AmFe°b.!i§9arm'}     Phenol  as  a  Test  for  Nitrites,  etc.  91 
I 
Phenol  and  nitrites : — A  200,000  N203  solution  yields  a  faint  pink 
band  at  first,  faint  green  below ;  pink  band  very  distinct  in  half  an 
hour,  but  after  five  hours  the  color  has  faded  away.  50,000  N203 
solution  gives  good  red  and  green  bands  at  once,  fading  after  five 
hours.  2000  N2Os  solution  very  intense  red  and  green  bands  in  24 
hours,  the  latter  changes  to  blue  and  the  red  to  dingy  purple.  Solu- 
tions of  intermediate  strengths  give  intermediate  reactions,  and  even 
400,000  N203  gives  a  very  faint  band.  Hydrochloric  acid  impedes  or 
retards  the  reaction,  so  that  1  drop  of  solution  a  renders  the  color  with 
200,000  N203  indistinct,  and  2  drops  upset  the  reaction  with  50,000 
N203  solution,  and  make  the  10,000  give  a  light  red  band  with 
yellowish-green  below.  Two  drops  of  sulphurous  acid,  on  the  other 
hand,  prevent  the  color  forming  even  with  the  2000  N203  solution ; 
but  with  the  addition  of  copper  sulphate  (see  below)  a  slight  pink 
band  is  obtained  after  two  or  three  hours,  presumably  due  to  a 
trace  of  N2Os. 
Orcinol  and  nitrites  : — One  of  nitrite  in  1,000,000  of  water  gives  a 
faint  but  uncertain  indication  after  some  time ;  500,000  N2Os  gives  a 
very  faint  pink  after  four  hours,  becoming  more  distinct  in  nine  hours  ; 
10,000  N203  solution  gives  a  strong  orange  band  at  once,  becoming 
almost  opaque  after  some  time.  The  color  is  permanent.  Hydro- 
chloric and  sulphurous  acids  both  interfere  with  the  reaction,  but  cop- 
per sulphate  with  hydrochloric  acid  increases  the  delicacy;  1,000,000 
N203  -|-  1  drop  of  copper  sulphate  -f-  2  drops  of  orcinol  -f  hydro- 
chloric acid,  sharp  orange  pink  band  in  10  minutes. 
Phenol  and  nitrates  : — ~No  reaction  with  1  N205  in  20,000  of  water, 
10,000  N2Os  faint,  pink  band  becoming  broader  and  less  definite,  ulti- 
mately fading .  1000  N205,  intense  red  band,  pale  green  below  becom- 
ing very  dense.  Intermediate  and  stronger  reactions  are  given  in  the 
original.  Phenol  alone  appears  to  be  but  a  poor  test  for  nitrates,  but 
in  the  presence  of  hydrochloric  acid  it  becomes  very  delicate ;  by  add- 
ing one  drop  of  solution  c  100,000  N2Os  gives  a  faint  pink  at  once, 
which  becomes  distinct  in  20  minutes,  spreads  and  ultimately  fades ; 
10,000  N2Os,  a  sharp,  crimson  band  at  once,  spreading  without  losing 
intensity  for  some  hours,  and  faint  green  below,  soon  fading.  1000 
N205,  very  dense  red,  green  below.  Sulphurous  acid  does  not  inter- 
fere with  this  test,  or  with  the  corresponding  one  with  orcinol. 
Neither  ferric  chloride  nor  copper  sulphate  disturbs  this  reaction,  in 
fact,  the  latter  rather  improves  it  with  highly  dilute  N2Os  solutions ; 
