Metal Finishing Guide Book

2012-2013

Issue link: https://metalfinishing.epubxp.com/i/98750

Contents of this Issue

Navigation

Page 550 of 903

tities���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 maintenance. 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. Acidbase titrations are performed with a glass indicator electrode and a calomel reference 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, turbid, 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 measurement 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 current, 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 available 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 interferants, 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, C2H2O4, CrO3, Cl Brass solutions: Cu, Zn, NH3, CO3 Bronze solutions: Cu, Sn, NaOH, NaCN, Na2CO3 Chromium solutions: Cr, Cl Cadmium solutions: Cd, NaOH, NaCN, Na2CO3 Acid copper solutions: Cl Alkaline copper solutions: NaOH, NaCN, Na2CO3 Gold solutions: Au, Ag, Ni, Cu Lead and tin/lead solutions: Pb, Sn, HBF4 Nickel solutions: Co, Cu, Zn, Cd, Cl, H3BO3 Silver solutions: Ag, Sb, Ni Acid tin solutions: Sn, HBF4, H2SO4 Alkaline tin solutions: Sn, NaOH, NaCO3, Cl Zinc solutions: Zn 541

Articles in this issue

view archives of Metal Finishing Guide Book - 2012-2013