14 
Ammonium  Benzoate. 
(Am.  Jour.  Pharm 
\    January-,  1910. 
12.24  Per  cent.)  in  the  compound  the  multiplication  of  the  error  in 
calculating  to  benzoate  is  considerable,  and  therefore  the  accuracy 
of  the  method  is  probably  within  only  about  0.5  per  cent,  of  the 
amount  of  the  salt  present.  It  is  interesting  to  note  that  of  the 
four  commercial  samples  analyzed  they  all  gave  results  above  99 
per  cent.  The  Pharmacopceial  purity  rubric  of  98  per  cent,  for  this 
compound  might,  therefore,  be  raised  to  99  per  cent,  without  requir- 
ing a  change  in  the  present  market  conditions  of  this  product. 
The  Formaldehyde  Method  for  the  Analysis  of  Ammonium 
Benzoate. — This  method,  which  depends  upon  the  formation  of 
hexamethylenetetramine  and  simultaneous  liberation  of  the  acid 
of  any  ammonium  salt  when  an  excess  of  a  neutral  solution  of  for- 
maldehyde is  added  to  it,  was  suggested  by  Schiff  1  and  subsequently 
by  Ronchese.2  Dr.  B.  Herstein,  of  the  Drug  Laboratory,  Bureau 
of  Chemistry,  Department  of  Agriculture,  recently  tested  this 
method  (private  communication)  upon  a  large  number  of  ammo- 
nium salts,  including  the  sulphate,  nitrate,  phosphate,  oxalate,  citrate, 
molybdate,  halogen  salts,  etc.,  and  found  that  entirely  satisfactory 
results  were  obtained. 
The  determination  may  be  very  conveniently  made  as  follows : 
about  5  c.c.  of  the  ordinary  40  per  cent,  formaldehyde  is  just  neu- 
tralized in  an  Erlenmeyer  flask  with  dilute  alkali  solution,  using 
phenolphthalein  as  indicator,  an  aliquot  portion  of  the  ammonium 
salt  solution  corresponding  to  0.5  Gm.  is  then  added,  and  the  lib- 
erated acid  titrated  to  the  first  appearance  of  the  pink  color  of  the 
phenolphthalein,  with  0.1  N  NaOH ;  the  solution  is  then  heated  just 
to  the  boiling  point  and  a  further  amount  of  alkali  added  to  the 
reappearance  of  the  faint  pink  color.  Our  experiments  showed  that 
satisfactory  results  were  not  obtained  when  less  than  3.0  c.c.  or 
more  than  10  c.c.  of  40  per  cent,  formaldehyde  were  used  per 
0.5  Gm.  of  ammonium  benzoate.  The  analytical  results  are  given 
in  the  following  Table  II. 
These  results  confirm  the  general  conclusion  that  the  formal- 
dehyde method  is  very  satisfactory  for  the  determination  of  the  acid 
radicle,  and  should,  no  doubt,  be  adopted  as  the  quantitative  method 
for  most  of  the  ammonium  compounds  of  the  Pharmacopoeia.  It 
happens,  however,  in  the  present  case  that,  since  the  most  common 
impurity  of  ammonium  benzoate  is  free  benzoic  acid  resulting  from 
1Liebig's  Annalen,  319,  75,  1901 ;  Chem.  Ztg.,  27,  14,  1903. 
2  Jour,  pharm.  et  chim.  (6),  25,  611,  1906. 
