 12; the assistance of Jim Anderson, Vice-President, and Julia Poncher, Publications Director, is greatly appreciated. Fred Soster and Marilyn Culler, both of DePauw University, provided assistance with some of the photographs. The editorial staff at McGraw-Hill has helped guide a novice through the process of developing this text. I am particularly thankful for the encouragement and confidence shown by Jim Smith, Publisher for Chemistry, and Kent Peterson, Sponsoring Editor for Chemistry. Shirley Oberbroeckling, Developmental Editor for Chemistry, and Jayne Klein, Senior Project Manager, patiently answered my ques- tions and successfully guided me through the publishing process. Finally, I would be remiss if I did not recognize the importance of my family’s support and encouragement, particularly that of my parents. A very special thanks to my daughter, Devon, for gifts too numerous to detail. How to Contact the Author Writing this textbook has been an interesting (and exhausting) challenge. Despite my efforts, I am sure there are a few glitches, better examples, more interesting end- of-chapter problems, and better ways to think about some of the topics. I welcome your comments, suggestions, and data for interesting problems, which may be addressed to me at DePauw University, 602 S. College St., Greencastle, IN 46135, or electronically at harveydepauw.edu.1400-CH01 9/9/99 2:20 PM Page 1 C Ch ha ap pt te er r 1 Introduction Chemistry is the study of matter, including its composition, structure, physical properties, and reactivity. There are many approaches to studying chemistry, but, for convenience, we traditionally divide it into five fields: organic, inorganic, physical, biochemical, and analytical. Although this division is historical and arbitrary, as witnessed by the current interest in interdisciplinary areas such as bioanalytical and organometallic chemistry, these five fields remain the simplest division spanning the discipline of chemistry. Training in each of these fields provides a unique perspective to the study of chemistry. Undergraduate chemistry courses and textbooks are more than a collection of facts; they are a kind of apprenticeship. In keeping with this spirit, this text introduces the field of analytical chemistry and the unique perspectives that analytical chemists bring to the study of chemistry. 11400-CH01 9/9/99 2:20 PM Page 2 2 Modern Analytical Chemistry 1A What Is Analytical Chemistry? “Analytical chemistry is what analytical chemists do.” We begin this section with a deceptively simple question. What is analytical chem- istry? Like all fields of chemistry, analytical chemistry is too broad and active a disci- pline for us to easily or completely define in an introductory textbook. Instead, we will try to say a little about what analytical chemistry is, as well as a little about what analytical chemistry is not. Analytical chemistry is often described as the area of chemistry responsible for characterizing the composition of matter, both qualitatively (what is present) and quantitatively (how much is present). This description is misleading. After all, al- most all chemists routinely make qualitative or quantitative measurements. The ar- gument has been made that analytical chemistry is not a separate branch of chem- 1 istry, but simply the application of chemical knowledge. In fact, you probably have performed quantitative and qualitative analyses in other chemistry courses. For ex- ample, many introductory courses in chemistry include qualitative schemes for identifying inorganic ions and quantitative analyses involving titrations. Unfortunately, this description ignores the unique perspective that analytical chemists bring to the study of chemistry. The craft of analytical chemistry is not in performing a routine analysis on a routine sample (which is more appropriately called chemical analysis), but in improving established methods, extending existing methods to new types of samples, and developing new methods for measuring 2 chemical phenomena. Here’s one example of this distinction between analytical chemistry and chemi- cal analysis. Mining engineers evaluate the economic feasibility of extracting an ore by comparing the cost of removing the ore with the value of its contents. To esti- mate its value they analyze a sample of the ore. The challenge of developing and val- idating the method providing this information is the analytical chemist’s responsi- bility. Once developed, the routine, daily application of the method becomes the job of the chemical analyst. Another distinction between analytical chemistry and chemical analysis is that analytical chemists work to improve established methods. For example, sev- 2+ eral factors complicate the quantitative analysis of Ni in ores, including the presence of a complex heterogeneous mixture of silicates and oxides, the low con- 2+ centration of Ni in ores, and the presence of other metals that may interfere in the analysis. Figure 1.1 is a schematic outline of one standard method in use dur- 3 ing the late nineteenth century. After dissolving a sample of the ore in a mixture 2+ of H SO and HNO , trace metals that interfere with the analysis, such as Pb , 2 4 3 2+ 3+ Cu and Fe , are removed by precipitation. Any cobalt and nickel in the sample are reduced to Co and Ni, isolated by filtration and weighed (point A). After dissolving the mixed solid, Co is isolated and weighed (point B). The amount of nickel in the ore sample is determined from the difference in the masses at points A and B. mass point A – mass point B %Ni = × 100 mass sample Attributed to C. N. Reilley (1925–1981) on receipt of the 1965 Fisher Award in Analytical Chemistry. Reilley, who was a professor of chemistry at the University of North Carolina at Chapel Hill, was one of the most influential analytical chemists of the last half of the twentieth century.1400-CH01 9/9/99 2:20 PM Page 3 3 Chapter 1 Introduction Original Sample 1:3 H SO /HNO 100°C (8–10 h) 2 4 3 dilute w/H O, digest 2–4 h 2 2+ 3+ PbSO Cu , Fe 4 2+ 2+ Sand Co , Ni dilute bubble H S(g) 2 3+ 2+ 2+ Fe , Co , Ni CuS cool, add NH 3 digest 50°–70°, 30 min 2+ 2+ Fe(OH) Co , Ni 3 HCl slightly acidify w/ HCl heat, bubble H S (g) 2 3+ Fe CoS, NiS Waste neutralize w/ NH 3 Na CO , CH COOH 2 3 3 aqua regia heat, add HCl until strongly acidic Basic bubble H S (g) 2 ferric acetate 2+ 2+ CuS, PbS Co , Ni heat add Na CO until alkaline 2 3 NaOH Co(OH) , Ni(OH) Waste 2 2 heat CoO, NiO Solid heat, H (g) 2 Key Solution Co, Ni A HNO 3 K CO , KNO 2 3 3 CH COOH 3 digest 24 h 2+ Ni K Co(NO ) 3 3 5 H O, HCl 2 2+ Waste Co as above Co B Figure 1.1 3 Analytical scheme outlined by Fresenius for the gravimetric analysis of Ni in ores.1400-CH01 9/9/99 2:20 PM Page 4 4 Modern Analytical Chemistry Original sample HNO , HCl, heat 3 Residue Solution 20% NH Cl 4 10% tartaric acid take alkaline with 1:1 NH 3 take acid with HCl 10% tartaric acid take alkaline with 1:1 NH 3 Is Yes solid present? Solid No Key take acid with HCl 1% alcoholic DMG Solution take alkaline with 1:1 NH 3 Ni(DMG) (s) 2 A Figure 1.2 Analytical scheme outlined by Hillebrand and 4 Lundell for the gravimetric analysis of Ni in ores (DMG = dimethylgloxime). The factor of 0.2031 in the equation for %Ni accounts for mass A · 0.2031 the difference in the formula weights of %Ni = · 100 g sample Ni(DMG) and Ni; see Chapter 8 for more 2 details. 2+ The combination of determining the mass of Ni by difference, coupled with the need for many reactions and filtrations makes this procedure both time-consuming and difficult to perform accurately. The development, in 1905, of dimethylgloxime (DMG), a reagent that selec- 2+ 2+ tively precipitates Ni and Pd , led to an improved analytical method for deter- 2+ 4 2+ mining Ni in ores. As shown in Figure 1.2, the mass of Ni is measured directly, requiring fewer manipulations and less time. By the 1970s, the standard method for 2+ the analysis of Ni in ores progressed from precipitating Ni(DMG) to flame 2 5 atomic absorption spectrophotometry, resulting in an even more rapid analysis. Current interest is directed toward using inductively coupled plasmas for determin- ing trace metals in ores. In summary, a more appropriate description of analytical chemistry is “. . . the science of inventing and applying the concepts, principles, and . . . strategies for 6 measuring the characteristics of chemical systems and species.” Analytical chemists typically operate at the extreme edges of analysis, extending and improving the abil- ity of all chemists to make meaningful measurements on smaller samples, on more complex samples, on shorter time scales, and on species present at lower concentra- tions. Throughout its history, analytical chemistry has provided many of the tools and methods necessary for research in the other four traditional areas of chemistry, as well as fostering multidisciplinary research in, to name a few, medicinal chem- istry, clinical chemistry, toxicology, forensic chemistry, material science, geochem- istry, and environmental chemistry.

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