Interaction parameter

Molten steel contains several dissolved solutes in dilute scale. For example, molten steel contains C, S, P, Si, Mn etc. This steel is a multi-component solution. In multi-component solution solutes interact with one another and thus influence activities of other solutes. If Fe is the solvent, and 1, 2….k are solutes in dilute state, then

(11)

The term  is known as interaction parameter describing the influence of solute j on the activity coefficient of solute i. The value of interaction parameter can be found in any book on thermodynamics.

The concept of interaction parameter is very important in estimating the activity of a solute element in presence of other solute elements. For example we want to calculate the activity of sulphur in hot metal of composition C = 4%, Si = 1.5%, Mn =1% and S = 0.04%  .By assuming infinite dilute solution as the standard state, the activity of sulphur is given by



   

Substituting the value of  and  we get  and activity of sulphur is 0.43.

References
A.Ghosh and A. Chatterjee: Ironmaking and steel making