64  The  Purity  Rubric  and  US.P.  Tests.  {^S^uST' 
as  the  standard  for  purity  and  strength  of  the  medicines  enumerated 
in  its  pages,  the  requirements  should  be  attainable  and  the  tests 
necessary  to  establish  the  identity,  strength,  and  purity  of  these 
medicinal  substances  should  be  such  as  can  be  followed  by  all  of 
the  directly  responsible  persons  engaged  in  the  medicine  supply 
business. 
In  other  words,  given  a  reasonable  standard  for  a  substance,  the 
identity,  purity,  and  activity  of  this  compound  should  be  controlled 
and  guaranteed  by  each  person  handling  it.  It  follows,  therefore, 
that  the  passage  from  the  producer  to  the  consumer  should  be  safe- 
guarded from  cupidity  and  ignorance  in  such  a  way  that  at  no  point 
will  there  be  opportunity  for  deterioration  or  sophistication  without 
at  least  a  fair  chance  of  the  shortcomings  being  detected  before 
the  medicine  reaches  the  consumer. 
A  prescribed  purity  rubric  which  is  not  accompanied  by  a  clearly 
described  analytical  method  gives  an  opportunity  for  differences  in 
the  results  which  may  be  obtained  by  even  the  most  careful  analysis 
working  with  different  methods.  As  a  general  principle  it  would, 
therefore,  appear  that  in  so  far  as  it  is  possible  quantitative  analytical 
methods  be  selected  for  all  substances  of  the  Pharmacopoeia  for 
which  a  purity  rubric  is  given.  Furthermore,  it  is  desirable  that, 
other  things  being  equal,  the  method  applicable  to  the  largest 
number  of  compounds  containing  a  given  constituent  should  be 
selected.  Such  general  methods  could  then  be  described  at  some 
length  in  the  Appendix,  and  simply  referred  to  under  the  description 
of  the  compound,  in  much  the  same  way  as  the  now  official  titri- 
metric  processes  are  referred  to  in  the  U.S. P. 
As  a  practical  demonstration  of  the  possibility  of  elaborating 
such  efficient  yet  simple  quantitative  methods,  the  following  labora- 
tory notes  on  some  of  the  Pharmacopceial  compounds  examined  in 
the  Division  of  Pharmacology  of  the  Hygienic  Laboratory  during 
the  past  year  are  herewith  presented. 
Red  Mercuric  Iodide. — The  Pharmacopceial  purity  rubric  for 
this  salt  requires  that  it  contain  not  less  than  98.5  per  cent,  of  pure 
mercuric  iodide.  Tests  for  the  identity  and  for  the  presence  of 
certain  impurities  are  given  by  the  Pharmacopoeia  but  no  quanti- 
tative method  by  which  the  required  purity  may  be  determined. 
There  are  a  number  of  processes  which  probably  could  be  adapted 
to  the  present  purpose,  but  the  one  which  has  been  found  most 
satisfactory  is  as  follows : 
A  weighed  quantity  of  the  sample  (about  1.0  Gm.)  is  mixed 
