274 
ASSAY  OF  COMMERCIAL  ACETIC  ACID. 
According  to  arrangements  between  the  buyer  and  seller,  the 
standard  of  this  acid  is  taken  either  bj  volume  or  by  weight.  In 
the  former  case  1  c.c.  of  acetic  acid  is  saturated,  in  the  latter 
1  grm. 
In  order  to  take  the  standard  by  weight,  a  small  glass,  con- 
taining in  it  a  10-grm.  weight,  is  tared  on  a  sensitive  balance. 
Equilibrium  being  established,  the  10-grm.  weight  is  taken  out, 
and  the  acetic  acid  to  be  tested  is  then  gradually  poured  into 
the  glass  by  means  of  a  small  tube,  so  as  to  regain  the  equili- 
brium. We  have  thus,  by  double  weighing,  obtained  10  grms. 
of  acetic  acid,  which  are  to  be  diluted  with  water  so  as  to  give  a 
volume  of  100  c.c. 
After  this  solution  has  been  rendered  homogeneous  by  shak- 
ing, 10  centimetres  of  it  are  removed,  corresponding  to  1  grm. 
of  acetic  acid.  Instead  of  directly  weighing  1  grm.  of  the  acid 
to  be  tested,  it  is  better  to  weigh  10  grms.  of  it,  as  we  have  in- 
dicated ;  because,  in  case  of  a  verification  being  needed,  it  is 
easier  to  measure  10  c.c.  than  to  commence  a  fresh  weighing. 
The  10  centimetres  deducted  are  placed  in  a  precipitating  jar, 
litmus  is  added,  and  the  alkaline  soda  liquid  poured  in,  drop  by 
drop,  till  the  litmus  becomes  clearly  blue.  If  150  divisions  have 
been  used,  we  shall  obtain  the  acidimetric  standard  of  pyrolige- 
neous  acid  by  the  following  equation — 
500  divisions  saturate       .        .    1-224  grs.  of  C^H^O^ 
150  will  saturate .        .  x. 
whence — 
X  =  0-3672. 
300  grms.  of  the  assayed  acetic  acid  contain,  then,  36-72  grms. 
of  C4H4O4,  which  is  expressed  by  saying  that  its  standard  is 
36-72°. 
When  the  standard  is  taken  by  volume  the  operation  is  the 
same  ;  only,  instead  of  weighing  10  grms.  of  the  acid,  10  c.c.  are 
measured. 
Examination  for  Mineral  Acids — Acetic  acids  are  sometimes 
adulterated  with  mineral  acids  (chlorhydric  and  sulphuric,  &c.), 
which  augment  their  standard. 
To  detect  their  presence,  50  c.c.  of  acetic  acid  are  heated  to 
the  boiling-point  with  1  or  2  centigrms.  of  starch,  and  left  to 
