Am.  Jour.  Pharm.) 
February,  1920.) 
Ionic  Dissocia  Hon . 
93 
appearance  of  hydroxidion  and  its  replacement  by  the  less  mobile 
anion  of  the  acid:  but  when  all  the  hydroxidion  has  been  removed 
by  combination  with  hydrion  from  the  acid  added,  then  any  further 
addition  of  acid  causes  the  conductance  to  increase,  owing  to  the 
addition  of  the  solution  of  the  very  mobile  hydrion.  Since  the 
velocity  of  hydrion  is  much  greater  than  that  of  any  other  ion,  the 
presence  of  a  slight  excess  of  free  acid  causes  a  marked  increase  on  the 
conductance.  Similar  results  are  obtained  when  a  solution  of  a  base 
is  added  to  a  solution  of  an  acid,  provided  both  base  and  acid  are 
strong.  When  the  acid  is  weak  it  must  be  added  to  the  alkali,  which 
in  this  case  must  be  a  strong  base. 
The  advantages  that  conductivity  methods  have  over  colorimetrio 
methods  are  chiefly  these: 
1.  No  great  skill  is  required,  as  delicate  galvanometers  are  used 
which  give  an  accurate  measurement  of  potential. 
2.  No  complicated  calculations  are  necessary. 
3.  The  personal  equation  counts  for  almost  nothing,  and  color 
blindness  is  no  bar,  as  in  the  case  of  the  use  of  indicators. 
4.  Precipitates,  opacity  or  colors  of  solutions  do  not  interfere. 
5.  The  method  is  fundamental,  as  colorimetric  methods  are 
referred  to  it. 
6.  Potentiometers  are  now  made  which  cover  the  entire  range 
of  Ph  values  while  any  one  indicator  is  useful  over  a  limited  range 
only. 
Conductivity  methods  are  largely  replacing  volumetric  methods 
in  many  laboratories  connected  with  large  industries  for  making 
determinations  other  than  those  involving  neutralization.  Par- 
ticularly is  this  the  case  with  determinations  depending  upon  oxida- 
tion and  reduction,  as  in  many  operations  connected  with  iron  and 
steel  analysis.  The  determination  of  the  percentage  of  chromium, 
manganese,  vanadium  and  carbon  in  steel  and  in  presence  of  one 
another  are  relatively  simple  matters,  with  these  advantages  over 
volumetric  methods: 
Accuracy.  If  chromium  is  present  in  steel  to  the  extent  of  20 
per  cent.,  it  may  be  determined  to  within  o.i  per  cent.;  if  present 
to  extent  of  0.2  to  0.3  per  cent,  it  may  be  determined  to  within 
o.oi  per  cent.;  if  less  than  0.2  per  cent,  it  may  be  determined  to 
within  0.002  to  0.003  per  cent. 
