412 
Estimatio7i  of  Hydrocyanic  Acid, 
r  Am.  Jour.  Pharm. 
(    August,  1883. 
HCN+ AgN03=AgCNH-HN03 ; 
or  with  the  addition  of  magnesia — 
2HCy + MgO=MgC72 + OH^ 
MgCy,+2AgN03=2AgCy  +  Mg(^^03), 
That  is  to  say,  2*7  grams  of  hydrocyanic  acid  are  equivalent  to  1000  cc. 
of  decinormal  sokition  of  nitrate  of  silver,  and  therefore  -0027  of  a 
gram  equals  1  cc. — Pharm.  Jour,  and  Trans.,  May  18,  1883,  p.  917. 
Note  on  the  above  paper.— Mr.  Cripps'  deductions  are  essentially  cor- 
rect, although  some  statements  contained  in  his  concluding  remarks  require 
slight  modification.  The  method  adopted  by  the  U.  S.  Pharmacopoeia  for 
the  estimation  of  diluted  hydrocyanic  acid  is  the  same  as  that  of  the  new 
Pharmacopoea  Germanica  for  the  estimation  of  hydrocyanic  acid  in  bitter 
almond  water,  with  the  exception  that  while  in  the  latter  the  factors  are 
correctly  stated,  the  U.  S.  P.  has  inadvertently  erred  in  stating  that  50  cc. 
of  the  volumetric  solution  of  nitrate  of  silver  should  be  required.  With 
the  employment  of  chromate  of  potassium  as  an  indicator  all  the  cyanogen 
of  the  acid  will  become  converted  into  cyanide  of  silver  before  the  red 
color  of  chromate  of  silver  remains  permanent.  The  amount  of  nitrate 
of  silver  sohition  required  in  the  application  of  the  Pharmacopoeia  test  will 
therefore  be  100  cubic  centimeters,  instead  of  50  cc,  or  by  the  use  of  half  of 
the  stated  amount  of  diluted  hydrocyanic  acid  (6'75  grams,  instead  of  13-5 
grams)  the  subsequent  text  may  remain  unaltered. 
Mr.  Cripps  assumes,  under  his  fourth  conclusion,  "  that  the  statement  of 
'  the  U.  S.  P.  (p.  397)  that  1  cubic  centimeter  of  solution  of  nitrate  of  silver  is 
equal  to  0-0054  gram  HON  is  also  incorrect,  and  that  this  statement  is  also 
based  upon  the  assumption  that  chromate  of  potassium  will  indicate  the 
complete  formation  of  a  double  salt."  The  factor  0'0054,  as  stated  by  the 
U.  S.  P.  is  correct,  with  evident  reference  to  the  original  method  of  Liebig 
for  the  estimation  of  cyanides,  whereby  but  half  of  the  cyanogen  present  in 
the  compound  is  indicated  by  the  amount  of  nitrate  of  silver  solution  em- 
ployed, the  other  half  remaining  in  combination  with  the  alkali  in  the  form 
of  a  soluble  double  salt ;  for  the  purpose  of  calculation,  therefore,  either 
the  number  of  cubic  centimeters  of  the  nitrate  of  silver  solution  employed 
must  be  doubled,  or  as  the  Pharmacoj^oeia  has  quite  as  correctly  expressed 
it,  the  fiictors  derived  from  the  molecular  weights  of  hydrocyanic  acid  and 
cyanide  of  potassium  must  be  doubled  in  order  to  represent  (by  Liebig's 
method)  the  amount  of  hydrocyanic  acid  or  cyanide  actually  present. 
That  the  above  statement  of  the  Pharmacopoeia  is  not  based  upon  a  false 
assumplion,  as  Mr.  Cripps  concludes,  is  evident  from  the  text  relating  to 
the  estimation  of  cyanide  of  potassium  (p.  266),  where  the  original  method 
of  Liebig  is  employed  and  the  quantities  are  correctly  stated  in  accordance 
with  the  factors  given  on  page  397.  F.  B.  Power. 
