552  U.  S.  Pharmacopoeial  Revision.       [  iSgus^rQ™: 
impurities  should  be  made  of  a  more  definite  character  wherever 
possible.  Such  expressions  as  "slightly  acid,"  "slightly  turbid," 
etc.,  have  always  been  deprecated.  They  have  been  a  source  of 
uncertainty  and  worry  to  the  conscientious,  and,  on  the  other  hand, 
a  good  excuse  for  foisting  on  the  public  inferior  products.  With  the 
number  of  persons  engaged  in  testing  U.  S.  P.  preparations  rapidly 
increasing,  the  possibilities  of  attaching  different  interpretations 
to  these  expressions  is  multiplied  and  it  becomes  very  necessary  to 
guard  against  it. 
Quantitative  or  semi-quantitative  tests  should  therefore  be 
adopted  for  permissible  impurities  wherever  such  tests  are  avail- 
able, when  they  are  not  difficult  of  execution,  and  can  be  made 
without  the  use  of  elaborate  apparatus.  For  instance,  instead  of 
"slightly  acid  or  slightly  alkaline"  a  titrimetric  limit  for  acidity  or 
alkalinity  should  be  given.  Turbidimetric  comparison  tests  can 
likewise  be  used  for  chlorides,  sulphates,  and  similar  impurities. 
These  tests  should  preferably  be  described  under  a  general  heading 
in  Part  II,  as  the  test  for  arsenic  in  the  present  edition.  The  ad- 
vantage of  this  method  is  very  obvious.  The  tests  can  be  described 
with  more  detail  and  greater  clearness,  and  thus  promote  greater 
accuracy  and  uniformity. 
In  describing  tests  for  impurities,  no  detail  should  be  omitted 
which  is  conducive  to  more  harmonious  results.  As  an  example, 
the  method  of  preparation  of  saturated  solution  of  the  substance  to 
be  tested  may  be  cited.  Not  infrequently  diametrically  opposed 
reports  have  been  made  because  of  the  different  methods  used  in 
preparing  the  saturated  solution. 
In  salts  containing  water  of  crystallization,  the  U.  S.  P.  assays 
frequently  fail  in  their  object.  A  deficiency  in  purity  may  be  con- 
cealed by  a  deficiency  in  water.  For  example,  sodium  phosphate 
or  thiosulphate  of  sub-standard  purity  may  still  show  the  required 
purity  of  the  assay  and  thus  defeat  the  object  of  the  U.  S.  P.  To 
remedy  this  defect,  it  is  suggested  that  suitable  tests,  limiting  the 
proportion  of  such  impurities  as  are  likely  to  be  present  and  are  not 
taken  care  of  by  the  assay,  be  introduced.  In  the  instances  cited 
above,  a  test  for  limit  of  sulphate  will  be  appropriate.  Another 
method  to  overcome  this  is  to  assay  the  dried  (anhydrous)  salt. 
The  latter  procedure,  however,  is  not  always  practicable. 
Assays. — ^The  introduction  in  the  Ninth  Revision  of  the  U.  S. 
P.  of  general  assays  for  alkali  salts  of  organic  acids  and  for  halogen 
