[Note 6:  Barium sulphate requires about 400,000 parts of water for its solution.  It is not decomposed at a red heat but suffers loss, probably of sulphur trioxide, at a temperature above 900 deg.C.]

DETERMINATION OF SULPHUR IN BARIUM SULPHATE

Procedure.—­Weigh out, into platinum crucibles, two portions of about 0.5 gram of the sulphate.  Mix each in the crucible with five to six times its weight of anhydrous sodium carbonate.  This can best be done by placing the crucible on a piece of glazed paper and stirring the mixture with a clean, dry stirring-rod, which may finally be wiped off with a small fragment of filter paper, the latter being placed in the crucible.  Cover the crucible and heat until a quiet, liquid fusion ensues.  Remove the burner, and tip the crucible until the fused mass flows nearly to its mouth.  Hold it in that position until the mass has solidified.  When cold, the material may usually be detached in a lump by tapping the crucible or gently pressing it near its upper edge.  If it still adheres, a cubic centimeter or so of water may be placed in the cold crucible and cautiously brought to boiling, when the cake will become loosened and may be removed and placed in about 250 cc. of hot, distilled water to dissolve.  Clean the crucible completely, rubbing the sides with a rubber-covered stirring-rod, if need be.

When the fused mass has completely disintegrated and nothing further will dissolve, decant the solution from the residue of barium carbonate (Note 1).  Pour over the residue 20 cc. of a solution of sodium carbonate and 10 cc. of water and heat to gentle boiling for about three minutes (Note 2).  Filter off the carbonate and wash it with hot water, testing the slightly acidified washings for sulphate and preserving any precipitates which appear in these tests.  Acidify the filtrate with hydrochloric acid until just acid, bring to boiling, and slowly add hot barium chloride solution, as in the preceding determination.  Add also any tests from the washings in which precipitates have appeared.  Filter, wash, ignite, and weigh.

From the weight of barium sulphate, calculate the percentage of sulphur (S) in the sample.

[Note 1:  This alkaline fusion is much employed to disintegrate substances ordinarily insoluble in acids into two components, one of which is water soluble and the other acid soluble.  The reaction involved is: 

BaSO_{4} + Na_{2}Co_{3}, —­> BaCO_{3}, + Na_{2}so_{4}.

As the sodium sulphate is soluble in water, and the barium carbonate insoluble, a separation between them is possible and the sulphur can be determined in the water-soluble portion.

It should be noted that this method can be applied to the purification of a precipitate of barium sulphate if contaminated by most of the substances mentioned in Note 3 on page 114.  The impurities pass into the water solution together with the sodium sulphate, but, being present in such minute amounts, do not again precipitate with the barium sulphate.]