Quantitative Analysis

Quantitative analysis

Quantitative analysis is a chemical analysis performed to find the amount of each component present in a material. It is done by either a classical or instrumental procedure.

A quantitative investigation means that the amount (quantity) or relative amount of each component present is determined. In a pure substance, the entire mass, or 100%, is composed of a single component. In materials composed of two or more substances, a quantitative investigation would determine the mass or relative mass present for each component within the sample. It is not always necessary to find quantitative values for all components that make up a substance. In most cases it is sufficient to analyze the material for one or perhaps more components of interest. The amount of active medicine within an antacid tablet, for example, is significant, whereas the fillers, binders, colorants, and flavoring agents present are of lesser importance.

A quantitative analysis involves more than simply measuring the amount of a component present in a sample

The sample must first be prepared for measurement, usually by placing it in solution if it is not already in soluble form. With complex substances a preliminary separation of the desired component is often necessary to prevent other substances present from interfering with the selected analytical method.

An analyst is one who measures the components of a material quantitatively as a percent or amount present in a sample. Analysts are employed by manufacturing industries to test the reliability of their products. If an automobile manufacturer, for example, specifies that the iron content of the steel used in an automobile is of a certain percentage, then this value must be checked constantly by the manufacturer to see that the automobile meets specifications. This repeated checking is known as quality control and manufacturing facilities have a quality control department employing analytical chemists. Hospitals, too, employ analytical chemists to test patients for proper amounts of medication. Athletes are subjected to quantitative testing to determine the presence and amount of possible illicit drugs in their bodies. The federal government carries out frequent quantitative measurements of environmental samples. Should, for example, a company generate greater amounts of a pollutant than is allowed by law, then the government can fine the company or force it to close until it meets government regulations. Legislators at the local, state and national level use quantitative results to formulate laws that prevent the general public from coming into contact with dangerous amounts of harmful chemicals in food, medicine, the environment, and other areas.

Various methods are employed to undertake a quantitative investigation. These methods are broadly classified as classical and instrumental methods.

Classical methods

Classical methods, employed since the beginning of modern chemistry in the nineteenth century, use balances and calibrated glass containers to directly measure the amounts of chemicals combined with an unknown substance. A classical gravimetric analysis utilizes an appropriate chemical reagent to combine with the analyte in a sample solution to form an insoluble substance, a precipitate.

Instrumental methods

The presence of many chemical substances can often be found by their response to some external signal. The magnitude of this response is proportional to the amount of substance present. Because electronic equipment is often necessary to generate the external signal and/or to detect the chemical response, these methods of quantitative analysis are called instrumental methods. Instrumental methods are indirect, so the detecting instrument requires calibration to measure the response initially from a sample with a known concentration of analyte. This is necessary to relate the response, which is often electrical, to the quantity of chemical substance. Standard solutions, containing known amounts of analyte, are first studied to calibrate the measuring instrument.

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