158  Nitrates  and  Ammonia  in  Water.  {Am-/KKrm* 
adding  37  grammes  sulphuric  acid  to  a  mixture  of  6  grammes  pure 
phenol  and  3  grammes  distilled  water)  and  heated  on  the  water-bath 
for  five  minutes;  the  liquid  is  then  diluted  with  5  c.c.  water,  an  excess 
of  water  of  ammonia  added,  drop  by  drop  (about  5  c.c.)  and  the  alka- 
line solution  finally  suitably  diluted  for  comparison  with  a  standard 
N 
solution.    This  standard  solution  I  have  always  made  from  a  _ 
100 
solution  of  potassium  nitrate  (roio  grammes  of  the  fused  salt  per 
litre) ;  1  c.c.  of  this  solution  is  evaporated  to  dryness  and  the  residue 
treated  as  above,  finally  diluting  to  100  c.c.  (each  cubic  centimetre 
of  this  solution  represents  0-0000014  gramme  nitrogen  as  nitrates). 
It  will  very  often  be  found  necessary  to  dilute  a  portion  of  this 
standard  solution,  so  as  to  match  the  color  of  the  solution  obtained 
from  the  sample  under  examination. 
The  phenol-sulphonic  acid  and  the  standard  solution  keep  very 
well  if  protected  from  light  ;  samples  at  least  seven  years  old  have 
been  examined  recently  and  were  found  practically  unchanged  ;  the 
the  phenol-sulphonic  acid  has  deposited  a  considerable  quantity  of 
a  white  precipitate,  but  by  agitation  can  be  incorporated  with  the 
fluid  portion  and  measured  without  difficulty. 
The  only  objection  to  this  method  of  estimating  nitrates  is  the 
difficulty  experienced  at  times  in  matching  the  color,  and  for  a  long 
time  no  modification  could  be  devised  which  satisfactorily  enabled 
the  color  comparison. 
Several  years  ago,  while  working  with  salicylic  acid,  the  idea 
suggested  itselt  to  use  a  saturated  solution  of  salicylic  acid  in  sul- 
phuric acid  instead  of  the  phenol-sulphonic  acid  ;  various  samples 
of  water  examined  with  these  two  reagents  indicated  the  superiority 
of  the  salicylic  acid  reagent  because  of  a  deeper  colored  solution 
and  the  decreased  number  of  cases  in  which  difficulty  was  experi- 
enced in  the  matching.  Several  months  ago,  however,  a  sample  of 
water  was  examined,  which  with  phenol-sulphonic  acid,  practically 
showed  no  nitrates,  but  giving  with  the  salicylic  acid  reagent  an 
intense  coloration  which  moreover  was  very  difficult  to  match.  An 
examination  disclosed  the  presence  of  manganese  and  iron  salts 
in  the  water  and  the  fact  that  the  difficulty  was  caused  by  the 
manganese. 
This  experience  led  me  to  abandon  the  salicylic  acid  solution  as 
a  test  for  nitrates  and  turned  my  attention  again  to  the  phenol-sul- 
