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ANALYSIS OF METAL FINISHING BATHS
A key to Quality, Productivity, Cost-Savings and a Cleaner Environment
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3. Electroanalytical Methods * or **
3.2 Current Methods
Today, at least a dozen electroanalytical techniques are in use, with many more still having been described in the literature. Outside the academic world, most of these are of no more than textbook interest. In virtually all such methods, the current response to an applied potential, or v.v forms the basis of the technique. These techniques may be steady-state (DC), or AC or use current or potential pulses. This form of electroanalysis can be said to have begun with the development, by Jan Heyrovsky, of Polarography, in Prague, for which he was awarded a Nobel Prize. In subsequent decades, Heyrovsky's simple DC polarographic technique was extensively refined. Various forms of polarography were quite widely used in the Metal Finishing industry around the 1970's. Since then, its use appears to have declined. For analysis of metallic species, the Atomic Absorption method has largely replaced it. The related electroanalytical technique now most widely used in Metal Finishing is CVS (Cyclic Voltammetric Stripping). This can broadly be described as the generation of a triangular potential ramp, which starts out cathodically, so that (in a plating bath) metal is deposited. The polarity is then reversed, as the ramp goes anodic, during which the deposited metal is anodically dissolved. The current response to these potential ramps is recorded. In a perfect (perfectly reversible) system, the initial cathodic peak would perfectly mirror the subsequent anodic one. In practice this is hardly ever the case. From a study of the peak shapes, peak potentials and most important, the area below the peaks (which has the dimensions of coulombic charge), a wealth of analytical information can be directly or indirectly obtained. The technique can be used to determine metal ion concentrations, but perhaps its greatest value is in analysis of organic additives. The method normally uses a platinum or similar rotating disk electrode. The current-potential transients ("scans") are recorded, and then analysed. Prior to the widespread use of computers, a chart-recorder was used for this, and there followed a fairly tedious procedure for integrating the area below the curves. Today, using an on-line computer, such integrations, not to mention use of first or seconf differential operations, are carried out instantaneously. This, or very similar methods are also known as CV (cyclic voltammetry) or potentiodynamic scanning. CVS is used in Metal Finishing both on a batch analysis basis and in the on-line mode. Around half-a-dozen manufacturers offer CVS equipment for Metal Finishing. A Specialist Report "Cyclic Voltammetric Stripping in Metal Finishing for Process Control & Analysis" will be released shortly .