A January ri906m  }      Inorganic  Chemistry  of  the  U.S. P.  19 
The  old  assay  method,  in  which  the  acid  is  mixed  with  magnesia 
and  water  and  titrated  to  the  point  where  the  indicator  (potassium 
chromate)  produces  a  red  tint,  is  replaced  by  the  better  method  of 
W.  J.  Sharwood,  which  is  a  modification  of  Denige's. 
In  this  method  5  grammes  of  diluted  hydrocyanic  acid  are  diluted 
with  distilled  water  to  measure  50  c.c.  Then  26.9  c.c.  of  this  solution, 
after  the  addition  of  5  c.c.  of  ammonia  water  and  3  drops  of  potas- 
sium iodide  T.  S.  are  titrated  with  tenth-normal  silver  nitrate  V.  S., 
until  a  slight  permanent  precipitate  occurs.  The  ammonia  water 
and  potassium  iodide  in  this  process  act  as  indicator. 
The  reactions  may  be  expressed  thus  : 
(1)  2NH4CN  +  AgNOs  =  NH4Ag(CN)2  +  NH4N03 
(2)  NH4Ag(CN)2  +  AgN03  =  NH4HOs  +  2AgCN 
(3)  KI  -j-  AgN03  =  KNO3  -f  Agl 
The  silver  nitrate  forms  with  the  cyanide  a  double  salt  which  is 
soluble,  no  precipitate  occurring  until  after  all  of  the  cyanide  has 
entered  into  combination  as  the  double  salt ;  then  the  further  addi- 
tion of  silver  nitrate  decomposes  the  double  salt,  and  a  precipitate 
of  silver  cyanide  occurs.  In  the  presence  of  ammonia  water,  how- 
ever, as  in  the  above  assay,  the  precipitation  of  silver  cyanide  is 
prevented,  but  the  iodide  is  now  (not  before)  acted  upon  by  the 
silver  solution  and  a  precipitate  of  silver  iodide  occurs,  which  very 
delicately  indicates  the  end  reaction.  As  regards  the  other  mineral 
acids  very  few  changes  are  noted. 
Bettendorf  s  test  for  arsenic  is  replaced  in  every  instance  by  the 
modified  Gutzeit's  test,  and  the  sulphide  method  of  testing  for 
metals  as  impurities  is  replaced  by  the  "  time-limit  test."  The 
volumetric  assay  is  modified.  Instead  of  a  definite  weight  of  the 
acid  being  taken  for  analysis,  it  is  directed  in  most  cases  to  take 
3  c.c.  in  a  stoppered  weighing  bottle  and  weigh  accurately;  the 
standard  solution  used  is  normal  potassium  hydroxide,  and,  except 
in  the  case  of  phosphoric  acid,  methyl  orange  is  the  indicator;  in 
this  case  phenolphthalein  is  used. 
For  the  detection  of  free  Chlorine  or  Bromine  in  hydrochloric  acid,, 
it  is  recommended  to  dilute  1  c.c.  of  the  acid  with  5  c.c.  of  water 
and  add  1  c.c.  of  potassium  iodide  T.  S.  and  I  c.c.  of  chloroform, 
then  to  shake  the  mixture  and  note  if  the  chloroform  shows  a  violet 
coloration.    This  test  replaces  the  old  and  less  reliable  zinc-iodide 
