Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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the same solution. Polarography has wide applicability to inorganic, organic, ion- ic, or molecular species. Disadvantages of polarography include the interferences caused by large con- centrations of electropositive metals in the determination of low concentra- tions of electronegative metals. The very narrow capillary of the DME occa- sionally becomes clogged. Polarographic methods are available for the following solutions: Anodizing solutions: Cu, Zn, Mn Brass solutions: Pb, Cd, Cu, Ni, Zn Bronze solutions: Pb, Zn, Al, Cu, Ni Cadmium solutions: Cu, Pb, Zn, Ni Chromium solutions: Cu, Ni, Zn, Cl, SO4 Acid copper solutions: Cu, Cl Alkaline copper solutions: Zn, Fe, Pb, Cu Gold solutions: Au, Cu, Ni, Zn, In, Co, Cd Iron solutions: Mn Lead and tin-lead solutions: Cu, Cd, Ni, Zn, Sb Nickel solutions: Cu, Pb, Zn, Cd, Na, Co, Cr, Mn Palladium solutions: Pd, Cr3+ , Cr6+ Rhodium solutions: Rh Silver solutions: Sb, Cu, Cd Acid tin solutions: Sn4+ , Cu, Ni, Zn Alkaline tin solutions: Pb, Cd, Zn, Cu Acid zinc solutions: Cu, Fe, Pb, Cd Alkaline zinc solutions: Pb, Cd, Cu Wastewater: Cd, Cu, Cr3+ , Ni, Sn, Zn Amperometry Amperometric titrations involve the use of polarography as the basis of an elec- trometric titration. Voltage applied across the indicator electrode (e.g., DME or platinum) and reference electrode (e.g., calomel or mercury) is held constant and the current passing through the cell is measured as a function of titrant volume added. The endpoint of the titration is determined from the intersection of the two straight lines in a plot of current against volume of titrant added. Polarograms are run to determine the optimum titration voltage. Amperometric titrations can be carried out at low analyte concentrations at which volumetric or potentiometric methods cannot yield accurate results. They are temperature independent and more accurate than polarographic meth- ods. Although amperometry is useful for oxidation-reduction or precipitation reac- tions, few acid-base reactions are determined by this method. Some of the reactions that can be analyzed by amperometric methods are giv- en in Table II. Electrogravimetry In electrogravimetry, the substance to be determined is separated at a fixed poten- tial on a preweighed inert cathode, which is then washed, dried, and weighed. Requirements for an accurate electrogravimetric analysis include good agita- tion, smooth adherent deposits, and proper pH, temperature, and current density. Advantages of electrogravimetry include its ability to remove quantitatively most common metals from solution. The method does not require constant supervision. Disadvantages include long electrolysis times. Some of the metals that have been determined electrogravimetrically are cad- mium, cobalt, copper, gold, iron, lead, nickel, rhodium, silver, tin, and zinc. 481

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