172    PREPARATION  AND  PURIFICATION  OP  HIPPURIC  ACID. 
ON  THE   PREPARATION  AND  PURIFICATION  OF  HIPPURIC 
ACID  AND  ITS  COMPOUND  WITH  OXIDE  OF  ZINC. 
By  D.  J.  Lowe. 
In  order  to  obtain  hippuric  acid  directly  from  the  urine,  the 
author  mixes  fresh  urine  with  an  excess  of  sulphate  of  zinc,  and 
evaporates  it,  together  with  the  precipitate  of  phosphate  and  basic 
carbonate  of  zinc  produced  by  the  addition  of  the  zinc-salt,  to 
one-sixth  or  one-eighth  of  the  original  volume.  This  is  quickly 
filtered ;  the  precipitate  is  washed  on  the  filter  with  a  little  hot 
water,  and  the  filtrate  decomposed  by  means  of  dilute  sulphuric 
or  muriatic  acid.  After  this  operation,  the  entire  fluid  solidifies 
into  a  white  jelly  of  hippuric  acid,  in  a  state  of  purity  which  is 
not  obtained  by  any  other  known  method.  It  is  collected  upon 
a  filter,  and  washed  with  cold  water  as  long  as  the  drops  passing 
are  of  a  yellowish  color;  it  is  afterwards  dried  by  pressing 
between  several  folds  of  blotting-paper.  By  this  process  all  that 
is  necessary  to  obtain  the  acid  in  large  crystals,  is  to  recrystal- 
lize  it.  As  it  is  not  always  possible  to  obtain  large  quantities 
of  fresh  urine,  it  is  advisable,  especially  in  the  hot  season,  to 
pour  it  into  tall  vessels  of  stone-ware,  into  which  a  sufficient 
quantity  of  a  solution  of  sulphate  of  zinc  in  water,  or  dilute 
muriatic  acid,  has  been  previously  put. 
To  purify  a  colored  hippuric  acid,  we  may  avail  ourselves  of 
its  behaviour  towards  metallic  zinc,  which  is  cut  into  small  pieces, 
and  boiled  with  the  solution  of  the  acid.  The  metal  is  strongly 
attacked  with  evolution  of  hydrogen.  The  solution  acquires  a 
slight  yellowish  color,  and  when  the  salt  is  completely  formed,  it 
solidifies  on  cooling  into  a  thick  crystalline  jelly.  If  towards 
the  close  of  the  operation,  when  the  metallic  zinc  is  but  faintly 
attacked,  a  little  freshly-calcined  animal  charcoal  be  added  to 
the  boiling  fluid,  and  this  is  then  filtered,  and  the  filtrate  allowed 
to  run  into  dilute  sulphuric  or  muriatic  acid,  the  hippuric  acid 
separates  in  beautiful  white  crystals.  It  is  only  when  a  con- 
siderable portion  of  the  acid  remains  uncombined,  that  it  still 
retains  a  slight  yellowish  color  after  the  precipitation. 
When  the  quantity  of  acid  to  be  purified  is  considerable,  this 
operation  is  rather  tedious,  even  when  a  small  addition  of  an 
electro-negative  metal  (such  as  platinum  or  lead)  has  been  made 
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