120 
The  Assay  of  Zinc  Stearate. 
{Am.  Jour.  Pharm. 
March,  1914. 
labelled  sodium  thiosulphate  was  not  what  it  was  labelled,  and  then 
he  spent  several  hours  more  in  finding  out  that  it  was  potassium 
nitrate. 
I  could  tell  you  of  many  more  such  incidents.  It  is  almost 
unbelievable  that  men  who  have  been  graduated  by  leading  institu- 
tions of  learning  can  be  so  helpless.  The  problems  of  the  com- 
petition of  life  bring  us  to  our  senses  and  then  we  realize  the  op- 
portunities that  have  been  thrown  away. 
Coming  back  to  my  opening  question,  "  What  are  you  going  to 
do  one  year  from  to-day?"  If  you  take  advantage  of  the  oppor- 
tunities before  you  during  the  year,  you  will  be  well  equipped  to 
consider  with  intelligence  any  problem  that  may  be  presented  to  you 
one  year  from  to-day. 
THE  ASSAY  OF  ZINC  STEARATE. 
By  Hans  Gesell. 
In  the  last  few  years  the  use  of  Zinc  Stearate  as  an  antiseptic  ■ 
and  astringent  has  been  constantly  increasing;  it  is  rather  difficult, 
however,  to  obtain  this  salt  free  from  impurities,  namely  alkalies, 
alkali  earths,  chlorides,  and  oleates.  The  tests  in  the  Pharmacopoeia 
do  not  give  concordant  results,  so  there  is  no  question  of  the  de- 
sirability of  assay  methods  which  will  be  accurate,  yet  simply  and 
rapidly  carried  out  by  analysts.  An  excess  of  Zinc  Oxide  and  Zinc 
Oleate  seem  to  be  the  most  frequent  admixtures. 
The  usual  analytic  method  by  which  the  stearic  acid  is  liberated 
by  means  of  hydrochloric  acid  and  floats  on  the  surface  of  the  hot 
liquid,  also  holds  good  for  any  Zinc  Oleate  present,  and  if  not  in 
excess,  will  even  congeal  with  the  stearic  acid  on  cooling.  This 
difficulty  can  easily  be  overcome  as  follows : 
Take  I  gm.  of  Zinc  Stearate  and  heat  with  10  c.c.  of  distilled 
water  and  i  c.c.  of  hydrochloric  acid.  The  Stearic  and  oleic  acid 
will  be  liberated  and  float  as  an  oily  layer.  Let  cool  and  this  layer 
will  solidify.  Pour  off  acid  liquid  and  wash  the  cake  several  times 
with  water.  Let  dry.  Determine  the  melting  point.  Pure  Sfearic 
acid  melts  at  690,  but  as  the  Stearic  acid  of  the  market  usually  con- 
tains Palmitic  acid,  the  melting  point  is  usually  55— 560.  Therefore, 
the  melting  point  of  this  cake  should  not  be  below  55 0 — a  lower 
melting  point  would  surely  point  to  the  presence  of  oleic  acid. 
