Analytical chemistry

Chemists use two main types of analytical tech­niques. Qualitative analysis reveals which elements or compounds are present in a substance or mixture of substances. Quantitative analysis determines how much of each component there is. Certain techniques, such as some forms of spec-trographic and chromato­graphic analysis, can make both determinations at the same time. These tech­niques tell the chemist both what is there and in what amounts.

Mixture of

unknown

substances

QUALITATIVE

Mixture of known substances

QUANTITATIVE

Analytical chemistry determines the properties of chemical substances, as well as the struc­ture and composition of compounds and mix­tures. Much of the science of modern chemis­try is built on results obtained from analyses. Many of chemistry's important uses, whether in helping to solve a murder case or discover­ing if a river is polluted, rely on analytical tech­niques.

The primary aim of analysis is to find out how a material is constituted. Thus, a chemist may analyze a mixture to find which com­pounds are present. Or, he may analyze a pure compound to find from which elements it is formed. All of the structures of chemical com­pounds illustrated in this book were originally worked out from various types of analysis.

The discovery of the different chemical ele­ments also depended largely on develop­ments in analytical techniques. For example, four elements (all relatively uncommon) derive their names from the Swedish town of Ytterby. Toward the end of the eighteenth century, the Finnish chemist Johan Gadolin identified what he thought was a new element. He had found it in a mineral (later called gadolinite) that he discovered near Ytterby. The element was named yttrium. It is one of a group of elements called the rare earth (lanthanides), which have very similar chemical properties. As analytical techniques improved during the nineteenth

century, it was shown that Gadolin's yttrium was a mixture of several elements. First, two other elements—erbium and terbium—were isolated from it. Then these two were found to be mixtures of several elements. One of these elements was subsequently called ytterbium, thus celebrating Ytterby for the fourth time.

Separation

Analytical techniques are methods for finding out something specific about an element or compound. They are based on differences in the chemical and physical properties of mate­rials. Sometimes, these differences are very small. This makes analytical work difficult and often time-consuming. An important set of an­alytical techniques are those that separate dif­ferent compounds or elements. It was refine­ments in such techniques that led, for example, to the isolation and identification of the full range of rare earth elements.

Very sophisticated separation techniques have been developed during the twentieth century. Often, as in the case of chromato­graphic methods, they have roots in nineteenth-century research. Chromatography is discussed later in this section. It is now pos­sible to separate very small amounts of com­plex molecules from one another. Much of the progress made in recent years in understand­ing the chemistry of biological processes has depended on the development of such tech­niques.

Detection

Analysis does not always depend on separa­tion, however. One of the triumphs of modern analytical chemistry has been the development of other techniques. These can detect very small quantities of a particular substance in a complex mixture. Such techniques may be qualitative or quantitative. They may reveal only what material is there (the "quality"). They may also be able to tell how much of it is pres­ent (the "quantity"). For example, it is possible to detect very low concentrations of particular pollutants in the atmosphere. In general terms, the atmosphere is a simple mixture of nitrogen and oxygen gases. But these make up only about 99 per cent of its total volume. The re­maining 1 per cent is a complex mixture that makes quantitative analysis of trace elements a demanding task. It can also be an essential one. The build-up of some pollutants could have a serious effect on world climate. This could make life as we know it more difficult than it is already.

These techniques, which have been devel­oped for measuring very small quantities of particular molecules, often depend on their physical rather than their chemical behavior. In general, this means how they behave in the presence of different types of radiation—infra­red, radio waves, or laser light, for example.

There is one problem facing chemists in re-

Date: 2015-12-11; view: 1249