Yielding unexpected results: Precipitation of Ba₃(PO ₄)₂ and implications for teaching solubility principles in the general chemistry curriculum

Precipitation of barium phosphate from aqueous solutions of a barium salt and a phosphate salt forms the basis for a number of conclusions drawn in general chemistry. For example, the formation of a solid white precipitate is offered as evidence that barium phosphate is insoluble. Furthermore, analysis of the supernatant is used to illustrate the concept of limiting reagent. In this paper, we present X-ray diffraction and gravimetric evidence that nabaphite is the species formed when aqueous solutions of barium and phosphate are mixed. In addition, our evidence suggests that nabaphite converts to barium apatite before barium phosphate is ultimately observed. This barium phosphate chemistry has implications when teaching solubility principles in general chemistry.