This method is most widely employed. The effect of various interfering ions (e.g. calcium, strontium, lead nitrate, etc., which contaminate the precipitate) is dealt with in Section 11.72 The solubility of barium sulphate in cold water is about 2.5mgL-1 it is. however, greater in hot water or in dilute hydrochloric or nitric acid, and less in solutions containing a common ion.
The barium sulphate may be precipitated either by the use of sulphuric acid or from homogeneous solution by the use of sulphamic acid solution which produces sulphate ions on boiling:
NH2SO3H + H2O = NH4+ + SO42- + H+
The solution (100mL) should contain not more than 0.15 g of barium and not more than 1 percent by volume of concentrated hydrochloric acid. Heat to boiling, add a slight excess of hot 0.5M sulphuric acid slowly and with constant stirring. Digest on the steam bath until the precipitate has settled, filter, wash with hot water containing two drops of sulphuric acid per litre, and then with a little water until the acid is removed. Dry, and ignite at 900-1000 °C to constant weight in a platinum crucible. Weigh as BaSO4.
The sample solution may contain up to lOOmg of barium, preferably present as the chloride. A solution prepared from about 0.18 g of accurately weighed barium chloride may be used to obtain experience in the determination. Dilute the solution to about 100 mL; add 1.0 g sulphamic acid. Heat the covered beaker on an electric hotplate at 97-98 °C; continue the heating for 30 minutes after the first turbidity appears. Filter through a weighed porcelain filtering crucible and wash with warm distilled water. Ignite to constant weight at 900 °C in an electric muffle furnace. Weigh as BaSO4.