178

QUANTITATIVE AGRICULTURAL ANALYMfi

iodine monobromide or iodine monochloridc in glacial acetic;
acid. The monobromide solution was proposed by Hanus, that
of monochloride by Wijs. As the former is somewhat more
easily prepared its preparation and use will be described.

The following solutions will be required for the determination of iodine
number:

(a) Potassium Bichromate.— -A tenth-normal solution, made by dissolving
exactly the calculated weight of a salt of known purity, or standardize an
directed on page 74. Five hundred cubic centimeters of this solution will
be sufficient.

(6) Potassium Iodide.—— Prepare 200 cc of a solution containing approxi-
mately 25 gm of the solid.

(c) Starch.— -Moisten 1 gm of potato starch with enough cold water to
make a thick paste. Heat 100 cc of water to boiling and pour it into the
starch paste. Boil gently, with constant stirring, for about a minute.
The solution does not'keep well and it should be made each day, as required.
The addition of preservatives, such as chloroform or zinc chloride, has been
tried but the solution deteriorates, even with such additions.

(d) tiodium 7Tto«wZ;p/i<zfe.— -Prepare an approximately tenth-normal solu-
tion, calculating the equivalent weight from the following equation :

2Na«SaO,

4- 2NaI.

In weighing the crystallized salt, calculations must include 10 molecules of
water of crystallization, the formula being NazSaOa.lOHgO.
Standardize the thiosulphate solution as follows: Pipette 25 cc of the
dichromatc solution into an Krlenmeyer flask and acid 50 cc of potassium
iodide and 10 cc of concentrated hydrochloric acid. Iodine is liberated
according to the equation:
K2Cr207 + 6KI + 14HC1 -+ 8KC1 + 2CrCli + 711*0 + 31*
Titrate immediately with sodium thiosulphate, adding 1 ee of starch notation
after most of the iodine has disappeared. If starch IK added too noon a
blue precipitate will be produced and the end point will be readied too
early in the titration.
The solution of chromium chloride, formed by the reduction of potassium
dichromatc, is green. The solution has an amber tint as long its free iodine
is present. Upon addition of starch the solution acquires a blue-green color
and the change to emerald green at the end point may be difficult, to judge
at first trial. By setting aside for comparison a solution that has been over-
titrated, the detection of the color change will be made easier.
(0) Iodine Monobnmi'ide..-—Yimi test the* laboratory stock of glacial
acetic acid to insure the absence of reducing matter. Add a drop of sul-
phuric acid and two or three drops of potassium dichrornate solution to 10
cc of acetic acid, and warm. The yellow color should persist, without the
appearance of green chromium salts,