Page:Text-book of Electrochemistry.djvu/286

 In many cases it is necessary to prolong the electrolysis for five times as long as the time indicated by Faraday's law. The reason for this is that a large part of the current is used up for other processes than the metal deposition. To determine when the deposition is complete, it is, therefore, advisable to withdraw small portions of the solution from the cell, and by means of some delicate reaction ascertain when all the metal has been precipitated from the solution. The quantity of solution taken out must, of course, be exceedingly small, so that the solution may not be robbed of an appreciable amount of the metal.

The solution to be electrolysed should not be more than about 100 c.c, and should be placed in a perfectly clean platinum basin of 10 cm. diameter, which serves as cathode. If the basin is not clean, a good deposit cannot be obtained. In order to cause the deposit to cling well to the basin, this is oft6n provided with a matt surface. The basin is placed on a metal ring, which is carried by an upright stand, the two being united by means of an insulator, and the ring is connected with the negative pole of the battery. The form of the anode may be a perforated plate, a wire spiral, a cylinder of foil or a foil cone of platinum. This anode is held by the same stand which carries the cathode ring, by means of a metal arm connected with the positive pole of the battery. If the electrolysis be carried out in a glass vessel, the cathode should have the form of a cylinder or cone. The anode must not possess any surface to which large bubbles of gas may cling, because when these are ultimately evolved they might easily carry away some of the liquid to be electrolysed. For safety a glass funnel should always be inverted over the basin to catch any drops which may be spirted out of the liquid.

Most of the processes for electrolytic analysis proceed better at a somewhat elevated temperature (50 — 60°) than at the ordinary temperature. For this purpose a small burner is placed below the basin, so that a current of hot air ascends and warms the solution. It is convenient to have the burner fixed to the stand carrying the celL The

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