Now  it  has  been  discovered  that  weak  acids,  like  carbonic  and  phos-
phoric, possess  the  remarkable  property  of  maintaining  the  reaction
constant  when  they  are  present  in  a  solution  which  also  contains  an
excess  of  their  salts.  Under  these  circumstances  the  concentration  of
ionized  hydrogen  is  almost  exactly  equal  to  the  product  of  the  dissocia-
tion constantt  of  the  acid  (see  page  10)  multiplied  by  the  ratio  be-
tween free  acid  and  salt;  in  other  words,

K  V[HA]
7K  X[BA]'

If  carbonic  acid  is  present  in  a  solution  of  bicarbonates  so  that  there


'According  to  circumstances,  proteins  may  act  either  as  acids  or  as  alkalies.  They  are  there-
fore called  amphoteric.
tThe  ionization  constant  has  already  been  referred  to  as  a  figure  which  expresses  the  tendency
of  a  weak  acid  or  base  to  dissociate  in  an  aqueous  solution.  "It  expresses  the  proportion  in  which
the  nondissociated  part  is  capable  of  existing  in  the  presence  of  its  ions,"  and  therefore  is  a  gauge
of  the  strength.  The  dissociation  constant  amounts  to  about  0.000,000,5  for  carbonic  acid  ;  that  is,
the  dissociation  of  HoCOs  into  H'-f-HCQg'  at  room  temperature  will  be  such  that  the  concentra-
tion of  H-ion  equals  a  0.000,000,5  N  solution.
