Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

DETEKMINATION OF SULPHUE 77
tube. Boil the platinum-wire gauze catalysts with dilute nitric acid
(1 : 1), ignite them strongly and push them into the tube until they
rest close to the constriction. Round this part of the tube slip a
roll of iron-wire gauze 20 cm. long and, in order further to retain the
heat, place a roof of iron-wire gauze on the combustion stand above
the roll. The delivery tube clips into a receiver of the type shown
in Fig. 41. The receiver contains 12-15 c.c. of the acetate-acetic
acid solution and 10-12 drops of bromine.
For the determination of iodine weigh out 20-30 mg. of substance
in a small platinum boat and carry out the combustion in the manner
described above for the other halogens. Adjust the rate of flow of
the oxygen to 4-5 c.c. per minute.
After the tube has cooled remove the boat and catalysts and
pour into the tube, holding it obliquely, about 4 c.c. of solution of
bromine in acetic acid in order to oxidise the iodine deposited in
the constriction. The constriction retains the solution at this point.
After ten minutes wash the contents of the receiver and tube
quantitatively into a clean conical flask, containing a solution of
2 g. of sodium acetate in a little water.
In order to use up the excess of bromine let a few drops (up to 0-5
c.c.) of formic acid run down the wall of the flask, shaking vigorously
meanwhile so that the bromine in the gaseous phase is also absorbed.
Wait a fewseconds until thesolution has become decolorised and then
add at once some dilute sulphuric acid and 1 -5 g. of potassium iodide.
Shake, leave for five minutes, and titrate the liberated iodine with
0-1 ./y thiosulphate solution from a micro-burette with 0-02 c.c. scaledivisions,
adding starch when the colour has diminished to yellowand
then continuing the titration until the colour disappears.
Limits of error of the method : ±0-3 per cent.
4. DETERMINATION OF SULPHUR BY THE CARIUS METHOD
The determination of sulphur by the Carius method is carried
out in the same way as is the determination of halogen, but, in place
of the silver nitrate, anhydrous barium chloride is here used.
The following are required for the determination :
Hard glass Carius tube.
Red fuming nitric acid (d 1-5).
Solid barium chloride.
Porcelain Gooch crucible for ignition with a protective saucer.
(Crucible Al, depth 2-7 cm., capacity 6 c.c. Berlin State Porcelain.)