272 ELECTRO-ANALYSIS. - chap.
efficacious action of the heat may be in part due to the convection current, to which it gives rise in the liquid.
When the electrolysis is complete the cathode basin is ^washed out The deposit should be washed three times with about 50 c.c. of cold water, then three times with about the same quantity of alcohol, after which it is placed for about five minutes in an air bath at 80"', dried in a desiccator, and finally weighed when cold. It frequently happens that the liquid in the basin (acid liquids particularly) exerts a dissolving action on the deposited metal so soon as the current is stopped. In these cases the liquid must be removed from the basin by means of a syphon while the current is still passing. Occasionally some substance is added to the liquid in the basin in order to diminish the rate of solution, e.g, sodium acetate to a sulphuric acid solution from which copper has been deposited. The sulphuric acid is replaced by the acetic acid, and in this wise the solution process is as good as prevented. Sometimes the adjuncts for washiug out are mounted on the stand along with the electrodes.
After the deposit has been weighed it is removed by some chemical solvent, which, as a rule, offers no difficulty. In certain cases the deposit adheres closely to the platinum surface, e.g, with zinc or tin, and the platinum becomes more or less porous when the deposit is dissolved off. To prevent this the basin may first be coated with a thin film of copper or silver. Precipitated gold is best removed by chromic anhydride dissolved in a saturated solution of sodium chloride.
Should the level of the liquid sink during the electro- lysis, part of the deposited metal will be exposed, and probably suffer oxidation. This may easily be avoided by replacing any water lost by evaporation.
It is characteristic of the depositions used for analytical purposes that they almost all result from secondary pro- cesses (see p. 282). So much substance should be taken for the analysis that a deposit of 01 to 0*4 gram of metal
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