3^4 
Analysis  of  Ointments. 
J  Am.  Jour.  Pharm. 
(       July,  1910. 
tative  analysis,  using  a  weighed  quantity  of  the  ointment,  shaking  out 
twice  with  the  solvent.  Drive  oft  the  latter  on  the  water  bath,  dry 
in  steam  oven  and  weigh. 
Soap. — Soluble  soap,  for  example  castile  soap,  may  be  shaken 
out  twice  with  hot  water,  filtered  after  cooling,  the  filtrate  evaporated 
and  the  soap  dried  and  weighed.  Soaps  insoluble  in  water,  zinc 
stearate,  lead  plaster,  etc.,  can  be  estimated  with  tolerable  accuracy 
by  quantitating  the  metal  (see  quantitative  analysis  under  metals). 
Stearic  Acid. — In  the  absence  of  other  free  fatty  acids  and 
alcohol  soluble  material,  stearic  acid  may  be  estimated  when  in  ad- 
mixture with  oils  by  introducing  a  weighed  sample  into  a  flask, 
adding  about  50  c.c.  methyl  alcohol  and  a  few  drops  phenolphthalein. 
The  flask  is  immersed  in  hot  water,  its  contents  thoroughly  agitated 
and  half  normal  caustic  alkali  added  until  after  vigorous  shaking 
the  liquid  retains  a  faint  pink  color.  The  alcoholic  liquid  is  sepa- 
rated from  the  oil,  the  alcohol  evaporated  and  the  residue  taken 
up  with  water.  The  aqueous  solution  is  agitated  with  a  little  petro- 
leum spirit  to  remove  any  oil  or  fat,  then  separated  and  the  stearic 
acid  precipitated  by  the  addition  of  diluted  sulphuric  acid.  On  shak- 
ing with  ether,  separating  the  ethereal  solution  and  evaporating 
to  dryness,  the  remaining  stearic  acid  can  be  weighed. 
Resinous  Substances. — Proceed  as  given  under  substances  solu- 
ble in  alcohol  (qualitative  analysis),  using  a  weighed  quantity  of 
the  ointment,  shaking  out  twice  with  hot  alcohol  (20+10  c.c), 
cool  on  ice,  filter,  evaporate  filtrate,  dry  and  weigh.  A  fair  approxi- 
mate estimation  of  colophony  in  admixtures  with  neutral  fats  or  oils 
can  be  effected  by  mixing  the  ointment  with  a  mixture  of  alcohol 
and  ether  and  titrating  with  standard  decinormal  sodium  hydroxide 
solution.  Each  c.c.  of  decinormal  alkali  represents  0.034  colophony, 
taking  165  as  the  average  acid  number  of  colophony.  Free  fatty 
acids  (stearic  acid)  when  present  in  quantity,  will  vitiate  the  re- 
sults.9 This  method  may  be  used  for  estimating  the  proportions  of 
colophony  in  a  mixture  with  linseed  oil  used  as  bird  or  fly  lime.10 
Acid  resin,  such  as  colophony,  can  be  separated  from  the  neutral 
fats  by  boiling  the  substance  with  a  strong  solution  of  sodium 
bicarbonate  or  borax.  After  cooling,  the  aqueous  liquid  is  separated 
from  the  fat  and  the  resin  precipitated  from  its  solution  by  adding 
hydrochloric  acid.11 
Qlvceriu  —It  may  be  approximately  estimated  after  separating 
the  fatty  acids  (saponification  of  the  ointment  with  alcoholic  potash 
