8 A PON IFI ABLE OILS, FATS AND WAXES 179
Dissolve 13.6 gin of powdered iodine in 825 cc of glacial acetic, acid.
The mixing machine shown in Fig. 51, page 230, will bo, found useful for
hastening solution. Cool, decant to insure that no particles of iodine
remain undissolved, and mix. Measure from a burette 25 ec of the solution
into a 250-cc Erlerimeyer flask, add 15 cc of potassium iodide nolution
(6) and 100 cc of water, and mix. Titrate at once with tenth-normal
sodium thiosulphate solution.
From a small burette measure 3 cc of bromine into 200 ce of glacial
acetic acid. Mix arid titrate 5 cc of the solution against sodium thiomil-
phate solution, adding potassium iodide and water as in the iodine titration.
From these titrations calculate the volume of bromine solution that would
1)0 equivalent to 800 cc of iodine solution. Add this quantity of bromine
solution to the iodine in a glass stoppered bottle and mix well. This should
produce a solution of iodine monobrornide, containing only a very slight
excess of cither bromine or iodine.
The addition of potassium iodide, both before and after absorption by the
oil, gives a titration which may be calculated as though iodine were the only
halogen present, since this element is titrated at the end, in both cases:
IBr + KI -> KBr + I2.
(Of course the iodine is then present as Klj.)
Determination of Iodine Number.—Half fill a 20-ce weighing bottle
with oil, place in it a piece of glass rod and weigh without the stopper.
Carefully pour about 0.25 gm of the oil into a 500-ec bottle or flask having
a ground glass stopper, using the glass rod to assist in the transference.
Reweigh and prepare another sample in the same manner.
Dissolve the weighed sample of oil in 10 ce of chloroform then add
25 cc of iodine moriobromide solution, measuring from a pipette. Stop-
per, mix and allow to stand for 30 minutes, shaking occasionally. The*
bottle should riot be left in strong light.
At the time that the iodine rnonobromide solution is measured into
the oil solution, measure the same amount of solution into two bottles,
containing the chloroform but no oil. Treat those in exactly tho sumo
manner as the solution containing oil. Thin in for the, "blank "determination.
At the end of the absorption period add 15 cc of potassium iodide solution
(b). Add 100 cc of water, washing down any iodine that may be on the
stopper. Titrate the unabsorbod iodine; with standard sodium thioHulphuto,
shaking constantly. When only a faint yellow remains add 1 ec, of Htiiroh
solution and finish the titration. At the la«t the bottle* should be cloned
and shaken until all iodine remaining in the chloroform has boon extracted
by the potassium iodide. The temperature should be kept as nearly con-
stant as possible throughout the experiment.
From the volume of sodium thiosulphate required for the iodine solu-
tion alone subtract that required for the oil and iodine solutions. Tho
remainder is the volume corresponding to the absorbed iodine. Calcu-
late the por cent of iodine absorbed.