Metal Finishing Guide Book


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TITRIMETRIC METHODS The active component of a plating bath sample is stiochiometrically titrated with a standard solution of a required titrant. The end point of a titration may be visually determined with a color change of the indicator or by an electrometric method. Indicators are auxiliary reagents added to samples and aid in end-point determination. For low concentrations, volumetric methods yield inaccurate results due to obscured end points. For accurate determinations, the method should be selective and free from interferences, with crisp end points. The details of volumetric titrations are simple, and supply houses provide procedures for analysis. For accuracy and precision, standardized reagent-grade solutions in conjunction with A-grade pipettes and burettes should be used. Automatic titrators (Metrohm, Fisher) and digital burettes are gaining popularity for their reproducibility and accuracy. The role of chemical interferences should be considered for a given multi-component bath. GRAVIMETRIC METHODS Gravimetric methods involve the separation of the desired component from other constituents by chemical precipitation, isolation, washing, and weighing after drying. These methods are time consuming, but for precious metals the gravimetric method is considered a referee method. Some metals (Cu, Ag) are determined by electrodepositing on pre-weighed platinum cathodes. The gravimetric methods are employed for chloride, sulfate, carbonate, phosphate, and certain metals. INSTRUMENTAL TECHNIQUES In wet chemical methods, the chemical property of the component is utilized in its determination, whereas instrumental methods utilize the physical property of the component. The analyst should weigh the cost, degree of precision, and accuracy for a given instrumental method. Plating solutions can be analyzed using the following instrumental methods: 1. Spectroscopic methods: A given substance is analyzed by the measurement of emitted light from the excited atoms by radiant energy, AC, or DC arc. Each element has characteristic wavelengths depending on its electronic configuration. A distinct set of wavelengths are generated and separated by a monochromator, and intensities of various wavelengths are measured by a spectrograph or photoelectric detectors (spectrophotometry). Spectroscopic methods are unique and specific and are employed for trace quantitative analysis. The accuracy of spectrographic methods is not very high, with limit of detection at about 3%. Sensitivities are much smaller for high-energy elements, such as zinc, than for elements of low energy, such as sodium. 2. Flame photometry (FP): A liquid sample is atomized at constant air pressure and aspirated into a flame (1,800–3,100 K) as fine mist. At high temperatures the solvent evaporates, forming a solid, which then vaporizes, dissociating the atoms into a ground state. The valence electrons of the ground state are excited by the flame energy to their higher energy level and fall back to the ground state. The intensities of emitted spectrum lines are 464

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