286 ELECTRO-ANALYSIS. chap.
Voltameter. — The principle of the voltameter, used for the measurement of current, is based on the separation of gases or metals. The oldest of these instruments is the electrolytic gas voltameter, in which hydrogen and oxygen are separated, and collected either singly in calibrated tubes or together in one tube. Formerly the electrolyte used was dilute sulphuric acid. Secondary reactions, however, arise in this case inasmuch as, at the expense of the oxygen, persulphuric acid, ozone, and hydrogen peroxide are formed. The sulphuric acid was first replaced by phosphoric acid, but later, potassium hydroxide solutions were introduced, and the electrodes were made of nickel instead of platinum. Using sulphuric acid, only the hydrogen should be collected, since the disturbances occur at the pole where the oxygen is separated. The gas volume must be reduced to normal temperature and pressure, and due allowance made for the water vapour present. One coulomb corresponds with 0174 c.c. of electrolytic gas, or 01 16 c.c. of hydrogen. One ampere evolves 6*96 c.c. of hydrogen per minute.
On account of the inaccuracies of the electrolytic gas voltameter, its place has now been taken by silver or copper voltameters. In the silver voltameter a platinum crucible is used as cathode, and a rod of silver in the centre serves as anode. In order to prevent pieces (particularly of peroxide) of the anode from falling into the crucible, the rod should be wrapped in filter paper, or a small glass basin should be suspended beneath it. The formation of peroxide can be most judiciously prevented by adding some alcohol to the silver nitrate solution (15-30 per cent.) used as electrolyte. The current density may be very variable.
In the copper voltameter two thick copper plates serve as anode, and a thin sheet of copper hung between them is the cathode ; the electrodes are suspended in a solution of about 15 per cent, copper sulphate, 5 per cent, sulphuric acid, and 5 per cent, alcohol, contained in a beaker. If the cun^ent density is less than 0*4 amp./sq. dm., oxidation by the air has a disturbing effect. When the current density is small, the
�� �
