152 
Phosphoric  Acid. 
Am.  Jour.  Pharm. 
April,  1908. 
Normal  Potassium  Absolute 
Hydroxide,  V.  S.         Phosphoric  Acid 
(N/i  KOH,  V.  S.)  (H3P04) 
Per  Cent. 
I. 
10  c.c. 
16-3  c.c. 
corresponding- to  8172 
2. 
10  " 
169  " 
82-21 
3- 
10  " 
i8-i  " 
"  88-05 
4, 
10  " 
"     +  10    "      "      "  "   
i8'i  " 
88-05 
5- 
10  " 
l8'2  " 
8853 
6. 
10  " 
"    +  10    "     "     "     "      +  90  c.c.  HoO. 
18-3  " 
"  89-04 
7- 
10  " 
"     +  10    "      "      "      "        "  " 
18-35  " 
89-25 
8. 
10  " 
"     +  10     "       "       "       "         "         "         "  . 
18-35  *' 
8925 
9- 
10  " 
20-6  " 
I00'2O 
IO. 
10  " 
"    +      "        "       "     +100C.C.  H20  . 
20'2  " 
98-33 
II. 
IO  " 
I9-2  " 
93  40 
12. 
10  " 
19*3  " 
93  88 
u- 
IO  " 
20  2  " 
9826 
14. 
IO  " 
20*2  ". 
98-26 
15- 
IO  " 
"    +  20    "     "     "      "    +  80  c.c.  H20  . 
226  " 
109-9 
When  the  acid  is  titrated  without  dilution,  the  end-reaction  is  not 
sharp,  phenolphthalein  giving  a  rose  tint  when  two  thirds  of  the 
acid  has  been  neutralized,  thus  : 
H3P04  +  2KOH  =  K2  HP04  +  2H20. 
This  irregularity  in  the  alkalimetric  estimation  of  phosphoric  acid 
is  brought  about  through  the  variable  valency  of  the  H  atoms  of 
the  acid,  influenced  by  its  dissociation.  Although  H3P04  possesses 
three  replaceable  hydrogen  atoms,  yet  it  is  only  a  feeble  acid,  that 
is,  there  are  but  few  H  ions  present  in  its  dilutions.  As  shown  by 
its  relative  conductivity,  only  H  and  H2P04  ions  are  present  in  any 
quantity.  In  the  alkalimetric  titrations  the  kations  HP04  and  P04 
have  the  tendency  to  go  over  into  the  stabler  H2P04  ion,  according 
to  the  equations : 
Na3  —  P04  +  H  —  OH  =  Na2—  HP04  +  Na  —  OH. 
Na2  —  HP04  +  H  —  OH  =  Na  —  H,P04  +  Na  —  OH. 
Since  NaOH  dissociates  completely,  free  OH  kations  are  present 
in  its  solution.  The  color  change  of  the  indicator  is  dependent  on 
the  degree  of  concentration  of  the  dissociated  — OH  ions,  which 
in  turn  is  governed  by  the  extent  of  dilution  and  the  presence  of 
an  ionizable  salt.  This  explains  the  irregularities  among  the  results 
of  experiments  3  to  15,  inclusive. 
The  differences  between  the  lower  gravimetric  results  (84  84  per 
cent.)  and  those  of  experiments  3,  4,  5,  etc.  v'88  to  89  25)  might  be 
explained,  according  to  the  preceding  ionic  theory,  by  the  conver- 
sion of  eleven-twelfths  of  the  acid  into  Na2HP04  and  one-twelfth 
