Am.  Jour.  Pharm.  | 
April,  1910.  j" 
The  U.S.P.  Melting  Points. 
the  manufacturers'  committee  to  the  Committee  on  Revision  of 
the  U.S.P. 
The  so-called  "  personal  factor  "  or  "  personal  equation  "  (or 
other  synonymous  phrase)  is  doubtless  a  material  and  legitimate 
cause  of  divergence  to  some  slight  extent  in  any  equally  conscien- 
tious work ;  but  it  is  also  readily  adapted  to  the  service  of  a  screen  or 
shield  covering  careless,  indifferent,  and  hurried  manipulation.  The 
amount  of  divergence  honestly  due  to  the  "personal  factor,"  I  am 
strongly  inclined  to  believe,  would,  in  this  instance,  at  least  come 
well  within  the  limits  of  a  reasonably  rigid  standard,  and  for  prac- 
tical purposes  may  therefore  be  disregarded. 
In  so  far  as  divergence  is  due  to  manipulation  of  the  apparatus 
we  believe  it  to  be  due  largely,  if  not  entirely,  to  differences  in  rate 
of  heating  and  the  variable  application  of  stirring — entirely  omitted 
in  most  cases ;  very  irregular  in  others. 
In  describing  the  official  method  I  would  recommend,  as  a 
remedy  for  such  defects,  that  the  rate  of  heating  for  different  stages 
of  the  determination  be  definitely  prescribed  and  that  constant 
stirring  be  required  throughout  the  experiment. 
Under  differences  in  physical  condition,  as  a  cause  of  divergent 
values,  there  are  three  main  considerations :  ( i )  the  size  of  the 
individual  particles  of  the  compound;  (2)  the  moisture  content; 
and  (3)  the  presence  of  impurity. 
Pawloff  2  has  shown  experimentally,  working  mainly  with  salol, 
that  the  more  finely  divided  a  solid  is  the  lower  is  the  melting  point — 
the  magnitude  of  difference  depending  in  some  measure  upon  the 
purity.  He  finds  that  a  powder  composed  of  particles  less  than  2  jx 
(in  diameter)  melts,  in  the  case  of  salol,  70,  in  the  case  of  antipyrine, 
5-70,  and  in  the  case  of  phenacetin,  40,  lower  than  particles  of 
0.5-2  mm.  diameter. 
Considering  the  wide  range  in  size  of  the  particles  of  the  same 
product,  as  found  in  the  market  under  different  labels,  the  experi- 
mental results  of  Pawloff  make  it  obvious  that  in  order  to  eliminate 
this  cause  of  divergence  it  is  necessary  to  officially  prescribe  that 
all  substances  shall  be  finely  powdered  before  being  subjected  to 
the  melting  point  test. 
The  work  of  Tyrer  and  Levy,  previously  referred  to  in  this 
paper,  offers  conclusive  evidence,  if  any  were  needed,  01  the  marked 
2 Zeit.  Physikal.  Chan.,  65,  1-35  (1908). 
