Analytical Chemistry for Technicians

John Kenkel

• Éditeur: CRC Press
• Année: 2013
• ISBN:
  9781482225266
• Édition: 4
• Sujet: Naturwissenschaften
• Sous-sujet: Chemie

Written as a training manual for chemistry-based laboratory technicians, this thoroughly updated fourth edition of the bestselling Analytical Chemistry for Technicians emphasizes the applied aspects rather than the theoretical ones. The book begins with classical quantitative analysis and follows with a practical approach to the complex world of so

1 Introduction to Analytical Science

1.1 Analytical Science Defined

Imagine yourself strolling down the aisle in your local grocery store to select your favorite foods for your lunch. You pick up a jar of peanut butter, look at the label, and read that there are 190 mg of sodium in one serving. You think to yourself: “I wish I knew how they knew that for sure.” After picking up the lunch items you want, you proceed to the personal hygiene aisle to look for your toothpaste. Again you look at the label and notice that the fluoride content is 0.15% w/v. “How do they know that?,” you again say to yourself. Finally, you stop by the pharmaceutical shelves and pick up a bottle of your favorite vitamin. Looking at the label, you see that there are 1.7 mg of riboflavin in every tablet and marvel how the manufacturer can know that that is really the case.

There is a seemingly endless list of examples of scenarios like the above that one can think of without even leaving the grocery store! We could also visit a hardware store and look at the labels of cleaning fluids, adhesives, paint or varnish formulations, paint removers, garden fertilizers, and insecticides and make similar statements. Although you may question how the manufacturers of these products know precisely the content of their products in such a quantitative way, you yourself may have undertaken exactly that kind of work at some point in your life right in your own home. If you have an aquarium, you may have come to know that it is important to not let the ammonia level in the tank get too high and may have purchased a kit to allow you to monitor the ammonia level. Or, you may have purchased a water test kit to determine the pH, the hardness, or even the nitrate concentration in the water that comes from your tap. You may have a soil test kit to determine the nitrate, phosphate, and potassium levels of the soil in your garden. Then you think: “Gee, it’s actually pretty easy.” But when you sit down and read the paper or watch the evening news, you are baffled again by how a forensic scientist determines that a criminal’s DNA was present on a murder weapon, or how someone determined the ammonia content in the atmosphere of the planet Jupiter without even being there, or how it can be possible to determine the ozone level high above the North Pole.

The science that deals with the identification and/or quantification of the components of material systems such as these is called analytical science. It is called that because the process of determining the level of any or all components in a material system is called analysis. It can involve both physical and chemical processes. If it involves chemical processes, it is called chemical analysis or, more broadly, analytical chemistry. The sodium in the peanut butter, the nitrate in the water, and the ozone in the air in the above scenarios are the substances that are the objects of analysis. The word for such substances is analyte, and the word for the material in which the analyte is found is called the matrix of the analyte.

Another word often used in a similar context is the word “assay.” If a material is known by a particular name and an analysis is carried out to determine the level of that named substance in the material, the analysis is called an assay for that named substance. For example, if an analysis is being carried out to determine what percent of the material in a bottle labeled “aspirin” is aspirin, the analysis is called an assay for aspirin. In contrast, an analysis of the aspirin would imply the determination of other minor ingredients as well as the aspirin itself.

The purpose of this book is to discuss in a systematic way the techniques, the methods, the equipment, and the processes of this important, all-encompassing science.

1.2 Classifications of Analysis

Analytical procedures can be classified in two ways: first, in terms of the goal of the analysis, and second, in terms of the nature of the method used. In terms of the goal of the analysis, classification can be based on whether the analysis is “qualitative” or “quantitative.” Qualitative analysis is identification. In other words, it is an analysis carried out to determine only the identity of a pure analyte, the identity of an analyte in a matrix, or the identity of several or all components of a mixture. Stated another way, it is an analysis to determine what a material is or what the components of a mixture are. Such an analysis does not report the amount of the substance. If a chemical analysis is carried out and it is reported that there is mercury present in the water in a lake and the quantity of the mercury is not reported, then the analysis was a qualitative analysis. Quantitative analysis, on the other hand, is the analysis of a material for how much of one or more components is present. Such an analysis is undertaken when the identity of the components is already known and when it is important to also know the quantities of these components. It is the determination of the quantities of one or more components present per some quantity of the matrix. For example, the analysis of the soil in your garden that reports the potassium level as 342 parts per million would be classified as a quantitative analysis. The major emphasis of this text is on quantitative analysis, although some qualitative applications will be discussed for some techniques (see also Application Note 1.1).

Analysis procedures can be additionally classified into procedures that involve physical properties, wet chemical analysis procedures, and instrumental chemical analysis procedures. Analysis using physical properties involves no chemical reactions and at times relatively simple devices (although possibly computerized) to facilitate the measurement. Physical properties are especially useful for identification, but may also be useful for quantitative analysis in cases where the value of a property, such as specific gravity or refractive index (Chapter 7), varies with the quantity of an analyte in a mixture.

Wet chemical analysis usually involves chemical reactions and/or classical reaction stoichiometry, but no electronic instrumentation beyond a weighing device. Wet chemical analysis techniques are classical techniques, meaning they have been in use in analytical laboratory for many years, before electronic devices came on the scene. If executed properly, they have a high degree of inherent accuracy and precision, but they take more time to execute.

Instrumental analysis can also involve chemical reactions, but always involves modern sophisticated electronic instrumentation. Instrumental analysis techniques are “high-technology” techniques, often utilizing the ultimate in complex hardware and software. Although sometimes not as precise as a carefully executed wet chemical method, instrumental analysis methods are fast and can offer a much greater scope and practicality to the analysis. In addition, instrumental methods are generally used to determine the minor cons...