Determination of tungsten in niobium-tantalum, vanadium and molybdenum bearing geological samples using derivative spectrophotometry and ICP-AES

Two different procedures, one using derivative spectrophotometry and another using inductively coupled plasma atomic emission spectrometry (ICP-AES) have been developed for the determination of tungsten in niobate-tantalates, tin slag samples, ores, concentrates and vanadium and molybdenum bearing geological materials. In the first method involving derivative spectrophotometry, 0.05-0.5 g of the sample is fused with sodium hydroxide, the tungsten is extracted by leaching the melt with distilled water and estimated as thiocyanate using a second derivative spectrophotometric method in the presence of interferents, i.e. Nb, Mo and V, without separating them. Mixtures of tungsten with V, Nb and Mo are used for standardizing the various parameters like zero-crossing wavelength, wavelength range, etc. Tolerance limits for V, Nb and Mo have also been evaluated. In the second method involving ICP-AES, 0.05-0.5 g of sample is fused with KHSO(4) to a clear melt and dissolved in ammonium oxalate solution. Ammonium hydroxide precipitation is then carried out to separate Nb and Ta as hydroxides and the filtrate is boiled with nitric acid to destroy the oxalates before aspiration into the plasma for measurement of tungsten values by ICP-AES using the 207.911 nm emission line. Both methods have been applied to niobate-tantalate and tin slag samples and the results obtained are reported in this paper. The values obtained by both methods are in good agreement with each other. The proposed methods have also been applied to the determination of tungsten in two Canadian Certified Reference Standards (CT-1 and MP-2) and the values obtained are in good agreement with the certified values and the R.S.D.% in case of the ICP-AES method varied from 1-2% at >1000 mug g(-1) level to 9.4% at the 20 mug g(-1) level whereas the R.S.D.% in case of the derivative method varied from 1 to 7.8%.     

Unusual effects of anisotropic bonding in Cu(II) and Ni(II) oxides with K2NiF4 structure

Geometric constraints present in A₂BO₄ compounds with the tetragonal-T structure of K₂NiF₄ impose a strong pressure on the BO_IIB bonds and a stretching of the AO_IA bonds in the basal planes if the tolerance factor is $\text{t ?}\text{R}{}_{\mathrm{<mtext>AO</mtext>}}\text{√2 R}{}_{\mathrm{<mtext>BO</mtext>}}\text{< 1}$, where R_AO and R_BO are the sums of the AO and BO ionic radii. The tetragonal-T phase of La₂NiO₄ becomes monoclinic for Pr₂NiO₄, orthorhombic for La₂CuO₄, and tetragonal-T′ for Pr₂CuO₄. The atomic displacements in these distorted phases are discussed and rationalized in terms of the chemistry of the various compounds. The strong pressure on the BO_IIB bonds produces itinerant bands and a relative stabilization of localized d_z² orbitals. Magnetic susceptibility and transport data reveal an intersection of the Fermi energy with the d²_z² levels for half the copper ions in La₂CuO₄; this intersection is responsible for an intrinsic localized moment associated with a configuration fluctuation; below 200 K the localized moment smoothly vanishes with decreasing temperature as the d²_z² level becomes filled. In La₂NiO₄, the localized moments for half-filled d_z² orbitals induce strong correlations among the electrons above $\text{T}{}_{\mathrm{<mtext>d</mtext>}}\text{? 200}\text{K}$; at lower temperatures the electrons appear to contribute nothing to the magnetic susceptibility, which obeys a Curie-Weiss law giving a μ_eff corresponding to $\text{S =}\text{1}\text{2}$, but shows no magnetic order to lowest temperatures. These surprising results are verified by comparison with the mixed systems La₂Ni_1?xCu_xO₄ and La_2?2xSr_2xNi_1?xTi_xO₄. The onset of a charge-density wave below 200 K is proposed for both La₂CuO₄ and La₂NiO₄, but the atomic displacements would be short-range cooperative in mixed systems. The semiconductor-metallic transitions observed in several systems are found in many cases to obey the relation $\text{E}{}_{\mathrm{<mtext>a</mtext>}}\text{? kT}{}_{\mathrm{<mtext>min</mtext>}}$, where $\text{? = ?}{}_0\text{exp}\text{(}\text{?E}{}_{\mathrm{<mtext>a</mtext>}}\text{kT}\text{)}$ and T_min is the temperature of minimum resistivity ?. This relation is interpreted in terms of a diffusive charge-carrier mobility with $\text{E}{}_{\mathrm{<mtext>a</mtext>}}\text{? ΔH}{}_{\mathrm{<mtext>m</mtext>}}\text{? kT}$ at T = T_min.     

Deprotonation energetics of adenine,adenosine, 5′-adenosine monophosphate and adenosine triphosphate in water from EMF and spectrophotometric measurements

The deprotonation constants of adenine (ADE) [K₁ and K₂], adenosine (ADO) [K₁ and K₂], 5-adenosine monophosphate (5-AMP) [K₁, K₂ and K₃] and adenosine triphosphate (ATP) [K₃ and K₄] have been obtained for aqueous soutions from emf measurements on cells such as Pt, H₂ (g, 1 atm)/HA(m₁), NaA(m₂), KI(m₃)/AgI-Ag (where HA is the corresponding acid of ADE, ADO, 5-AMP and ATP) at different temperatures. The pK values were fitted by the temperature equation: pK=AT¹+B+CT by the least squares method and the related thermodynamic quantities viz. G ^o , TS ^o and H ^o were calculated from the coefficients A, B and C.     

Encapsulation of paramagnetic 3d1-vanadium(IV) in an antiferromagnetically coupled dodecanuclear copper(II) cage

A mixed metal cluster [Cu12VO5L6] of a pentadentate Schiff base (H3L) containing vanadium(IV) in a dodecanuclear copper(I) cage is prepared by vanadyl templated self assembly of dicopper(II) precursor and the structurally characterized complex shows antiferromagnetic coupling involving copper(II) centers, which leads close to diamagnetism for the Cu(II) cage below 40 K in the presence of an encapsulated paramagnetic 3d1-V(IV) atom.     

Über Pterinchemie. 77. Mitteilung)Das (6R,S)-5-Formyl-6-methyl-5,6,7,8-tetrahydropterin: Synthese,chemische und physikalisch-chemische Eigenschaften

(6R,S)-5-Formyl-6-methyl-5,6,7,8-tetrahydropterine: Synthesis, Chemical and Physico-chemical Properties (6R, S)-5-Formyl-6-methyl-5,6,7,8-tetrahydropterine ( III ), a model substance for the biologically important 6-substituted 5-formyl-5,6,7,8-tetrahydropterines, was obtained for the first time by formylation of the corresponding tetrahydropterine with formic acid. Compound III , analogously to the other known 5-acyltetrahydropterines, exists in solution (except in DMSO) as a mixture of two rotamers IIIa and IIIb . The two H-C(7) in III have the same chemical shift and the same coupling constant to H-C(6), giving rise to a A,A',X,C-system. Conformational analysis of III , based on its ¹H-NMR,-spectrum, shows that the methyl group is pseudo-axial and the formyl group pseudo-equatorial. Moreover, the H(X)-C(6) bond lies nearly in the plane bisecting the C(6)-C(7)-H(A), C(6)-C(7)-H(A') dihedral angle; the C(6) and C(7) lie outside the N(5)-C(4a)-C(8a) plane and the tetrahydropyrazine cycle must be extremely strained, according to Dreiding models.     

A new motif in the formation of peptide nanotubes: the crystallographic signature

Terminally protected acyclic tripeptides Boc-Tyr(1)-Val(2)-Tyr(3)-OMe 1 and Boc-Tyr(1)-Ile(2)-Tyr(3)-OMe 2 self-assemble into nanotubes in crystals through various noncovalent interactions with an average internal diameter of 5 A (0.5 nm), and the tubular ensemble is developed through the hydrogen-bonded side chains of tyrosine residues. The inside of the hollow nanotubular structures is hydrophilic; however, no solvent molecules have been crystallographically detected. [structure: see text]