76       PROCESS  FOR  THE  ESTIMATION  OF  RESIN  IN  SOAPS. 
PROCESS  FOR  THE  ESTIMATION  OF  RESIN  IN  SOAPS. 
By  J.  Sutherland. 
Having  often  had  occasion  to  analyze  samples  of  soap,  the 
author  has  found  the  great  want  of  a  process  for  the  correct 
estimation  of  the  resinous  acids  contained  in  admixture  with  the 
fatty  acids.  A  process  has  been  given  in  which  spirit  of  turpen- 
tine is  recommended  for  this  purpose,  but  resin  being  so  soluble 
in  that  menstruum,  it  is  at  once  apparent  that  a  correct  result 
cannot  be  obtained.  The  following  process  depends  on  the  fact 
that  resin,  when  subjected  to  the  action  of  nitric  acid  at  212°, 
is  converted  into  a  soluble  substance  known  as  terebic  acid, 
(C7H10O4,)  with  liberation  of  nitrous  acid ;  while  fatty  acids  are 
unacted  upon,  or  at  least  inappreciably  so.  Of  course,  the  oleic 
acid  present  is  converted  into  elaidic  acid  ;  but,  as  these  acids 
are  isomeric,  the  reaction  does  not  interfere  with  the  accuracy  of 
the  result  obtained. 
Three  hundred  grains  of  the  soap  cut  into  small  pieces  are 
placed  in  a  capsule,  and  covered  with  strong  hydrochloric  acid, 
the  capsule  being  covered  with  a  piece  of  glass,  and  the  contents 
kept  gently  boiling  till  the  soap  is  dissolved  and  thoroughly  de- 
composed. Three  or  four  ounces  of  hot  water  are  then  added, 
and  the  capsule  is  set  aside  to  cool. 
When  cold,  the  cake  of  fatty  and  resinous  acids  is  carefully 
removed  and  re-melted  on  pure  water  to  remove  any  acid  solu- 
tion adhering.  After  cooling,  it  is  dried  on  bibulous  paper,  and 
again  very  gently  re-melted  and  carefully  brought  to  the  boiling 
point  for  a  minute  or  two  to  expel  the  last  traces  of  moisture. 
This  cake,  containing  the  fatty  and  resinous  acids,  must  now 
be  weighed,  and  the  weight  carefully  noted. 
One  hundred  grains  of  the  mixed  acids  are  placed  in  a  six  or 
eight  ounce  capsule.  It  is  covered  with  strong  nitric  acid,  and 
the  temperature  gradually  raised  to  the  boiling  point,  when  a 
powerful  action  takes  place  with  violent  evolution  of  nitrous 
acid  fumes.  The  heat  is  withdrawn  till  the  violence  of  the 
action  subsides,  and  is  then  again  applied  to  maintain  gentle 
ebullition  for  some  minutes  with  frequent  stirring. 
Small  portions  of  nitric  acid  are  successively  added  till  no 
