Methods of separation

are important in noninstru-mental chemical analysis. Solvent extraction methods are often used in organic analysis. Components in a mixture of substances are dissolved out of solution. This is done selectively in turn by different solvents. Liquids of different boiling points can be separated by distillation. The mixture is first heated. The component liquids are then distilled off one at a time near their boil­ing points.

cent years—the ability to measure very small amounts of different substances has out­stripped the ability to understand their signifi­cance. Thus, it is now possible to measure the presence of almost unbelievably small con­centrations of certain impurities in food. What is not understood, in many cases, is whether or not there is a threshold level. Below such a level, an impurity would exert no harmful ef­fect. Its presence would therefore be of no sig­nificance. This, however, is not really a prob­lem for the analytical chemist, whose basic task is to find ways of obtaining the informa­tion.

Three types of analysis

An analytical chemist can perform one of three types of analysis on a given substance. Qualita­tive analysis identifies the various types of ele­ments and compounds that are in a substance. Quantitative analysis measures the amounts of the different chemicals in the substance being analyzed. Radiochemistry involves the identifi­cation and production of radioactive elements and their use in the study of chemical proc­esses.

The modelof the double helix structure of a mole­cule of DNA (deoxyribonu­cleic acid) was "drawn" by a computer. Laboratory com­puters have become an es­sential part of modern in­strumental analysis. They can store and compare data or present results as print­outs, displays on a screen, or colored computer graph­ics.

Routine testingfor quality control, particularly in the petrochemical and pharma­ceutical industries, often employs the methods of classical analysis. The test­ing involves modern appa­ratus and laboratory facili­ties such as this enclosed cabinet, which can be used for hazardous materials.

Classical analysis

There have been considerable advances in re­cent years in instrumental methods of chemi­cal analysis. However, many analyses are still carried out by what are termed "classical methods." These employ techniques and pro­cedures that have proved to be reliable and reproducible over many years. Such methods are of particular value for use in laboratories that lack the more advanced instruments. Clas­sical methods fall into two main groups: quali­tative analysis is used to find out what is pres­ent in a given substance; quantitative analysis deals with the procedures for determining how much of a substance is present in that substance.

Date: 2015-12-11; view: 1887