514  Inorganic  Chemistry  of  the  U.S. P.      { ^o/JS^SoT 
a  bath  of  ice-water  for  several  hours  and  then  strained  through 
cotton  to  separate  the  crystalline  precipitate  of  potassium  bitartrate. 
The  reaction  may  be  explained  by  the  equation  KI  -f-  H2C4H406  = 
HI  -f-  KHC4H406.  The  use  of  alcohol  and  of  the  ice  bath  are  to 
facilitate  the  separation  of  the  potassium  bitartrate. 
In  the  Pharmacopoeia  of  i860,  diluted  hydriodic  acid  was  made 
by  passing  hydrogen  sulphide  gas  through  iodine  suspended  in 
water.  2l2  -f  H20  -f  2H2S  =  4HI  +  H20  -f  S2.  The  product  was 
a  purer  one  than  that  of  the  present  Pharmacopoeia,  but  the  use  of 
sulphuretted  hydrogen,  because  of  its  offensive  odor,  was  decidedly 
objectionable. 
The  Pharmacopoeia  describes  tests  for  the  following  impurities: 
Sulphuric  acid,  barium,  non-volatile  residue,  heavy  metals,  by  the 
"  Time-limit  Test;  "  and  arsenic  by  "  Modified  Gutzeit's  Test."  To 
these  might  have  been  added  tests  for  free  iodine  and  tartrate. 
The  assay  method  recommended  for  the  diluted  acid,  as  well  as 
for  the  syrup,  is  that  employing  Volhard's  solution,  as  in  the  case  of 
Syrupus  ferri  iodidi,  in  the  1 890  U.S. P.  This  is  an  improvement 
over  the  former  method  for  Syrupus  acidi  hydriodici,  in  which  direct 
titration  with  decinormal  silver  nitrate  v.  s.  was  employed,  and  potas- 
sium chromate  as  the  indicator. 
Acidum  Hypophosphorosum,  Hypophosphorous  Acid  (HPH202). — ■ 
Acidum  hypophosphorosum  and  Acidum  hypophosphorosum  dilu- 
tum  are  both  official,  the  former  being  newly  added.  It  is  li  a  liquid 
composed  of  30  per  cent,  by  weight  of  absolute  hypophosphorous 
acid,  POH2(OH),  and  70  per  cent,  of  water." 
No  method  for  the  preparation  of  this  acid  is  official,  though  the 
National  Formulary  gives  a  method  for  preparing  the  diluted  acid. 
This  may  be  concentrated  by  evaporation,  but  if  heated  to  1300 — 
1400  C,  it  decomposes,  forming  the  spontaneously  combustible 
hydrogen  phosphide  and  other  compounds.  Hypophosphorous  acid 
is  a  powerful  reducing  agent,  reducing  silver  nitrate  to  metallic 
silver,  mercuric  chloride  to  mercurous  chloride  and  finally  to  metallic 
mercury.  It  also  precipitates  yellow  copper  hydride  when  heated 
with  copper  sulphate. 
Tests  for  the  detection  of  the  following  possible  impurities  are 
described  :  Phosphoric,  phosphorous,  sulphuric,  oxalic  and  tartaric 
acids,  and  potassium,  the  tests  being  the  same  as  those  described 
for  the  diluted  acid  in  the  1890  U.S.P.    A  test  for  barium  is  given, 
