Electrogravimetric methods

There is a whole gamut of electrochemical methods available for the determination of the transition elements. Electrogravimetric methods are available for large numbers of metals (e.g. Cu, Ag, Cd, Co, Ni, Sn, Zn, Pb, and Tl) provided these are available in weighable amounts. Controlled potential electrolysis at a mercury pool electrode is best suited for separations (e g. Cu, Cd, and Pd from uranium) or removing traces of metalUc impurities when preparing very pure electrolytes for use in polarography. ... 

There are two general types of electrogravimetric methods. In one, no control of the potential of the working electrode is exercised, and the applied cell potential is held at a more or less constant level that provides a large enough current to complete the electrolysis in a reasonable length of time. The second type of electrogravimetric method is the controlled-potential or potentiostatic method. 

Controlled Current Electrolysis Simulation Exploring the Electrogravimetric Method. 

Table 22-2 lists some other separations performed by controlled-potential electrolysis. Because of limited sensitivity and the time required for washing, drying, and weighing the electrodes, many electrogravimetric methods have been replaced by the coulometric methods discussed in the next section. 

It is generally desirable for bulk electrolytic processes to be carried out with high current efficiency. This requires that the working electrode potential and other conditions be chosen so that no side reactions occur (e.g., reduction or oxidation of solvent, supporting electrolyte, electrode material, or impurities). In electrogravimetric methods, 100% current efficiency is usually not necessary, as long as the side reactions do not produce insoluble products. In coulometric titrations at constant current, 100% titration efficiency (rather than current efficiency) is required the distinction is discussed in Section 11.4.2. 

The sensitivity of an electrogravimetric method is limited by the difficulty in determining the small difference in weight between the electrode itself and the electrode plus... 

Some metals determined by electrogravimetric methods and their deposition potentials are given in Table 11.3.1. Detailed discussions of the methods and applications of electrogravimetric methods are available (1, 3, 19, 20). Electrogravimetry can also be carried out with the quartz crystal microbalance as described in Section 17.5. 

The three methods generally have moderate selectivity, sensitivity, and speed in many instances, they are among the most accurate and precise methods available, with uncertainties of a few tenths of a percent relative being common. Finally, in contrast to all of the other methods discussed in this text, these three require no calibration against standards that is, the functional relationship between the quantity measured and the mass of analyte can be calculated from theory. Applications of electrogravimetric methods are found in many elementary textbooks, so we will not discuss them in any detail here. However, before discussing the two coulometric methods, we explore the processes that occur in an electrolytic deposition. 

Three electroanalytical methods are based on electrolytic oxidatiorior reduction of an anqlyte fora sufficient period to assure its quantitatwg. conversion to a new oxidation state. These methods are constant-potential coulomelry constant-current coulometry, or coulometric. titrationsf imd electiogravimeiiry. In electrogravimetric methods, the product of the electrolysis is weighed as a dep osit on one of the electrodes. In the two coulometric procedures, on the other hand, the quantity of electricity needed to complete the electrolysis is a measure of the amount of analyte present. 

The emergence of modem electroanalytical sciences is tightly bound to the implication of specific conductive materials as electrode material. Lubert and Kalcher recently provided a history of electroanalytical methods in which the reader could find a wider view of the field. First electroanalytical determination of copper in copper-nickel coins was accomplished by electrogravimetric method on platinum electrodes in the nineteenth century. Later, polagraphy, rewarded by a Nobel Prize for J. Heyrovsky, was developed with dropping mercury electrode and important developments have been then conducted on both analytical methods and materials. 

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