A™' May  mi™'}     Scientific  and  Technical  Abstracts  355 
SCIENTIFIC  AND  TECHNICAL 
ABSTRACTS 
Color  Test  for  Oxalic  Acid.— A  few  crystals  of  resorcinol 
are  added  to  about.  5  mils  (Cc.)  of  the  unknown  solution  in  a  test- 
tube,  and  the  mixture  is  warmed  slightly  to  dissolve  the  resorcinol. 
It  is  then  cooled  and  5  mils  (Cc.)  of  concentrated  sulphuric  acid  are 
carefully  and- slowly  poured  in  along  the  side  of  the  tube,  so  as  to 
form  a  layer.  A  blue  ring  will  be  formed  at  the  junction  of  the  two 
layers,  if  oxalic  acid  is  present.  The  color  is  best  seen  if  held  to  the 
light  in  front  of  a  sheet  of  white  paper.  Care  must  be  taken  that  the 
mixture  does  not  warm  up  appreciably.  If  the  blue  color  does  not 
appear  in  a  few  minutes,  the  mixture  is  shaken  thoroughly,  and  after 
cooling  somewhat  5  mils  (Cc.)  more  of  sulphuric  acid  are  added. 
Should  the  color  still  fail  to  appear,  the  mixed  contents  of  the  tube 
should  be  gently  warmed  over  a  flame  (not  boiled)  when  an  indigo 
blue  color  will  diffuse  throughout  the  liquid.  If  the  mixture  be  cooled 
with  ice-water,  the  color  will  disappear  only  to  reappear  again  on 
heating.  If  the  mixture  be  boiled  a  few  minutes,  the  color  will  turn 
a  deep  dark  green,  which  will  become  a  light  yellow-green  on  cool- 
ing. If  to  the  cold  yellow-green  solution  an  equal  volume  of  sul- 
phuric acid  be  added  so  as  to  form  two  layers,  the  blue  color  will 
again  appear.  It  is  believed  that  all  these  reactions  taken  together 
are  characteristic  of  oxalic  acid  alone. 
This  test  may  be  made  sensitive  to  one  milligramme  if  the  dry 
unknown  substance  be  warmed  with  two  drops  of  a  10  per  cent,  aque- 
ous resorcinol  solution  and  the  sulphuric  acid  added  drop  by  drop. 
The  blue  color  then  appears  immediately.  For  very  dilute  solutions  of 
oxalic  acid  or  its  salts,  it  is  best  to  evaporate  to  a  concentration  of 
about  10  per  cent. 
If  interfering  substances  are  present  the  oxalic  acid  may  be 
precipitated  in  ammoniacal  solution  as  the  calcium  salt,  washed  with 
water,  and  the  test  applied  directly  to  an  aqueous  suspension  of  the 
salt.—  (Journal  of  the  American  Chemical  Society,  through  Merck's 
Report,  April,  1921.) 
