Am.  Jour.  Pharm. 
April,  19 1 7. 
Assay  Processes. 
=  205   +  K2S04  +   MnS04   +  8H20. 
Potassium  Manganese 
Sulphate  Sulphate 
There  is  only  one  assay  in  which  potassium  dichromate  is  used, 
saccharated  ferrous  carbonate.  The  salt  is  dissolved  in  diluted  sul- 
phuric acid,  and  immediately  titrated  with  potassium  dichromate, 
potassium  f  erricyanide  being  used  as  an  indicator : 
6FeS04   +   K2Cr207   +  7H2S04 
Ferrous  Potassium  Sulphuric 
Sulphate         Dichromate  Acid 
=3Fe2(SOJ3   +   K2S04   +   Cr2(S04)3   +  7H20. 
Ferric  Sulphate       Potassium  Chromium 
Sulphate  Sulphate 
There  are  but  few  of  the  compounds  and  preparations  of  the 
U.  S.  P.  in  which  potassium  sulphocyanide  is  used :  silver  nitrate, 
solution  arsenic  and  mercury  iodide,  mass  of  mercury,  mercury  and 
chalk  and  mercuric  oxide. 
In  the  assay  of  mercuric  oxide  the  mercuric  compound  is  treated 
with  nitric  acid,  whereby  it  is  converted  into  mercuric  nitrate.  This 
is  diluted  with  water,  ferric  alum  added  as  an  indicator,  and  titrated 
with  sulphocyanide  of  potassium.  In  this  process,  the  mercury  is 
precipitated  by  the  sulphocyanide  as  the  insoluble  mercuric  sulpho- 
cyanide, the  completion  of  the  reaction  is  shown  by  the  formation  of 
a  yellowish-red  color. 
The  mercury  of  solution  of  arsenic  and  mercury  iodide,  is  deter- 
mined by  adding  to  the  solution  KOH  and  then  formaldehyde,  by 
which  the  mercury  is  reduced  to  the  metallic  condition.  The  mer- 
cury is  washed  by  decantation,  and  dissolved  by  nitric  acid.  The 
same  process  is  followed  as  under  the  oxide. 
In  determining  mass  of  mercury,  the  substance  is  treated  with 
sulphuric  and  nitric  acids  whereby  mercuric  nitrate  is  formed,  and 
the  organic  matter  is  destroyed.  Potassium  permanganate  is  also 
added  until  a  pink  tint  is  produced,  making  certain  that  no  organic 
matter  remains,  and  the  color  discharged  by  oxalic  acid  T.S.  The 
sulphocyanide  method  is  then  followed. 
GASOMETRIC  METHODS. 
But  three  substances  of  the  Pharmacopoeia  are  assayed  gaso- 
metrically :  oxygen,  amyl  nitrite,  and  spirits  ethyl  nitrite.  The  de- 
termination of  the  two  latter  is  carried  out  in  a  nitrometer,  supplied 
