164 CULTURE MEDIA 



they dissociate into two parts called ions. One part, the H 

 or metal, is called the positive ion or cation and the other, 

 the negative ion or anion . The cation carries a positive charge 

 of electricity and the anion a negative charge. The move- 

 ment of these ions is the conduction of the current. In a 

 solution of HCl there will be H+ ions and CI" ions and in 

 dilute solution practically no undissociated HCl. On account 

 of the free H ions in excess the reaction is acid. In a solu- 

 tion of NaOH there will be Na+ ions and OH" ions. The 

 reaction will be alkaline on account of the excess OH ions. 



If to a solution of HCl in water a solution of NaOH in 

 water is added the amount of free H ions in relation to the 

 OH ions will be diminished in proportion to the NaOH added. 

 If an equal volume of a normal solution of NaOH be added 

 to a given volume of normal HCl the mixture will become 

 neutral because the relative concentration of H ions and 

 OH ions will be the same. 



Weak electrolytes comprise weak acids and bases and salts 

 of these— acetic acid and acetates, carbonic acid and bicar- 

 bonates and acid phosphates are examples. 



In solutions of weak electrolytes dissociation is relatively 

 slight, so that there is always considerable undissociated 

 weak electolyte. In a solution of sodium acetate, NaC2H302, 

 in acetic acid solution, there will be H ions, C2H3O2 ions, 

 undissociated acetate and undissociated acetic acid. The 

 addition of NaOH to such solution results largely in increas- 

 ing the amount of undissociated sodium acetate and diminish- 

 ing the amount of undissociated acetic acid, but affecting the 

 relative concentration of free H ions very little unless com- 

 paratively large amounts of NaOH are added. If to this 

 acetic acid solution of sodium acetate a strong acid be added 

 the result is to increase the relative amount of undissociated 

 acetic acid with little effect on the free H ions, unless large 

 amounts of acid are added. 



It thus appears that in solutions of weak electrolytes 

 either acid or alkali may frequently be added icithout affect- 

 ing apyreciably the free H-ion concentration, i. e., the reaction, 

 and that the change in H-ion concentration is not necessarily 

 proportional to the acid or alkali added. 



In solutions of non-electrolytes there is no dissociation 



