In industrial practice ionic definition of basicity is not useful; the molecular approach is more useful. The molecular approach assumes slag to consist of chemical compounds. The basicity of slag is

(6)

                                                                  
In presence of different basic oxides, the different strength of the basic oxides should be considered. In a slag which contains CaO, MgO,  and , the basicity is

(7)

                                                                                            
In slag/metal reactions which involve desulphurization and dephosphorization, the concept of free lime in slag is useful. Free lime in CaO,  and  slag is that amount which is available after the formation of neutral compound like 2CaO., 3CaO.

(8)

For 100 ton hot metal with 1% silicon and 0.2% P the calculation shows that free CaO in slag would be available when CaO content exceeds 4540Kg.

Oxidation and reduction potential of slag

It refers to the capability of slag to transfer oxygen to and from the molten steel bath. FeO content of slag determines the oxidation potential of slag. Thus activity of FeO in slag is an important parameter. The equilibrium between FeO of slag and oxygen of steel is

 

(9)

The activity of oxygen in metal is proportional to the activity of FeO in slag.