The hydrolysis and interaction of Np(IV) and Pu(IV) with orffiosilicic acid, Si(OH)(4), were studied in 0.1-1.0M ionic strength aqueous solutions. Spectrophotometry was used to study these reactions at about 10(-4) M Np(IV) and Pu(IV) concentrations. The first hydrolysis constants, K-hydr, agree with the majority of earlier spectrophotometric and potentiometric data. The absorption spectra of NpOH3+ and PuOH3+ were obtained by spectral deconvolution. Reasons to explain the overestimation of K-hydr, obtained by other methods [by extraction of trace amounts of Np(IV) and Pu(IV) and by solubility] are discussed. Formation of the complexes NpOSi(OH)(3)(3+) and PuOSi(OH)(3)(3+) is demonstrated in the presence of 0.005-0.016 M Si(OH), in the p[H+] range 1.0-2.2 and 0.3-1.4, respectively. Measured values of equilibrium constants of the reaction M4+ +Si(OH)(4) <----> MOSi(OH)(3)(3+) +H+ at ionic strength I = 1.0 are Ig K-1 = 0.41 +/- 0.02 and 1.04 +/- 0.04, respectively, for Np(IV) and Pu(IV). The stability constants of the NpOSi(OH)(3)(3+) and PuOSi(OH)(3)(3+) complexes, recalculated to zero ionic strength, are 1g beta(1)(0) = 11.2 and 11.8, respectively. The correlation between K-hydr, and K-1, as observed for all earlier studied metal ions, also occurs for both Np(IV) and Pu(IV).
