^Aiig^ust  1883°^}         Estimation  of  Hydrocyanic  Acid.  411 
Corresponding  to — 
Beal  hydrocyanic  acid   -030348  of  a  gram. 
HCy  required  to  form  AgCy  in  double  salt..    -015217  of  a  gram. 
HCy  left  in  combination  with  Mg  in  double 
salt   '015131  of  a  gram. 
Silver  required  to  form  double  salt   -0607     of  a  gram. 
Thus  agreeing  within  the  limit  of  experimental  error  with  the  formula 
MgCy/AgCy),. 
The  salt  was  prepared  in  filamentous  crystals  by  filling  an  ordinary 
vial  with  hydrocyanic  acid,  adding  excess  of  calcined  magnesia, 
securely  corking  and  heating  in  a  water-bath  for  about  twenty  min- 
utes, filtering  and  dissolving  in  the  filtrate  cyanide  of  silver  until  satu- 
rated, again  filtering  and  evaporating  over  sulphuric  acid. 
From  these  experiments  the  following  conclusions  have  been  drawn : 
1st.  That  the  United  States  Pharmacopoeia  is  at  fault  in  the  direc- 
tions given  for  the  estimation  of  hydrocyanic  acid,  arising  from  a  false 
assumption,  viz. :  that  chromate  of  potassium  will  indicate  the  com- 
plete formation  of  double  cyanide  of  silver  and  magnesium. 
2d.  That  in  order  to  use  this  method  for  the  practical  estimation  of 
hydrocyanic  acid,  the  clause  "  should  require  50  cc.  of  the  volumetric 
solution  of  nitrate  of  silver  in  the  U.  S.  P.  must  be  read,  should 
require  100  cc.  of  the  volumetric  solution  of  nitrate  of  silver.'' 
3d.  That  the  method  is  not  so  satisfactory  as  that  in  vogue  in  this 
country ;  for  if  it  be  carried  on  till  the  formation  of  red  chromate  of 
silver,  double  the  amount  of  silver  solution  will  be  required,  and  if  it 
be  filtered  before  estimation,  the  length  of  time  required  for  complete 
union  of  MgO  with  the  HCy,  and  for  filtration  and  washing,  would 
generally  exclude  the  method  from  use. 
4th.  That  a  double  cyanide  of  magnesium  and  silver  does  exist,  hav- 
ing the  formula  MgCy2(AgCy)2,  analogous  to  that  of  silver  and  sodium 
]SraCy,AgCy. 
Since  writing  the  above  I  find  that  the  U.  S.  Pharmacopoeia  in 
another  place  states  that  1  cc.  of  solution  of  nitrate  of  silver  is  equiva- 
lent to  '0054  of  a  gram  of  HCy,  which  is  also  incorrect,  being  also 
based  upon  the  assumption  that  chromate  of  potassium  will  indicate 
the  complete  formation  of  a  double  salt.  The  correct  statement  would 
be,  that  1  cc.  of  dec! normal  solution  of  nitrate  of  silver  is  equivalent 
to  '0027  of  a  gram  of  real  HCv  as  shown  by  the  following  equations. 
