Colorimetric  Determination  of  Mediums. 
593 
following  Salm,3  in  determining  the  so-called  half-transformation 
points  of  indicators.  Within  the  range  of  its  transition  from  red  to 
yellow,  we  may  regard  the  observed  color  of  a  phenolsulphone- 
phthalein  solution  as  composed  of  a  definite  amount  of  red  plus  a 
definite  amount  of  yellow,  and  such  a  color  may  be  exactly  dupli- 
cated by  superimposing  the  extreme  red  and  the  extreme  yellow 
of  the  indicator  in  proper  concentrations.  Thus,  if  to  one  test  tube 
we  add  5  Cc.  of  dilute  acid,  and  to  another  similar  tube  5  Cc.  of 
dilute  alkali,  and  to  each  add  5  drops  of  phenolsulphonephthalein 
solution,  a  bright  yellow  will  be  produced  in  the  first  tube  and  a 
bright  red  in  the  other.  But  if  we  look  toward  the  light  through 
both  tubes,  a  color  will  be  observed  that  is  half  way  between  the 
yellow  and  the  red.  In  fact,  it  will  be  identical  with  the  color  pro- 
duced by  10  drops  of  the  phenolsulphonephthalein  solution  in  5  Cc. 
of  a  standard  solution  having  a  pH  7.9.  This  is  the  half-transfor- 
mation point,  and  is  a  definite  constant  for  this  indicator.  But  if 
instead  of  using  equal  amounts  of  indicator  in  each  of  the  two  tubes 
we  vary  the  partition  of  the  10  drops  of  indicator  between  them, 
then  by  superimposing  each  pair  and  viewing  them  by  transmitted 
light,  a  series  of  colors  will  be  produced  which  will  cover  the  range 
of  usefulness  of  the  indicator;  and  once  such  a  series  is  "cali- 
brated" against  solutions  of  known  hydrogen-ion  concentration,  it 
may  be  used  as  a  standard  series  for  the  determination  of  unknown 
reactions. 
Results  obtained  by  such  a  procedure  in  the  case  of  phenolsul- 
phonephthalein, comparison  being  made  with  phosphate  solutions 
prepared  according  to  Sorensen,4  are  as  shown  in  the  accompanying 
table. 
Outline  of  Method. — Apparatus  and  Chemicals  Required. — 1. 
Clean  test  tubes.    These  must  be  of  approximately  the  same  diam- 
Results  with  Phenolsulphonephthalein. 
Acid  tubes,  phenolphoneph-     Alkali  tubes,  phenolsulphoneph- 
thalein solution,  drops.  thalein  solution,  drops. 
pH. 
6.9 
7-2 
7-5 
7-7 
7-9 
8.1 
9  1 
8  2 
7  3 
6  4 
5  5 
4  6 
3  Salm,  Ztschr.  f.  phys.  Chem.,  1906,  57,  471. 
4  Sorensen  and  Palitzsch,  Biochem.  Ztschr.,  1910,  24,  387. 
