Am.  Jour.  Pharm.) 
February,  1920.) 
Ionic  Dissociation. 
91 
Now  the  same  principle  can  be  applied  to  the  determination 
of  hydrogen-ion  concentration  (acidity  or  alkalinity),  if  for  elec- 
trodes we  use  a  noble  metal,  like  platinum  or  palladium  saturated 
with  hydrogen  gas  under  definite  pressure.  These  electrodes  are 
usually  of  platinum  or  gold  in  thin  sheets  previously  covered  with 
platinum  black  by  electrolytic  methods,  and  so  disposed  in  a  glass 
holder  that  they  can  be  kept  saturated  with  the  gas.  Their  action 
is  the  same  as  if  the  hydrogen  were  in  solid  form.  If  we  use  such  an 
electrode  dipping  into  a  solution  containing  normal  hydrion  so  that 
Ci—  If  and  measure  the  difference  of  potential  between  it  and  another 
similar  electrode  dipping  into  a  solution  of  which  the  hydrogen-ion 
concentration  is  unknown,  the  two  solutions  being  connected,  let 
us  say,  by  a  siphon  containing  concentrated  potassium  nitrate  or 
chloride  solution,  then  the  unknown  concentration  and  the  difference 
of  potential  are  related  by  the  expression 
E  =  0.058  log  i/c. 
In  practice  a  calomel  electrode  (consisting  of  pure  mercury  in  con- 
tact with  a  saturated  solution  of  Hg2Cl2)  is  used  in  place  of  one  of  the 
hydrogen  electrodes.  This  involves  the  making  of  a  correction  in 
the  final  calculations,  but  this  disadvantage  is  more  than  balanced 
by  the  advantages  to  be  gained  by  the  substitution.  For  measuring 
hydrogen-ion  concentrations  a  calomel  electrode,  a  hydrogen  elec- 
trode and  a  potentiometer  for  measuring  the  voltage  between  them 
are  employed. 
We  have  seen  that  all  acids  contain  hydrogen  atoms  which 
when  the  acid  is  diluted  with  water,  become  hydrogen  ions.  Some 
acids,  the  so-called  "strong"  acids,  are  more  highly  dissociated  in  a 
solution  of  given  concentration  than  are  others,  or,  in  other  words, 
yield  solutions  of  greater  hydrogen-ion  concentration.  The  organic 
acids  are,  relatively,  "weak"  acids,  but  differ  from  each  other  quite 
widely  at  times  in  this  respect.  In  many  vital  processes,  such  as, 
for  example,  those  in  which  chemical  changes  are  brought  about  by 
the  presence,  in  solution  or  suspension,  of  ferments  or  bacteria, 
the  rapidity  of  the  action,  the  nature  of  the  substances  formed, 
and  other  factors,  are  apparently  dependent  in  large  part  upon  the 
hydrogen-ion  concentration  of  the  medium  in  which  the  changes 
are  taking  place,  rather  than  upon  the  total  acidity  or  alkalinity, 
as  was  for  a  long  time  supposed  to  be  the  case.  The  total  acidity 
of  a  solution  depends  upon  the  amount  of  acidic  hydrogen  present 
