The stability constants for tracer concentrations of Co(II) complexes with the red earth fulvic acid were determined at pH
3.8–6.8 and ionic strength 0.0010–1.0 mol/l by using the cation exchange equilibrium method and the radiotracer60Co. The effects of ionic strength and pH on the stability constants of 1∶1 Co(II) complexes were investigated, and it was
found that the stability constants of complexes of humic substances do not vary with ionic strength and pH in a manner similar
to that of simple complexes.