Studies were made on the rate of mass transfer to single spheres, with the aid of a diffusioncontrolled electrode reaction:
Microelectrodes used as anode were platinum spheres having 1.66mm and 2.74mm diameters respectively, each with PVC-coated platinum wire of 0.6mm diameter. The reference electrode used as cathode was a thin platinum plate of 50×120mm. The experimental apparatus employed was as shown in Fig. 1. To obtain uniform distribution of liquid velocity over the entire column cross-section, glass beads of 5mm diameter were packed to a height of 30mm, topped with 10 sheets of Saran screen of about 12 meshes. Below the packing and 8mm above the spherical electrode, a Saran screen of about 150 meshes was placed.
From the measurements of limiting current _{6), 8)} at various liquid flow rates, the mass transfer coefficients, kf, were calculated by the equation:
kf=i/nFAc (7)
The experimental results are shown in Table 1. It was found that neither the results thus obtained nor those by previous investigators_{3), 9), 10), 14), 16)} showed good agreements with the Ranz-Marshall's eq. (2)₁₄₎.
Thereupon, assuming that the empirical equation might be presented as:
Sh-2=kRe_pSc_q (8)
the authors determined the values of the constants k, p and q (Figs. 3, 4 and 5), obtaining an equation:
Sh=2.0+0.52Re^0.54Sc^0.35 (9)
The value of the exponent of the Schmidt group, 0.35, agrees well with 0.348 obtained by Frossling³⁾ from his experimental data.