90 
Ionic  Dissociation 
(Am.  Jour.  Pharm. 
\     February,  1920. 
N/i  (acid) 
1 .0 
1 .0 
1 .0 
1 .0 
N/io 
0. 1 
0. 1 
lo-i 
Phi 
iV/ioo 
O.OI 
O.OI 
10-2 
PH2 
N / looo 
0.001 
0.02I 
10-3 
PH3 
iV/ioooo 
0.0001 
0.03I 
10-4 
Ph4 
N / looooo 
0.00001 
0.04I 
IO~5 
PH5 
iV/ioooooo  (acid) 
0.000001 
o.Osi 
IO~6 
Phs 
iV/iooooooo 
(neutrality) 
0.0000001 
o.oei 
10-7 
PH7 
iV/ioooooo  (alkali) 
0.000001 
0.05I 
IO~^ 
Phs 
iV/iooooo 
0.00001 
0.O4I 
10-9 
PH9 
iV/ioooo 
0.0001 
0.O3I 
10-10 
Phi, 
iV/ looo 
0.001 
0.O2I 
10-11 
Phi, 
iV/ioo 
O.OI 
O.OI 
10-12 
Ph,, 
iV/io 
0. 1 
0. 1 
IO-13 
Phu 
N/i  (alkali) 
1 .0 
1 .0 
IO-14 
Phh 
It  will  be  noted  that  half  of  the  numbers  given  in  the  first  three 
columns  represent  acidity,  and  the  other  half  represent  alkalinity. 
The  data  given  in  the  last  two  columns  represent  relative  acidity, 
an  N/ioo  solution  of  alkali)  for  example,  having  a  hydrogen-ion 
concentration  of  Phi2,  or  lo"^^. 
If  two  electrodes  of  the  same  metal  are  placed  in  solutions  of  the 
ions  of  the  same  metal  in  different  concentrations,  Ci  and  C2,  re- 
spectively, and  these  solutions  are  placed  in  electrical  contact, 
either  through  a  porous  partition  or  by  means  of  a  siphon,  or  even 
separated  by  gravity  alone,  there  exists  between  the  two  electrodes 
a  difference  of  potential,  expressed  fairly  closely  by  the  formula,^ 
E  =  0.000198  T/n  log  C1/C2 
where  B  denotes  the  difference  of  potential,  T  the  absolute  tempera- 
ture and  n  the  valence  of  the  ions  of  the  metal  in  solution.  At  room 
temperature,  about  i8°  C,  the  formula  becomes 
E  =  0.058/;^  log  Ci/c2.    [0.000198  X  (273  +  18)  =  0.058.] 
If  the  metal  is  silver  (when  w  =  i)  and  if  Ci  and  C2  are  o.i  and 
0.001  normal,  respectively,  then  log  Ci/cz  =  log  100,  or  2,  and  E  = 
0.116.  Conversely,  when  E  is  measured,  either  concentration  can 
be  easily  calculated  if  the  other  is  known.  It  is  thus  possible  to 
determine  quite  accurately  even  very  small  ionic  concentrations, 
and  such  determinations  as  the  solubility  of  the  silver  halides  may 
be  easily  made. 
1  Hildebrand,  Jour.  Am.  Chem.  Soc,  35:  848,  1913. 
