Robert J. Silva

Standard Gibbs free energies of formation at the temperature 303.15 K of four uranyl silicates: soddyite, uranophane, sodium boltwoodite, and sodium weeksite

The solubility products and standard molar Gibbs free energies of formation were obtained from solubility measurements at 303.15 K for the following synthetic uranium-silicate phases: soddyite, (UO2)2SiO4·2H2O; uranophane, Ca(H3O)2(UO2)2(SiO4)2·3H2O; sodium boltwoodite, Na(H3O)(UO2)SiO4·H2O; and sodium weeksite, Na2(UO2)2(Si2O5)3·7H2O. These phases were identified as uranium precipitates likely to form near a high-level nuclear waste repository. The experimental standard molar Gibbs free energies of formation for soddyite, uranophane, sodium boltwoodite, and sodium weeksite are: −(3658.0±4.8) kJ·mol−1, −(6210.6±7.6)kJ·mol−1, −(2966.0±3.6)kJ·mol−1, and −(9088.5±18.4) kJ·mol−1, respectively. The value for sodium boltwoodite is a lower bound and therefore should be used with caution.

Collinear photothermal deflection spectroscopy with light-scattering samples

An analytical model that incorporates effects of light scattering was developed for dual-beam photothermal deflection spectroscopy. Thermal gradients induced by a modulated excitation beam deflect an optical probe beam which was treated as being of finite dimensions. Mechanisms by which thermal gradients produce refractive index gradients are discussed, with an explicit expression for dn/dT being derived. Experimental studies with suspensions of small latex particles in Nd(3+) solutions demonstrated that the model accurately predicts both the shape of the deflection signal and the attenuation of the signal due to light scattering. The absolute magnitude of the observed signal is approximately predicted by theory.

Temperature Effects on a Fiber-Optic Evanescent Wave Absorption Sensor

A coiled fiber-optic chemical sensor has proven to be effective for the remote detection of volatile organic compounds, such as trichloroethylene (TCE), 1,1-dichloroethylene (DCE), and gasoline, in aqueous solutions. The analyte diffuses into the hydrophobic cladding and evanescent wave absorption spectra are measured in the near-infrared (1600–1850 nm) without the presence of the water absorption bands. In order for fiberoptic chemical sensors to operate effectively in remote environments, the influence of temperature on the sensor response must be known. The C-H bonds of the polysiloxane cladding material also have absorption bands in the near-infrared (NIR). Changes in temperature will change the density (i.e., concentration of C-H bonds) and refractive index of the cladding. Due to these effects, a temperature change of only 3°C from the reference has been shown to significantly alter the background absorbance. The temperature-dependent background absorption is found to be linear with the slope, and the values are proportional to the absorption coefficient of the cladding material. The intercept of the absorbance vs. temperature plot is found to follow the first derivative of the fiber sensor transmission spectrum. Evanescent wave absorption spectra of TCE solutions have been corrected for temperature.

Cation-cation complexes of PuO2+ and NpO2+ with Th4+ and UO22+

Cation-cation complexes of NpO2+ and PuO2+ with UO22+ and Th4+ were studied using laser-induced photoacoustic spectroscopy (LIPAS). An equilibrium constant of 2.4±0.2 M-1 was measured at an ionic strength of 6.0 M for the NpO2+ · UO22+ complex which is in good agreement with previous measurements using conventional absorption spectroscopy and Raman spectroscopy. Equilibrium constants of 1.8±0.9 and 2.2±1.5 M-1 were measured for NpO2+ · Th4+ and PuO2+ · UO22+ complexes at an ionic strength of 6.0 M, respectively. This is the first time that LIPAS has been used to study cation-cation complexes and the first time that the PuO2+ · UO22+ cation-cation complex has been reported.

Remote photoacoustic measurements in aqueous solutions using an optical fiber

A photoacoustic spectrometer was developed for remote optical absorption measurements in aqueous solutions using an 85‐m optical fiber to deliver pulsed tunable dye laser radiation to a sample cuvette located in a glove box. The spectrometer was tested using aqueous solutions of praseodymium ions. Beer’s law was verified down to a concentration of 8×10−6M for an equivalent absorptance of 3.2×10−5 cm−1.

Collinear Photothermal Deflection Spectroscopy of Liquid Samples at Varying Temperature

The method of photothermal beam deflection was demonstrated for measuring the absorption spectrum of an aqueous Nd3+ solution at temperatures between 25°C and 90°C. Changes in the magnitude of the observed deflection can be correlated with the temperature dependence of both the absorptivity of the Nd3+ ions and the physical properties of the solvent, most notably the thermo-optic coefficient dn/dT.

Differential Photothermal Deflection Spectroscopy Using a Single Position Sensor

A differential dual-beam photothermal deflection spectrometer using a single position sensor is described. An excitation and a probe laser beam are directed simultaneously into a sample solution and a reference solution to provide real-time background correction. The probe beam that passes through the reference cell is folded back into the path of the sample-cell probe beam in such a way that only one position sensor is needed to monitor the deflection in both cells. Nd3+ solutions are used to demonstrate the effect of background correction and the sensitivity of this dual-beam photothermal deflection arrangement.

Stability quotients of neodymium acetate complexes from 20 to 70°C by laser-induced photoacoustic spectroscopy

The formation of Nd(III) acetate complexes in aqueous solution is studied by laser-induced photoacoustic spectroscopy and spectrophotometry. Stability quotients for the complexes Nd(CH3COO)2+ and Nd(CH3COO)2 and dissociation quotients for acetic acid are determined at 20, 35, 50 and 70‡C in NaClO4 medium of ionic strength 2.2m (2.0M at 20‡C). The 20‡C results are consistent with previous values determined by potentiometry and energy transfer from Tb(III). The temperature dependence of the stability quotients is consistent with the measured enthalpies of reaction at 25‡C.

Atom-at-a-time radiochemical separations of the heaviest elements: Lawrencium chemistry

The isotope260Lr, produced in reactions of18O with249Bk, was used to perform chemical experiments on lawrencium to learn more about its chemical properties. These experiments involved extractions with thenoyl trifluoroacetate, elutions from cation exchange resin columns with ammonium alpha-hydroxyisobutyrate, and reverse-phase chromatography using hydrogen di(2-ethylhexyl) orthophosphoric acid to investigate the chemical properties of Lr. The results from the elutions gave information about the ionic radius of Lr(III) which was found to elute very close to Er. An attempt to reduce Lr(III) with hydroxylamine hydrochloride was unsuccessful.

Methods and results of99Tc analysis of Nevada test site groundwaters

Samples of the long-lived fission product99Tc were isolated from liter quantities of selected groundwaters from the Cheshire, Bilby, Nash, Bourbon, Cambric and Faultless event sites and the99Tc concentrations determined by liquid scintillation counting. Although99Tc was detected in several of the samples, the concentrations did not exceed the MPCw of 3·10−4 μCi/ml and the results indicated that the Tc was present primarily in solution rather than sorbed on colloidal-size material.