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. 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.

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.

The type of instrumental method used for quantitative analysis varies with the nature of the substance being analyzed and with the amount of analyte thought to be present. While classical analytical methods are suitable for major amounts of analyte present in a sample, 1% or greater, instrumental methods are generally employed for amounts of analyte which may be less than 1% of the sample's total mass. Modern instrumental techniques are capable of analyzing the presence of a component which can comprise 0.0001% or less of its mass.

Table 1 names the more common instrumental techniques used for quantitative analysis and the type of signal they invoke from a chemical system.

A thorough understanding of chemistry is necessary in selecting the proper method for the quantitative determination of a substance. Lastly, the necessary calculations to convert the data obtained into its desired form must be carried out. Computer programs have helped considerably with this last step.

See also Nuclear magnetic resonance; Spectroscopy.

Resources

books

Harris, Daniel C. Quantitative Chemical Analysis. 4th ed. New York: W.H. Freeman & Company, 1995.

Skoog, Douglas A., and James J. Leary. Principles of Instrumental Analysis. 4th ed. Philadelphia: Saunders College Publishing, 1992.

Gordon A. Parker

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 suitable chemical 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.

Quantitative Analysis

Resources

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

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.

Resources

BOOKS

Berk, Kenneth N. and Patrick Carey. Data Analysis with Microsoft Excel: Updated for Office XP. Belmont, CA: Duxbury Press, 2003.

Quinn, Gerry P. and Michael J. Keough. Experimental Design and Data Analysis for Biologists. Cambridge: Cambridge University Press, 2002.

Rice, John A. Mathematical Statistics and Data Analysis. Belmont, CA: Duxbury Press, 2006.

Gordon A. Parker

Quantitative analysis

The term quantitative analysis is used to described any procedure by which the percentage composition of any compound or mixture is determined. For example, chemists might want to know the exact composition of some new compound that has been discovered. Or they might want to find out what the percentage of gold is in a new ore that has been discovered. Both of these questions can be answered by the procedures of quantitative analysis. Quantitative techniques can be divided into two general categories: wet or classical techniques and instrumental methods.

Classical methods

Classical methods have been used since the beginning of modern chemistry in the nineteenth century. They generally make use of balances and calibrated glass containers to measure the percentage composition of a compound or mixture. For example, the procedure known as gravimetric analysis involves the addition of some chemical to the unknown compound or mixture to produce a precipitate. A precipitate is a solid formed during a chemical reaction—usually in water—that eventually settles out of the solution. In a gravimetric analysis, the precipitate is filtered, washed, dried, and weighed. The composition of the original unknown can then be calculated from the weights of the precipitate and sample and original unknown.

Words to Know

Classical (or wet) analysis: Those procedures in which a suitable chemical reagent is reacted with some unknown, either by precipitate formation or titration.

Gravimetric analysis: A classical quantitative technique in which a chemical added to an unknown forms an insoluble precipitate with some part of the unknown. The precipitate is then collected and weighed.

Instrumental analysis: Any quantitative technique in which some property of the unknown material (electrical, optical, thermal, etc.) is measured and related to the amount of the unknown present.

Volumetric analysis: A classical quantitative technique in which a solution of known concentration is reacted exactly with an unknown and a calculation is performed to find the amount of the unknown present.

Another classical form of quantitative analysis is known as volumetric analysis. Volumetric analysis uses a procedure known as titration, in which a solution whose concentration is known precisely is caused to react with an unknown sample. The amount of the known solution needed to react precisely with the sample of the unknown can be used to calculate the percentage composition of the unknown.

Instrumental methods

Suppose that you shine a beam of X rays at a sample of gold metal. Those X rays will cause electrons in gold atoms to become excited and give off light. The same result can be produced with any one of the 100 or so chemical elements. The only difference is that the electrons of each

element respond differently to the beam of X rays. This means that a chemist can decide which element or elements are present in a substance by shining X rays on the substance and observing the light pattern that is produced.

Forms of energy other than X rays can be used to produce the same results. For example, different elements conduct an electric current more or less effectively. In some cases, the presence of various elements in an unknown sample can be discovered, then, simply by passing an electric current through the sample and measuring the electrical conductivity in the sample.

Instrumental techniques have made possible a much greater sensitivity in the analysis of unknown materials. In most classical forms of analysis, an accuracy of about one part per thousand is not unusual (meaning one milligram of some substance is detectable in a one-gram sample).

Instrumental techniques have the capability to detect concentrations of one part per million, one part per billion, and, in the very best cases, one part per trillion. At that level of sensitivity, it would be possible to detect a grain of sand in an volume of water equal to about three typical high school swimming pools.

[See also Spectroscopy ]

quantitative analysis Identification of the amount of chemical constituents in a substance or mixture. Chemical methods use reactions such as precipitation (the suspension of small particles in a liquid), neutralization, and oxidation, and measure volume (volumetric analysis) or weight (gravimetric analysis). Physical methods measure qualities such as density and refractive index (how much light is refracted by a medium). A qualitative analysis identifies the chemical elements or ions.