1.4367

_Bichromate Method._--This process was originally devised by Hehner
(_Journ. Soc. Chem. Ind._, 1889, 4-9), but the modification suggested by
Richardson and Jaffe (_ibid._, 1898, 330) is preferred by the authors,
and has been practised by them for several years with perfectly
satisfactory results.

Twenty-five grammes of the crude glycerine are weighed out in a beaker,
washed into a 250 c.c. stoppered flask, and made up to the graduation
mark with water. Twenty-five c.c. of this solution are then measured
from a burette into a small beaker, a slight excess of basic lead
acetate solution added to precipitate organic matter, the precipitate
allowed to settle, and the supernatant liquid poured through a filter
paper into another 250 c.c. flask. The precipitate is washed by
decantation until the flask is nearly full, then transferred to the
filter, and allowed to drain, a few drops of dilute sulphuric acid being
added to precipitate the slight excess of basic lead acetate solution,
and the contents of the flask made up with water to 250 c.c. This
solution is filtered, 20 c.c. measured from a burette into a conical
flask of about 150 c.c. capacity, 25 c.c. of a standard potassium
bichromate solution containing 74.86 grammes bichromate per litre added,
together with 50 c.c. of 50 per cent. sulphuric acid, and the whole
placed in a boiling water-bath for one hour, after which it is allowed
to cool, diluted with water to 250 c.c., and this solution run in to 20
c.c. of a 3 per cent. ferrous ammonium sulphate solution until the
latter is completely oxidised, as shown by no blue coloration being
produced when one drop is brought into contact with one drop of a
freshly prepared solution of potassium ferricyanide on a spot-plate. The
ferrous ammonium sulphate solution is previously standardised by
titration with a potassium bichromate solution of one-tenth the above
strength, made by diluting 10 c.c. of the strong solution to 100 c.c.
with water.

The reaction taking place in the oxidation may be represented by the
equation:--

3C_{3}H_{5}(OH)_{3} + 7K_{2}Cr_{2}O_{7} + 28H_{2}SO_{4} =
9CO_{2} + 40H_{2}O + 7K_{2}SO_{4} + 7Cr_{2}(SO_{4})_{3}.

Now the strong potassium bichromate solution above mentioned is of such
a strength that 1 c.c. will oxidise 0.01 gramme glycerine, and 20 c.c.
of the ferrous ammonium sulphate solution should require about 10 c.c.
of the one-tenth str