The interaction of UO 2 2+ ions with orthosilicic acid Si(OH) 4 and polymeric silicic acids (PSAs) was studied spectrophotometrically. The equilibrium constant of the reaction UO 2 2+ + Si(OH) 4 = UO 2 OSi(OH) 3 + + H + in solutions with the ionic strength I = 0.13 0.2 is log K = 32.56 + 0.09 (log K 0 = 32.29 + 0.09 recalculated to I = 0); the stability constant of the complex UO 2 OSi(OH) 3 + (I = 0) is log b 0 = 7.52 + 0.09. Formation of small oligomers (degree of polymerization n < 4) has virtually no effect on the apparent constant K. When high polymers are formed (n > 100), the apparent equilibrium constant decreases by a factor of 233, and the [true] equilibrium constant recalculated to the actual concentration of silanol groups increases by a factor of 33 4. The absorption spectrum of the complex UO 2 OSi(OH) 3 + was obtained by treatment of the experimental spectra; it has an absorption maximum in the visible range at 422.5 nm, e 422.5 = 35 + 2 l mol !1 cm !1 . At pH higher than 53 6, complexes of UO 2 2+ with PSAs of the composition UO 2 (=SiO) 2 (=SiOH) m !2 are formed. The absorption spectrum of such a complex was obtained.Much attention is given today to the interaction of f-element ions with orthosilicic acid Si(OH) 4 and other silicate species. It has been shown that silicate ions appreciably affect the state and behavior of UO 2 2+ and other actinide ions under widely varied conditions [138]. The interaction of uranyl ions with orthosilicic acid at pH 33 4.5 is described by the equation [13 6, 9] UO 2 2+ + Si(OH) 4 = UO 2 OSi(OH) 3 + + H + (1) with the equilibrium constant [UO 2 OSi(OH) 3 + ][H + ] K = ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ. (2) [UO 2 2+ ][Si(OH) 4 ] The values given in the literature for log K range from 31.37 [5] to 32.92 [3] in 0.13 0.2 M NaClO 4 . Jensen and Choppin [3] believe that the major cause of this inconsistency is the polymerization of Si(OH) 4 , which took place in some studies. Polymeric silicic acids (PSAs) were believed, without due proof, to interact with the UO 2 2+ ion more strongly. The value recommended by the thermodynamic database of the year 2003 [9] is log K = 32.11 + 0.10 at the ionic strength I = 0.13 0.2, whence log K 0 = 31.84 + 0.10 at I = 0 (T = 293.15 K). The interaction of Si(OH) 4 with the PuO 2 2+ ion [10] appeared to be considerably weaker: log K = 33.91 + 0.17 at I = 0.2. Relevant data for the NpO 2 2+ ion are lacking.Thus, the following questions concerning the interaction of hexavalent actinides with Si(OH) 4 remain to be answered. (1) What is the cause of major disagreement between the published values of the constant K (without understanding these causes, the choice of the constant recommended in [9] seems rather arbitrary)?(2) How does the polymerization of Si(OH) 4 affect K?(3) What is the trend in variation of K in the series U(VI)3Np(VI)3Pu(VI)?Here and in the subsequent paper [11] we report on a direct spectrophotometric study of the interaction of Si(OH) 4 and PSAs with the UO 2 2+ and NpO 2 2+ ions.EXPERIMENTAL Uranyl solutions were prepared by dilution of...