ASSAY  OF  COMMERCIAL  ACETIC  ACID. 
27B 
ASSAY  OF  COMMERCIAL  ACETIC  ACID, 
By  M.  Gaston  Tissandier. 
Acetic  acid,  sometimes  called,  in  commerce,  pjroligneous  acid, 
generally  contains  about  40  per  cent,  of  acetic  acid,  C^H^O^ ;  it 
is  sold  by  the  acidimetric  standard,  which  is  determined  by  means 
of  a  titrated  alkaline  liquid. 
Acidimetric  Standard. — Preparation  of  the  Alkaline  Liquid. — 
We  have  already  shown  how  the  alkalimetric  sulphuric  acid  may 
be  prepared  accurately;  this  well-verified  solution,  containing 
100  grms.  of  sulphuric  acid  per  litre,  is  the  basis  of  the  prepara- 
tion of  the  alkalimetric  liquid  which,  in  laboratories,  is  used  to 
effect  nitrogen  determinations  or  to  take  acidimetric  standards. 
Any  quantity  whatever  of  pure  caustic  soda — 15  or  18  grms., 
for  instance — is  dissolved  in  a  litre  of  water.  10  centimetres 
of  the  normal  sulphuric  acid  liquor  (containing  1  grm.  of 
SO3HO)  are  taken  and  poured  into  a  small  precipitating  glass ; 
to  this  is  added  some  sensitive  tincture  of  litmus,  and  the  solu- 
tion of  caustic  soda  poured  into  it  drop  by  drop  by  means  of  a 
graduated  burette,  divided  into  tenths  of  c.c,  till  the  red  litmus 
becomes  blue — that  is  to  say,  till  the  acid  is  saturated.  Let  us 
suppose  that  50  c.c,  or  500  divisions,  of  our  alkaline  solution  are 
required  to  saturate  10  c.c.  of  the  titrated  sulphuric  acid  liquid. 
We  know^  that  500  divisions  saturate  1  grm.  of  sulphuric  acid, 
and  we  can  calculate,  according  to  the  equivalents,  what  quantity 
of  acetic  acid  will  saturate  a  certain  volume  of  our  liquid;  w^e 
know,  for  example,  that  500  divisions  ought  to  saturate  1*224 
49(S03,HO)  1. 
grms.  of  acetic  acid,  C^H^O^ : — in  fact,   =  — 
60  (CJI.O,)  X 
Before  thus  standardizing  the  alkaline  liquor  of  caustic  soda, 
it  is  well  to  add  some  slaked  lime,  to  prevent  it  from  carbonat- 
ing,  w^hich  would  interfere  with  the  sharpness  of  the  coloration 
of  the  red  litmus  into  blue.  B<ifore  using  this  liquor,  the  bottle 
which  contains  it  is  shaken,  and  left  to  settle,  so  that  the  lime 
may  be  deposited  at  the  bottom  ;  the  solution,  becoming  clear  in 
a  few  minutes,  is  then  poured  into  the  graduated  burette,  which 
is  used  to  take  the  standard  of  pyroligneous  acid  under  assay. 
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