46 research outputs found
Guaiacol as a new reagent for the spectrophotometric determination of uranium
Guiacol, i.e. o-hydroxyanisole, gives a distinct color reaction with U(VI)
suitable for spectrophotometric determination of the metal. The complex formed in the reaction has an
absorption maximum at 352 nm. Optimum pH for the color development ranges from 6.5 to 8.5. The molar
absorptivity and Sandell's sensitivity of the method were found to be 3.75×103
l.mol-1.cm-1 and 0.063 μg.cm-2, respectively. Many anions and
cations do not interfere up to 100 ppm. The method has been made very specific by selective
extraction of U(VI) with TBP from a mixture of different cations and anions in the presence of 60%
NH4NO3 as salting out agent followed by developing the color in the non-aqueous
phase by adding quaiacol in methanol at pH 6.5 to 8.5 An amount as low as 30 μg of uranium (VI)
per 10 ml of the solution could be satisfactorily determined with an RSD of ±2.0%. The method
was applied to rock samples after U(VI) had been extracted from a sample solution into 25% TBP in
hexane. Results obtained by the new method compare very well with those of conventional fluorimetric
and radiometric assays. The features of the method include excellent precision, rapidity, good
selectivity, and ease of performance
Consequences of local gauge symmetry in empirical tight-binding theory
A method for incorporating electromagnetic fields into empirical
tight-binding theory is derived from the principle of local gauge symmetry.
Gauge invariance is shown to be incompatible with empirical tight-binding
theory unless a representation exists in which the coordinate operator is
diagonal. The present approach takes this basis as fundamental and uses group
theory to construct symmetrized linear combinations of discrete coordinate
eigenkets. This produces orthogonal atomic-like "orbitals" that may be used as
a tight-binding basis. The coordinate matrix in the latter basis includes
intra-atomic matrix elements between different orbitals on the same atom.
Lattice gauge theory is then used to define discrete electromagnetic fields and
their interaction with electrons. Local gauge symmetry is shown to impose
strong restrictions limiting the range of the Hamiltonian in the coordinate
basis. The theory is applied to the semiconductors Ge and Si, for which it is
shown that a basis of 15 orbitals per atom provides a satisfactory description
of the valence bands and the lowest conduction bands. Calculations of the
dielectric function demonstrate that this model yields an accurate joint
density of states, but underestimates the oscillator strength by about 20% in
comparison to a nonlocal empirical pseudopotential calculation.Comment: 23 pages, 7 figures, RevTeX4; submitted to Phys. Rev.
Age determination of plutonium material by alpha spectrometry and thermal ionization mass spectrometry
Investigating the role of compression rates in pressure induced polymerization of crystalline acrylamide using ab initio molecular dynamics
Varying the rate at which pressure is applied on a crystal is experimentally known to yield different pressure induced polymorphic structures. Herein, we explore the effect of pressure increase rate on pressure induced polymerization in crystalline acrylamide, using a density functional theory based approach. While quasi-static compression at 0 K stabilizes a 3-dimensional topochemical polymer, Pol-I, at 23 GPa, rapid compression optimizations suggest the presence of multiple polymeric intermediates in the system. Room temperature ab initio molecular dynamics performed with two different compression rates - 0.4 GPa/ps and 2 GPa/ps - revealed very different structural evolution of the system. While both rates ultimately yielded a metastable 1-dimensional polymer at pressures beyond 64 GPa, rapid compression resulted in many disordered polymers at lower pressures with unanticipated linkages. The mechanisms leading to polymerization as well as the structure and electronic properties of the various polymer polymorphs obtained in the two compression routes are described. While large kinetic barriers delay the formation of the thermodynamically favored polymer Pol- I, our simulations suggest a hierarchical route for the pressure induced polymerization of solid acrylamide towards the thermodynamically favorable Pol-I
Neutron activation analysis method for the determination of neptunium in sub-microgram quantities in process solutions
Pressure Induced Topochemical Polymerizationof Solid Acryalmide Facilitated by Anisotropic Response of Hydrogen Bond Network
The pressure induced polymerization of molecular
solids is an appealing route to obtain pure,
crystalline polymers without the need for radical
initiators. Here, we report a detailed density
functional theory (DFT) based study of the
structural and chemical changes that occur in
defect free solid acrylamide, a hydrogen bonded
crystal, when it is subjected to hydrostatic pressures.
Our calculations predict a polymerization
pressure of 94 GPa, in contrast to experimental
estimates of 17 GPa, while being able
to reproduce the experimentally measured pressure
dependent spectroscopic features. Interestingly,
we find that the two-dimensional hydrogen
bond network templates a topochemical
polymerization by aligning the atoms through
an anisotropic response at low pressures. This
results not only in conventional C-C, but also
unusual C-O polymeric linkages, as well as a
new hydrogen bonded framework, with both NH...
O and C-H...O bonds.</p