AmApXimrru  }  Phosphoric  Acid.  153 
into  Na3?04.  Again,  as  in  experiment  9,  a  mixture  of  32  per  cent, 
of  K3P04  and  78  per  cent,  of  K2HP04  will  have  required  20  6  c.c.  of 
N/i  KOH,  V.S.  for  the  neutralization  of  the  acid  used.  The  results 
of  these  titrations,  while  corresponding  quite  satisfactorily  when  car- 
ried out  under  the  same  conditions,  are  invariably  high  and  fluctu- 
ating, through  variations  in  dilution  and  ionizable  salts.  A  few 
titrations,  with  methyl  orange  as  indicator,  are  added : 
N/i  KOH,  V.S.  H3  PO4. 
Per  Cent. 
10  c.c.  H3PO4  (10  per  cent.)  10  c.c.  sat.  sol.  NaCl  8.9  c.c.  KOH  corresponding  to  86*59 
10   "  "  10   "     "     "      "      9-05    "     "  "  88*05 
10   "  '•  10   "     "     "      "      9*05   "     !<  "  8805 
10   "  "  10   "     "     "      "      9*05   "     "  "  88-05 
Among  other  methods  possibly  available  for  pharmicopoeial  pur- 
poses, the  following  iodometric  one,  proposed  by  A.  Christensen 
(Jahresberichte  d.  Phar.y  96,  338)  has  been  found  to  be  very  satisfac- 
tory. The  method  is  simple  and  gives  uniform  results,  agreeing 
accurately  with  gravimetric  determinations.  This  method  is  based 
on  the  reaction  taking  place  between  phosphoric  acid,  potassium 
iodide  and  bromate  in  solution,  whereby  a  molecular  equivalent 
quantity  of  iodine  is  liberated  for  each  molecule  of  phosphoric  acid 
present.    Thus : 
6H3P04  +  6KI  +  KBrOs  =  6KH2P04  -f  l\  +  KBr  +  3H20 
6  x  97  29  p.  H3P04  =6x  125-9  p.  I  =  6  x  24646  p.  Na2S203,  or 
97  29  p.  H3P04  =       125  9  p.  I  =       246-46  p.  Nd2S203,  or 
icc.N /ioNa2S203,  V.S.  (0-024646  gm.)  =  0-009729  gm.  H3P04. 
A  5  per  cent,  solution  of  the  original  sample  of  phosphoric  acid 
was  employed  in  these  estimations  and  the  results  calculated  to  the 
strong  acid,  for  comparison.  Like  all  iodometric  methods  of  this 
class,  the  operation  is  carried  out  in  a  securely  stoppered  bottle.  In 
an  accurately  fitted  glass-stoppered  bottle  of  about  150  c.c.  capacity, 
5  c.c.  of  the  sample  of  phosphoric  acid  (measured  from  a  burette) 
was  introduced,  followed  by  approximately  2  grammes  of  potassium 
iodide,  5  c.c.  of  a  saturated  solution  of  potassium  bromate  and 
30  c.c.  of  water.  After  securing  the  stopper,  the  bottle  and  con- 
tents were  nearly  immersed  in  a  bath  of  water  at  the  temperature 
of  650  C.  for  ten  minutes;  ater  cooling  thoroughly,  removing 
(rinsing  the  neck  and  stopper),  the  liberated  iodine  was  estimated 
as  usual  with  sodium  thiosulphate,  V.  S. 
