Chemical Analysis - chemistry.

Chemical Analysis - chemistry. 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 sampleconstituent 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. 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. 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. 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 8 CH3 and 8 CH2Cl 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. Contributed By: Royce W. 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