270 ELECTRO-ANALYSIS. chap.
quickly and most suddenly, just when the last quantity of mercury is precipitated. The electrometer then indicates a spring of about 0*1 volt^ and may thus be used as an indicator. This kind of analysis, which can be applied in other similar cases, has not been used in practice to any extent.
Analysis by Metal Deposition. — ^The electrolytic de- position of metal, on account of the ease with which it can be carried out, is the most frequent electrical method of analysis. For the separation of a metal a certain electro- motive force (see p. 257) and a definite quantity of electricity are required. The former can be ascertained from the polarisation, the latter by means of Faraday's law, according to which 1 gram-equivalent of any substance is precipitated hours 48 minutes. If we used an electromotive force for the separation exactly equal to the back electromotive force of polarisation, it would require an infinitely long time to carry out the deposition. In practice, therefore, it is customary to use an appreciably higher electromotive force (higher by 1 to 2 volts). The potential difiference between the cathode and anode determines what electromotive force must be used, and between these an electrometer is interposed in a branch circuit. In order to obtain a good, coherent deposit, the current density (given below in amperes per square deci- metre) must be judiciously chosen. In order to be able to control this an ammeter is introduced before the decomposi- tion cell. To regulate the current density, a metal wire rheostat is used, and arranged so that diflTerent lengths of wire can be interposed at will. The electromotive force is best obtained from a battery of accumulators (only few elements are required, for the electromotive force never exceeds 5 volts). The instruments do not need to be very exact, for an approxi- mate measurement of the electromotive force and the current density sufiftces.
When the times required for the deposition are calculated by Faraday's law, it is always found that they are too small.
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