A solution containing several metal ions is treated with dilute HCl; no precipitate forms. The pH is adjusted to about 1, and H₂S is bubbled through. Again, no precipitate forms. The pH of the solution is then adjusted to about 8. Again, H₂S is bubbled through. This time a precipitate forms. The filtrate from this solution is treated with (NH₄)₂HPO₄. No precipitate forms. Which of these metal cations are either possibly present or definitely absent: Al³⁺, Na⁺, Ag⁺, Mg²⁺?