28  US.P.  Chemicals  and  Tests.  {A j^/uTry  1910™" 
requires  99  per  cent.  Na2HP04  +  I2H20  and  allows  small  quantities 
of  innocuous  impurities  which  in  no  way  impair  its  therapeutic 
value. 
However,  the  official  description  must  be  made  ro  clear  that 
there  can  be  no  question  concerning  poisonous  and  undesirable  im- 
purities, and  when  a  chemical  in  question  complies  with  the  purity 
rubric,  it  certainly  fulfils  the  Pharmacopceial  requirements.  A  num- 
ber of  questions  have  been  raised  concerning  this  very  point,  and  it 
undoubtedly  was  the  intention  of  the  Committee  of  Revision,  that 
the  purity  rubric  should  cover  the  case,  that  when  a  substance  is  free 
from  deleterious  matters  and  meets  the  percentage  required  by  the 
rubric,  it  complies  fully  with  the  meaning  of  the  text. 
Melting  and  boiling  points  have  already  been  freely  discussed, 
and  there  is  no  doubt  that,  as  a  criterion  for  purity  of  many  sub- 
stances, the  Pharmacopoeia  must  insist  on  their  use  for  this  purpose. 
However,  it  is  very  necessary  that  uniform  methods  should  be 
required  for  their  determination,  and  this  also  applies  to  solubilities, 
as  experience  has  shown  that  cases  have  occurred  where  variations 
have  been  noted.  Is  the  solubility  determined  by  dissolving  the 
specified  quantity  of  the  chemical  in  the  required  amount  of  the 
solvent,  or  is  it  determined  by  the  amount  of  the  chemical  remaining 
in  the  saturated  filtered  mother-liquor?  In  the  case  of  some  alka- 
loids and  their  salts  discordant  results  have  been  found,  and  there 
is  evidently  no  question  but  that  there  should  be  careful  revision  to 
meet  such  instances. 
Ash  and  residue  determinations  have  been  a  source  of  consider- 
able discussion  in  the  laboratory.  When  it  is  stated  that  there 
should  be  no  weighable  residue,  and  there  is  no  definite  quantity 
given,  it  depends  a  great  deal  on  the  operator  whether  or  not  this 
requirement  is  fulfilled.  It  is  also  necessary  in  cases  where  evap- 
oration is  required  to  take  the  actions  of  the  various  chemicals  on 
glass  and  porcelain  into  consideration,  as  the  residue  may  be  in- 
creased manifold  if  this  necessary  precaution  is  neglected.  A  plati- 
num crucible  for  these  determinations  is  not  mentioned  in  any  in- 
stance in  the  U.S. P.,  and  the  reason  is  obvious,  as  all  of  us  are  not 
in  possession  of  such  a  valuable  piece  of  apparatus,  but  for  accurate 
work  platinum  is  required.  There  have  been  instances  reported 
of  examinations  of  chemicals,  during  which  porcelain  crucibles  were 
used,  with  the  result  that  finally  impurities  were  in  evidence  that 
were  not  present  in  the  original  substance. 
