Metal Finishing Guide Book

2013

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arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, iron, lead, manganese, molybdenum, nickel, selenium, silver, and tin. Cold vapor atomic absorption (CVAA) involves the chemical reduction of mercury or selenium by stannous chloride and its subsequent analysis. The reduced solution is vigorously stirred in the reaction vessel to obtain an equilibrium between the element in the liquid and vapor phases. The vapor is then purged into an absorption cell located in the light path of a spectrophotometer. The resultant absorbance peak is recorded on a strip chart recorder. The extremely sensitive CVAA procedure is subject to interferences from some organics, sulfur compounds, and chlorine. Metallic ions (e.g., gold, selenium), which are reduced to the elemental state by stannous chloride, produce interferences if they combine with mercury. Hydride atomic absorption (HAA) is based on chemical reduction with sodium borohydride to selectively separate hydride-forming elements from a sample. The gaseous hydride that is generated is collected in a reservoir attached to a generation flask, and is then purged by a stream of argon or nitrogen into an argon-hydrogen-air flame. This permits high-sensitivity determinations of antimony, arsenic, bismuth, germanium, selenium, tellurium, and tin. The HAA technique is sensitive to interferences from easily reduced metals such as silver, copper, and mercury. Interferences also arise from transition metals in concentrations greater than 200 mg/L and from oxides of nitrogen. Ion Chromatography In ion chromatography (IC), analytes are separated with an eluent on a chromatographic column based on their ionic charges. Because plating solutions are water based, the soluble components must be polar or ionic; therefore, IC is applicable to the analysis of plating and related solutions. Ion chromatographs consist of a sample delivery system, a chromatographic separation column, a detection system, and a data handling system. IC permits the rapid sequential analysis of multiple analytes in one sample. The various detectors available, such as UV-visible, electrochemical, or conductivity, allow for specific detection in the presence of other analytes. IC is suitable for the analysis of metals, anionic and cationic inorganic bath constituents, and various organic plating bath additives. It is also used for continuous on-line operations. Interferences arise from substances that have retention times coinciding with that of any anion being analyzed. A high concentration of a particular ion may interfere with the resolution of other ions. These interferences can be greatly minimized by gradient elution or sample dilution. IC has been applied to the analysis of the following analytes in plating and related solutions: Metals: Aluminum, barium, cadmium, calcium, trivalent and hexavalent chromium, cobalt, copper, gold, iron, lead, lithium, magnesium, nickel, palladium, platinum, silver, tin, zinc. Ions: Ammonium, bromide, carbonate, chloride, cyanide, fluoborate, fluoride, hypophosphite, nitrate, nitrite, phosphate, potassium, sodium, sulfate, sulfide, sulfite. Acid Mixtures: Hydrofluoric, nitric, and acetic acids. Organics: Brighteners, surfactants, organic acids. ELECTROANALYTICAL METHODS Electroanalytical methods involve the use of one or more of three electrical quantities—current, voltage, and resistance. These methods are useful when indicators for a titration are unavailable or unsuitable. Although trace analysis may be done 488

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