458 
Theory  of  Indicators,  etc. 
J  Am.  Jour.  Pharm. 
I     October,  1904. 
in  the  formation  of  a  salt  of  the  indicator,  whether  the  indicator  be 
acid  or  basic,  than  when  the  change  in  question  results  in  the  libera- 
tion of  a  free  acid  or  base. 
An  attempt  will  be  made  in  the  course  of  the  succeeding  para- 
graphs to  show  to  what  extent  acids  and  bases  possessed  both  of 
strong  and  weak  affinities  may  be  differentiated  by  means  of  the 
range  of  indicators  at  our  disposal.  It  must,  of  course,  always  be 
remembered  that,  except  in  the  case  of  those  indicators  which 
are  exceptionally  sensitive  to  acids,  and  therefore  not  appreciably  to 
bases  and  vice  versa,  a  marked  interference  must  necessarily  be  in- 
troduced wherever  there  is  a  tendency  on  the  part  of  an  indicator  to 
be  affected  by  both  acids  and  bases. 
This  is  not  the  place  to  deal  further  with  the  constitution  of  indi- 
cators and  the  theory  ©f  their  dissociation  in  solution  with  the  for- 
mation of  ions  and  the  effects  of  dilution,  etc.,  upon  this  degree  of 
dissociation.  It  is  merely  necessary  to  state  that  both  practically 
and  theoretically  certain  indicators  are  particularly  sensitive  to  ex- 
treme dilution,  and  consequently  should  be  employed  in  as  concen- 
trated solutions  as  is  possible. 
III.  BEHAVIOR  OF  INDICATORS  TOWARDS  MINERAL  AND  ORGANIC  ACIDS. 
In  these  experiments  titrations  were  carried  out  by  means  of  n  /  io 
NaOH  and  n/10  H2S04.  In  addition,  n/ 10  ammonia,  n /  10 
oxalic  acid  were  employed  when  required.  N  /  io  solutions  of  the 
following  acids,  hydrochloric,  phosphoric,  oxalic,  formic,  lactic,  pro- 
pionic, butyric,  aspartic  and  malic,  were  prepared  with  the  greatest 
accuracy.  In  the  case  of  the  mineral  acids,  gravimetric  methods 
were  employed  for  the  determination  of  the  strength  of  the  solution. 
In  the  case  ©f  the  organic  acids,  the  greatest  precaution  was  taken  to 
obtain  pure,  ammonia-free  preparations,  and  the  exact  strength  of  the 
solution  was  in  each  case  determined  by  the  use  of  phenolphthalein 
and  in  the  majority  of  cases  confirmed  by  the  use  of  Porrier's 
blue.  Both  these  indicators  are  extremely  weak  acids  and  belong  to 
Group  3,  referred  to  above.  The  following  indicators,  dissolved  in 
the  most  suitable  solvents,  were  then  tested  in  conjunction  with 
each  of  the  acids  enumerated  above,  effect  of  time  and  temperature 
being  especially  noted.  Of  the  first  group,  dimethyl-amido-azo- 
benzol,  methyl-orange,  benzo-purpurin  and  cochineal  in  solution,  and 
congo-red  and  lakmoid,  both  in  solution  and  on  strips  of  paper,  also 
