n6  Assay  Processes  of  US.P.  IX.     { Am^f  *£*Tm' 
cient  carbon  tetrachloride  is  added  to  cover  the  precipitate.  After 
an  hour  the  precipitate  is  drained  and  further  washed  with  carbon 
tetrachloride  until  the  nitrate  upon  evaporation  no  longer  shows  any 
visible  residue.  It  is  further  washed  with  alcohol,  dried  and 
weighed.  The  mercuric  sulphide  found  multiplied  by  1.167  gives 
the  mercuric  chloride. 
The  third  method  of  gravimetric  assay,  precipitation  and  fur- 
ther decomposition  of  the  precipitate  by  heat,  is  used  in  the  follow- 
ing: alum,  gold  and  sodium  chloride,  zinc  acetate,  sulphate,  phenol- 
sulphonate  and  valerate. 
Alum  is  determined  by  precipitation  as  hydroxide,  igniting,  and 
weighing  as  oxide. 
Gold  and  sodium  chloride  is  assayed  by  the  addition  of  KOH 
in  excess.  The  potassium  meta-aurate,  KAu02,  formed  is  reduced 
to  metallic  gold  by  the  addition  of  peroxide  of  hydrogen.  The 
precipitate  is  filtered,  washed  and  ignited. 
The  zinc  compounds  are  dissolved  in  water  made  slightly  alka- 
line with  ammonia  water,  and  then  precipitated  as  sulphide  by  the 
addition  of  ammonium  sulphide.  The  precipitate,  after  washing, 
is  dissolved  in  nitric  acid,  the  solution  evaporated,  the  residue  ig- 
nited, and  weighed  as  oxide. 
Miscellaneous  gravimetric  methods  of  assay  are  used  for  sul- 
phur, magnesium  citrate  and  sulphate,  and  uranium  nitrate. 
To  assay  sulphur,  1  Gm.  of  sulphur  is  added  to  50  mils  of  10 
per  cent.  NaOH.  The  resultant  solution  of  sodium  sulphide  is 
boiled,  cooled  and  made  up  to  250  mils  with  water.  25  mils  of  the 
above  solution  is  oxidized  by  the  addition  of  hydrogen  peroxide, 
and  heated.  HC1  and  BaCl2  are  then  added,  and  from  the  precipi- 
tated BaS04,  after  washing  and  igniting,  the  amount  of  sulphur  is 
calculated.  With  this  method  a  blank  is  run,  using  the  same  quan- 
tity of  solution,  but  omitting  the  sulphur. 
The  solution  of  magnesium  citrate  is  first  evaporated  to  dryness, 
then  carbonized,  and  the  carbonized  mass  treated  with  HC1.  To 
the  well-washed  filtrate,  sodium  phosphate  is  added  and  ammonium 
water,  and  the  precipitated  magnesium  ammonium  phosphate,  after 
washing  and  drying,  is  ignited,  and  from  the  magnesium  pyrophos- 
phate obtained  the  citrate  is  determined.  Magnesium  sulphate  is 
also  determined  in  a  similar  manner.  In  this  case  carbonization 
is  unnecessary. 
Uranium  nitrate.    The  salt  is  dissolved  in  water,  and  to  the 
