The complexation behavior of Co2+with ortho-silicic acid (o-SA) has been studied as a function of ionic strength (I) from 0.20 to 1.00M (NaClO4) at pH 4.96±0.03 and 25 °C by solvent extraction with bis(2-ethylhexyl) phosphoric acid (HDEHP) as the extractant. The stoichiometry
of the extracted species was determined to be Co(DEHP)2(HDEHP)2. Co2+forms a 1:1 complex, CoOSi(OH)3+, as the predominant species witho-SA concentrations of 3.00. 10-4to 4.00. 10-3M. The stability constant (logb1) values for CoOSi(OH)3+complex decrease with the increase in ionic strength. These values were fitted with the extended Debye-Huckel expression to
obtain the value of logb1at I=0.00M. The effect of aging time of the o-SA solution on logb1values for CoOSi(OH)3+complex was investigated and compared with those of the UO2OSi(OH)3+complex.