It is indispensable to know values of the activities of coexistent elements, C, Si, Mn, and etc., in molten iron, in order to obtain the exact equilibrium relation of the fundamental reaction in iron and steelmaking. Recently some studies have been performed on this field but the detailed researches are left remained in future.
So that, in order to know the behaviour of carbon in molten iron, authors have constructed the following concentration cell and measured the electromotive force, E corresponding to the change of carbon content by potentiometer. The theoretical relation between, E and the activity of carbon, a_c was as follows. where a_c and a'_c were respectively the activities of carbon in Fe-C melt (I), (II). a_cⁱ was the activity of carbon ion in molten slag.
Because the carbon saturated system was taken as a standard state, a'_c was equal to unity and the following equation was obtained from eq. (1): n was approximately determined as n≅2 by the calculation from authors' data.
The activities of carbon at about 1450°C, 1500°C and 1550°C were obtained from eq. (2).
This result being judged, mole fraction of carbon, N_c was considered to be approximately equal to a_c up to about 0.04 N_c, i.e. about 1%C but a_c increased rapidly as the carbon content increases above 1%C.
Using the values of a_c thus obtained, authors examined the equilibrium relation of Fe-C-O system and performed some calculation from the view point of the statistical thermodynamics.