20 



PHYSICAL CHEMISTRY 



surface and reduce electric polarization. In doing this the cell is 

 filled with about 2 per cent platinic chloride solution containing a 

 trace of basic lead acetate. An electric current of about four volts 

 is passed through the cell until one electrode is blackened and the 

 current is reversed until the other pole is blackened. The plati- 

 nizing solution is returned to the bottle and the cell rinsed and 

 filled with dilute sulphuric acid. A current is now passed in both 

 directions as before. In this way hydrogen is produced, first on 

 one electrode and then on the other, and reduces any chlorine 

 that may have been absorbed by the platinum. The cell is now 

 rinsed many times, finally with conductivity water, is filled with 

 the latter and left until used. Conductivity water is made by 

 adding sulphuric acid and potassium bichromate to distilled water 

 and redistilling it, then adding barium hydrate to it and distilling 

 it the third time with the exclusion of C0 2 of the air. Both 

 distillations may be done at once (Jones, Hulett). 



In order to j reduce polarization still further, a rapidly alter- 

 nating current is used. This is best obtained from a special 

 electric generator, or Vreeland oscillator, but owing to its cost 

 most people are content with a small induction coil. The vibrator 

 of the coil should be very stiff or very short, in order to produce 

 a high pitch resembling the sound of a mosquito. A great change 

 in the frequency may change the resistance measured 3 per cent 

 (W. A. Taylor, 1915). The coil should be enclosed in a sound 

 proof box. 



A Wheatstone bridge is built up of a resistance box, the con- 



Fig. 4. Wheatstone bridge arrangement for conductivity apparatus. 



