VITAL PHENOMENA 37 



of electricity in the metal for charging the ions as illustrated by 

 the behavior of radioactive metals. If a piece of metal is placed 

 in pure water it gives off ions, but these ions taking positive 

 charges from the metal leave the metal negative, and when the 

 attraction of the negative metal for the positive ions equals the 

 diffusion pressure (=r osmotic pressure) of the ions no more 

 ions will be given off. If instead of placing the metal in pure 

 water we place it in a solution containing ions of the metal, the 

 metal will give out a less number of ions and will become less 

 negative. If the concentration of the metallic ions in the solu- 

 tion is increased, a point will be reached at which the metal will 

 give out no ions and hence remain electrically neutral, and if the 

 ionic concentration of the solution is still further increased ions 

 will be deposited on the metal and give it a positive charge. If 

 a piece of metal is placed in a solution containing 1 gram equiva- 

 lent of its ions per liter it may become negative or positive ac- 

 cording to the metal used. The following table is prepared in 

 this way, but the neutral point (taken arbitrarily) is not the 

 potential of the earth but the potential of a hydrogen electrode 

 in a gram equivalent solution of hydrogen ions (see Abegg Auer- 

 bach & Luther, 1911). The potential that the electrode assumes 

 is called the electrolytic solution tension. 



Electrolytic Solution Tension. 

 Potential of metal (or non-metal) when immersed in a normal solution 

 of its ions at 25°, as measured against a normal hydrogen electrode. Some 

 American writers use reverse signs (+, — ). 



•Os 



ILi* — 3-305 



Rb* — 3.205 



K" — 3-203 



Na* — 2.993 



Ba" — 2.7 



Sr*" — 2.767 



Ca" - 2.55 



Mg" - 1.55 



Al — 1.276 



Mn — 1.075 



Zn" — .760 



Cd" — .40 



It may be seen from the table that the baser metals have a 

 greater tendency to dissolve and produce ions than the noble 

 metals. 



Electrolytic solution tension is reduced by certain impurities 



