48 
ON  SOLUTION  OF  ACETATE  OF  AMMONIA. 
pie  of  which  is,  neutralizing  the  acid  with  a  normal  solution  of 
ammonia ;  the  quantity  of  the  ammonia  employed  indicating 
directly  the  percentage  of  the  acetic  acid, — assuming  that  100 
grains  of  the  officinal  dilute  acetic  acid  must  saturate  7*6  grains 
of  cryst.  bi-carbonate  of  potassa.  This  will  be  equivalent  to 
4*55  per  cent,  of  mono-hydrated  acetic  acid.  (Wood  &  Bache 
give  5  per  cent,  as  the  strength).  Acid  of  this  percentage  has 
a  specific  gravity  of  1*0068  at  60°  F.,  when  accurately  taken. 
Having  the  acetic  acid  properly  diluted,  the  directions  of 
the  Pharmacopoeia  are  to  add  the  carb.  ammonia  gradually  to 
the  acid,  until  this  is  saturated.  It  is  just  here  that  a  practical 
difficulty  occurs,  in  determining  the  exact  point  of  saturation. 
At  first  the  taste  will  sufficiently  indicate  if  the  solution  is  yet 
acid,  but  as  the  point  of  saturation  is  approached,  litmus  paper 
or  solution  o^  litmus  must  be  resorted  to.  These,  if  proper 
precautions  are  taken,  will  give  sufficiently  accurate  results. 
It  will  be  observed,  in  testing  liquids  containing  free  carbonic 
acid,  that  litmus  will  be  reddened,  even  after  the  solution  is 
neutralized  by  an  alkali.  This  reddening,  however,  is  not  per- 
manent, but  will  disappear  upon  drying  the  paper,  the  carbonic 
acid  going  off  with  the  moisture.  This  source  of  fallacy  may 
also  be  overcome  by  gently  warming  a  portion — say  half  an 
ounce — of  the  solution,  to  which  a  few  drops  of  the  solution  of 
litmus  has  been  added.  The  red  color  of  the  solution  will 
gradually  change  to  a  violet,  indicating  the  fact  of  its  alkalin- 
ity. If  now  dilute  acetic  acid  be  dropped  in  until  the  color  is 
slightly  reddened,  the  number  of  drops  required  will  give  the 
proportionate  amount  of  acid  to  be  added  to  the  remainder  of 
the  solution.  If,  on  the  contrary,  the  solution  when  heated 
should  remain  red,  aqua  ammonias  should  be  dropped  in  until 
the  color  just  begins  to  change,  the  number  of  drops  indicating, 
as  before,  the  amount  requisite  for  the  whole  solution. 
It  has  been  proposed,  in  order  to  avoid  in  a  measure  the  un- 
certainty above  alluded  to,  to  saturate  the  strong  acid  No.  8 
directly  with  carbonate  of  ammonia,  and  afterwards  to  dilute 
to  the  proper  degree. 
Made  in  this  manner,  there  is  much  less  difficulty  in  ascer- 
taining the  point  of  saturation,  no  carbonic  acid  being  retained 
in  the  solution. 
