Hydrofluoplumbic Acid, H₄PbF₈

Hydrofluoplumbic Acid, H₄PbF₈, is probably formed in solution when lead tetra-acetate is dissolved in concentrated hydrofluoric acid. When this solution is evaporated, lead dioxide separates (Brauner). The potassium salt K₃HPbF₈ was prepared by Brauner by acting on the freshly precipitated oxide Pb₅O₇.3H₂O with a solution of potassium hydrogen fluoride in aqueous hydrofluoric acid, by fusing together lead dioxide and potassium fluoride and mixing hydrofluoric acid with the product, and by bringing together lead tetra-acetate potassium hydrogen fluoride and hydrofluoric acid. The solution obtained in either of these ways is evaporated in a vacuum desiccator, when monoclinic needles of the required salt are obtained. These are isomorphous with the corresponding tin salt K₃HSnF₈. The salt is stable in dry air, but is decomposed by water with separation of hydrogen fluoride, thus:

K₃HPbF₈ + 2H₂O = 3KF + PbO₂ + 5HF.

At 230° C. hydrogen fluoride is evolved, and at a higher temperature fluorine, thus:

K₃HPbF₈ → K₃PbF₇ → 3KF + PbF₂ + F₂.

It is noteworthy that this reaction furnishes a chemical method of preparing fluorine.