Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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by a color change, formation of a turbid solution, or the solubilization of a tur- bid solution. Some volumetric methods require little sample preparation, whereas others may require extensive preparation. Accuracy decreases for volumetric analyses of components found in low concentrations, as endpoints are not as easily observed as with the components found in high concentrations. Volumetric methods are limited in that several conditions must be satisfied. Indicators should be available to signal the endpoint of the titration. The com- ponent-titrant reaction should not be affected by interferences from other sub- stances found in the solution. GRAVIMETRIC METHODS In gravimetric methods, the component being determined is separated from other components of the sample by precipitation, volatilization, or electroana- lytical means. Precipitation methods are the most important gravimetric meth- ods. The precipitate is usually a very slightly soluble compound of high purity that contains the component. The weight of the precipitate is determined after it is filtered from solution, washed, and dried. Gravimetric methods are used to supplement the available volumetric methods. Limitations of gravimetric methods include the requirement that the pre- cipitated component has an extremely low solubility. The precipitate must also be of high purity and be easily filterable. Species that are analyzed gravimetrically include chloride, sulfate, carbonate, phosphate, gold, and silver. INSTRUMENTAL METHODS Instrumental methods differ from wet methods in that they measure a physical property related to the composition of a substance, whereas wet methods rely on chemical reactions. The selection of an instrument for the analysis of plating solu- tions is a difficult task. Analysts must decide if the cost is justified and if the ana- lytical instrument is capable of analyzing for the required substances with a high degree of accuracy and precision. Instruments coupled to computers can auto- matically sample, analyze, and record results. Mathematical errors are minimized and sample measurements are more reproducible than with wet methods. Instrumental methods are also extremely rapid when compared with wet methods. Unlike humans, instruments cannot judge. They cannot recognize improper sam- ple preparation or interfering substances. Erroneous results are sometimes produced by electronic and mechanical malfunctions. Analytical instruments frequently used in the analysis of plating solutions can be categorized as spectroscopic, photometric, chromatographic, and electroanalytical. Spectroscopic methods (flame photometry, emission spectrometry, X-ray fluores- cence, mass spectrometry, and inductively coupled plasma) are based on the emis- sion of light. Photometric methods (spectrophotometry, colorimetry, and atomic absorption) are based on the absorption of light. Chromatographic methods (ion chromatography) involve the separation of substances for subsequent identification. Electroanalytical methods (potentiometry, conductometry, polarography, amper- ometry, and electrogravimetry) involve an electric current in the course of the analysis. The instrumental methods, comprehensively reviewed below, are most applic- able to plating environments. SPECTROSCOPIC METHODS Spectroscopy is the analysis of a substance by the measurement of emitted 472

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