634 
Improved  Deniges  Test. 
5  Am.  Jour.  Pharm. 
(      Sept.,  1921. 
strict  adherence  to  a  standard  set  of  conditions  under  which  it  is 
assumed  that  a  certain  concentration  of  methanol  originally  present 
results  in  a  certain  concentration  of  formaldehyde  at  the  end. 
Likewise,  for  the  demonstration  of  formaldehyde  there  appears 
to  be  no  reagent  any  more  convenient  or  reliable  than  SchifFs 
reagent,  prepared  according  to  the  Elvove7  formula.  Its  chief  com- 
parative disadvantage  is  the  slowness  of  development  of  the  final 
color. 
QUANTITATIVE  METHOD. 
The  Deniges'  method  has  been  used  with  more  or  less  modifi- 
cation by  a  considerable  number  of  investigators.  Since  in  routine 
analyses  following  the  procedure  of  Elvove  the  observed  margin  of 
precision  seemed  unnecessarily  large,  the  whole  process  has  been 
subjected  to  close  scrutiny  with  a  view  to  attaining  greater  pre- 
cision. It  was  decided  that  0.04  cc.  of  total  alcohol  should  be  the 
standard  quantity  for  each  test,  which,  including  the  necessary  acid, 
should  be  made  to  a  volume  of  5  cc.  The  nature  and  proportion 
of  the  acid  is  of  very  great  importance.  The  highest  yield  of  for- 
maldehyde results  from  slow  action  of  permanganate  in  presence 
of  low  hydrion  concentration;  but  practical  considerations  prohibit 
an  inordinately  long  reaction  time,  while  the  total  acid  must  be  kept 
up  to  a  safely  high  figure.  The  conditions  finally  chosen  were  the 
addition  of  0.2  cc.  of  phosphoric  acid  (C.  P.,  85  per  cent.),  previ- 
ously diluted  to  1  cc.  for  accuracy  in  measurement,  and  an  oxidation 
period  of  30  min.,  instead  of  the  0.2  cc.  of  concentrated  sulfuric 
acid  and  oxidation  period  of  3  min.  employed  by  Elvove.  Next, 
after  deciding  that  the  necessary  permanganate  should  be  added 
in  a  volume  of  2  cc,  it  remained  merely  to  find  a  concentration  of 
the  permanganate  solution  such  that  either  more  or  less  than  2  cc. 
of  it  would  give  a  lower  yield  of  formaldehyde  than  exactly  2  cc. 
The  desired  strength  was  found  to  be  3  per  cent.  In  a  similar  way 
the  volumes  of  sulfuric  acid  and  S  chirr"- Elvove  reagent  were  tested. 
Directions  for  the  method  may  be  given  as  follows : 
7  This  Journal,  9  (1917),  295.  Fuchsin  (0.2  g.)  is  dissolved  in  120  cc.  hot 
water.  After  cooling  to  room  temperature  there  are  added  2  g.  of  anhydrous 
sodium  sulfite  dissolved  in  20  cc.  water,  followed  by  2  cc.  concentrated  hydro- 
chloric acid.  The  solution  is  diluted  to  200  cc.  and  is  allowed  to  stand  1  hour 
before  use.  If  well  stoppered  in  an  amber  bottle  it  may  remain  fit  for  use  for 
several  weeks.  The  Schiff -Elvove  reagent  appears  decidedly  superior  to  the 
original  Schiff  reagent,  and  should  supersede  the  latter. 
