Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 480 of 707

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 quite well by spectroscopic or photometric methods, electroanalytical methods offer ease of operation and relatively lower costs of purchase and main- tenance. Potentiometry Potentiometry involves an electrode that responds to the activity of a particular group of ions in solution. Potentiometric methods correlate the activity of an ion with its concentration in solution. In potentiometric titrations, titrant is added to a solution and the potential between an indicator and reference electrode is measured. The reaction must involve the addition or removal of an ion for which an electrode is available. Acid- base titrations are performed with a glass indicator electrode and a calomel ref- erence electrode. The endpoint corresponds to the maximum rate of change of potential per unit volume of titrant added. Advantages of potentiometric titrations include its applicability to colored, tur- bid, or fluorescent solutions. It is also useful in situations where indicators are unavailable. The sensitivity of potentiometric titrations is limited by the accuracy of the mea- surement of electrode potentials at low concentrations. Solutions that are more dilute than 10-5 N cannot be accurately titrated potentiometrically. This is because the experimentally measured electrode potential is a combined potential, which may differ appreciably from the true electrode potential. The difference between the true and experimental electrode potentials is due to the residual cur- rent, which arises from the presence of electroactive trace impurities. The direct potentiometric measurement of single ion concentrations is done with ion selective electrodes (ISEs). The ISE develops an electric potential in response to the activity of the ion for which the electrode is specific. ISEs are avail- able for measuring calcium, copper, lead, cadmium, ammonia, bromide, nitrate, cyanide, sulfate, chloride, fluoride, and other cations and anions. Cation ISEs encounter interferences from other cations, and anion ISEs encounter interferences from other anions. These interferences can be eliminated by adjusting the sample pH or by chelating the interfering ions. ISE instructions must be reviewed carefully to determine the maximum allowable levels of inter- ferants, the upper limit of the single ion concentration for the ISE, and the type of media compatible with the particular ISE. Some of the solutions that can be analyzed by potentiometric methods are: Anodizing solutions: Al, H2SO4 Brass solutions: Cu, Zn, NH3, CO3 , C2 H2 O4 , CrO3 Bronze solutions: Cu, Sn, NaOH, NaCN, Na2 Chromium solutions: Cr, Cl Cadmium solutions: Cd, NaOH, NaCN, Na2 Gold solutions: Au, Ag, Ni, Cu Lead and tin/lead solutions: Pb, Sn, HBF4 , Cl CO3 Acid copper solutions: Cl Alkaline copper solutions: NaOH, NaCN, Na2 CO3 CO3 479

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