Am 'ir i9warm' }  Determination  of  Hydrocyanic  Acid.  261 
1868,  the  firm,  finding  that  more  room  was  necessary,  moved  to  528 
Arch  Street,  where  the  business  was  conducted  until  its  dissolution 
after  the  demise  of  the  partners.  Mr.  Crenshaw  died  in  1894  and 
Mr.  Bullock  in  1900. 
It  is  interesting  to  add  that  Thomas  H.  Powers  was  a  one-time 
apprentice  of  Smith  &  Hodgson,  leaving  them  to  become  associated 
with  William  Weightman  in  the  firm  of  Powers  &  Weightman,  the 
world-famous  firm  of  manufacturing  chemists,  at  Ninth  and  Parrish 
Streets,  Philadelphia. 
ON  THE  DETERMINATION  OF  SMALL  QUANTITIES  OF 
HYDROCYANIC  ACID.1 
By  Arno  Viehoever  and  Carl  O.  Johns. 
In  our  work  on  cyanogenetic  plants  we  found  it  necessary  to  esti- 
mate small  quantities  of  hydrocyanic  acid.  The  various  titration 
methods  as  well  as  the  silver  gravimetric  methods  had  to  be  excluded 
because  we  were  working  with  plant  distillates  which  usually  contain 
reducing  compounds.  Furthermore,  the  quantities  to  be  determined 
were  often  too  small  to  permit  the  use  of  the  above  methods.  We 
frequently  had  to  deal  with  less  than  0.5  mg.  of  hydrocyanic  acid. 
Chapman  2  has  shown  that  the  picric  acid  colorimetric  method  of 
Waller  3  is  not  applicable  to  plant  distillates  which  usually  contain 
reducing  substances  other  than  hydrocyanic  acid.  This  left  two 
colorimetric  methods  to  be  examined,  namely,  the  thiocyanate  method 
and  the  Prussian  blue  method. 
In  the  thiocyanate  method  of  Francis  and  Connell 4  the  hydro- 
cyanic acid  is  distilled  into  a  solution  of  potassium  hydroxide,  yellow 
ammonium  sulphide  is  added,  and  the  solution  is  evaporated  to 
dryness.  The  residue  is  dissolved  in  water,  acidified  with  hydro- 
chloric acid,  and  the  mixture  is  filtered  to  remove  sulphur,  after 
which  the  filtrate  is  further  acidified  and  boiled  to  cause  precipitation 
of  the  free  sulphur.    The  process  of  boiling  and  filtering  is  repeated 
1  Reprinted  from  the  Journal  of  the  American  Chemical  Society,  vol. 
xxxvii,  No.  3,  March,  1915. 
2  The  Analyst,  35,  471  (1910)  ;  36,  269  (1911). 
3 Proc.  Royal  Soc.  (B),  82,  574,  1910;  The  Analyst,  35,  406  (1910). 
4  Journal  of  the  American  Chemical  Society,  35,  1624  (1913). 
