636 
Improved  Deniges  Test. 
5  Am.  Jour.  Pharm. 
(      Sept.,  1921. 
thereto  0.5  cc.  of  the  4  per  cent,  ethyl  alcohol  and  sufficient  water 
to  make  4  cc.  Approximate  readings  may  be  made  after  30  minutes, 
precise  ones  after  1  hour,  but  best  under  2  hours,  for  the  colors 
fade  later.  The  limit  of  detection  is  0.2  cc.  of  the  standard  0.04 
per  cent,  methanol. 
Tests  on  four  "unknown"  mixtures  of  methanol,  ethyl  alcohol, 
and  water  prepared  by  an  assistant  indicated  that,  including  the 
necessary  determination  of  total  alcohol  via  specific  gravity,  the 
results  need  not  be  in  error  by  more  than  1  part  in  20. 
QUALITATIVE  METHOD. 
A  modification  of  Deniges'  method  is  official  as  a  qualitative 
test  in  the  U.  S.  Pharmacopeia  IX.  The  U.  S.  P.  test  has  been 
criticized  as  unreliable  because  a  false  reaction  sometimes  occurs. 
Ehman8  attributes  the  fault  to  temperature  and  overcomes  it  by 
running  a  blank  with  pure  ethyl  alcohol,  adjusting  the  temperature 
until  the  blank  remains  colorless.  In  the  judgment  of  the  present 
writer  the  difficulty  is  primarily  due  to  an  undesirably  high  con- 
centration of  total  alcohol.  Since  the  substitution  of  phosphoric 
acid  for  sulfuric  acid  considerably  more  than  doubles  the  yield  of 
formaldehyde  from  a  given  amount  of  methanol,  the  concentration 
of  the  sample  in  the  test  here  proposed  need  be  only  half  that  em- 
ployed in  the  U.  S.  P.  test  and  still  leave  the  proposed  test  more 
delicate  than  the  U.  S.  P.  test  at  its  best.  The  proposed  test  has 
been  run  at  temperatures  of  150  and  350  C.  without  experiencing' 
difficulty  with  false  reactions.9    It  may  be  conducted  as  follows : 
Dilute  the  liquid,  purified  as  necessary,  to  a  content  of  5  per 
cent,  by  volume  of  total  alcohol.  To  5  cc.  add  0.3  cc.  of  phosphoric 
acid  (C.  P.;  85  per  cent.)  mix,  add  2  cc.  of  a  31  per  cent,  solution 
of  potassium  permanganate  mix  and  let  stand  until  the  perman- 
ganate is  entirely  decomposed  (about  10  min.).  Add  1  cc.  of  10 
per  cent,  oxalic  acid  mix,  and  let  stand  till  a  clear  brown  (about  2 
min.).  Add  1  cc.  concentrated  sulfuric  acid,  mix,  add  5  cc.  Schiff- 
Elvove  reagent,  immediately  mix  well,  and  observe  the  color  after 
exactly  10  min.  The  solution  may  then  possess  a  pale  greenish  tint, 
but  should  show  no  distinct  blue  or  violet  color  against  a  white 
background  (less  than  0.2  per  cent,  methanol  in  the  total  alcohol). 
8  Am.  J.  Pharm.,  91  (1919),  594. 
9  It  may  be  that  in  the  U.  S.  P.  test  the  presence  of  sulfuric  acid  promotes 
oxidation  of  ethyl  alcohol  to  formaldehyde  at  an  elevated  temperature. 
