VIEWS: 210 PAGES: 12 POSTED ON: 11/14/2009
Analytical Chemistry Chapter 1 Analytical chemistry is a school of science consisting of a set of powerful ideas and methods that are useful in all fields of science and medicine. “The world was captivated by the Pathfinder mission. As a result, the numerous World Wide Web sites tracking the mission were nearly overwhelmed by millions of Internet surfers who closely monitored the progress of tiny Sojourner in its quest for information on the na-ture of the Red Planet” (Skoogs and West). The key experiment on Sojourner was the APXS, or alpha proton X-ray spectrometer, which combines the three advanced instrumen-tal techniques of Rutherford backscattering spectroscopy, proton emission spec-troscopy, and X-ray fluorescence. 1A THE ROLE OF ANALYTICAL CHEMISTRY 1B CLASSIFYING QUANTITATIVE ANALYTICAL METHODS 1C STEPPING THROUGH A TYPICAL QUANTITATIVE ANALYSIS C-1 Picking a Method C-2 Acquiring the Sample C-3 Processing the Sample C-4 Eliminating Interferences C-5 Calibration and Measurement C-6 Calculating Results C-7 Evaluating Results by Estimating Their Reliability ID. AN INTEGRAL ROLE FOR CHEMICAL ANALYSIS: FEEDBACK CONTROL SYSTEMS D-1 Eliminating Interferences D-2 Calculating the Concentration D-3 Measuring the Amount Of The Analyte The Role of Analytical Chemistry “Analytical chemistry plays a vital role in the development of science. In 1894, Friedrich Wilhelm Ostwald wrote: Analytical chemistry, or the art of recognizing different substances and determin-ing their constituents, takes a prominent position among the applications of sci-ence, since the questions which it enables us to answer arise wherever chemical processes are employed for scientific or technical purposes. Its supreme impor-tance has caused it to be assiduously cultivated from a very early period in the his-tory of chemistry, and its records comprise a large part of the quantitative work which is spread over the whole domain of science” (Skoog andwest) Analytical chemistry has evolved from an art of court magicians to alchemist’s into a science with applications throughout industry, medicine, and all the sciences. Examples Of uses •The concentrations of oxygen and of carbon dioxide are determined in millions of blood samples every day and used to diag-nose and treat illnesses. •Smog-control is done by the measurement of quantities of hydrocarbons, nitrogen oxides, and carbon monoxide in automobile exhaust. •Analytical chemistry helps diagnose parathyroid diseases in humans measurements of ionized cal-cium in blood serum. •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 chromium to achieve a desired strength, hardness, corrosion resistance, and ductility. Classifying Quantitative Analytical Methods Analytical Chemists compute the results of a typical quantitative analysis from two measurments. One is the mass or the volume of sample to be analyzed. The second is the measurement 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 measurement usually completes the analysis, and we classify analytical methods. according to the nature of this final measurement. •Gravimetric methods determine the mass of the analyte or some compound chemically related to it. •Volumetric method, the volume of a solution containing sufficient reagent to re-act completely with the analyte is measured. •Electroanalytical methods involve the measurement of such electrical properties as voltage, current, resistance, and quantity of electrical charge. •Spectroscopic methods are based on measurement of the interaction between electromagnetic radiation and analyte atoms or mole-cules or on the production of such radiation by analytes. Finally, there is a group of miscellaneous methods that includes the measurement of such quantities as mass-to-charge ratio of molecules by mass spectrometry, rate of radioactive de-cay, heat of reaction, rate of reaction, sample thermal conductivity, optical activ-ity, and refractive index. Stepping through a typical Quantitative anslysis Figure Flow diagram showing the steps in a quantitative analysis. There are a number of pos-sible paths through the steps in a quantitative analysis. In the simplest example represented by the central vertical pathway, we select a method, acquire and process the sample, dissolve the sample in a suitable solvent, measure a property of the analyte, calculate the results, and estimate the reliability of the results. Depending on the complexity of the sample and the chosen method, various other pathways may be necessary. (Skoogs, West) A typical quantitative analysis involves the sequence of steps shown in the flow diagram. 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. The first 23 chapters of this book focus on the last three steps in Figure 1-2. In the measurement step, we measure one of the physical properties men-tioned in Section IB. 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 re-sults and estimate their reliability. We then present a case study to illustrate these steps in solving an important and practical analytical problem. The details of the case study foreshadow many of the methods and ideas you will explore as you study analytical chemistry. Section C Picking a Method The essential first step in any quantitative analysis is the selection of a method as depicted in. The choice is sometimes difficult and requires experience as well as intuition. One of the first questions to be considered in the selection process is the level of accuracy required. Unfortunately, high reliability nearly always requires a large investment of time. The selected method usually repre-sents a compromise between the accuracy needed and the time and money that are available for the analysis. A second consideration related to economic factors is the number of samples to be analyzed. If there are many samples, we can afford to spend a good deal of time in preliminary operations such as assembling and calibrating instruments and equipment and preparing standard solutions. If we have only a single sample or 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 sample always influence the choice of method to some degree. Section C Acquiring the Sample This step is a quantitative analysis it is done to acquire the sample • To produce meaningful information, an analysis must be performed on a sample whose composition represents the bulk of material from which it was taken. • Where the bulk is large and heterogeneous, great effort is required to get a representative sample. • Sample problem below. •A railroad car containing 25 tons of silver ore. Buyer and seller must agree on a price, which will he based primarily on the silver content of the shipment . •The ore itself is in-herently heterogeneous, consisting of many lumps that vary in size as well as in silver content. Explanation of procedure •The assay of this shipment will be performed on a sample that weighs about one gram. For the analysis to have significance, this small sample must have a composition that is representative of the 25 tons (or approximately 22.700,000 g) of ore in the shipment. •Isolation of one gram of material that accurately represents the average composition of the nearly 23,000,000 g of bulk sample is a difficult undertaking that requires a careful, systematic manipu-lation of the entire shipment. Sampling involves obtaining a small mass of a ma-terial whose composition accurately represents the bulk of the material being sampled. (Skoogs, West) Processing the Sample Section C The third step in an analysis is to process the sample as shown in Figure. Under certain circumstances, no sample processing is required prior to the mea-surement 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 any of a variety of different ways. The first step in processing the sample is often the preparation of a laboratory sample. Preparing a Laboratory Sample Solid samples •A solid laboratory sample is ground to decrease particle size, mixed to ensure homogeneity, and stored for various lengths of time before analysis begins. •(dry samples just before starting an analysis, to avoid absorption or loss of water) Liquid sample •Liquid samples present a slightly different but related set of problems during the preparation step. •(Liquid sample container must be kept inside a second sealed container, loss of liquid through evaporations change the concentration of the analyte .) Defining Replicate Samples•We perform most chemical analyses on replicate samples whose masses or vol-umes have been determined by careful measurements with an analytical balance or with a precise volumetric device. •Replication improves the quality of the re-sults and provides a measure of their reliability. Quantitative measurements on replicates are usually averaged, and various statistical tests are performed on the results to establish their reliability. Processing the Sample Preparing Solutions: Physical and Chemical Changes •Most analyses are performed on solutions of the sample made with a suitable solvent. •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 or is minimized. •Unfortunately, many materials that must be analyzed are insoluble in common solvents. • Examples include silicate minerals, high-molecular-weight polymers, and specimens of animal tissue. •Under this circumstance, we must follow the flow diagram to the box on the right and carry out some rather harsh chemistry. •Conversion of the analyte in such materials into a soluble form is often the most difficult and time-consuming task in the analytical process. •The sample may require heating with aqueous solutions of strong acids, strong bases, oxidiz-ing agents, reducing agents, or some combination of such reagents. •It may be necessary to ignite the sample in air or oxygen or 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 solution has a property that is pro-portional to analyte concentration and that we can measure. •For example, in the determination of manganese in steel, manganese must be oxidized to MnO before the absorbance of the colored solution is measured. •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 measurements as illustrated in the flow diagram. Processing the Sample Eliminating Interferences •Once we have gotten the sample into solution and converted the analyte to an appropriate form for the measurement step, the next step is to eliminate substances from the sample that may interfere with the measurement step. •Few chemical or physical properties of importance in chemical analysis are unique to a single chemical species. •Instead, the reactions used and the properties measured are characteristic of a group of elements or compounds. •Species other than the analyte that affect the final measurement are called interferences, or interferents. •A scheme must be devised to isolate the analytes from interferences before the final measurement is made. •There are no stead fast rules can be given for eliminating interferences; indeed, resolution of this problem can be the most demanding aspect of an analysis. Calibration and Measurement •All analytical results depend on a final measurement A' of a physical or chemical property of the analyte. •This property must vary in a known and reproducible way with the concentration ca of the analyte. •Ideally, the measurement of the property is directly proportional to the concentration. •That is, where k is a proportionality constant. With two exceptions, analytical methods require the empirical determination of k with chemical standards for which ca is known. The process of determining k is thus an important step in most analyses; this step is called a calibration. Calculating Results •Computing analyte concentrations from experimental data is usually relatively easy, particularly with modern calculators or computers. •These computations are based on the raw experimental data collected in the measurement step, the characteristics of the measurement instruments, and the stoichiometry of the analyti-cal reaction. Samples of these calculations appear throughout this book. Evaluating Results by Estimating Their Reliability •Analytical results are incomplete without an estimate of their reliability. •The experimenter must provide some measure of the uncertainties associated with computed results if the data are to have any value. An Integral Role For Chemical Analysis: Feedback Control System Analytical chemistry is usually not an end in itself, but is part of a bigger picture in which we may use analytical results to help control a patient's health, to control the amount of mercury in fish, to control the quality of a product.. Chemical analysis is the measurement elements 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. Patients suffering from insulin-dependent diabetes mellitus develop hyperglycemia, which manifests itself in a blood glucose concentration above the normal concentration of 60 to 95 mg/dL. We begin our example by determining that the desired state is a blood glucose level below 95 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. Process of Analysis •The first step in the monitoring process is to determine the actual state by collecting a blood sample from the patient and measuring the blood glucose level. •If the measured blood glucose level is above 95 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 blood glucose level, is maintained at or below the critical threshold, which keeps the metabolism of the patient in control. I The process of continuous measurement and control is often referred to as a feedback system, and the cycle of measurement, comparison, and control is called a feedback loop. These ideas find wide application in biological and bio-medical systems, mechanical systems, and electronics. From the measurement and control of the concentration of manganese in steel to maintaining the proper level of chlorine in a swimming pool, chemical analysis plays a central role in a broad range of systems. An Integral Role For Chemical Analysis: Feedback Control System ELIMINATING INTERFERENCES Arsenic can be separated from other substances that might interfere in the analysis by converting it to arsine, AsH3, a toxic, colorless gas that is evolved when a solution of H3AsO3 is treated with zinc. The solutions resulting from the deer and grass samples were combined with Sn-+, and a small amount of iodide ion was added to catalyze the reduction of H3AsO4 to H3AsO3 according to the following reaction: H3AsO4 + SnCl2 + 2HC1 H2AsO3 + SnCl4 + H2O The H3AsO3 was then converted to AsH3 by the addition of zinc metal as follows: H3AsO3 + 3Zn + 6HC1 AsH3 (g) + 3ZnCl2 + 3H2O The entire reaction was carried out in flasks equipped with a stopper and delivery tube so that the arsine could be collected in the absorber. The arrangement ensured that interferences were left in the reaction flask and that only arsine was collected in the absorber in special transparent containers called cuvettes. Arsine bubbled into the solution in the cuvette, reacted with silver diethyldithiocarbamate to form a colored complex compound according to the following equation: An Integral Role For Chemical Analysis: Feedback Control System MEASURING THE AMOUNT OF THE ANALYTE •The amount of arsenic in each sample was determined by measuring the intensity of the red color formed in the cuvettes with an instrument called a spectrophotometer. •Spectrophotometer provides a number called absorbance that is directly proportional to the color intensity, which is also proportional to the concentra-tion of the species responsible for the color. •To use absorbance for analytical purposes, a calibration curve must be generated by measuring the absorbance of several solu-tions that contain known concentrations of analyte. The color becomes more intense as the arsenic content of the standards increases from 0 to 25 parts per million (ppm). CALCULATING THE CONCENTRATION •The absorbance for the standard solutions containing known concentrations of arsenic are plotted to produce a calibration curve. •The color intensity of each solution is represented by its absorbance, which is plotted on the vertical axis of the calibration curve. Note that the absorbance increases from 0 to about 0.72 as the concentration of arsenic increases from 0 to 25 parts per million. •The concentration of arsenic in each standard solution corresponds to the vertical grid lines of the calibration curve as shown. •This curve is then used to determine the concentration of the two unknown solutions shown on the right. We first find the absorbance of the unknowns on the absorbance axis of the plot and then read the corresponding concentrations on the concentration axis. •The lines leading from the cuvettes to the calibration curve show that the concentrations of arsenic in the two deer were 16 ppm and 22 ppm, respectively. •Arsenic in kidney tissue of an animal is toxic at levels above about 10 ppm, so it was probable that the deer were killed by ingesting an arsenic compound. •The tests also showed that the samples of grass contained about 600 ppm arsenic. This very high level of arsenic suggested that the grass had been sprayed with an arsenical herbicide. The investigators concluded that the deer had probably died as a result of eating the poisoned grass.
Pages to are hidden for
"Analytical Chemistry Analytical Chemistry Chapter"Please download to view full document