Am.  Jour.  Pharm.) 
February,  1920.) 
Ionic  Dissociation. 
85 
which  are  easily  attacked  by  dilute  solutions  of  the  same  acids. 
A  familiar  example  is  the  behavior  of  sulphuric  acid  on  iron.  The 
diluted  acid  attacks  the  metal  readily  with  the  liberation  of  hydrogen 
gas,  while  the  highly  concentrated  form  of  the  acid  has  so  little  action 
on  the  metal  that  iron  is  used  in  the  construction  of  tanks,  tank 
cars  and  other  containers  for  the  storage  and  transportation  of  this 
acid.    The  same  is  true  of  hydrochloric  and  nitric  acids. 
Reference  has  been  made  to  the  fact  that  the  extent  to  which  a 
compound  dissociates  in  solution  may  be  determined  by  conductivity 
methods.  The  more  ions  in  a  given  solution  the  better  it  will  con- 
duct the  electric  current.  As  a  solution  containing  molecules  and 
ions  is  diluted  its  conductivity  increases  until  a  certain  point  is 
reached,  complete  ionization,  after  which  the  conductivity  decreases 
with  further  dilution,  as  the  number  of  ions  in  a  unit  volume  of  solu- 
tion becomes  less. 
The  relative  conductivity  of  several  substances  in  solution  are 
here  given  for  study. 
Gram-Molecules  in  1000  Gms.  Water. 
HNO3 
HCl 
KCl 
NaCl 
I  .  00  
2770 
2780 
919 
695 
0.50  
•  2991 
3017 
958 
757 
0.  10  
•  3225 
3244 
1047 
865 
0 . 05  
•  3289 
3330 
1083 
895 
O.OI  
•  3395 
3416 
I  147 
962 
Reference  to  the  table  shows  that  solutions  of  acids  are  much 
better  conductors  than  solutions  of  salts  of  the  same  concentra- 
tion, and  it  has  been  shown  that  bases  give  solutions  intermediate 
in  conductivity.  Among  the  electrolytes  "strong"  acids  are  the 
best  conductors,  with  "strong"  bases  next  and  salts  the  slowest. 
This  is  explained  when  one  compares  the  relative  velocities  of  their 
ions,  hydrion  being  the  swiftest,  hydroxidion  next  in  speed,  and 
other  ions  slower,  in  any  given  solvent  and  under  any  given  driving 
force.  Compare  the  following  figures  which  show  the  relative 
velocities  of  several  common  ions  at  25°  C: 
H  ion,  325;  OH  ion,  170;  Na  ion,  49.2;  K  ion,  70.6;  CI  ion,  70.2. 
The  velocity  of  an  ion  in  any  given  solution  is  independent  of  the 
nature,  number  or  condition  of  other  ions  present  in  the  same  solu- 
tion, an  important  fact  in  hydrogen-ion  concentration  determina- 
tions. 
While  metals  are  better  conductors  at  high  temperatures  than 
at  low  temperatures,  the  reverse  is  the  case  with  electrolytes.  It 
