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Chapter 1 anachem - chemistry
organic- bio chemistry (CHEM153L)
Xavier University - Ateneo de Cagayan
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2 Modern Analytical Chemistry
*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.
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- istry, but simply the application of chemical knowledge. 1 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 chemical phenomena. 2 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- eral factors complicate the quantitative analysis of Ni2+in ores, including the presence of a complex heterogeneous mixture of silicates and oxides, the low con- centration of Ni2+in ores, and the presence of other metals that may interfere in the analysis. Figure 1 is a schematic outline of one standard method in use dur- ing the late nineteenth century. 3 After dissolving a sample of the ore in a mixture of H 2 SO 4 and HNO 3 , trace metals that interfere with the analysis, such as Pb2+, Cu2+and Fe3+, 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.
%Ni =
mass point A – mass point B mass sample
× 100
Chapter 1 Introduction 3
Original Sample
PbSO 4 Sand
Basic ferric acetate
CuS
1:3 H 2 SO 4 /HNO 3 100 °C (8–10 h) dilute w/H 2 O, digest 2–4 h
Cu2+, Fe3+ Co2+, Ni2+
Fe3+, Co2+, Ni2+
Fe(OH) 3
CoS, NiS
CuS, PbS
Co(OH) 2 , Ni(OH) 2
CoO, NiO
cool, add NH 3 digest 50°–70°, 30 min
Co2+, Ni2+
Fe3+ Waste
Waste
Co2+, Ni2+
aqua regia heat, add HCl until strongly acidic bubble H 2 S (g)
Co2+ Waste
Solid Key Solution
H 2 O, HCl
heat add Na 2 CO 3 until alkaline NaOH
Ni2+ K 3 Co(NO 3 ) 5
neutralize w/ NH 3 Na 2 CO 3 , CH 3 COOH
slightly acidify w/ HCl heat, bubble H 2 S (g) HCl
heat
Co
as above
Co, Ni
heat, H 2 (g)
HNO 3 K 2 CO 3 , KNO 3 CH 3 COOH digest 24 h
dilute bubble H 2 S(g)
A
B
Figure 1. Analytical scheme outlined by Fresenius 3 for the gravimetric analysis of Ni in ores.
Chapter 1 Introduction 5
You will come across numerous examples of qualitative and quantitative meth- ods in this text, most of which are routine examples of chemical analysis. It is im- portant to remember, however, that nonroutine problems prompted analytical chemists to develop these methods. Whenever possible, we will try to place these methods in their appropriate historical context. In addition, examples of current re- search problems in analytical chemistry are scattered throughout the text. The next time you are in the library, look through a recent issue of an analyti- cally oriented journal, such as Analytical Chemistry. Focus on the titles and abstracts of 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 Perspective
Having noted that each field of chemistry brings a unique perspective to the study of chemistry, we now ask a second deceptively simple question. What is the “analyt- ical perspective”? Many analytical chemists describe this perspective as an analytical approach to solving problems. 7 Although there are probably as many descriptions of the analytical approach as there are analytical chemists, it is convenient for our purposes to treat it as a five-step process:
- 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 shows an outline of the analytical approach along with some im- portant considerations at each step. Three general features of this approach de- serve attention. First, steps 1 and 5 provide opportunities for analytical chemists to collaborate with individuals outside the realm of analytical chemistry. In fact, many problems on which analytical chemists work originate in other fields. Sec- ond, 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 a reevaluation of the other two steps. Finally, the solution to one problem often suggests a new problem. Analytical chemistry begins with a problem, examples of which include evalu- ating the amount of dust and soil ingested by children as an indicator of environ- mental exposure to particulate based pollutants, resolving contradictory evidence regarding the toxicity of perfluoro polymers during combustion, or developing rapid and sensitive detectors for chemical warfare agents.* At this point the analyti- cal approach involves a collaboration between the analytical chemist and the indi- viduals responsible for the problem. Together they decide what information is needed. It is also necessary for the analytical chemist to understand how the prob- lem relates to broader research goals. The type of information needed and the prob- lem’s context are essential to designing an appropriate experimental procedure. Designing an experimental procedure involves selecting an appropriate method of analysis based on established criteria, such as accuracy, precision, sensitivity, and detection limit; the urgency with which results are needed; the cost of a single analy- sis; 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 regular feature in the journal Analytical Chemistry since 1974.
Figure 1. Flow diagram for the analytical approach to solving problems; modified after Atkinson
analysis. Finding an appropriate balance between these parameters is frequently complicated by their interdependence. For example, improving the precision of an analysis may require a larger sample. Consideration is also given to collecting, stor- ing, and preparing samples, and to whether chemical or physical interferences will affect the analysis. Finally, a good experimental procedure may still yield useless in- formation if there is no method for validating the results. The most visible part of the analytical approach occurs in the laboratory. As part of the validation process, appropriate chemical or physical standards are used to calibrate any equipment being used and any solutions whose concentrations must be known. The selected samples are then analyzed and the raw data recorded. The raw data collected during the experiment are then analyzed. Frequently the data must be reduced or transformed to a more readily analyzable form. A statistical treatment of the data is used to evaluate the accuracy and precision of the analysis and to validate the procedure. These results are compared with the criteria estab- lished during the design of the experiment, and then the design is reconsidered, ad- ditional experimental trials are run, or a solution to the problem is proposed. When a solution is proposed, the results are subject to an external evaluation that may re- sult in a new problem and the beginning of a new analytical cycle.
6 Modern Analytical Chemistry
Identify the problem Determine type of information needed (qualitative, quantitative, characterization, or fundamental) Identify context of the problem
Design the experimental procedure Establish design criteria (accuracy, precision, scale of operation, sensitivity, selectivity, cost, speed)
Identify interferents
Select method
Establish validation criteria
Establish sampling strategy Feedback loop
- Conduct an experiment Calibrate instruments and equipment
Standardize reagents
Gather data
- Analyze the experimental data Reduce or transform data
Analyze statistics
Verify results
Interpret results
- Propose a solution Conduct external evaluation
8 Modern Analytical Chemistry
qualitative analysis An analysis in which we determine the identity of the constituent species in a sample.
Nd in samples. Unfortunately, mass spectrometry is not a selective technique. A mass spectrum provides information about the abundance of ions with a given mass. It cannot distinguish, however, between different ions with the same mass. Consequently, the choice of TIMS required developing a procedure for separating the tracer from the aerosol particulates. Validating the final experimental protocol was accomplished by running a model study in which 148 Nd was released into the atmosphere from a 100-MW coal utility boiler. Samples were collected at 13 locations, all of which were 20 km from the source. Experimental results were compared with predictions determined by the rate at which the tracer was released and the known dispersion of the emissions. Finally, the development of this procedure did not occur in a single, linear pass through the analytical approach. As research progressed, problems were encoun- tered and modifications made, representing a cycle through steps 2, 3, and 4 of the analytical approach. Others have pointed out, with justification, that the analytical approach out- lined here is not unique to analytical chemistry, but is common to any aspect of sci- ence involving analysis. 8 Here, again, it helps to distinguish between a chemical analysis and analytical chemistry. For other analytically oriented scientists, such as physical chemists and physical organic chemists, the primary emphasis is on the problem, with the results of an analysis supporting larger research goals involving fundamental studies of chemical or physical processes. The essence of analytical chemistry, however, is in the second, third, and fourth steps of the analytical ap- proach. Besides supporting broader research goals by developing and validating an- alytical methods, these methods also define the type and quality of information available to other research scientists. In some cases, the success of an analytical method may even suggest new research problems.
1C Common Analytical Problems
In Section 1A we indicated that analytical chemistry is more than a collection of qualitative and quantitative methods of analysis. Nevertheless, many problems on which analytical chemists work ultimately involve either a qualitative or quantita- tive measurement. Other problems may involve characterizing a sample’s chemical or physical properties. Finally, many analytical chemists engage in fundamental studies of analytical methods. In this section we briefly discuss each of these four areas of analysis. Many problems in analytical chemistry begin with the need to identify what is present in a sample. This is the scope of a qualitative analysis, examples of which include identifying the products of a chemical reaction, screening an athlete’s urine for the presence of a performance-enhancing drug, or determining the spatial dis- tribution of Pb on the surface of an airborne particulate. Much of the early work in analytical chemistry involved the development of simple chemical tests to identify the presence of inorganic ions and organic functional groups. The classical labora- tory courses in inorganic and organic qualitative analysis, 9 still taught at some schools, are based on this work. Currently, most qualitative analyses use methods such as infrared spectroscopy, nuclear magnetic resonance, and mass spectrometry. These qualitative applications of identifying organic and inorganic compounds are covered adequately elsewhere in the undergraduate curriculum and, so, will receive no further consideration in this text.
Perhaps the most common type of problem encountered in the analytical lab is a quantitative analysis. Examples of typical quantitative analyses include the ele- mental analysis of a newly synthesized compound, measuring the concentration of glucose in blood, or determining the difference between the bulk and surface con- centrations of Cr in steel. Much of the analytical work in clinical, pharmaceutical, environmental, and industrial labs involves developing new methods for determin- ing the concentration of targeted species in complex samples. Most of the examples in this text come from the area of quantitative analysis. Another important area of analytical chemistry, which receives some attention in this text, is the development of new methods for characterizing physical and chemical properties. Determinations of chemical structure, equilibrium constants, particle size, and surface structure are examples of a characterization analysis. The purpose of a qualitative, quantitative, and characterization analysis is to solve a problem associated with a sample. A fundamental analysis, on the other hand, is directed toward improving the experimental methods used in the other areas of analytical chemistry. Extending and improving the theory on which a method is based, studying a method’s limitations, and designing new and modify- ing old methods are examples of fundamental studies in analytical chemistry.
Chapter 1 Introduction 9
characterization analysis An analysis in which we evaluate a sample’s chemical or physical properties.
fundamental analysis An analysis whose purpose is to improve an analytical method’s capabilities.
quantitative analysis An analysis in which we determine how much of a constituent species is present in a sample.
1D KEY TERMS
characterization analysis ( p. 9 ) fundamental analysis ( p. 9 )
qualitative analysis ( p. 8 ) quantitative analysis ( p. 9 )
Analytical chemists work to improve the ability of all chemists to make meaningful measurements. Chemists working in medicinal chemistry, clinical chemistry, forensic chemistry, and environ- mental chemistry, as well as the more traditional areas of chem- istry, need better tools for analyzing materials. The need to work with smaller quantities of material, with more complex materi- als, with processes occurring on shorter time scales, and with species present at lower concentrations challenges analytical
chemists to improve existing analytical methods and to develop new analytical techniques. Typical problems on which analytical chemists work include qualitative analyses (what is present?), quantitative analyses (how much is present?), characterization analyses (what are the material’s chemical and physical properties?), and funda- mental analyses (how does this method work and how can it be improved?).
1E SUMMARY
1. For each of the following problems indicate whether its solution requires a qualitative, quantitative, characterization, or fundamental study. More than one type of analysis may be appropriate for some problems. a. A hazardous-waste disposal site is believed to be leaking contaminants into the local groundwater. b. An art museum is concerned that a recent acquisition is a forgery. c. A more reliable method is needed by airport security for detecting the presence of explosive materials in luggage.
d. The structure of a newly discovered virus needs to be determined. e. A new visual indicator is needed for an acid–base titration. f. A new law requires a method for evaluating whether automobiles are emitting too much carbon monoxide. 2. Read a recent article from the column “Analytical Approach,” published in Analytical Chemistry, or an article assigned by your instructor, and write an essay summarizing the nature of the problem and how it was solved. As a guide, refer back to Figure 1 for one model of the analytical approach.
1F PROBLEMS
Chapter 1 anachem - chemistry
Course: organic- bio chemistry (CHEM153L)
University: Xavier University - Ateneo de Cagayan
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