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Basic Chemistry Concepts

Chemical Analysis

Qualitative Organic Analysis, element iodine, conductometry, reaction test, neutron activation analysis

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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.

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).

Qualitative Organic Analysis

Organic analysis relies on certain chemical reactions to detect particular functional groups, such as alcohol, amine, aldehyde, olefin, ester, carboxylic acid, and ether. The test reactions are usually employed without prior separation. As an example, olefins (compounds containing carbon-carbon double bonds) can be identified by the bleaching effect they have on a colored bromine solution. For both organic and inorganic qualitative analysis, instrumental methods are currently preferred because they are more sensitive and specific.

Quantitative Wet Methods

These are mainly gravimetric and titrimetric procedures for inorganic substances. An example of a gravimetric analysis is the determination of chloride ion concentration in a solution by causing the precipitation of insoluble silver chloride (AgCl). The precipitate is then collected and weighed. The analysis yields very accurate results.

Titrimetric procedures are commonly based on acid-base reactions such as the titration of acetic acid with a solution of sodium hydroxide. Another common reaction employed is that of a complexing agent, such as ethylenediaminetetraacetic acid (EDTA), with solutions of metal ions, such as lead or mercury. Reactions suitable for titrations must proceed rapidly to completion, without side reactions that tend to obscure the results. This requirement is more often satisfied by inorganic reactions than by organic functional group chemistry.

Radiochemical Techniques

These methods rely on the detection of radioactivity in the form of alpha and beta particles and gamma rays that result from nuclear disintegrations. Radioactivity can be induced in the sample by bombarding it with neutrons. Such a procedure, called neutron activation analysis, is commonly used in industry to identify certain metals in a sample. Neutron activation analysis has the advantage of being rapid and highly automated, and it does not destroy the sample.

Electrochemical Techniques

When a positive and a negative electrode are placed in a solution containing ions, and an electric potential is applied to the electrodes, the positively charged ions (cations) move toward the negative electrode, or cathode, and the negatively charged ions (anions) to the positive electrode, or anode. As a result, electric current flows between the electrodes. The strength of the current depends on the electric potential between the electrodes and the concentration of ions in the solution. Hence, this instrumental quantitative method, called conductometry, is often used to measure the ion concentration in a solution.

In a related technique, electrodes specially constructed to accept only specific ions are used to determine the sodium ion or calcium ion concentration or the pH of the solution being analyzed. Such ion-selective electrodes are important in several types of clinical analysis.

Contributors

Murray, Rpyce W., Ph.D.

Kenan Professor of Chemistry, University of North Carolina. Editor-in-Chief of the "Journal of Analytical Chemistry". Contributor to many research Publications.



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