510  Chemistry  of  Tartaric  and  Citric  Acid.  | 
Am.  Jour.  Pharm. 
Oct.,  1883. 
metatartaric,  and  at  a  higher  temperature,  or  by  longer  heating,  into 
ditartaric  acid,  is  well  known.  According  to  Gmelin,  the  change  into 
metatartaric  acid  may  be  effected  in  the  presence  of  a  small  quantity 
of  water ;  2  or  3  parts  to  60  of  acid.  Some  of  my  own  experiments 
with  the  tartaric  liquors  of  the  factory  ("  Chem.  Soc.  J.,"  1875,  988) 
had  made  it  probable  that  the  same  change  may  to  some  extent  occur 
when  solutions  of  tartaric  acid  are  concentrated  in  the  ordinary  way 
over  steam,  as  it  was  found  that  two  such  liquors,  when  diluted  and 
boiled  for  three  hours,  yielded  3'36  and  3*68  per  cent,  more  of  tartaric 
acid  than  before  boiling.  Mr.  Grosjean  has  now  amply  proved  that 
solutions  of  tartaric  acid,  concentrated  on  a  water-bath,  may  undergo 
conversion  into  metatartaric  acid  to  a  very  considerable  extent.  He 
has  also  shown  that  in  a  dilute  solution,  the  reverse  change,  even  in 
the  cold,  will  slowly  take  place. 
I.  40  grams  of  tartaric  acid  were  dissolved  in  water,  concentrated 
in  a  beaker  over  a  water-bath  till  a  crust  formed  on  the  surface,  and 
maintained  four  hours  in  this  condition ;  the  temperature  of  the  tar- 
taric solution  was  93°.  When  cold,  the  whole  was  redissolved  and 
brought  to  a  litre.  The  acidity  was  now  97*9  per  cent,  of  the  original; 
while  the  tartaric  acid,  found  by  precipitation  with  citrate  of  potas- 
sium, was  only  74*6  per  cent,  of  the  tartaric  acid  taken.  On  boiling 
a  portion  of  the  diluted  solution  for  two  hours,  the  acidity  was  raised 
to  99*9  per  cent.,  and  the  tartaric  acid  also  to  99*9  per  cent,  of  the 
original  acid,  showing  that  a  complete  reversal  of  the  reaction  had  been 
effected. 
The  remainder  of  the  diluted  solution  which  had  not  been  boiled 
was  mixed  with  a  little  phenol,  and  the  amount  of  tartaric  acid  deter- 
mined from  time  to  time  with  the  following  results  : 
August      24,  1861,  tartaric  acid,  74*6  per  cent,  of  original  acid, 
u  25,    "  77-2         "  " 
31,    "  "  81-7 
September  16,    "  "         88*0         "  *' 
October     18,    "  "         90*0         "  " 
The  experiment  now  ceased  for  want  of  material. 
II.  80  grams  of  tartaric  acid  were  treated  as  before,  the  heating 
being  continued  3 J  hours  after  a  crust  had  formed ;  the  temperature 
of  the  solution  was  92°.  When  cold,  the  beaker  was  weighed. 
Assuming  that  it  still  contained  80  grams  of  tartaric  acid,  the  water 
present  was  24*75  grams.    Redissolved  and  diluted  to  one  liter,  the 
