Strontium (30-200 mg) may be precipitated as SrHPO4 using potassium dihydrogenphosphate; precipitation commences at pH 4 and is quantitative at pH 5.7-6. The flocculent precipitate soon becomes crystalline. It may be weighed as SrHPO4 after drying at 120 °C; alternatively, it may be ignited and weighed as Sr2P2O7. Ions which yield insoluble phosphates should be absent; the sodium, potassium, or ammonium ion concentration should not exceed 0.2M.
Procedure: Treat the
sample solution, containing about 0.1g strontium with 10-20 mL 0.5M potassium
dihydrogenphosphate and heat to the boiling point. Add 1M potassium hydroxide
from a dropper pipette until an appreciable precipitate is formed. The final pH
should be about 6; this can be detected by adding the base until bromocresol
purple indicator in the solution just turns purple, or a pH meter may be used.
Boil until the initial flocculent precipitate becomes crystalline (30-60
minutes). Allow to stand for 1 hour. Collect the precipitate in a
sintered-glass (porosity No. 4) or porcelain filtering crucible; remove any
precipitate adhering to the walls of the beaker with a rubber-tipped stirring
rod, and wash the precipitate with a little cold water. Dry at 120°C. Weigh as
SrHPO4.