198 NON-METALS AND THEIR COMBINATIONS. 



ductivity is A v , which means the conductivity shown by the molecular weight 

 of the substance in v liters of solution. 



The conductivity increases with dilution. This is what we should expect, 

 since conductivity is proportional to the number of ions, which also increases 

 with dilution. By measuring the conductivity in a given dilution and also in 

 very large dilution, it is a simple matter to calculate the degree of ionization 

 of the substance in the given dilution. This method of study is called the 

 conductivity method. 



Electromotive force required in electrolysis. The amount of work 

 that can be done by any form of energy depends not merely on the quantity, 

 but also on the intensity of the energy. Thus, the quantity of steam in a loco- 

 motive boiler, however large, will not cause the driving-wheels to turn if the 

 pressure (intensity factor) is not sufficiently large. Likewise, in electrolysis, 

 different substances require for decomposition currents of different electro- 

 motive force. If the latter is less than the required minimum, there is no 

 liberation of ions at the electrodes, that is, no electrolysis. The quantity of 

 current only controls the amount of material liberated, but the electromotive 

 force decides whether there will be any decomposition at all. The reason for 

 the latter fact is that as soon as the electrodes are coated with the products of 

 electrolysis, a reverse electromotive force and current tend to develop which 

 oppose the original current. The electrodes are then said to be polarized. To 

 overcome this polarization current requires a certain minimum electromotive 

 force in the electrolyzing current. The electromotive force required for a few 

 common electrolytes are as follows : 



Hydriodic acid 0.53 volts. 



Silver nitrate 0.7(5 " 



Hydrochloric acid 1.41 " 



Sulphuric acid 1.92 " 



Zinc sulphate . . 2.70 " 



Electrochemical series of the metals. If the metals are arranged in 

 the order of the electromotive force required to liberate them in electrolysis, 

 we have the following electropositive series : 



Electrochemical series of metals. 



Potassium Gold 



Sodium Platinum 



Lithium Palladium 



Calcium Silver 



Strontium Mercury 



Barium Bismuth 



Magnesium Antimony 



Aluminum Copper 



Manganese Arsenic 



Zinc Hydrogen 

 Chromium Lead 

 Cadmium Tin 

 Iron Nickel 

 Cobalt 



