92 
Ionic  Dissociation. 
(Am.  Jour.  Pharnr. 
\     February,  1920. 
both  as  hydrion  and  in  undissociated  molecules  of  acid,  and  the  total 
alkalinity  of  a  solution  depends,  likewise,  upon  the  amount  of  hy- 
droxyl  (OH)  present  both  as  hydroxidion  and  in  undissociated 
molecules  of  base.  Hydrogen-ion  concentration,  on  the  other  hand, 
depends  upon  the  H  ions  alone,  and  its  value  for  a  given  solution  may 
be  low  when  compared  to  the  total  acidity  as  obtained  by  titrametric 
methods.  It  is  clear,  then,  that  volumetric  methods  for  determining 
acidity  of  a  medium  in  which  the  concentration  in  H  ions  is  the 
important  factor  have  practically  no  value.  This  probably  accounts 
for  many  of  the  discordant  results  often  obtained,  by  workers  in 
bacteriological  fields,  for  instance,  where  relative  acidity  or  alkalinity 
have  so  much  to  do  with  the  success  or  failure  of  such  a  process,  for 
example,  as  that  of  growing  a  culture  of  a  certain  micro-organism 
on  some  particular  culture  medium.  Two  lots  of  culture  media  may 
have  the  same  degree  of  acidity  or  alkalinity,  as  determined  by 
volumetric  methods,  and  may  seem  the  same  in  other  particulars, 
yet  one  may  serve  admirably  for  growing  cultures  of  a  certain  micro- 
organism, while  the  other  is  worthless.  Often  it  has  been  found  that 
two  such  media  differed  quite  radically  in  hydrogen-ion  concentra- 
tion, and  those  workers  who  are  preparing  their  media  in  such  a 
manner  as  to  make  them  approximately  uniform  in  the  matter  of 
hydrogen-ion  concentration  seem  to  be  obtaining  concordant  re- 
sults more  often  than  formerly. 
The  hydrogen-ion  concentration  of  a  solution  may  be  determined 
fairly  accurately  by  colorimetric  methods,  in  which,  by  adding  to 
portions  of  the  solution  under  examination  various  indicators,  color 
changes  are  brought  about,  which  are  dependent  upon  the  nature 
of  the  indicator  and  its  sensitiveness  to  the  concentration  of  H  ions 
present.  Other  methods  have  been  used,  but,  next  to  conductivity 
methods,  the  colorimetric  method  is  preferred,  though  it  leaves 
much  to  be  desired  in  processes  where  results  closely  approaching 
accuracy  are  sought. 
Measurements^  of  electrical  conductance  can  be  employed  in  the 
determination  of  the  point  of  neutralization  of  a  base  by  an  acid, 
or  an  acid  by  a  base;  and  the  method  is  of  especial  importance  when 
dealing  with  colored  or  turbid  solutions,  in  which  the  change  of  color 
of  an  indicator  would  be  more  or  less  masked. 
When  a  solution  of  a  strong  acid  is  added  to  a  solution  of  an  alkali 
the  conductance  of  the  latter  solution  decreases,  owing  to  the  dis- 
^  Findlay:  "Practical  Phys.  Chem.,"  p.  199.    Longmans,  Green  &  Co.,  1914. 
