Am.  Jour.  Phaem  ) 
Mar.,  1874.  Suppl.J 
Laboratory  Notes. 
145 
LABORATORY  NOTES. 
By  E.  B.  Shuttleworth. 
Use  of  Glycerin  in  the  Estimation  of  Tannin. — The  estimation  of 
tannic  aeid  by  means  of  a  solution  of  gelatin  is  generally  a  tedious 
and  troublesome  process.  The  precipitate  formed  is  so  slowly  depos- 
ited that,  without  resorting  to  a  filtration  almost  as  inconvenient,  it  is 
difficult  to  determine  the  exact  point  when  a  sufficient  quantity  of  the 
precipitant  has  been  added,  as  also  to  separate  the  precipitate  at  the 
close  of  the  operation.  In  order  to  ascertain  the  completion  of  the 
process,  Wheeler*  suggests  that  a  tube,  loosely  closed  at  the  bottom 
with  sponge,  be  dipped  into  the  solution ;  the  filtered  liquid  which 
enters  the  tube  is  then  tested  with  a  further  quantity  of  the  gelatin 
solution.  Mullerf  accelerates  the  clarification  of  the  liquid  by  add- 
ing a  certain  proportion  of  alum.  I  have  found  that  both  these  ends 
may  be  more  easily  accomplished  by  adding  to  the  tannin  solution  a 
small  quantity  of  glycerin.  The  precipitate  by  gelatin  subsides  more 
or  less  readily  according  to  the  concentration  of  the  solution. 
As  I  have  noted  in  a  previous  paper,J  the  precipitation  of  tannin 
may  be  altogether  prevented  by  employing  a  very  large  proportion  of 
glycerin,  so  that  it  is  probable  that  in  using  a  lesser  quantity  a  small 
proportion  of  the  precipitate  is  retained  in  solution.  In  comparative 
examinations — and  it  is  chiefly  in  this  manner  that  estimations  are 
made — this  is  of  no  consequence,  as  the  loss  may  be  determined  when 
making  the  standard  solution.  Even  with  simple  water  the  indica- 
tions are  not  altogether  reliable,  and  a  certain  allowance  must  be  made, 
as  the  precipitate  is  not  absolutely  insoluble  in  water  without  the  tan- 
nin is  in  considerable  excess. 
Fluorescence  of  the  Acid  Residue  from  the  Manufacture  of  Ether. 
— Those  who  are  practically  acquainted  with  the  preparation  of  ether 
may  have  noticed  the  extraordinary  fluorescent  appearance  of  the  sul- 
phuric acid  remaining  at  the  close  of  the  process.  I  am  not,  how- 
ever, aware  that  this  property  has  ever  been  noted  in  any  of  the 
journals,  and  to  those  engaged  in  researches  on  fluorecence  the  fact 
may  be  of  interest. 
*  Mem.  Chem.  Soc  iii,  319. 
f  Chem  Oentr.  1859,  42.    Watts'  Diet,  ii,  70,"). 
%  Can  Pharm.  Jour,  vii,  229. 
9a 
