:88 
Ionic  Dissociation. 
(Am.  Jour.  Pharm. 
\     February,  1920. 
sible  easily  to  determine  just  what  particular  acidic  or  alkalinic 
compound  may  be  present  in  a  given  solution  to  govern  one  in  the 
choice  of  indicator  to  be  used,  it  is  not  surprising  that  methods  by 
which  the  hydrogen-ion  concentration  of  a  solution  may  be  deter- 
mined electrometrically  should  be  receiving  so  much  attention  as  they 
are  in  certain  quarters  and  among  certain  workers  at  the  present 
time.  There  are  other  reasons,  which  we  expect  to  give  in  this  paper, 
for  the  favorable  reception  that  electrometric  processes  are  getting. 
Before  going  into  any  explanation  of  electrometric  methods  for 
determining  the  hydrogen-ion  concentration  of  solutions  it  is  per- 
tinent to  look  into  some  of  the  facts  underlying  them. 
Theoretically,  it  is  impossible  to  get  or  make  any  substance  ab- 
solutely pure,  since  every  substance  is  soluble  in  some  degree  in 
every  other  substance  with  which  it  comes  into  contact,  hence  ab- 
solutely pure  water  is  not  obtainable.  By  rather  elaborate  methods 
of  purification  water  has  been  prepared  so  nearly  pure  that  a  milli- 
meter cube  of  it  was  calculated  to  give  the  same  resistance  to  the 
passage  of  an  electric  current  through  it  as  would  a  copper  wire  a 
square  millimeter  in  cross  section  and  long  enough  to  encircle  the 
earth  at  the  equator  a  thousand  times.  Such  water,  called  con- 
ductivity water,  has  a  specific  conductivity  of  about  io~^,  and  is 
sufficiently  near  pure  to  be  used  in  making  solutions  for  hydrogen- 
ion  determinations.  Water  ionizes  according  to  the  following 
equation : 
H2O  =  H+  +  0H~,  but  the  degree  of  ionization  is  very  slight, 
so  that  absolutely  pure  water  at  22°  to  23°  C.  is  estimated  to  con- 
tain H  ions  of  a  concentration  expressed  at  io~^  and  OH  ions  of  the 
same  concentration,  which  corresponds  to  one  gram  of  H  ions 
and  17  grams  of  OH  ions  in  ten  million  liters.  This  is  equivalent 
to  a  ten-millionth  normal  solution  of  either  acid  or  alkali  (N/ 10,000,- 
000). 
In  any  solution  containing  ions  from  a  dissociated  compound 
the  product  of  the  concentration  of  the  positive  ions  (cations)  and 
the  concentration  of  the  negative  ions  (anions)  is  a  constant.  The 
constant  at  22°  C.  for  pure  water  is,  accordingly,  lo"''  (for  H) 
times  10-7  (for  OH),  or  lo-^^ 
If  to  a  solution  containing  ions  of  a  certain  kind  there  be  added  a 
substance  yielding  ions  of  the  same  kind,  the  ionization  of  the  com- 
pound which  furnished  the  original  ions  will  be  "pushed  back" 
sufficiently  to  make  the  product  of  the  new  concentrations  the  same 
