Am.  Jour.  Fharm. 
October,  1911. 
I       Note  on  Assay  of  Formaldehyde. 
461 
molecular  union.  The  KCN  solution  was  prepared  from  a  sample 
of  Kahlbaum's  potassium  cyanide,  and  its  value  in  terms  of  tenth- 
normal silver  nitrate  was  determined  simultaneously  with  each  series 
of  experiments,  the  determination  being  carried  out  in  a  manner 
similar  to  that  used  for  determining  the  uncombined  cyanide  in  the 
experiments.  The  mixing  of  the  formaldehyde  and  potassium  cya- 
nide solutions  was  generally  effected  in  Erlenmeyer  flasks ;  and  im- 
mediately after  mixing  (the  latter  operation  consuming  about  half 
a  minute),  the  resulting  solution  was  added  to  a  mixture  of  a  known 
amount  of  silver  nitrate  and  nitric  acid  in  a  200  c.c.  measuring 
flask.  The  amount  of  silver  nitrate  used  was  sufificient  in  every 
case  to  combine  with  all  of  the  excess  of  KCN  and  calculated  to 
leave  in  the  filtrate,  for  the  subsequent  titration,  the  equivalent  of 
about  2  c.c.  of  n/io  KCNS  per  100  c.c.  of  the  filtrate.  The  amount 
of  nitric  acid  used  was  10  or  20  c.c.  of  U.S. P.  dilute  (10  per  cent.) 
nitric  acid.  After  thoroughly  washing  the  Erlenmeyer  flask  in  which 
the  KCN  and  formaldehyde  solutions  were  mixed  and  adding  the 
washings  tO'  the  flask  containing  the  acidified  silver  nitrate  solution, 
the  whole  was  made  up  to  200  c.c.  with  distilled  water,  thoroughly 
shaken,  filtered  through  a  dry  filter,  and  100  c.c.  of  the  filtrate 
titrated  for  the  excess  of  silver  by  means  of  n/io  KCNS,  using 
2  c.c.  of  a  10  per  cent,  ferric  alum  solution  as  indicator.  The  results 
obtained  are  given  in  Table  I. 
Table  I. 
Effect  of  Varying  the  Excess  of  KCN. 
Amount  of 
No.  of  formalde- 
experiment    hyde  solu- 
tion taken 
(  c.c. ) 
15 
15 
15 
15 
15 
15 
15 
15 
15 
I.^ 
15 
15 
Amount  of 
N/io  KCN 
added  a 
(c.c.) 
19.7 
20.0 
21.0 
22.0 
23.0 
24.0 
25.0 
27.0 
30.0 
40.0 
50.0 
60.0 
Amount  of 
N/io  AgNOs 
used 
(c.c.) 
4 
4 
5 
6 
7 
8 
9 
TI 
H 
24 
34 
44 
«  The  KCN  solution  was  only  approximately  tenth-normal 
to  47.4  c.c.  n/io  AgNOj. 
N/io  KCNS 
required 
for  A  of 
filtrate 
(c.c.)  ■ 
1.50 
1-45 
1-55 
1.67 
175 
175 
1.80 
T.85 
1.83 
T.90 
1.98 
2.02 
48  c.c. 
Found  equivalent 
of  the  formal- 
dehyde solu- 
tion in  terms 
of  n/io 
AgNOs 
( c.c. ) 
18.45 
18.65 
18.84 
19.07 
19.21 
19.20 
19.29 
19.36 
19.29 
19.30 
19.34 
19.29 
'.being'^equivalent 
