Am'j^neri9i3arm*}  M^ods  for  Analysis  of  Castile  Soap.  251 
eter  in  a  beaker  of  water  and  heat  the  water  slowly.  The  melting 
point  is  regarded  as  the  time  when  the  material  forms  a  clear  drop 
on  the  tip  of  the  thermometer.  Allen's  organic  analysis  gives  the 
melting  -points  of  various  fatty  acids. 
Tests  for  Silica  and  other  Insoluble  Matter .—The  pharmaco- 
poeia determines  these  by  dissolving  20  grammes  of  soap  in  hot 
alcohol,  washing  with  hot  alcohol,  then  with  hot  water  and  weigh- 
ing the  insoluble  as  silica.  A  better  plan  is  to  take  5  grammes  of 
soap,  dissolve  in  about  150  c.c.  hot  water  and  collect  the  insoluble 
on  a  tared  ashless  filter  paper  and  after  washing  with  hot  water 
drying  at  105 0  C.  and  weighing.  The  increase  in  weight  of  the 
filter  paper  indicates  the  insoluble.  After  igniting  this  the  residue 
represents  insoluble  mineral  matter. 
Sodium  Carbonate. — The  pharmacopoeia  dissolves  20  grammes 
of  soap  in  hot  alcohol  and  pours  on  a  tared  filter  paper ;  the  increase 
in  weight  of  the  paper  after  washing  with  hot  alcohol  is  regarded 
as  sodium  carbonate,  silica,  etc. ;  after  pouring  water  on  this  the 
residue  is  silica  and  other  insoluble  matter ;  the  difference  between 
the  two  being  regarded  as  Sodium  Carbonate. 
Free  Alkali. — If  upon  adding  a  few  drops  of  alcoholic  solution 
of"  phenolphtalein  to  the  freshly  cut  surface  of  the  soap  a  pink  color 
is  not  developed  the  absence  of  free  caustic  alkali  is  indicated. 
The  pharmacopceial  method  of  determining  alkalinity  is  inexact 
and  indefinite. 
The  method  of  determining  sodium  carbonate  as  above  directed 
can  with  slight  modification  be  employed  for  the  quantitative  de- 
termination of  free  alkali. 
In  place  of  taking  20  grammes  take  2  grammes  of  soap,  dissolve 
in  about  150  c.c.  of  hot  neutral  alcohol  and  filter.  After  washing 
the  filter  thoroughly  with  hot  neutral  alcohol  add  phenolphthalein 
to  filtrate  and  titrate  with  N/10  H2S04.  The  Alkalinity  found  is 
calculated  as  free  alkali  due  to  sodium  hydroxide.  The  material 
insoluble  in  alcohol  is  then  dissolved  in  water  and  titrated  with 
N/10  H2S04,  using  Methyl  orange  as  indicator.  This  alkalinity 
is  calculated  as  free  alkali  due  to  sodium  carbonate. 
If  the  soap  contains  both  free  alkali  and  free  fat  the  heating 
with  alcohol  will  influence  the  result,  and  for  that  reason  it  is  often 
advantageous  to  follow  Devines'  method  of  determining  free  alkali 
(Journal  American  Chem.  Soc,  1900,  page  693),  which  is  carried 
out  as  follows : 
