AmMJarch•,T91o6.1*m•}  Estimation  of  Caseine.  123 
earlier.  This  phenomenon  is  evidently  due  to  the  fact  that  the 
albumin  of  the  milk  forms  a  soluble  iron  albuminate,  which  does 
not  respond  to  ferrocyanide  until  decomposed  by  acid.  This  may 
eventually  lead  to  a  quick  method  of  estimating  total  proteids  of 
milk,  but  so  far,  results  obtained  have  been  too  variable  for  practical 
use. 
Failing  along  all  these  lines  for  results  trial  was  made  of  precipi- 
tating milk  with  a  definite  excess  of  decinormal  ferric  alum  solution, 
filtering  off  the  precipitated  caseine  and  estimating  amount  of  ferric 
alum  remaining  in  the  filtrate  by  titration  with  potassium  iodide, 
hydrochloric  acid  and  decinormal  sodium  thiosulphate.  The  pro- 
cess at  present  employed  is  as  follows:  20  c.c.  decinormal  ferric  alum 
solution  (48-1  grammes  to  the  liter)  is  mixed  with  the  desired 
amount  of  milk — 5,  10,  20  or  30  c.c. — at  ordinary  temperature, 
some  water  added,  the  mixture  shaken  and  allowed  to  stand  a  few 
minutes  and  then  filtered,  the  precipitation  being  washed  until  the 
washings  are  free  from  iron.  Either  the  whole  filtrate  or  half  of  it 
is  titrated  with  potassium  iodide,  acid  and  thiosulphate,  and  thus  the 
amount  of  unused  ferric  alum  is  ascertained.  The  difference  in  the 
amount  of  ferric  alum  in  the  original  solution  and  the  amount  used 
gives  the  quantity  of  ferric  alum  required  for  precipitation  of  caseine. 
Figures  on  this  work  are  here  given  : 
Tabi<e  I. 
All  samples  treated  with  20  c.c.  decinormal  ferric  alum  solution. 
Cubic    centimetres   Cubic  centimetres  deci- 
Cubic  centimetres 
decinormal  thiosul- 
normal   ferric  alum 
Milk  sample. 
milk  used. 
phate  required. 
used  for  milk. 
I  a 
IO 
I6'2 
3-8 
1  b 
IO 
l6'2 
3-8 
1  c 
5 
l8'2 
i-8 
1  d 
5 
18-4 
r6 
2  a 
10 
16*4 
3-6 
2  b 
10 
164 
3-6 
2  c 
5 
18-0 
2'0 
2  d 
5 
18-0 
2*0 
2  e 
5 
18-0 
2*0 
3  a 
5 
17-9 
2'l 
3b 
5 
18-0 
2-0 
3  c 
10 
16*2 
3-8 
3d 
10 
i6*o 
4-0 
3  e 
15 
14-4 
5-6 
3f 
15 
14-4 
5-6 
4a 
25 
9-8 
IO'2 
