594         Colorimetric  Determination  of  Mediums.    {Al^uJ°g['  ^|rm' 
eter.  An  equal  volume  of  water  is  measured  into  a  number  of 
tubes,  and  fifteen  or  twenty  are  selected  for  use  in  which  the  water 
stands  at  about  the  same  level. 
2.  A  5  Cc.  pipette. 
3.  A  medicine  dropper  drawn  out  to  a  fairly  fine  point. 
4.  A  burette. 
5.  An  indicator  solution.  A  convenient  solution  (0.01  per  cent.) 
of  phenolsulphonephthalein  is  prepared  by  diluting  1  Cc.  of  the  usual 
solution  used  for  kidney  function  tests  (1  Cc.  =  6  Mg.)  to  60  Cc. 
with  distilled  water.  No  accuracy  is  necessary,  provided  the  same 
solution  is  used  throughout. 
6.  Roughly  normal  and  twentieth  normal  sodium  hydroxide. 
7.  Hydrochloric  or  sulphuric  acid. 
Preparation  of  Standard  Color  Series. — Twelve  test  tubes 
are  placed  in  two  rows  of  six.  Into  each  tube  of  one  row  5  Cc.  of 
dilute  alkali  are  placed.  (The  twentieth-normal  sodium  hydroxide 
may  be  used,  or  any  solution  sufficiently  alkaline  to  bring  out  the 
maximum  red  color  of  the  indicators.)  Into  each  tube  of  the  other 
row,  5  Cc.  of  very  dilute  acid  are  placed.  (One  drop  of  concen- 
trated hydrochloric  or  sulp'huric  acid  to  100  Cc.  of  distilled  water 
is  sufficiently  strong.  Strong  acid  is  to  be  avoided  in  the  case  of 
phenolsulphonephthalein,  on  account  of  its  secondary  color  change.) 
Into  the  six  tubes,  9,  8,  7,  6,  5  and  4  drops,  respectively,  of  indi- 
cator are  placed.  Into  the  six  corresponding  alkali  tubes,  1,  2,  3,  4,  5 
and  6  drops  of  indicator  are  placed.  If  the  dropper  is  held  ver- 
tically, the  drops  will  be  practically  of  a  size.  Each  pair  of  tubes 
thus  contain  10  drops  of  indicator  between  them,  and  the  series  of 
six  pairs,  when  viewed  by  transmitted  light,  will  correspond  to  pH 
values  of  6.9,  7.2,  7.5,  7.7,  7.9,  and  8.1  (as  in  the  table)  when  com- 
pared with  5  Cc.  volumes  of  any  solution  containing  10  drops  of  the 
same  indicator  solution.  In  order  to  determine  the  hydrogen  ion 
concentration  of  an  unknown  solution  whose  reaction  lies  within 
this  range,  5  Cc.  of  it  are  placed  in  a  test  tube,  10  drops  of  indicator 
are  added,  and  its  color  is  compared  with  those  of  the  six  pairs  of 
tubes.  The  use  of  a  second  tube  containing  distilled  water  to  form 
a  pair  with  the  unknown  is  to  be  recommended,  but  does  not  appear 
to  affect  the  results  appreciably.  The  color  series  prepared  in  this 
manner  is  as  accurate  as  that  yielded  by  standard  phosphate  or 
other  solutions  when  used^in  a  similar  apparatus,  and  can  be  pre- 
pared anywhere  in  a  few  minutes  without  the  use  of  graduated  ap- 
