52  Examination  of  Commercial  Dextrin.  {  A™aJuaury  ^i™* 
pose  the  maximum  surface  to  the  air,  after  which  the  dish  is  trans- 
ferred to  a  well-regulated  oven  and  dried  to  constant  weight  at  2000 
C,  below  which  temperature  crystallized  barium  chloride  is  de- 
hydrated. Dextrin  gum  being  hygroscopic,  precautions  must  be 
taken  accordingly.  The  dish  is  now  heated  strongly,  but  not  beyond 
the  temperature  necessary  for  the  complete  combustion  of  the  or- 
ganic matter.  During  ignition  the  sand  should  be  well  stirred  at  in- 
tervals to  allow  full  exposure  to  the  air.  The  dish  and  contents  are 
then  cooled  and  weighed.  <•  The  difference  in  weight  before  and  after 
ignition  represents  the  dextrin  gum  in  50  Cc.  of  nitrate,  i.  e.,  in  half 
the  weight  of  sample  taken. 
Limit  of  Accuracy. — The  phenolphthalein  and  the  barium  hy- 
droxide added  in  excess  of  neutrality  are  obvious  sources  of  ir- 
regularity, but  if  the  amounts  laid  down  above  are  not  exceeded  this 
is  unimportant.  A  blank  determination  showed  that  the  total  re- 
sultant error  from  this  source 'was  11  milligrams  or  0.22  per  cent, 
on  the  basis  of  a  i-Gm.  sample. 
The  precipitation  of  starch  by  barium  hydroxide  is  not  quite 
complete,  and  that  portion  which  remains  in  solution  must  ultimately 
be  weighed  as  dextrin  gum.  The  error  likely  to  arise  from  this 
cause  was  found  by  making  blank  determinations  of  dextrin  gum 
in  two  samples  of  starch.  No.  1  was  arrowroot  and  No.  2  a  corn 
starch. 
No. 
Weight  of  Sample, 
Grams. 
Weight  of  Dextrin  Gum 
Found,  Grams 
Dextrin  Gum  Found, 
Per  Cent. 
I 
2 
1. 00 
1. 00 
0.0053 
O.OO87 
1.06 
1.74 
Varying  weights  of  arrowroot  starch  were  added  to  measured 
volumes  of  a  well-standardized  dextrin  solution  and  these  mixtures 
were  examined  by  this  method,  with  results 'shown  in  the  following 
table : 
Mixture 
Number. 
Weight  of 
Starch  Taken, 
Grams. 
Weight  of  Dextrin 
in  Solution,  Grams. 
Dextrin, 
Per  Cent. 
Total  Weight  of 
Dextrin  Gum  Found, 
Grams. 
Dextrin  Gum 
Found, 
Per  Cent. 
I 
1.50 
2.4879 
62.3 
2.4430 
6l.2 
2 
1. 00 
2.4879 
71-3 
2.3700 
67.9 
3 
1. 00 
2.4879 
71-3 
2.5044 
71.8 
4 
0.50 
2.4879 
83-2 
2.4858 
83.I 
