Biologia e química

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Selected Methods of Analysis
Chemistry is primarily an experimental science. This chapter presents a variety of laboratory experiments, from classical titrations and gravimetry to instrumental methods such as chromatography and spectroscopy. Detailed directions are given for each experiment.

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his chapter contains detailed directions for performing avariety of chemical analyses. The methods have been chosen to introduce you to analytical techniques that are widely used by chemists. For most of these analyses, the composition of the samples is known to the instructor. Thus, you will be able to judge how well you are mastering these techniques. Your chances of success in the laboratory will greatly improve if you take time before you enter thelaboratory to read carefully and understand each step in the method and to develop a plan for how and when you will perform each step. The discussion in this section is aimed at helping you develop efficient work habits in the laboratory and also at providing you with some general information about an analytical chemistry laboratory. Before you start an analysis, you should understand thesignificance of each step in the procedure to avoid the pitfalls and potential sources of error that are inherent in all analytical methods. Information about these steps can usually be found in (1) preliminary discussion sections, (2) earlier chapters that are referred to in the discussion section, and (3) the “Notes” that follow many of the procedures. If, after reading these materials, you still do notunderstand the reason for doing one or more of the steps in the method, consult your instructor before you begin laboratory work. The Accuracy of Measurements In looking over an analytical procedure, you should decide which measurements must be made with maximum precision, and thus with maximum care, as opposed to those that can be carried out rapidly with little concern for precision. Generally,measurements that appear in the equation used to compute the results must be performed with maximum precision. The remaining measurements can and should be made less carefully to conserve time. The words about and approximately are frequently used to indicate that a measurement does not have to be done carefully. For example, you should not waste time and effort to measure a volume to 0.02 mL whenan uncertainty of 0.5 mL or even 5 mL will have no discernible effect on the results. In some procedures, a statement such as “weigh three 0.5-g samples to the nearest 0.1 mg” is encountered. Here, samples of perhaps 0.4 to 0.6 g are acceptable, but their masses must be known to the nearest 0.1 mg. The number of significant

37A An Introductory Experiment


figures in the specificationof a volume or a mass is also a guide to the care that should be taken in making a measurement. For example, the statement “add 10.00 mL of a solution to the beaker” indicates that you should measure the volume carefully with a buret or a pipet, with the aim of limiting the uncertainty to perhaps 0.02 mL. In contrast, if the directions read “add 10 mL,” the measurement can be made with agraduated cylinder. Time Utilization You should study carefully the time requirements of the several unit operations involved in an analysis before work is started. This study will reveal operations that require considerable elapsed, or clock, time but little or no operator time. Examples of such operations include drying a sample in an oven, cooling a sample in a desiccator, or evaporating liquid on ahot plate. Efficient workers use such periods to perform other operations or perhaps to begin a new analysis. Some people find it worthwhile to prepare a written time schedule for each laboratory period to avoid dead time. Time planning is also needed to identify places where an analysis can be interrupted for overnight or longer, as well as those operations that must be completed without a break....
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