502  Titration  of  Ammonium  Carbonate.  {Am'octu,ri8S9.arm* 
The  volumetric  estimation  of  carbonate  of  ammonia  can  not  be 
accomplished  by  direct  titration  with  the  oxalic  acid  solution,  owing 
to  the  decomposition  of  the  carbonate  on  heating  to  expel  the  carbonic 
anhydride  liberated.  The  method  used  by  the  writer  was  practi- 
cally the  same  as  that  described  by  Dr.  Squibb  (Ephemeris,  698.) 
The  sample  for  examination  was  obtained  by  selecting  a  piece  of  salt 
free  from  the  outer  white  deposit  of  acid  carbonate  and  making  a 
clearage  through  the  centre  of  the  piece,  so  as  to  obtain  a  section  con- 
taining the  several  layers.  This  was  quickly  powdered,  and  the  de- 
sired quantity  weighed  and  placed  in  a  beaker  with  50  cc.  distilled, 
the  beaker  covered  with  a  watch  crystal  or  plate  until  the  salt  is 
dissolved  and  then  carefully  run  in  55  or  60  cc.  of  the  oxalic 
acid  solution,  raising  the  cover  sufficiently  to  admit  the  end  of 
the  pipette.  After  the  effervescence  has  ceased,  warm  the  solution 
until  the  C02  is  entirely  dispelled,  and  then  titrate  with  volumetric 
soda  solution,  previously  standardized  with  the  oxalic  acid  solution, 
using  the  same  indicator  as  used  in  the  titration  of  the  ammonium 
carbonate.  By  deducting  the  number  of  cc.  of  soda  solution  re- 
quired from  the  number  of  cc.  of  oxalic  solution  used,  the  number 
of  cc.  of  oxalic  acid  solution  required  by  the  ammonium  carbonate 
is  indicated.  As  an  indicator,  I  prefer  litmus  solution,  prepared 
by  first  extracting  the  litmus  with  80  per  cent,  alcohol,  and  then 
dissolving  in  water  and  adding  to  the  filtered  solution  a  few  drops 
of  nitric  acid,  until  the  bright-blue  is  changed  to  a  purplish 
tint. 
It  is  apparent  that  much  of  the  commercial  salt  is  unfit  for  phar- 
maceutical purposes,  although  excellent  in  appearance.  The  import- 
ance of  the  volumetric  test  can  not  be  overestimated.  A  definite  salt 
answering  the  pharmaceutical  requirements,  can  be  obtained  by  re- 
sublimation  and,  no  doubt,  if  the  pharmacists  would  demand  a 
a  resublimed  carbonate  of  ammonium  the  manufacturers  would  soon 
meet  the  demand.  According  to  Dr.  Divers  (loc.  cit.  265)  "It  ap- 
pears that  when  the  normal  carbonate  is  distilled  slowly  enough,  the 
products  are  carbamate  and  water;  and  that  when  the  acid  carbonate 
is  similarly  distilled  the  product  is  the  acid  carbonate  again;  but  that 
when  either  the  normal,  the  acid  or  the  half  acid  carbonate  is  distilled 
faster  than  the  whole  of  the  products  of  distillation  can  condense  in 
the  receiver,  partial  condensation  takes  place  in  such  a  way  that  the 
carbonate  of  commerce  is  formed. 
