Metal Finishing Guide Book


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ters to isolate the desired wavelength region. Filters isolate a wider band of light. In spectrophotometric titrations, the cell containing the analyte solution is placed in the light path of a spectrophotometer. Titrant is added to the cell with stirring, and the absorbance is measured. The endpoint is determined graphically. Applications of this titration include the analysis of a mixture of arsenic and antimony and the analysis of copper with ethylene diamine tetra acetic acid (EDTA). The possibility of errors in spectrophotometric analyses is increased when numerous dilutions are required for an analysis. Colorimetry involves comparing the color produced by an unknown quantity of a substance with the color produced by a standard containing a known quantity of that substance. When monochromatic light passes through the colored solution, a certain amount of the light, proportional to the concentration of the substance, will be absorbed. Substances that are colorless or only slightly colored can be rendered highly colored by a reaction with special reagents. In the standard series colorimetric method, the analyte solution is diluted to a certain volume (usually 50 or 100 ml) in a Nessler tube and mixed. The color of the solution is compared with a series of standards similarly prepared. The concentration of the analyte equals the concentration of the standard solution whose color it matches exactly. Colors can also be compared to standards via a colorimeter (photometer), comparator, or spectrophotometer. The possible errors in colorimetric measurements may arise from the following sources: turbidity, sensitivity of the eye or color blindness, dilutions, photometer filters, chemical interferences, and variations in temperature or pH. Photometric methods are available for the analysis of the following analytes: Anodizing solutions: Fe, Cu, Mn Brass solutions: Fe Cadmium solutions: Fe, Ti, Zn, Cu, Ni Chromium solutions: Cr, Fe, Ni, Cu, Se Acid copper solutions: Cl, Fe Alkaline copper solutions: Fe, Se Gold solutions: Au, Ni, In, Co, Cu, Fe, PO4 Iron solutions: Mn, NH3 Lead and tin-lead solutions: Pb Nickel solutions: Cr, Cu, Zn, Fe, Co, NH3 Palladium solutions: Pd, Cr, NH3 Platinum solutions: Pt Rhodium solutions: Rh Silver solutions: Ni, Cu, Sb Acid tin solutions: Fe, Cu Alkaline tin solutions: Cu, Pb, Zn Acid zinc solutions: Cu, Fe Alkaline zinc solutions: Cu, Fe Wastewater: Cr6+, Ni, Cu, Fe, Zn, Pb, Al, B, NO3, NO2, PO4, Cl, CN, wetting agents. Atomic Absorption Metals in plating and related solutions can be readily determined by AA spectrophotometry. Optimum ranges, detection limits, and sensitivities of metals vary with the various available instruments. In direct-aspiration atomic absorption (DAAA) analysis, the flame (usually air-acetylene or nitrous oxide-acetylene) converts the sample aerosol into atomic vapor, 486

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