Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 481 of 707

Nickel solutions: Co, Cu, Zn, Cd, Cl, H3 , H2 Silver solutions: Ag, Sb, Ni Acid tin solutions: Sn, HBF4 Zinc solutions: Zn Conductometry Alkaline tin solutions: Sn, NaOH, NaCO3 SO4 BO3 , Cl Electrolytic conductivity measures a solution's ability to carry an electric current. A current is produced by applying a potential between two inert metallic electrodes (e.g., platinum) inserted into the solution being tested. When other variables are held constant, changes in the concentration of an electrolyte result in changes in the conductance of electric current by a solution. In conductometric titrations, the endpoint of the titration is obtained from a plot of conductance against the volume of titrant. Excessive amounts of extra- neous foreign electrolytes can adversely affect the accuracy of a conductometric titration. Conductometric methods are used when wet or potentiometric methods give inaccurate results due to increased solubility (in precipitation reactions) or hydrolysis at the equivalence point. The methods are accurate in both dilute and concentrated solutions, and they can also be used with colored solutions. Conductometric methods have been applied to the analysis of Cr, Cd, Co, Fe, Ni, Pb, Ag, Zn, CO3 , Cl, F, and SO4 . Polarography In polarography, varying voltage is applied to a cell consisting of a large mercury anode (reference electrode) and a small mercury cathode (indicator electrode) known as a dropping mercury electrode (DME). Consequent changes in current are measured. The large area of the mercury anode precludes any polarization. The DME consists of a mercury reservoir attached to a glass capillary tube with small mercury drops falling slowly from the opening of the tube. A saturated calomel electrode is sometimes used as the reference electrode. The electrolyte in the cell consists of a dilute solution of the species being deter- mined in a medium of supporting electrolyte. The supporting electrolyte func- tions to carry the current in order to raise the conductivity of the solution. This ensures that if the species to be determined is charged, it will not migrate to the DME. Bubbling an inert gas, such as nitrogen or hydrogen, through the solution prior to running a polarogram, will expel dissolved oxygen in order to prevent the dissolved oxygen from appearing on the polarogram. Reducible ions diffuse to the DME. As the applied voltage increases, negligi- ble current flow results until the decomposition potential is reached for the metal ion being determined. When the ions are reduced at the same rate as they dif- fuse to the DME, no further increases in current occur, as the current is limited by the diffusion rate. The half-wave potential is the potential at which the current is 50% of the limiting value. Polarograms are obtained by the measurement of current as a function of applied potential. Half-wave potentials are characteristic of particular sub- stances under specified conditions. The limiting current is proportional to the concentration of the substance being reduced. Substances can be analyzed quan- titatively and qualitatively if they are capable of undergoing anodic oxidation or cathodic reduction. As with other instrumental methods, results are referred to standards in order to quantitate the method. Advantages of polarographic methods include their ability to permit simul- taneous qualitative and quantitative determinations of two or more analytes in 480

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