Chemical Analysis

I

Introduction

Chemical Analysis, body of procedures and techniques used to identify and quantify the chemical composition of a sample of a substance. A chemist executing a qualitative analysis seeks to identify the substances in the sample. A quantitative analysis is an attempt to determine the quantity or concentration of a specific substance in the sample. Thus, for example, determining whether a sample of salt contains the element iodine is a qualitative analysis; measuring the percentage by weight of any iodine in the sample is a quantitative analysis.

The measurement of chemical composition is necessary throughout commerce, regulatory government, and many fields of science. Chemical analysis thus takes on many specialized forms.

II

Preparation for Analysis

Chemists are commonly asked to analyze such diverse materials as stainless steel, beer, a fingernail, a rose petal, smoke, aspirin, and paper. The determination of the identity or quantity of a constituent of such materials is preceded by a sampling step—the selection of the amount and uniformity of material required for the analysis—and by the separation from the sample of either the desired constituent or the undesired, interfering constituents. The appropriate separation method depends on the nature of the constituent sought and of the overall sample.

Chromatography is the most generally applicable of the separation methods and has many variants according to the nature of the column packing and the sample-constituent interaction. The two most important types of chromatography are gel permeation chromatography, in which large molecules separate according to their size; and ion exchange chromatography, in which charged, or ionic, constituents are separated (see Ion Exchange). Gas chromatography separates the volatile constituents of a sample, and liquid/liquid chromatography separates small, neutral molecules in solution.

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The goal in conducting a separation is to produce a purified or partly purified form of the desired constituent for analytical measurement, or to eliminate other constituents that would interfere with the measurement, or both. Separation is often unnecessary when the method is highly specific, or selective, and responds to the desired constituent while ignoring others. Measuring the pH, or hydrogen ion content, of blood with a glass electrode is an example of a measurement that does not require a separation step.

Another step preparatory to both qualitative and quantitative analyses is standardization, or calibration. The response of the analytical method and the sensitivity of the mechanical and electronic equipment to the desired constituent must be calibrated, or standardized, using a pure constituent or a sample containing a known amount of constituent. A task of the National Institute of Standards and Technology is to develop and provide such standard samples.

III

Presentation of Results

The numerical result of a quantitative analysis may state the absolute quantity of the constituent or some percentage of it in the sample. The latter can be expressed as weight percent, molar concentration (moles of dissolved constituent per liter of solution), or ppm (parts per million by weight), among others (see Mole). The accuracy of the analytical result is reflected by how well it agrees with the true quantity of constituent. The precision of the result is reflected by its reproducibility, or repeatability. Results from repeated measurements are called precise if they all lie within a narrow range of values. Such results are termed highly reproducible. Precision does not necessarily mean that the results are accurate, however, because some part of the measuring process may bias the results toward values that are higher or lower than the true value. Standardization of the analysis often uncovers such systematic errors.

Random errors in a measurement tend to cancel one another out. Accuracy is nearly always improved by averaging multiple determinations. Depending on the method used, measurements may need to be repeated only three or four times. For procedures in which computers are connected to the analytical instruments, as many as 100,000 measurements may be made very quickly. This technique is referred to as signal averaging.

An actual analysis of a sample is commonly based on a chemical reaction of the constituent that produces an easily identifiable quality such as color, heat, or insolubility. Gravimetric analysis, which hinges on measuring the mass of precipitates of the constituent, and titrimetric analysis, which depends on measuring the volumes of solutions that react with the constituent, are referred to as “wet methods”; these are more labor intensive and less versatile than newer methods.

Instrumental methods of analysis, or analyses that rely on electronic instruments, became important in the 1950s, and today most analytical measurements are conducted with the aid of such devices.

IV

Qualitative Inorganic Analysis

A systematic “wet method” qualitative analysis of inorganic ions proceeds by separating the ions into groups by selective precipitation reactions, isolating individual ions in the groups by an additional precipitation reaction, and confirming the identity of the ion by a reaction test that gives a specific precipitate or color. Several schemes exist for doing this, with cations (positively charged ions) and with anions (negatively charged ions). Table 3 is an abbreviated scheme for the analysis of environmentally important cations of metallic elements.

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