Flame photometric determinations of diffusion coefficients. Part 4.—Results for lithium, potassium, rubidium and caesium
Diffusion coefficients of alkali metals are measured for trace quantities present in the burnt gases of atmospheric pressure flames of H2+ O2+ N2 in the temperature range 1920–2520 K. The results are considered in terms of two potential functions (the Lennard-Jones (12 : 6) and a purely repulsive inverse power one) for interactions between the diffusing and flame species. The latter potential function fits the experimental data better, indicating that repulsive forces are more important than attractive ones under these flame conditions. However, the Lennard-Jones function is almost as good as the purely repulsive one for Na and K, which exist in these flames as atoms, the other metals having relatively stable hydroxides. Also for Na and K, there is evidence that the coefficient for diffusion in the multicomponent flame gases is a linear combination of binary diffusion coefficients, in contrast to the usual reciprocal relationship, which holds well for the other three metals. Values of the Lennard-Jones parameters and constants characterising purely repulsive forces are presented for each alkali metal.