Am.  Jour.  Pharm.  ) 
October,  1911.  j 
Note  on  Assay  of  Formaldehyde. 
AS7 
ing  the  iodometric  inethod/''  the  method  of  Blank  and  Finkenbeiner 
and  the  ammonia  method;  while  the  potassium  cyanide  method  he 
recommends  only  in  the  case  of  very  dilute  solutions  such  as  are 
met  with  when  determining  formaldehyde  in  milk/^  The  impres- 
sion, therefore,  which  one  obtains  from  the  literature  on  the  subject 
as  has  been  referred  to,  is  that  while  the  potassium  cyanide  method 
is  admittedly  a  very  good  method  for  determining  formaldehyde  in 
dilute  solutions  it  is  not  recommended  in  the  case  of  the  concen- 
trated or  moderately  strong  solutions,  and  that  in  such  cases  it  is 
preferable  to  use  one  of  the  other  methods,  of  which  the  hydrogen 
peroxide  method  is  the  most  favored.  An  examination  of  the  lead- 
ing pharmacopoeias  as  to  the  methods  chosen  for  assaying  commer- 
cial formaldehyde  solutions  also  seems  to  confirm  the  conclusion 
that  the  potassium  cyanide  method  is  not  generally  regarded  as  a 
method  suitable  for  use  in  such  cases.  Thus  the  U.  S.  Pharmaco- 
poeia (1905)  adopts  the  hydrogen  peroxide  method,  the  German 
Pharmacopoeia  (1910)  gives  the  acidimetric  sodium  sulphite  method, 
the  British  Pharmaceutical  Codex  (1907)  recommends  the  am- 
monium chloride  and  iodometric  methods,  while  the  French  Pharma- 
copoeia (1908)  agrees  with  the  U.  S.  Pharmacopoeia  in  giving 
preference  to  the  hydrogen  peroxide  method.  Finally,  we  may 
mention  that  according  to  Fresenius  and  Griinhut  ^-  there  really 
are  only  four  methods  in  use  for  assaying  commercial  formaldehyde, 
namely,  the  ammonia  method  of  Legler,  the  method  in  which  the 
'^'^  Ibid.  It  may  be  noted  in  this  connection  that  both  here  and  in  the 
first  edition  (p.  665)  there  seems  to  be  an  error  in  some  of  the  figures  given. 
For  if  we  take  10  c.c.  of  a  3  per  cent,  formaldehyde  solution  (it  is  directed 
that  the  solution  is  to  be  diluted  "  to  a  strength  not  exceeding  3  per  cent." ) 
we  will  be  operating  on  about  0.3  Gm. ;  and  since  "  two  atoms  of  iodine  are 
equivalent  to  one  molecule  of  formaldehyde,"  0.3  Gm.  HCHO  will  require 
200  c.c.  of  n/io  iodine,  whereas  according  to  the  given  directions  only  25  c.c. 
of  n/io  iodine  are  to  be  taken.  The  same  error  is  found  also  in  Lefifmann 
and  Beam:  Food  Analysis,  2d  ed.,  p.  84  (1905). 
Ibid.,  p.  181.  There  seems  to  be  also  an  oversight  in  this  connectioi^ 
since  it  is  directed  to  use  ferric  chloride  as  the  indicator  in  titrating  the 
excess  of  silver. 
^'^  Zeit.  anal.  Chem.,  44,  p.  13  (1905):  "  Doch  sind  es — soweit  unsere 
Kenntnis  reicht — im  wesentlichen  nur  vier  Verfahren,  deren  man  sich  in  der 
Technik  zur  Bctriebskontrolle,  sowie  zur  Wertbestimmung  des  Handels- 
produktes,  bcdient :  die  Ammoniak-Methode  von  Legler,  die  Oxydation  mit 
Natronlauge  in  Druckflaschen,  die  Wasserstoffsuperoxyd-Methode  von  Blank 
und  Finkenbeiner,  und  endlich  die  jodometrische  Methode  von  Romijn." 
