Am.  Jour.  Pharm.) 
February,  1920.) 
Ionic  Dissociation. 
83 
The  hydroxidions  are  the  anions,  while  the  K,  Ca,  NH4,  etc.,  are 
the  cations,  sometimes  called  "metal  ions,"  for  reasons  similar  to 
those  given  under  acids. 
Salts  may  be  defined  ionically  as  compounds  which,  when  dis- 
solved in  dissociating  solvents,  yield  cations  other  than  hydrion 
and  anions  other  than  hydroxidion.  The  cations  of  salts  are  the 
cations  of  bases  and  the  anions  are  those  of  the  acids. 
Examples:  NaCl  =  Na+  +  CI";  Ca(N03)2  =  Ca++  +  NO3- 
+  NO3-;  (NH4)2S04  =  NH4+  +  NH4+  +  SO4— . 
From  the  examples  given  it  may  be  seen  that  ions  may  be  either 
single  atoms  or  groups  of  atoms,  but  they  differ  from  the  atoms  of 
elemental  molecules  in  having  radically  different  properties  because 
of  the  electric  charges  which  they  hold  when  in  the  ionic  form. 
Hydrogen,  for  example,  has  very  different  properties  as  an  ion  than 
it  has  as  a  gas.  Oxygen  (acid  former)  was  so  named  because  it  was 
once  thought  that  oxygen  was  necessary  for  acidity,  but  it  now  appears 
that  hydrogen  in  ionic  form  is  necessary  for  acidity,  and  the  greater 
the  degree  to  which  an  acid  ionizes  the  more  pronounced  its  acid 
properties.  In  like  manner  it  appears  that  hydroxidion  is  necessary 
for  alkalinity. 
The  extent  to  which  compounds  ionize  when  passing  into  solu- 
tion is  dependent  upon  temperature  and  the  degree  of  concentration 
of  solution,  and  varies  with  the  composition  of  the  substance.  Such 
acids  and  bases  as  ionize  freely  are  known  as  "strong  acids"  and 
"strong  bases,"  respectively,  while  those  which  ionize  but  slightly 
are  said  to  be  "weak."  Salts  of  "strong"  acids  with  "strong" 
bases,  those  of  "strong"  acids  with  "weak"  bases,  those  of  "weak" 
acids  with  "strong"  bases  all  ionize  freely;  and  even  those. of  "weak" 
acids  with  "weak"  bases  ionize  quite  appreciably.  With  but  few 
exceptions,  salts  ionize  more  freely  than  the  acids  and  bases  to 
which  they  are  related.  Salts  of  the  type  Me'X'  ionize  most  freely, 
those  of  the  types  Me2'X"  and  Me"X2'  ionize  less  freely,  and  those 
of  the  type  Me"X"  still  less  for  a  given  concentration  of  solution. 
In  concentrated  solutions  we  may  have  both  molecules  and  ions, 
the  former  predominating;  in  rather  dilute  solutions  we  may  have 
both,  with  the  ions  predominating;  while  in  very  dilute  solutions  we 
may  have  ions  only.  The  extent  to  which  a  compound  dissociates 
when  dissolved  in  a  dissociating  solvent  may  be  determined  from  the 
freezing  point,  the  boiling  point,  the  osmotic  pressure  and  the 
electrical  conductivity  of  the  solution  as  compared  with  the  same 
