Chemical Analysis

V

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 (see Chemistry, Organic). 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.

VI

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 (see Acids and Bases). 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.

VII

Spectroscopic Techniques

Spectroscopy, or the study of the interactions of electromagnetic radiation with matter, is the largest and most nearly accurate class of instrumental methods used in chemical analysis and indeed in all of chemistry (see Spectroscopy; Spectrum). The electromagnetic radiation (emr) spectrum is divided into the following wavelength regions: X ray, ultraviolet, visible, infrared, microwave, and radiowave. Emr interactions with matter involve absorption or emission of emr energy by means of transitions between quantized, or discrete, levels of energy for electrons, bond vibrations, molecular rotations, and electron and nuclear spins in atoms and molecules (see Atom; Quantum Theory). The matter-emr interactions take place in devices called spectrometers, spectrophotometers, or spectroscopes. The spectra produced in these devices are recorded graphically or photographically on spectrograms or spectrographs that permit convenient study of the wavelengths and intensities of the emr absorbed or emitted by the sample being analyzed.

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Absorption spectrophotometry in the visible and ultraviolet portions of the emr spectrum is a common quantitative spectral method for both organic and inorganic substances. This technique measures the relative transparency of a solution both before and after the solution has been made to react with a color-forming reagent. The resulting decrease in transparency of the solution is proportional to the concentration of the constituent being analyzed.

Infrared absorption spectrophotometry is useful for organic analysis because bonds for olefins, esters, alcohols, and other functional groups have very different strengths and therefore absorb infrared radiation of very different frequencies, or energies. Such absorption spectra appear as peaks when plotted on a spectrograph.

Nuclear magnetic resonance (nmr) spectroscopy depends on transitions between nuclear-spin energy states by absorption of radio-frequency emr energy. In nmr spectra of hydrogen, for example, chemically different hydrogen states absorb emr at different energies. For example, the organic groups 8CH₃ and 8CH₂Cl give very different, well-resolved peaks. Accordingly, nmr is a powerful qualitative analysis tool to deduce the structure of organic molecules.

Fluorescence spectroscopy is the reverse of absorption spectrophotometry. With this technique, molecules are induced to emit light, which they do at energies characteristic of their structure, and at intensities proportional to the sample concentration. This method yields extremely sensitive quantitative results for certain molecules.

In atomic emission and atomic absorption spectrophotometry the sample is heated to a high temperature and thereby decomposed into atoms and ions that absorb or emit visible or ultraviolet emr at energies characteristic of the elements involved. The yellowing of a flame by the addition of salt, for example, occurs because the sodium in salt emits strongly in the yellow portion of the visible light emr spectrum. These methods are especially useful for low concentrations of metallic elements in both qualitative and quantitative analysis.

In mass spectroscopy, the sample of an organic compound is placed in a vacuum, vaporized, ionized, and given extra energy, all of which cause the individual molecules to fragment. These molecular fragments are then sorted out according to their weight by the electric and magnetic fields in a mass analyzer. The spectral pattern, or mass spectrum, produced is a “fingerprint” of the molecule, in that organic molecules display unique fragmentation patterns.

X-ray fluorescence spectroscopy is useful for both qualitative and quantitative analyses of metallic elements, which emit X rays at characteristic energies when bombarded by a high energy X-ray source.

VIII

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.

IX

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.

See also Chemical Reaction; Chemistry.