172        kerr's  solution  of  pernitrate  of  iron. 
oxide  in  the  proper  quantity  of  nitric  acid,  diluted  with  water, 
so  as  to  produce  the  necessary  measure  of  30  fluid-ounces  for 
each  ounce  of  metallic  iron. 
Supposing  60  fluid-ounces  the  quantity  to  be  prepared  ;  make 
a  solution  of  perchloride  of  iron,  according  to  the  direction  of  the 
British  Pharmacopoeia,  dilute  with  2  or  3  gallons  of  cold  water, 
and  add  ammonia  or  a  weak  solution  of  carbonate  of  soda  till 
the  precipitation  is  complete.  The  precipitate  having  subsided, 
wash  by  decantation  or  siphoning  till  completely  free  from  saline 
taste.  As  the  oxide  of  iron,  after  entire  subsidence,  contains 
too  much  water,  the  excess  must  be  removed  by  pressure  in  a 
cloth,  so  that  the  remaining  pulp  may  not  occupy  more  space 
than  about  30  ounces. 
The  solution  of  the  oxide  of  iron  may  now  be  easily  made ; 
dilute  the  acid  to  within  a  little  of  30  ounces,  introduce  the  pulpy 
mass  gradually,  with  constant  stirring,  and  bring  the  whole  to 
the  proper  bulk. 
A  simple  method  of  accurately  arriving  at  the  proper  quantity 
is  to  measure  into  the  vessel,  before  use,  60  ounces  of  water, 
and  mark  with  a  slip  of  gummed  paper  the  level  of  the  liquid. 
In  order  to  avoid  the  risk  of  the  formation  of  a  basic  compound 
(which  the  after  addition  of  acid  might  fail  entirely  to  dissolve), 
it  is  preferable  to  add  the  oxide  to  the  acid  than  the  reverse. 
56  parts  or  2  eq.  of  iron  require  3  eq.,  or  162  parts  of  absolute 
nitiic  acid. 
For  2  oz.  of  iron  we  find  by  proportion — 
56  :  162  :  :  437*5  x  2  :  2531  parts  of  nitric  acid  necessary. 
Let  80  per  cent,  of  anhydrous  acid  be  present  in  the  acid  used, 
and  the  requisite  quantity  will  be — 
80  :  100  :  :  2531  :  3164  grains,  or,  as  the  measure  is  inversely 
as  the  density— 1500  :  1000  :  :  3164  :  2109  water  grs.,  or 
4*82  fl.  oz.,  or  4  oz.  6  drs.  33  ms. 
As  it  is  advisable  to  have  a  slight  excess  of  acid  in  the  perni- 
trate solution,  one  or  two  drachms  more  than  the  calculated 
quantity  should  be  used. 
Following  rigidly  the  process  given  in  the  British  Pharma- 
copoeia, we  have  been  unable  to  obtain  a  preparation  giving  no 
precipitate  with  fcrridcyanide  of  potassium. — London  Pharm. 
Jour  a.,  November,  1866. 
