Evaporate to dryness 100 to 1,000 cc. of the sample, or sufficient to
obtain a residue weighing 0.4 to 0.6 gram and containing preferably 0.05
to 0.2 gram of sodium. Acidify the residue with hydrochloric acid and
remove the silica, iron, and aluminium (pp. 56–57). Make acid and add a
hot solution of barium chloride in slight excess to the hot filtrate,
and warm it, stirring at intervals for one-half hour, until the
precipitate settles readily and leaves a clear supernatant liquid. Dry,
ignite, and weigh the precipitate of barium sulfate, 41.1 per cent of
which is equal to the content of sulfate (SO_{4}).