580 
A  Study  of  the  Use  of  Mayer's  Reagent.  {Am'$££;$£Tm- 
time  disappeared,  showing  that  the  reaction  is  not  instantaneously 
completed,  or  possibly  that  an  intermediate  soluble  compound  is  first 
formed,  which  subsequently  takes  up  a  larger  proportion  of  the  alkaloid, 
frequently  producing  in  this  manner  distinctly  crystallized  compounds. 
My  own  observations,  made  at  first  in  ignorance  of  Mr.  Groves' 
paper,  are  confirmatory  of  the  earlier  ones.  I  found  that  in  the  case 
of  some  of  the  alkaloids,  the  solution  would  begin  to  show  apparent 
excess  of  reagent  when  scarcely  half  the  quantity  had  been  added 
necessary  for  complete  precipitation.  The  larger  the  quantity  of  fluid 
present^  the  larger  the  amount  of  this  excess,  which,  it  seems,  must 
form  a  definite  proportion  of  the  entire  fluid. 
The  practical  deduction  from  this  observed  fact  is,  that  the  titra- 
tion equivalent  of  any  given  alkaloid  varies  with  the  degree  of  dilu- 
tion of  the  solution.  If  it  is  fixed,  e.  g.,  for  a  solution  of  1  :  200,  a 
correction  must  be  made  in  titrating  more  dilute  solutions,  not  addi- 
tive, as  most  writers  represent,  but  subtractive. 
The  experiments,  whose  results  are  given  in  the  following  tabulated 
statement,  aimed  to  gather  data  on  which  to  base  formulas  for  cor- 
recting the  results  of  titrations,  and  further,  to  study  the  influence  of 
other  variations  in  the  conditions,  such  as  the  presence  in  the  solution 
of  alcohol,  glycerin,  etc.  In  all  the  experiments  the  Mayer's  reagent 
used  was  1-20  normal,  i.  e.,  one-half  the  strength  of  the  original 
Mayer's  solution.  Two  modified  forms  of  the  reagent  were  prepared, 
]STo.  1  containing  5  molecules  instead  of  3  of  potassium  iodide,  No.  2 
containing  barely  enough  potassium  iodide  to  hold  in  solution  the 
mercuric  iodide.  The  quantity  of  mercury  in  all  was  the  same,  and 
in  each  experiment  0'10  gram  of  alkaloid  was  used. 
In  each  experiment  reagent  was  added  gradually  until  a  drop  of 
the  filtered  fluid  began  to  give  a  faint  cloud  with  a  fresh  portion  of 
the  alkaloidal  solution.  The  quantity  of  reagent  required  is  set  down 
in  column  3  of  the  table.  In  some  cases  this  point  was  passed  before 
any  reading  of  the  burette  was  taken ;  such  reading  was  then  entered 
in  column  4.  When  the  precipitation  was  nearly  complete,  so  that 
the  addition  of  a  drop  or  two  of  reagent  produced  only  a  faint  cloud, 
another  reading  of  the  burette  was  frequently  taken,  entered  in  col- 
umn 5.  The  table  otherwise  is  self-explanatory.  When  not  other- 
wise stated,  the  titrations  were  made  in  presence  of  a  little  free  sul- 
phuric acid — about  1  minim  of  6  per  cent,  acid  to  each  cc.  of  fluid 
titrated. 
