462 
Theory  of  Indicators,  etc. 
<  Am.  Jour.  Pharm 
I     October,  1904. 
as  phloroglucinvanillin  drops  were  concerned.  All  the  results  have, 
however,  been  reduced  to  the  equivalent  of  100  c.c.  of  a  n/io  solu- 
tion in  order  to  afford  a  uniform  means  of  comparison.  Thus,  if  in 
one  case  10  c.c.  of  a  solution  had  been  employed  on  account  of 
scarcity  of  material,  and  in  another  case  25  c.c,  the  former  would 
be  multiplied  by  10  and  the  latter  by  4  in  order  to  make  the  results 
comparable.  It  will  be  seen  on  examining  the  figures  that  Porrier's 
blue  is  sensitive  to  all  the  free  acid  groups  and  entirely  indifferent 
even  to  ammonia  and  certainly  to  NH2,  etc.  Phenolphthalein  lies 
next,  being  less  sensitive  to  acid  groups  and  far  more  sensitive  to 
TABLE  I. 
Substance  Titrated,  100  c.c. 
Porrier's 
Blue. 
Phenol- 
phthalein. 
Alizarin. 
Phloro- 
glucinva- 
nillin. 
Nitrogen. 
N/10  ammonia  
Nil 
—  IOO 
  IOO 
—  IOO 
IOO 
N/10  formic  acid  +  100  c.c.  n/io  am- 
+  105 
+  14 
+  3 
—  105 
100 
N/10  acetic  acid  +  100  c.c.  n/io  am- 
+  105 
+  8 
-f  10 
—  no 
IOO 
N/10  lactic  acid  +  ioo  c.c.  n/io  am- 
+  110 
+  8 
+  10 
— 105 
IOO 
N/io  malic  acid  
-f  IOO 
-f  IOO 
+  IOO 
Nil? 
Nil 
N/io  solution  pure  asparagin    .  .  . 
+  IOO 
+  25 
Nil 
—  100 
200 
IOO 
+  3 
Nil 
—  IOO 
IOO 
+  95 
+  5 
Nil 
-  95 
IOO 
+  200 
+  IOO 
+"100 
—  IOO 
IOO 
+  117 
+  40 
Nil 
—  "7 
115 
<+  100 
+  22 
+  1 
—  114 
220 
Nil 
Nil 
Nil 
-5 
IOO 
+  15 
+  3 
~    +  1 
—  15 
IOO 
Nil 
Nil 
Nil 
—  6 
200 
ammonia  than  is  Porrier's  blue.  Alizarin,  the  third  representative 
we  decided  to  use  after  trying  a  considerable  number  of  indicators, 
is  sensitive  both  to  ammonia  and  to  strong  organic  acid  groups,  so 
that  ammonium  acetate,  ammonium  formate,  etc.,  are  neutral. 
But  whilst  fully  sensitive  to  ammonia  in  all  cases,  alizarin  is  appar- 
ently indifferent  to  the  weak  acid  groups  such  as  are  present  in 
asparagin,  tyrosin,  etc.,  and  also  to  the  amido  group  of  those  same 
compounds.  Phloroglucinvanillin  drops  which,  after  employing 
various  indicators,  we  decided  upon  as  most  valuable  for  weak  bases, 
