D. Harvey - Modern Analytical Chemistry (794078), страница 5
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A very special thanks tomy daughter, Devon, for gifts too numerous to detail.How to Contact the AuthorWriting this textbook has been an interesting (and exhausting) challenge. Despitemy efforts, I am sure there are a few glitches, better examples, more interesting endof-chapter problems, and better ways to think about some of the topics.
I welcomeyour comments, suggestions, and data for interesting problems, which may beaddressed to me at DePauw University, 602 S. College St., Greencastle, IN 46135, orelectronically at harvey@depauw.edu.1400-CH01 9/9/99 2:20 PM Page 1Chapter 1IntroductionChemistry is the study of matter, including its composition,structure, physical properties, and reactivity. There are manyapproaches to studying chemistry, but, for convenience, wetraditionally divide it into five fields: organic, inorganic, physical,biochemical, and analytical. Although this division is historical andarbitrary, as witnessed by the current interest in interdisciplinary areassuch as bioanalytical and organometallic chemistry, these five fieldsremain the simplest division spanning the discipline of chemistry.Training in each of these fields provides a unique perspective to thestudy of chemistry.
Undergraduate chemistry courses and textbooksare more than a collection of facts; they are a kind of apprenticeship. Inkeeping with this spirit, this text introduces the field of analyticalchemistry and the unique perspectives that analytical chemists bring tothe study of chemistry.11400-CH01 9/9/99 2:20 PM Page 22Modern Analytical Chemistry1A What Is Analytical Chemistry?“Analytical chemistry is what analytical chemists do.”*We begin this section with a deceptively simple question. What is analytical chemistry? Like all fields of chemistry, analytical chemistry is too broad and active a discipline for us to easily or completely define in an introductory textbook.
Instead, wewill try to say a little about what analytical chemistry is, as well as a little about whatanalytical chemistry is not.Analytical chemistry is often described as the area of chemistry responsible forcharacterizing the composition of matter, both qualitatively (what is present) andquantitatively (how much is present). This description is misleading. After all, almost all chemists routinely make qualitative or quantitative measurements.
The argument has been made that analytical chemistry is not a separate branch of chemistry, but simply the application of chemical knowledge.1 In fact, you probably haveperformed quantitative and qualitative analyses in other chemistry courses. For example, many introductory courses in chemistry include qualitative schemes foridentifying inorganic ions and quantitative analyses involving titrations.Unfortunately, this description ignores the unique perspective that analyticalchemists bring to the study of chemistry. The craft of analytical chemistry is not inperforming a routine analysis on a routine sample (which is more appropriatelycalled chemical analysis), but in improving established methods, extending existingmethods to new types of samples, and developing new methods for measuringchemical phenomena.2Here’s one example of this distinction between analytical chemistry and chemical analysis.
Mining engineers evaluate the economic feasibility of extracting an oreby comparing the cost of removing the ore with the value of its contents. To estimate its value they analyze a sample of the ore. The challenge of developing and validating the method providing this information is the analytical chemist’s responsibility. Once developed, the routine, daily application of the method becomes thejob of the chemical analyst.Another distinction between analytical chemistry and chemical analysis isthat analytical chemists work to improve established methods.
For example, several factors complicate the quantitative analysis of Ni2+ in ores, including thepresence of a complex heterogeneous mixture of silicates and oxides, the low concentration of Ni2+ in ores, and the presence of other metals that may interfere inthe analysis. Figure 1.1 is a schematic outline of one standard method in use during the late nineteenth century.3 After dissolving a sample of the ore in a mixtureof H2SO4 and HNO3, trace metals that interfere with the analysis, such as Pb2+,Cu2+ and Fe3+, are removed by precipitation. Any cobalt and nickel in the sampleare reduced to Co and Ni, isolated by filtration and weighed (point A).
Afterdissolving the mixed solid, Co is isolated and weighed (point B). The amountof nickel in the ore sample is determined from the difference in the masses atpoints A and B.%Ni =mass point A – mass point B× 100mass sample*Attributed to C. N. Reilley (1925–1981) on receipt of the 1965 Fisher Award in Analytical Chemistry.
Reilley, who wasa professor of chemistry at the University of North Carolina at Chapel Hill, was one of the most influential analyticalchemists of the last half of the twentieth century.1400-CH01 9/9/99 2:20 PM Page 3Chapter 1 IntroductionOriginal Sample1:3 H2SO4/HNO3 100°C (8–10 h)dilute w/H2O, digest 2–4 hCu2+, Fe3+Co2+, Ni2+PbSO4Sanddilutebubble H2S(g)Fe3+, Co2+, Ni2+CuScool, add NH3digest 50°–70°, 30 minCo2+, Ni2+Fe(OH)3HClslightly acidify w/ HClheat, bubble H2S (g)Fe3+CoS, NiSWasteneutralize w/ NH3Na2CO3, CH3COOHaqua regiaheat, add HCl untilstrongly acidicbubble H2S (g)BasicferricacetateCo2+, Ni2+CuS, PbSheatadd Na2CO3 until alkalineNaOHWasteCo(OH)2, Ni(OH)2heatCoO, NiOSolidheat, H2 (g)Co, NiKeySolutionAHNO3K2CO3, KNO3CH3COOHdigest 24 hNi2+K3Co(NO3)5H2O, HClCo2+Wasteas aboveCoBFigure 1.1Analytical scheme outlined by Fresenius3 for the gravimetric analysis of Ni in ores.31400-CH01 9/9/99 2:20 PM Page 44Modern Analytical ChemistryOriginal sampleHNO3, HCl, heatResidueSolution20% NH4Cl10% tartaric acidtake alkaline with 1:1 NH3take acid with HCl10% tartaric acidtake alkaline with 1:1 NH3YesIssolidpresent?SolidNoKeySolutionAFigure 1.2Analytical scheme outlined by Hillebrand andLundell4 for the gravimetric analysis of Ni inores (DMG = dimethylgloxime).
The factor of0.2031 in the equation for %Ni accounts forthe difference in the formula weights ofNi(DMG)2 and Ni; see Chapter 8 for moredetails.%Ni =take acid with HCl1% alcoholic DMGtake alkaline with 1:1 NH3Ni(DMG)2(s)mass A × 0.2031× 100g sampleThe combination of determining the mass of Ni2+ by difference, coupled with theneed for many reactions and filtrations makes this procedure both time-consumingand difficult to perform accurately.The development, in 1905, of dimethylgloxime (DMG), a reagent that selectively precipitates Ni2+ and Pd2+, led to an improved analytical method for determining Ni2+ in ores.4 As shown in Figure 1.2, the mass of Ni2+ is measured directly,requiring fewer manipulations and less time. By the 1970s, the standard method forthe analysis of Ni 2+ in ores progressed from precipitating Ni(DMG) 2 to flameatomic absorption spectrophotometry,5 resulting in an even more rapid analysis.Current interest is directed toward using inductively coupled plasmas for determining trace metals in ores.In summary, a more appropriate description of analytical chemistry is “.
. . thescience of inventing and applying the concepts, principles, and . . . strategies formeasuring the characteristics of chemical systems and species.”6 Analytical chemiststypically operate at the extreme edges of analysis, extending and improving the ability of all chemists to make meaningful measurements on smaller samples, on morecomplex samples, on shorter time scales, and on species present at lower concentrations. Throughout its history, analytical chemistry has provided many of the toolsand methods necessary for research in the other four traditional areas of chemistry,as well as fostering multidisciplinary research in, to name a few, medicinal chemistry, clinical chemistry, toxicology, forensic chemistry, material science, geochemistry, and environmental chemistry.1400-CH01 9/9/99 2:20 PM Page 5Chapter 1 IntroductionYou will come across numerous examples of qualitative and quantitative methods in this text, most of which are routine examples of chemical analysis.
It is important to remember, however, that nonroutine problems prompted analyticalchemists to develop these methods. Whenever possible, we will try to place thesemethods in their appropriate historical context. In addition, examples of current research problems in analytical chemistry are scattered throughout the text.The next time you are in the library, look through a recent issue of an analytically oriented journal, such as Analytical Chemistry. Focus on the titles and abstractsof the research articles. Although you will not recognize all the terms and methods,you will begin to answer for yourself the question “What is analytical chemistry”?1B The Analytical PerspectiveHaving noted that each field of chemistry brings a unique perspective to the studyof chemistry, we now ask a second deceptively simple question.
What is the “analytical perspective”? Many analytical chemists describe this perspective as an analyticalapproach to solving problems.7 Although there are probably as many descriptionsof the analytical approach as there are analytical chemists, it is convenient for ourpurposes to treat it as a five-step process:1.2.3.4.5.Identify and define the problem.Design the experimental procedure.Conduct an experiment, and gather data.Analyze the experimental data.Propose a solution to the problem.Figure 1.3 shows an outline of the analytical approach along with some important considerations at each step. Three general features of this approach deserve attention.
First, steps 1 and 5 provide opportunities for analytical chemiststo collaborate with individuals outside the realm of analytical chemistry. In fact,many problems on which analytical chemists work originate in other fields. Second, the analytical approach is not linear, but incorporates a “feedback loop”consisting of steps 2, 3, and 4, in which the outcome of one step may cause areevaluation of the other two steps. Finally, the solution to one problem oftensuggests a new problem.Analytical chemistry begins with a problem, examples of which include evaluating the amount of dust and soil ingested by children as an indicator of environmental exposure to particulate based pollutants, resolving contradictory evidenceregarding the toxicity of perfluoro polymers during combustion, or developingrapid and sensitive detectors for chemical warfare agents.* At this point the analytical approach involves a collaboration between the analytical chemist and the individuals responsible for the problem.
Together they decide what information isneeded. It is also necessary for the analytical chemist to understand how the problem relates to broader research goals. The type of information needed and the problem’s context are essential to designing an appropriate experimental procedure.Designing an experimental procedure involves selecting an appropriate methodof analysis based on established criteria, such as accuracy, precision, sensitivity, anddetection limit; the urgency with which results are needed; the cost of a single analysis; the number of samples to be analyzed; and the amount of sample available for*These examples are taken from a series of articles, entitled the “Analytical Approach,” which has appeared as a regularfeature in the journal Analytical Chemistry since 1974.51400-CH01 9/9/99 2:20 PM Page 66Modern Analytical Chemistry1.
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