Quantitative Analysis

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Quantitative Analysis

Quantitative analysis is a form of analysis performed in a variety of scientific disciplines to find the amount of each component present in a material, or to numerically define a system being examined. Incontrast to qualitative analysis, it is not subjective. Rather, experimental findings provide actual numbers.

A quantitative investigation means that the amount (quantity) or relative amount of each component present is determined. In a pure substance, the entire mass is composed of a single component. In materials composed of two or more substances, a quantitative investigation would determine the mass or

Table 1. Instrumental Techniques of Quantitative Analysis . (Thomson Gale.)
Instrumental techniques
Method	Response
Potentiometry	Many chemical reactions produce electric energy, a battery for example. The amount of chemical to produce a measured potential is calculated.
Coulometry	The amount of electrical current and the duration over which it flows is a measure of the amount of chemical substance producing the current.
Conductimetry	The number of charged chemical components in a solution determine the resistance or conductance of a solution to the passage of electrical current.
Voltammetry	The magnitude of electric potential necessary to cause the breakdown of a chemical substance and the current resulting from that breakdown are related to the amount of chemical present.
Ultraviolet, visible, infrared, and x-ray spectometry	The extent to which these rays are absorbed by a sample depends upon the amount of sample present.
Thermogravimetry	The loss in weight of a substance as it decomposes upon heating is proportional to the amount of substance initially present.
Nuclear magnetic resonance	For chemicals showing magnetic properties the strength of the magnetism is related to the amount of substance present.
Nuclear activation analysis	The amount of radioactivity produced by a substance is proportional to the amount of material emitting radiation.
Mass spectrometry	The intensity of each component fraction present as a chemical is broken apart relates to the amount initially present.

relative mass present for each component within the sample, or for the compounds of most interest.

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. The precipitate is filtered, washed, dried and weighed. From the weight of the precipitate and sample and from the known chemical composition of the precipitate, the analyst calculates the percent of analyte in the sample.

A classical titrimetric, or volumetric, analysis uses titration, a procedure in which a solution of exactly known concentration reacts with the analyte in a sample solution. A chemical solution of known concentration, the titrant, is placed in a buret, a long calibrated tube with a valve at one end capable of dispensing variable known volumes of liquid. An indicator solution, a colored dye, is added to the unknown sample. Titrant is then delivered slowly from the buret. The indicator dye is chosen so that a color change occurs when exactly the proper amount of titrant to combine with the unknown has been added. This amount is called the equivalent point volume. From the strength of the titrant solution, the equivalent point volume, and the volume of unknown sample in the titration flask, the amount or percent of an analyte can be calculated.

KEY TERMS

Analyte— The component within a sample that is to be measured.

Classical analysis— Those procedures in which the desired component is reacted with a suitablechemical reagent, either by precipitate formation or titration.

Gravimetric analysis— A classical quantitative technique in which an added chemical forms an insoluble precipitate with the desired component. The precipitate is collected and weighed.

Instrumental analysis— A modern quantitative technique in which some property of the desired component (electrical, optical, thermal, etc.) is measured and related to the amount present.

Titrimetric analysis— A classical quantitative technique in which a solution of known concentration is reacted exactly with the desired component and a calculation preformed to find the amount present.

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. The type of instrumental method used for quantitative analysis varies with the nature of the substance being analyzed and with the amount of the target compound thought to be present.

Quantitative analysis is also a fundamentally important part of the biological and microbiological sciences. For example, determining the number of bacteria growing on solid nutrient or charting the speed at which a bacteria can move through a solution are quantitative measurments.