Usually, a basicity above 3.5 provides good absorption and holding capacity for calcium phosphates and calcium sulfides.
The basicity is measured in terms of the pKb value, which is equal to −logKb, where Kb is the equilibrium constant for the reaction B + H3O+ ⇌ BH+ + H2O (B is the amine).
The acidity of the solution represented by the first equation is due to the presence of the hydronium ion (H3O+), and the basicity of the second comes from the hydroxide ion (OH−).
Atmospheric conditions are characterized by low temperatures and pressures, and under such conditions stability fields of minerals can often conveniently be expressed in terms of Eh (oxidation potential) and pH (the negative logarithm of the hydrogen ion concentration [H+]; a pH of 0 7 indicates acidity, a pH of 7 14 indicates basicity, and neutral solutions have a pH of 7).
The basicity (a property of metals) of the elements also increases in proceeding down the group, as shown by the oxides they form: boric oxide (formula B2O3) is acidic; the next three oxides, of aluminum, gallium, and indium (formulas Al2O3, Ga2O3, and In2O3) are either acidic or basic depending on the environment (a property called amphoterism); and thallic oxide (Tl2O3) is wholly basic.
According to this equation, desulfurization is successful only when using a slag with plenty of calcium oxide in other words, with a high basicity.
This solvent must also possess some basic properties, because its ionic product is high ([H3SO4+] [HSO4−] = 1.7 × 10−4), but the basicity of the solvent is obscured normally by its very high acidity.
The basicity of a slag is often simply expressed by the ratio of CaO to SiO2, with CaO being the basic and SiO2 the acidic component.
Finally, the specific solvation (or close association with the solvent) of particular ions (excluding the solvation of the proton to give SH2+, which is already included in the basicity of the solvent) may be important.
Sodium ions replace calcium and other ions in clay complexes, transforming the clay to a sticky mass; water percolation is then drastically reduced, and the basicity of the soil rises markedly.
The lanthanide contraction also accounts for the decreased basicity from lanthanum to lutetium and is the basis of various separation techniques.
