The basicity of these acids is deduced from the formulas of double salts. Thus, there are three ortho-phosphates of sodium and hydrogen; they have the formulae H₂NaPO₄, HNa₂PO₄, and Na₃PO₄ ; the hydrogen calcium salts are: H₄Ca(PO₄)₂, HCaPO₄, and Ca₃(PO₄)₂. It is therefore argued that since the hydrogen atoms of ortho-phosphoric acid are replaceable in three stages by metals, there are therefore three atoms of hydrogen in the acid. These salts are made by mixture; 2H₃PO₄.Aq + Na₃PO₄.Aq = 3H₂NaPO₄.Aq, and so on. The acid is, therefore, said to be tri-basic. The arsenates are precisely similar ; but only simple vanadates are known, and no ortho-antimonates. A pyrophosphate is known of the formula HK₂(NH₄)P₂O₇, which demonstrates the tetra-basic character of pyrophosphoric acid, and the other pyro-acids are classified accordingly.