!68     Determination  of  Vanillin  in  Vanilla  Extract.  {AmMa°rch  ^i™" 
cipitate,  which  analysis  showed  to  contain  the  percentages  of  nitro- 
gen and  sulphur  required  for  the  simple  condensation  product,  3- 
methoxy-4-hydroxybenzalmalonylthiourea.  Since  the  reaction  ap- 
peared to  be  practically  quantitative,  we  decided  to  test  out  its  pos- 
sible application  as  a  means  of  quantitatively  estimating  vanillin. 
The  first  experiments  were  conducted  with  a  standard  solution 
of  pure  vanillin.  Aliquots  of  this  solution  were  precipiated  with 
thiobarbituric  acid  in  the  presence  of  12  per  cent,  hydrochloric  acid. 
The  total  volume  of  the  reaction  mixture  was  50  Cc.  The  precipi- 
tates were  allowed  to  stand  over  night,  then  filtered  on  Gooch 
crucibles,  washed  with  50  Cc.  12  per  cent,  hydrochloric  acid  in 
small  portions  and  finally  with  20  Cc.  of  water.  The  results  are 
set  forth  in  the  following  table: 
Van.  Taken,  G. 
Weight  of  Precip  ,  G. 
Van.  Recov. 
Error,  Mg. 
.0200 
.0330 
.0180 
—  2.0 
.0200 
•0337 
.0184 
-1.6 
.0200 
•0336 
.OI84 
-1.6 
.0200 
•0330 
.0180 
—  2.0 
.0500 
.0877 
.0480 
—  2.0 
.0500 
.O89O 
.0487 
-1-3 
.0500 
.0885 
.0484 
-1.6 
.0500 
.0890 
.0487 
—i-3 
.0500 
.0888 
.0486 
-1.4 
.OSOO 
.0906 
.0496 
-0.5 
.0500 
.09OO 
.0492 
-0.8 
.0500* 
.0890 
.0487 
-r-3 
.0500* 
.0890 
.0487 
-i-3 
In  each  case 
the  filtrate  was 
a  lemon-yellow 
color,  indicating 
that  a  small  amount  of  the  condensation  product  remained  in  solu- 
tion. This  observation  is  furter  substantiated  by  the  fact  that  the 
vanillin  recovered  is  somewhat  less  than  the  amount  taken.  The 
error  is,  however,  quite  uniform,  regardless  of  the  amount  of  vanillin 
taken,  averaging  2.6  Mg.  of  the  condensation  product  or  1.4  Mg. 
vanillin.  It  is  apparent  therefore  that  a  solubility  correction  must 
be  applied  just  as  in  the  determination  of  furfural  by  the  official 
phloroglucinol  method. 
Before  applying  the  method  to  the  determination  of  vanillin  in 
commercial  vanillin  extracts,  where  clarification  is  necessary,  the  Use 
of  lead  acetate  as  a  clarifying  agent  was  tested.  In  the  last  two 
determinations  in  the  above  table,  lead  acetate  was  added  and  the 
*  Clarified  with  lead  acetate. 
