556 
VALUATION  OF  CYANIDE  OF  POTASSIUM. 
water,  so  as  to  measure  2000,  3000,  or  more,  water  grain  mea- 
sures, at  60°F.  Supposing  390.62  grs.  of  the  pure  sulphate,  or 
100  grs.  of  metallic  copper,  in  the  form  of  nitrate,  to  have  been 
taken,  and  diluted  to  2000  grain  measures,  every  100  grs.  of 
such  solution  will,  of  course,  represent  5  grs.  metallic  copper,  or 
6.25  grs.  of  the  prot-oxide  of  copper.  100  grs.  of  each  of  the 
samples  of  cyanide  of  potassium  to  be  tested,  are  then  dissolved 
in  a  sufficient  quantity  of  water,  and  introduced  into  the  colori- 
meters ;  an  excess  of  ammonia  is  added,  and  the  standard  solu- 
tion of  copper  is  carefully  added,  out  of  a  graduated  burette,  to 
the  contents  of  each  colorimeter  in  turn,  until  a  faint  blue  color- 
ation makes  its  appearance  in  each  of  the  solutions.  The  quan- 
tities of  copper,  or  of  the  solutions  taken,  then  indicate  the  rela- 
tive strength  and  money  value  of  the  samples  of  cyanide  ex- 
amined. Suppose,  for  example,  one  specimen  took  100  measures, 
and  a  second  150  measures,  of  the  copper  solution,  the  relative 
strengths  and  values  of  such  specimens  are,  therefore,  as  100  to 
150,  or  2  to  3. 
In  order  to  render  this  process  available  in  the  determination 
of  the  actual  strength  of,  or  proportion  by  weight  of  pure  cyan- 
ide of  potassium  existing  in,  the  commercial  cyanides,  it  is  only 
necessary  to  ascertain  the  weight  of  pure  cyanide  of  potassium 
that  is  required  to  decolorise  1  gr.  of  copper,  in  the  form  of  am. 
monio-nitrate.  I  regret  that  the  temporary  loss  of  my  note- 
book prevents  me  from  giving  the  numerical  result  of  my  experi- 
ments on  this  point.  The  mode,  however,  by  which  this  may  be 
effected  is  as  follows  : — The  strength  of  a  solution  of  pure  hy- 
drocyanic acid  (Scheele's  acid)  having  been  determined  in  the 
usual  way,  either  by  precipitation  by  silver,  or  by  means  of 
oxide  of  mercury,  a  certain  known  quantity  of  such  acid,  equiva- 
lent to  100  grs.  of  cyanide  of  potassium,  is  taken,  and  supersa- 
turated by  ammonia ;  the  alkaline  solution  so  prepared,  is  then 
tested  with  the  standard  copper  solution,  in  the  manner  before 
described.  The  latter  can  then,  if  necessary,  be  diluted  with 
water,  so  as  to  render  it  of  uniform  and  convenient  strength. — 
The  Chemist,  April,  1856. 
