EXCRETION  OF  PHOSPHATES. — The  more  permanent  control  of  neutrality
depends  on  the  excretion  of  phosphates  by  the  kidney.  The  principle
governing  this  process  is  exactly  the  same  as  that  already  discussed  in
connection  with  carbonic  acid.  In  the  one  case  it  is  the  volatile  acid
C02,  and  in  the  other,  the  fixed  phosphoric  acid  that  is  concerned  in  the
reaction.  The  ratio  between  the  acid  salts  of  phosphoric  acid,  MH2P04,
and  the  alkaline  salts,  M2HP04,  in  blood  is  approximately  1  to  5,  but  in
the  urine  this  ratio  varies  according  to  the  amount  of  H  ion  that  must
be  eliminated  from  the  blood.  In  other  words,  a  definite  amount  of  phos-
phoric acid  is  enabled  to  carry  variable  amounts  of  H  ion  out  of  the  body
by  causing  the  amount  of  alkali  excreted  in  combination  with  it  to  be-
come altered.  For  example,  in  the  form  of  MH2P04  a  given  amount  of
P04  obviously  carries  out  more  H  ion  than  when  it  is  excreted  as
M2HP04.  The  adjustment  between  these  two  salts  is  a  function  of  the
kidney.  We  may  accordingly  measure  the  amount  of  alkali  retained  by
the  organism  by  finding  how  much  standardized  alkali  must  be  added
to  a  given  quantity  of  urine  until  the  reaction  of  the  blood  is  obtained.
Since  the  latter  value  is  constant,  the  titration  can  be  done  simply  by
titrating  the  urine  with  an  indicator  such  as  sulphonephenolphthalein,
which  changes  tint  at  about  PH  of  blood.
