Fundaments of Analytical Chemistry PDF

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 PERIODIC TABLE OF THE ELEMENTS Metals IA VIIA 0 1 Nonmetals 17 18 1 Metalloids 1 2 1 H IIA IIIA IVA VA VIA H He 1.008 2 13 14 15 16 1.008 4.0026 3 4 5 6 7 8 9 10 Li Be B C N O F Ne Unless otherwise noted, all content on this page is © Cengage Learning. 2 6.941 9.0122 10.81 12.011 14.007 15.999 18.9984 20.1797 11 12 VIIIB 13 14 15 16 17 18 3 Na Mg IIIB IVB VB VIB VIIB IB IIB Al Si P S Cl Ar 22.9898 24.3050 3 4 5 6 7 8 9 10 11 12 26.9815 28.085 30.9738 32.06 35.453 39.948 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 4 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 39.0983 40.078 44.9559 47.867 50.9415 51.9961 54.9380 55.845 58.9332 58.6934 63.546 65.38 69.723 72.63 74.9216 78.96 79.904 83.798 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 5 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe 85.4678 87.62 88.9058 91.224 92.9064 95.96 (98) 101.07 102.9055 106.42 107.8682 112.411 114.818 118.710 121.760 127.60 126.9045 131.293 55 56 57 * 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 6 Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 132.9055 137.327 138.9055 178.49 180.9479 183.84 186.207 190.23 192.217 195.084 196.9666 200.59 204.38 207.2 208.9804 (209) (210) (222) 87 88 89 ** 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 7 Fr Ra Ac Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv Uus Uuo (223) (226) (227) (265) (268) (271) (270) (277) (276) (281) (280) (285) (284) (289) (288) (293) (294) (294) *Lanthanide Series 58 59 60 61 62 63 64 65 66 67 68 69 70 71 Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 140.116 140.9076 144.242 (145) 150.36 151.964 157.25 158.9254 162.500 164.9303 167.259 168.9342 173.054 174.9668 deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Note: Atomic masses are 2009 ** Actinide Series IUPAC values (up to four decimal 90 91 92 93 94 95 96 97 98 99 100 101 102 103 places). More accurate values for some elements are given in the Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has table inside the back cover. 232.0381 231.0359 238.0289 (237) (244) (243) (247) (247) (251) (252) (257) (258) (259) (262) International Atomic Masses Atomic Atomic Atomic Atomic Element Symbol Number Mass Element Symbol Number Mass Actinium Ac 89 (227) Mendelevium Md 101 (258) Aluminum Al 13 26.9815386 Mercury Hg 80 200.59 Americium Am 95 (243) Molybdenum Mo 42 95.96 Antimony Sb 51 121.760 Neodymium Nd 60 144.242 Argon Ar 18 39.948 Neon Ne 10 20.1797 Arsenic As 33 74.92160 Neptunium Np 93 (237) Astatine At 85 (210) Nickel Ni 28 58.6934 Barium Ba 56 137.327 Niobium Nb 41 92.90638 Berkelium Bk 97 (247) Nitrogen N 7 14.007 Beryllium Be 4 9.012182 Nobelium No 102 (259) Bismuth Bi 83 208.98040 Osmium Os 76 190.23 Bohrium Bh 107 (270) Oxygen O 8 15.999 Boron B 5 10.81 Palladium Pd 46 106.42 Bromine Br 35 79.904 Phosphorus P 15 30.973762 Cadmium Cd 48 112.411 Platinum Pt 78 195.084 Calcium Ca 20 40.078 Plutonium Pu 94 (244) Californium Cf 98 (251) Polonium Po 84 (209) Carbon C 6 12.011 Potassium K 19 39.0983 Cerium Ce 58 140.116 Praseodymium Pr 59 140.90765 Cesium Cs 55 132.90545 Promethium Pm 61 (145) Chlorine Cl 17 35.45 Protactinium Pa 91 231.03588 Chromium Cr 24 51.9961 Radium Ra 88 (226) Cobalt Co 27 58.933195 Radon Rn 86 (222) Copernicium Cn 112 (285) Rhenium Re 75 186.207 Copper Cu 29 63.546 Rhodium Rh 45 102.90550 Curium Cm 96 (247) Roentgenium Rg 111 (280) Darmstadtium Ds 110 (281) Rubidium Rb 37 85.4678 Dubnium Db 105 (268) Ruthenium Ru 44 101.07 Dysprosium Dy 66 162.500 Rutherfordium Rf 104 (265) Einsteinium Es 99 (252) Samarium Sm 62 150.36 Erbium Er 68 167.259 Scandium Sc 21 44.955912 Europium Eu 63 151.964 Seaborgium Sg 106 (271) Fermium Fm 100 (257) Selenium Se 34 78.96 Flerovium Fl 114 (289) Silicon Si 14 28.085 Fluorine F 9 18.9984032 Silver Ag 47 107.8682 Francium Fr 87 (223) Sodium Na 11 22.98976928 Gadolinium Gd 64 157.25 Strontium Sr 38 87.62 Gallium Ga 31 69.723 Sulfur S 16 32.06 Germanium Ge 32 72.63 Tantalum Ta 73 180.94788 Gold Au 79 196.966569 Technetium Tc 43 (98) Hafnium Hf 72 178.49 Tellurium Te 52 127.60 Hassium Hs 108 (277) Terbium Tb 65 158.92535 Helium He 2 4.002602 Thallium Tl 81 204.38 Holmium Ho 67 164.93032 Thorium Th 90 232.03806 Hydrogen H 1 1.008 Thulium Tm 69 168.93421 Indium In 49 114.818 Tin Sn 50 118.710 Iodine I 53 126.90447 Titanium Ti 22 47.867 Iridium Ir 77 192.217 Tungsten W 74 183.84 Iron Fe 26 55.845 Ununoctium Uuo 118 (294) Krypton Kr 36 83.798 Ununpentium Uup 115 (288) Lanthanum La 57 138.90547 Ununseptium Uus 117 (294) Lawrencium Lr 103 (262) Ununtrium Uut 113 (284) Lead Pb 82 207.2 Uranium U 92 238.02891 Lithium Li 3 6.94 Vanadium V 23 50.9415 Livermorium Lv 116 (293) Xenon Xe 54 131.293 Lutetium Lu 71 174.9668 Ytterbium Yb 70 173.054 Magnesium Mg 12 24.3050 Yttrium Y 39 88.90585 Manganese Mn 25 54.938045 Zinc Zn 30 65.38 Meitnerium Mt 109 (276) Zirconium Zr 40 91.224 The values given in parentheses are the atomic mass numbers of the isotopes of the longest known half-life. From M. E. Wieser and T. B. Coplen, Pure Appl. Chem., 2011, 83(2), 359–96, DOI: 10.1351/PAC-REP-10-09-14. Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Molar Masses of Some Compounds Compound Molar Mass Compound Molar Mass AgBr 187.772 K3Fe(CN)6 329.248 AgCl 143.32 K4Fe(CN)6 368.346 Ag2CrO4 331.729 KHC8H4O4 (phthalate) 204.222 AgI 234.7727 KH(IO3)2 389.909 AgNO3 169.872 K2HPO4 174.174 AgSCN 165.95 KH2PO4 136.084 Al2O3 101.960 KHSO4 136.16 Al2(SO4)3 342.13 KI 166.0028 As2O3 197.840 KIO3 214.000 B2O3 69.62 KIO4 229.999 BaCO3 197.335 KMnO4 158.032 BaCl2 ? 2H2O 244.26 KNO3 101.102 BaCrO4 253.319 KOH 56.105 Ba(IO3)2 487.130 KSCN 97.18 Ba(OH)2 171.341 K2SO4 174.25 BaSO4 233.38 La(IO3)3 663.610 Bi2O3 465.958 Mg(C9H6NO)2 312.611 CO2 44.009 (8-hydroxyquinolate) CaCO3 100.086 MgCO3 84.313 CaC2O4 128.096 MgNH4PO4 137.314 CaF2 78.075 MgO 40.304 CaO 56.077 Mg2P2O7 222.551 CaSO4 136.13 MgSO4 120.36 Ce(HSO4)4 528.37 MnO2 86.936 CeO2 172.114 Mn2O3 157.873 Ce(SO4)2 332.23 Mn3O4 228.810 (NH4)2Ce(NO3)6 548.22 Na2B4O7 ? 10H2O 381.36 (NH4)4Ce(SO4)4 ? 2H2O 632.53 NaBr 102.894 Cr2O3 151.989 NaC2H3O2 82.034 CuO 79.545 Na2C2O4 133.998 Cu2O 143.091 NaCl 58.44 CuSO4 159.60 NaCN 49.008 Fe(NH4)2(SO4)2 ? 6H2O 392.13 Na2CO3 105.988 FeO 71.844 NaHCO3 84.006 Fe2O3 159.687 Na2H2EDTA ? 2H2O 372.238 Fe3O4 231.531 Na2O2 77.978 HBr 80.912 NaOH 39.997 HC2H3O2 (acetic acid) 60.052 NaSCN 81.07 HC7H5O2 (benzoic acid) 122.123 Na2SO4 142.04 (HOCH2)3CNH2 (TRIS) 121.135 Na2S2O3 ? 5H2O 248.17 HCl 36.46 NH4Cl 53.49 HClO4 100.45 (NH4)2C2O4 ? H2O 142.111 H2C2O4 ? 2H2O 126.064 NH4NO3 80.043 H5IO6 227.938 (NH4)2SO4 132.13 HNO3 63.012 (NH4)2S2O8 228.19 H2O 18.015 NH4VO3 116.978 H2O2 34.014 Ni(C4H7O2N2)2 288.917 H3PO4 97.994 (dimethylglyoximate) H2S 34.08 PbCrO4 323.2 H2SO3 82.07 PbO 223.2 H2SO4 98.07 PbO2 239.2 HgO 216.59 PbSO4 303.3 Hg2Cl2 472.08 P2O5 141.943 HgCl2 271.49 Sb2S3 339.70 KBr 119.002 SiO2 60.083 KBrO3 166.999 SnCl2 189.61 KCl 74.55 SnO2 150.71 KClO3 122.55 SO2 64.06 KCN 65.116 SO3 80.06 K2CrO4 194.189 Zn2P2O7 304.70 K2Cr2O7 294.182 Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Nature of Analytical Chemistry CHAPTER 1 A nalytical chemistry is a measurement science consisting of a set of powerful ideas and methods that are useful in all fields of science, engineering, and medicine. Some exciting illustrations of the power and significance of analytical chemistry have occurred, are occurring, and will occur during NASA’s rover explorations of the planet Mars. On July 4, 1997, the Pathfinder spacecraft delivered the Sojourner rover to the Martian surface. Analytical in- struments returned information on the chemical composition of rocks and soil. Investigations by the lander and rover suggested that Mars was at one time in its past warm and wet with liquid water on the surface and water vapor in the atmosphere. In January 2004, the Mars rovers Spirit and Opportunity arrived on Mars for a three-month mission. A major result from Spirit’s alpha particle X-ray spectrometer (APXS) and Mossbauer spectrometer was finding concentrated deposits of silica and, at a different site, high concentrations of carbonate. Spirit continued to explore and transmit data until 2010, outliving even the most optimistic predictions. Even more amazing, Opportunity continues to travel the surface of Mars and, by March, 2012, had covered more than 21 miles exploring and transmitting images of craters, small hills, and other features. In late 2011, the Mars Science Laboratory aboard the rover Curiosity was launched. It arrived on August 6, 2012 with a host of analytical instruments on board. The Chemis- try and Camera package includes a laser-induced breakdown spectrometer (LIBS, see Chapter 28) and a remote microimager. The LIBS instrument will provide determination NASA/JPL-Caltech NASA/JPL-Caltech Mars Science Laboratory aboard rover Curiosity Curiosity observing Martian landscape from Gale crater, August 2012 Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 2 CHAPTER 1 The Nature of Analytical Chemistry of many elements with no sample preparation. It can determine the identity and amounts of major, minor, and trace elements and can detect hydrated minerals. The sample analy- sis package contains a quadrupole mass spectrometer (Chapter 29), a gas chromatograph (Chapter 32), and a tunable laser spectrometer (Chapter 25). Its goals are to survey carbon compound sources, search for organic compounds important to life, reveal the chemical and isotopic states of several elements, determine the composition of the Martian atmosphere, and search for noble gas and light element isotopes.1 These examples demonstrate that both qualitative and quantitative information are Qualitative analysis reveals the iden- required in an analysis. Qualitative analysis establishes the chemical identity of the tity of the elements and compounds in species in the sample. Quantitative analysis determines the relative amounts of these a sample. species, or analytes, in numerical terms. The data from the various spectrometers on Quantitative analysis indicates the the rovers contain both types of information. As is common with many analytical instru- amount of each substance in a sample. ments, the gas chromatograph and mass spectrometer incorporate a separation step as Analytes are the components of a a necessary part of the analytical process. With a few analytical tools, exemplified here sample that are determined. by the APXS and LIBS experiments, chemical separation of the various elements con- tained in the rocks is unnecessary since the methods provide highly selective informa- tion. In this text, we will explore quantitative methods of analysis, separation methods, and the principles behind their operation. A qualitative analysis is often an integral part of the separation step, and determining the identity of the analytes is an essential ad- junct to quantitative analysis. 1A The Role of Analytical Chemistry Analytical chemistry is applied throughout industry, medicine, and all the sci- ences. To illustrate, consider a few examples. The concentrations of oxygen and of carbon dioxide are determined in millions of blood samples every day and used to diagnose and treat illnesses. Quantities of hydrocarbons, nitrogen oxides, and carbon monoxide present in automobile exhaust gases are measured to deter- mine the effectiveness of emission-control devices. Quantitative measurements of ionized calcium in blood serum help diagnose parathyroid disease in humans. Quantitative determination of nitrogen in foods establishes their protein content and thus their nutritional value. Analysis of steel during its production permits adjustment in the concentrations of such elements as carbon, nickel, and chro- mium to achieve a desired strength, hardness, corrosion resistance, and ductility. The mercaptan content of household gas supplies is monitored continually to ensure that the gas has a sufficiently obnoxious odor to warn of dangerous leaks. Farmers tailor fertilization and irrigation schedules to meet changing plant needs during the growing season, gauging these needs from quantitative analyses of plants and soil. Quantitative analytical measurements also play a vital role in many research areas in chemistry, biochemistry, biology, geology, physics, and the other sci- ences. For example, quantitative measurements of potassium, calcium, and sodium ions in the body fluids of animals permit physiologists to study the role these ions play in nerve-signal conduction as well as muscle contraction and re- laxation. Chemists unravel the mechanisms of chemical reactions through reac- tion rate studies. The rate of consumption of reactants or formation of products 1 For details on the Mars Science Laboratory mission and the rover Curiosity, see http://www.nasa.gov. Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  1A The Role of Analytical Chemistry 3 in a chemical reaction can be calculated from quantitative measurements made at precise time intervals. Materials scientists rely heavily on quantitative analyses of crystalline germanium and silicon in their studies of semiconductor devices whose impurities lie in the concentration range of 1 3 1026 to 1 3 1029 per- cent. Archaeologists identify the sources of volcanic glasses (obsidian) by measur- ing concentrations of minor elements in samples taken from various locations. This knowledge in turn makes it possible to trace prehistoric trade routes for tools and weapons fashioned from obsidian. Many chemists, biochemists, and medicinal chemists devote much time in the laboratory gathering quantitative information about systems that are im- portant and interesting to them. The central role of analytical chemistry in this enterprise and many others is illustrated in Figure 1-1. All branches of chemistry draw on the ideas and techniques of analytical chemistry. Analyti- cal chemistry has a similar function with respect to the many other scientific fields listed in the diagram. Chemistry is often called the central science; its top center position and the central position of analytical chemistry in the figure Chemistry Biochemistry Inorganic Chemistry Biology Organic Chemistry Botany Physical Chemistry Physics Genetics Astrophysics Microbiology Astronomy Molecular Biology Biophysics Zoology Geology Engineering Geophysics Civil Geochemistry Chemical Paleontology Electrical Paleobiology Mechanical Analytical Chemistry Environmental Medicine Sciences Clinical Chemistry Ecology Medicinal Chemistry Meteorology Pharmacy Oceanography Toxicology Agriculture Agronomy Materials Science Animal Science Metallurgy Crop Science Polymers Food Science Solid State Social Sciences Horticulture Archeology Soil Science Anthropology Forensics Figure 1-1 The relationship between analytical chemistry, other branches of chemistry, and the other sciences. The central location of analytical chemistry in the diagram signifies its importance and the breadth of its interactions with many other disciplines. Unless otherwise noted, all content on this page is © Cengage Learning. Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 4 CHAPTER 1 The Nature of Analytical Chemistry emphasize this importance. The interdisciplinary nature of chemical analysis makes it a vital tool in medical, industrial, government, and academic labora- tories throughout the world. 1B Quantitative Analytical Methods We compute the results of a typical quantitative analysis from two measurements. One is the mass or the volume of sample being analyzed. The second measurement is of some quantity that is proportional to the amount of analyte in the sample such as mass, volume, intensity of light, or electrical charge. This second measure- ment usually completes the analysis, and we usually classify analytical methods according to the nature of this final measurement. In gravimetric methods, we determine the mass of the analyte or some compound chemically related to it. In a volumetric method, we measure the volume of a solution containing sufficient reagent to react completely with the analyte. In electroanalytical methods, we measure electrical properties such as potential, current, resistance, and quantity of electrical charge. In spectroscopic methods, we explore the interaction between electromagnetic radiation and analyte atoms or molecules or the emission of radi- ation by analytes. Finally, in a group of miscellaneous methods, we measure such quantities as mass-to-charge ratio of ions by mass spectrometry, rate of radioac- tive decay, heat of reaction, rate of reaction, sample thermal conductivity, optical activity, and refractive index. 1C A Typical Quantitative Analysis A typical quantitative analysis includes the sequence of steps shown in the flow diagram of Figure 1-2. In some instances, one or more of these steps can be omitted. For example, if the sample is already a liquid, we can avoid the dissolu- tion step. Chapters 1 through 34 focus on the last three steps in Figure 1-2. In the measurement step, we measure one of the physical properties mentioned in Section 1B. In the calculation step, we find the relative amount of the analyte present in the samples. In the final step, we evaluate the quality of the results and estimate their reliability. In the paragraphs that follow, you will find a brief overview of each of the nine steps shown in Figure 1-2. We then present a case study to illustrate the use of these steps in solving an important and practical analytical problem. The details of this study foreshadow many of the methods and ideas you will explore as you study ana- lytical chemistry. 1C-1 Choosing a Method The essential first step in any quantitative analysis is the selection of a method as depicted in Figure 1-2. The choice is sometimes difficult and requires experience as well as intuition. One of the first questions that must be considered in the selec- tion process is the level of accuracy required. Unfortunately, high reliability nearly always requires a large investment of time. The selected method usually represents a compromise between the accuracy required and the time and money available for the analysis. Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  1C A Typical Quantitative Analysis 5 Select method Acquire sample Process sample Is No Carry out sample soluble chemical dissolution ? Yes Change chemical No Measurable form property? Yes Eliminate interferences Figure 1-2 Flow diagram showing the steps in a quantitative analysis. There are a number of possible paths Measure through these steps. In the simplest property X example represented by the central vertical pathway, we select a method, acquire and process the sample, dissolve the sample in a suitable Calculate results solvent, measure a property of the analyte, calculate the results, and estimate the reliability of the results. Depending on the complexity of the Estimate sample and the chosen method, reliability of results various other pathways may be necessary. A second consideration related to economic factors is the number of samples that will be analyzed. If there are many samples, we can afford to spend a significant amount of time in preliminary operations such as assembling and calibrating instru- ments and equipment and preparing standard solutions. If we have only a single sample or a just a few samples, it may be more appropriate to select a procedure that avoids or minimizes such preliminary steps. Finally, the complexity of the sample and the number of components in the sam- ple always influence the choice of method to some degree. 1C-2 Acquiring the Sample As illustrated in Figure 1-2, the second step in a quantitative analysis is to acquire the sample. To produce meaningful information, an analysis must be performed on a sample that has the same composition as the bulk of material from which it was Unless otherwise noted, all content on this page is © Cengage Learning. Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 6 CHAPTER 1 The Nature of Analytical Chemistry ScienceCartoonsPlus.com A material is heterogeneous if its taken. When the bulk is large and heterogeneous, great effort is required to get constituent parts can be distinguished a representative sample. Consider, for example, a railroad car containing 25 tons visually or with the aid of a micro- of silver ore. The buyer and seller of the ore must agree on a price, which will be scope. Coal, animal tissue, and soil are heterogeneous. based primarily on the silver content of the shipment. The ore itself is inherently heterogeneous, consisting of many lumps that vary in size as well as in silver con- An assay is the process of determin- tent. The assay of this shipment will be performed on a sample that weighs about ing how much of a given sample is the one gram. For the analysis to have significance, the composition of this small sam- material by its indicated name. For ple must be representative of the 25 tons (or approximately 22,700,000 g) of ore example, a zinc alloy is assayed for its in the shipment. Isolation of one gram of material that accurately represents the zinc content, and its assay is a particu- lar numerical value. average composition of the nearly 23,000,000 g of bulk sample is a difficult un- dertaking that requires a careful, systematic manipulation of the entire shipment. Sampling is the process of collecting a small mass of a material whose composition We analyze samples, and we determine substances. For ❯ accurately represents the bulk of the material being sampled. Sampling is discussed in more detail in Chapter 8. example, a blood sample is analyzed to determine the The collection of specimens from biological sources represents a second type concentrations of various of sampling problem. Sampling of human blood for the determination of blood substances such as blood gases gases illustrates the difficulty of acquiring a representative sample from a com- and glucose. We, therefore, speak plex biological system. The concentration of oxygen and carbon dioxide in blood of the determination of blood depends on a variety of physiological and environmental variables. For example, gases or glucose, not the analysis of blood gases or glucose. applying a tourniquet incorrectly or hand flexing by the patient may cause the blood oxygen concentration to fluctuate. Because physicians make life-and-death decisions based on results of blood gas analyses, strict procedures have been de- veloped for sampling and transporting specimens to the clinical laboratory. These procedures ensure that the sample is representative of the patient at the time it is collected and that its integrity is preserved until the sample can be analyzed. Many sampling problems are easier to solve than the two just described. Whether sampling is simple or complex, however, the analyst must be sure that the laboratory sample is representative of the whole before proceeding. Sampling is frequently the most difficult step in an analysis and the source of greatest error. The final analytical result will never be any more reliable than the reliability of the sampling step. Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  1C A Typical Quantitative Analysis 7 1C-3 Processing the Sample As shown in Figure 1-2, the third step in an analysis is to process the sample. Under certain circumstances, no sample processing is required prior to the measurement step. For example, once a water sample is withdrawn from a stream, a lake, or an ocean, the pH of the sample can be measured directly. Under most circumstances, we must process the sample in one of several different ways. The first step in processing the sample is often the preparation of a laboratory sample. Preparing a Laboratory Sample A solid laboratory sample is ground to decrease particle size, mixed to ensure homo- geneity, and stored for various lengths of time before analysis begins. Absorption or desorption of water may occur during each step, depending on the humidity of the environment. Because any loss or gain of water changes the chemical composition of solids, it is a good idea to dry samples just before starting an analysis. Alternatively, the moisture content of the sample can be determined at the time of the analysis in a separate analytical procedure. Liquid samples present a slightly different but related set of problems during the preparation step. If such samples are allowed to stand in open containers, the solvent may evaporate and change the concentration of the analyte. If the analyte is a gas dissolved in a liquid, as in our blood gas example, the sample container must be kept inside a second sealed container, perhaps during the entire analytical procedure, to prevent contamination by atmospheric gases. Extraordinary measures, including sample manipulation and measurement in an inert atmosphere, may be required to preserve the integrity of the sample. Defining Replicate Samples Most chemical analyses are performed on replicate samples whose masses or vol- Replicate samples, or replicates, are umes have been determined by careful measurements with an analytical balance or portions of a material of approximately with a precise volumetric device. Replication improves the quality of the results and the same size that are carried through an analytical procedure at the same provides a measure of their reliability. Quantitative measurements on replicates are time and in the same way. usually averaged, and various statistical tests are performed on the results to establish their reliability. Preparing Solutions: Physical and Chemical Changes Most analyses are performed on solutions of the sample made with a suitable sol- vent. Ideally, the solvent should dissolve the entire sample, including the analyte, rapidly and completely. The conditions of dissolution should be sufficiently mild that loss of the analyte cannot occur. In our flow diagram of Figure 1-2, we ask whether the sample is soluble in the solvent of choice. Unfortunately, many ma- terials that must be analyzed are insoluble in common solvents. Examples include silicate minerals, high-molecular-mass polymers, and specimens of animal tissue. With such substances, we must follow the flow diagram to the box on the right and perform some rather harsh chemistry. Converting the analyte in such materials into a soluble form is often the most difficult and time-consuming task in the ana- lytical process. The sample may require heating with aqueous solutions of strong acids, strong bases, oxidizing agents, reducing agents, or some combination of such reagents. It may be necessary to ignite the sample in air or oxygen or to perform a high-temperature fusion of the sample in the presence of various fluxes. Once the analyte is made soluble, we then ask whether the sample has a property that is pro- portional to analyte concentration and that we can measure. If it does not, other chemical steps may be necessary, as shown in Figure 1-2, to convert the analyte to a Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 8 CHAPTER 1 The Nature of Analytical Chemistry form that is suitable for the measurement step. For example, in the determination of manganese in steel, the element must be oxidized to MnO42 before the absorbance of the colored solution is measured (see Chapter 26). At this point in the analysis, it may be possible to proceed directly to the measurement step, but more often than not, we must eliminate interferences in the sample before making measure- ments, as illustrated in the flow diagram. 1C-4 Eliminating Interferences Once we have the sample in solution and converted the analyte to an appropriate form for measurement, the next step is to eliminate substances from the sample that may interfere with measurement (see Figure 1-2). Few chemical or physical proper- ties of importance in chemical analysis are unique to a single chemical species. In- stead, the reactions used and the properties measured are characteristic of a group of elements of compounds. Species other than the analyte that affect the final measure- An interference or interferent is a ment are called interferences, or interferents. A scheme must be devised to isolate species that causes an error in an the analytes from interferences before the final measurement is made. No hard and analysis by enhancing or attenuating fast rules can be given for eliminating interference. This problem can certainly be the (making smaller) the quantity being measured. most demanding aspect of an analysis. Chapters 31 through 34 describe separation methods in detail. 1C-5 Calibrating and Measuring Concentration The matrix, or sample matrix, is the All analytical results depend on a final measurement X of a physical or chemical collection of all of the components in property of the analyte, as shown in Figure 1-2. This property must vary in a known the sample containing an analyte. and reproducible way with the concentration cA of the analyte. Ideally, the measure- Techniques or reactions that work for ment of the property is directly proportional to the concentration, that is, only one analyte are said to be specific. Techniques or reactions that apply to cA 5 kX only a few analytes are selective. where k is a proportionality constant. With a few exceptions, analytical methods re- quire the empirical determination of k with chemical standards for which cA is known.2 The process of determining k is thus an important step in most analyses; this step is Calibration is the process of deter- called a calibration. Calibration methods are discussed in some detail in Chapter 8. mining the proportionality between analyte concentration and a measured quantity. 1C-6 Calculating Results Computing analyte concentrations from experimental data is usually relatively easy, particularly with computers. This step is depicted in the next-to-last block of the flow diagram of Figure 1-2. These computations are based on the raw experimental data collected in the measurement step, the characteristics of the measurement in- struments, and the stoichiometry of the analytical reaction. Samples of these calcula- tions appear throughout this book. 1C-7 Evaluating Results by Estimating Reliability As the final step in Figure 1-2 shows, analytical results are complete only when their reliability has been estimated. The experimenter must provide some measure of the uncertainties associated with computed results if the data are to have any value. 2 Two exceptions are gravimetric methods, discussed in Chapter 12, and coulometric methods, con- sidered in Chapter 22. In both these methods, k can be computed from known physical constants. Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  1D An Integral Role for Chemical Analysis: Feedback Control Systems 9 Chapters 5, 6, and 7 present detailed methods for carrying out this important final step in the analytical process. ❮ An analytical result without an estimate of reliability is of no value. A  n Integral Role for Chemical Analysis: 1D Feedback Control Systems Analytical chemistry is usually not an end in itself but is part of a bigger picture in which the analytical results may be used to help control a patient’s health, to control the amount of mercury in fish, to control the quality of a product, to determine the status of a synthesis, or to find out whether there is life on Mars. Chemical analysis is the measurement element in all of these examples and in many other cases. Consider the role of quantitative analysis in the determination and control of the concentration of glucose in blood. The system flow diagram of Figure 1-3 illustrates the process. Patients suffering from insulin-dependent diabetes mellitus develop hyperglycemia, which manifests itself in a blood glucose concentration above the normal concentration range of 65 to 100 mg/dL. We begin our example by determining that the desired state is a blood glucose level below 100 mg/dL. Many patients must monitor their blood glucose levels by periodically submitting samples to a clinical laboratory for analysis or by measuring the levels themselves using a handheld electronic glucose monitor. The first step in the monitoring process is to determine the actual state by col- lecting a blood sample from the patient and measuring the blood glucose level. The results are displayed, and then the actual state is compared to the desired state, as shown in Figure 1-3. If the measured blood glucose level is above 100 mg/dL, the patient’s insulin level, which is a controllable quantity, is increased by injection or oral administration. After a delay to allow the insulin time to take effect, the glucose level is measured again to determine if the desired state has been achieved. If the level is below the threshold, the insulin level has been maintained, so no insulin is required. After a suitable delay time, the blood glucose level is measured again, and the cycle is repeated. In this way, the insulin level in the patient’s blood, and thus the Start control system Figure 1-3 Feedback system flow diagram. The desired system state is defined, the actual state of the system is measured, and the two states are Determine desired state compared. The difference between the two states is used to change a control- lable quantity that results in a change in the state of the system. Quantitative Change control Measure measurements are again performed Delay quantity actual state on the system, and the comparison is repeated. The new difference between the desired state and the actual state is again used to change the state of Display