146 
Soap  Manufacture  and  the  Soap  of  Commerce,  j 
Am.  Jour.  Pharm. 
March,  1884. 
The  percentage  of  silicate  is  obtained  from  the  silicic  acid  found. 
To  obtain  this  I  prefer  to  ignite  about  2  grams  of  the  soap  in  a  plati- 
num dish  until  the  volatile  matters  are  dispersed.  After  cooling,  the 
ash  is  covered  with  a  glass  and  treated  with  an  excess  of  hydrochloric 
acid.  It  is  then  evaporated  to  dryness,  taken  up  with  dilute  acid,  well 
washed  and  then  ignited  and  weighed. 
These  are  the  constituents  which  it  is  usually  necessary  to  determine, 
but  it  is  sometimes  required  to  make  a  more  complete  analysis.  When 
this  is  desired  it  is  a  good  plan  to  dissolve  the  soap  in  alcohol  and 
filter.  By  this  means  most  ol  the  adulterating  materials  are  separated. 
The  chlorine  is  best  estimated  after  decomposing  the  soap  with  nitric 
acid  and  allowing  the  fat  to  solidify,  as  in  the  estimation  of  fatty  acids, 
by  precipitating  with  nitrate  of  silver  and  weighing  the  resulting 
chloride. 
The  percentage  of  free  alkali  is  important.  It  can  be  obtained  by 
precipitating  the  clear  alcoholic  solution  with  carbonic  acid,  but  I 
prefer  to  titrate  the  solution  writh  standard  acid,  using  phenolphthalein 
as  indicator.    The  results  are  good. 
In  making  out  the  analysis  of  a  soap  it  must  be  remembered  that 
the  fatty  constituents  actually  exist  as  fatty  anhydrides  and  not  as  fatty 
acids,  and  if,  therefore,  we  determine  the  whole  of  the  constituents  of 
a  soap  and  include  the  fatty  matters  as  the  estimated  acids  we  shall 
find  that  the  figures  will  add  up  to  about  103  per  cent.  This  is  due 
to  the  absorption  of  water  by  the  fatty  anhydrides  in  decomposition. 
The  actual  percentage  of  fatty  acids  should  always  be  placed  as  a  foot 
note. 
In  making  a  choice  of  the  soaps  usually  found  in  the  market  is  is 
difficult  to  know  which  to  take  as  representative,  but  it  will,  perhaps, 
be  sufficient  to  divide  them  into  two  classes,  the  pure  and  the  silicated. 
The  analyses  given  of  the  average  qualities  of  these  soaps  show  the 
the  following : 
Soaps. 
Fatty  Acids. 
Soda. 
Hydra,  ted 
Silicate  of  Soda. 
Water. 
- 
Highest. 
Lowest. 
Highest. 
Lowest. 
Highest. 
Lowest. 
Highest. 
Lowest. 
Pure  
63-18 
56  -91 
53*74 
26-26 
8-31 
7-45 
6-38 
5-30 
28-13 
31*41 
36  -89 
58-97 
Silicated  
8-58 
1-04 
— Phar.  Jour,  and  Trans.,  Jan.  5,  1884,  pp.  534-537. 
