20  Inorganic  Chemistry  of  the  U.S. P.      {  Klj'&i^y^mm' 
starch  T.  S.  No  test  for  copper  as  an  impurity  is  given.  Under 
sulphuric  acid  a  test  for  selenium  is  given. 
The  volumetric  assay  process  for  aromatic  sulphuric  acid  is  one 
of  residual  titration.  A  measured  excess  of  normal  potassium 
hydroxide  V.  S.  is  taken  and  retitrated  with  normal  sulphuric  acid. 
The  preparation  of  Acidum  Sulphurosum  is  the  same  as  before. 
It  differs  only  in  a  slight  detail,  which  is  that  the  distilled  water  into 
which  the  gas  (S02)  is  passed  is  kept  at  a  temperature  of  io°  C.  or 
below,  by  surrounding  the  containing  flask  with  ice.  The  gas  is 
then  completely  absorbed  and  the  use  of  an  additional  flask  con- 
taining sodium  carbonate  solution  to  take  up  the  excess  of  the  gas 
is  not  needed. 
Ihe  Assay  of  Sulphurous  Acid. — Of  this  acid  2  c.c.  are  measured 
into  a  stoppered  weighing  bottle  and  accurately  weighed.  A  meas- 
ured excess  of  tenth-normal  iodine  V.  S.  is  then  added  and  after 
five  minutes'  standing  the  mixture  is  titrated  with  tenth-normal 
sodium  thiosulphate  until  decolorized.  This  is  a  much  more  satis- 
factory way  than  the  old  one  of  direct  titration  with  the  iodine  V.  S. 
and  starch  as  indicator. 
ALUMINUM  SALTS. 
Alumen  should  contain  not  less  than  99-5  per  cent,  of  pure  alumi- 
num and  potassium  sulphate.  The  chemical  formula  is  given  as 
A1K(S04)2  -f  i2H20  instead  of  A12K2(S04)4  +  24H20. 
Alumen  hxsiccatum  should  contain  not  less  than  99  per  cent,  of 
the  pure  anhydrous  salt. 
Alumini  Sulphas,  AI2(S04)3  -f  i6H20,  should  contain  not  less  than 
99-5  per  cent,  of  the  pure  salt. 
Ammonium  Salts. — The  chemical  formula  for  ammonii  carbonasis 
given  as  C2HnN305,  (the  atoms  being  grouped)  instead  of  the  elon- 
gated NHJ1CO3,  NH4NH2C02 ;  the  latter  is,  however,  the  better 
way  of  writing  the  formula,  in  that  it  shows  the  constitution  of  the 
commercial  salt  more  readily  to  be  a  mixture  of  Ammonium  Bicarb, 
onate  and  Carbamate.  The  salt  should  contain  97  per  cent,  of  the 
above  mixture,  and  should  yield  not  less  than  31-58  per  cent,  of 
ammonia  gas.  For  the  assay,  the  residual  titration  process  is  em- 
ployed, litmus  being  employed  as  indicator  instead  of  rosolic  acid. 
The  other  ammonium  salts  are  as  before. 
Antimonii  et  Potassii  Tartras  is  the  only  official  salt  of  antimony. 
