Effect of gamma irradiation on the thermal decomposition of zinc bromate by dynamic thermogravimetry

The thermal decoposition of -irradiated zinc bromate has been studied by dynamic thermogravimetry. The reaction order, activation energy, frequency factor and entropy of activation were computed using the Coats-Redfern, Freeman-Carroll and Horowitz-Metzger methods and were compared with those of the unirradiated salt. Irradiation enhances the decomposition and the effect increases with the irradiation dose. The activation energy decreases on irradiation. The mechanism for the decomposition of unirradiated and irradiated zinc bromate follows the Avrami model equation, 1-(1-)^1/3=kt, and the rate-controlling process is a phase boundary reaction assuming spherical symmetry.     

Parameter choice by discrepancy principles for ill-posed problems leading to optimal convergence rates

Schock (Ref. 1) considered a general a posteriori parameter choice strategy for the Tikhonov regularization of the ill-posed operator equationTx=y which provides nearly the optimal rate of convergence if the minimal-norm least-squares solution belongs to the range of the operator (T ^* T) ^v , o<v1. Recently, Nair (Ref. 2) improved the result of Schock and also provided the optimal rate ifv=1. In this note, we further improve the result and show in particular that the optimal rate can be achieved for 1/2v1.The final version of this work was written while M. T. Nair was a Visiting Fellow at the Centre for Mathematics and Its Applications, Australian National University, Canberra, Australia. The work of S. George was supported by a Senior Research Fellowship from CSIR, India.     

New redox titrants in non-aqueous or partially aqueous media--VI: potentiometric determinations using dibromamine-T and some further applications of dichloramine-T

A new redox titrant, Dibromamine-T (N,N'-dibromo-p-toluenesulphonamide) is introduced for use in acetic acid media. Potentiometric titrations of Sb(III), As(III), Tl(I), Fe(II), ferrocyanide, iodide, ascorbic acid, hydroquinone, hydrazine, aniline, phenol and oxine are described. Direct potentiometric titrations of oxinates of Mn, Fe, Co, Ni, Cu and Zn with Dibromamine-T as well as with Dichloramine-T are also described.     

Extraction of uranium(VI) and plutonium(IV) with dihexylbutyramide and dihexylisobutyramide from nitric acid medium

The extraction of uranium(VI) and plutonium(IV) was carried out with two isomeric monoamides, dihexylbutyramide (DHBA) and dihexylisobutyramide (DHIBA) from nitric acid medium, usingn-dodecane as diluent. The possibility of separation of the two metal ions from each other without valency adjustment was attempted. U(VI) was extracted as its disolvate, while Pu(IV) was extracted as its trisolvate. From the variation of distribution ratio with temperature, it was shown that the extraction reaction was enthalpy controlled in all the cases.     

A DSC study of the effects of sugars on thermal properties of rice starch gels before and after aging

Thermal characterization of gelatinized binary rice starch-water and ternary starch-sugar-water gels before and after aging was carried out using differential scanning calorimetry. The glass transition temperature of the maximally freeze-concentrated solution (T_g) in both fresh and aged gels was observed to decrease progressively with increasing sugar concentration. Aging of the gels generally shiftedT_g to higher temperatures, but had little or no effect on the ice melting peak temperature (T _m). The presence of various sugars could either accelerate or retard starch (amylopectin) recrystallization, depending on the type and concentration of sugar, as well as on starch/water ratio. A hypothesis based on the dual antiplasticizing-plasticizing effects of sugars was postulated to explain the observed effects. Of the sugars studied, xylose and fructose appeared to display exceptional retardative and accelerative effects, respectively, on retrogradation.This study was funded by a Sixth Malaysia Plan R&D grant under the Intensification of Research Priority Areas (IRPA) Program of the Ministry of Science, Technology and Environment, Malaysia. We thank Dr. Harry Levine and Dr. Louise Slade for giving us the opportunity to contribute a paper to this special issue of JTA.     

Synthesis of cryptophycins via an N-acyl-beta-lactam macrolactonization

An efficient and concise approach to the synthesis of the macrolide core of the cryptophycins has been developed. A novel macrolactonization utilizing a reactive acyl-beta-lactam intermediate incorporates the beta-amino acid moiety within the 16-membered macrolide core. This modular approach, involving a cyanide-initiated acyl-beta-lactam ring opening followed by cyclization, was successfully applied to the total synthesis of cryptophycin-24. The strategy was also used in an efficient synthesis of the 6,6-dimethyl-substituted dechlorocryptophycin-52. In this case, the cyanide-initiated ring opening of the bis-substituted 2-azetidinone followed by macrolactonization was achieved through a catalytic process.     

A novel three-component reaction for the diastereoselective synthesis of 2H-pyrimido[2,1-a]isoquinolines via 1,4-dipolar cycloaddition

[reaction: see text] The 1,4-dipole derived from isoquinoline and DMAD has been shown to react readily with N-tosylimines resulting in the diastereoselective synthesis of 2H-pyrimido[2,1-a]isoquinoline derivatives.     

Cumulative yields of the short-lived ruthenium isotopes in the spontaneous fission of252Cf

⁶⁷Ga is produced by the⁶⁶Zn(d,n) reaction at 9 MeV bombarding energy. Deuterons of about 30 A obtained from the T-11 Tandem accelerator are used to bombard targets of natural Zinc prepared by electroplating onto thick copper disks. Radiochemical separation of gallium is performed by using a cation exchange column. The optimum conditions for an efficient separation are given in detail.     

Thermogravimetric analysis of PVC/ELNR blends

The thermal behaviour of a series of solution-cast blends of polyvinyl chloride/epoxidised liquid natural rubber (ELNR) of different mole percentage of epoxidation has been examined using thermogravimetric analysis. Thermal degradation is found to occur through a two-step route in which the first step corresponds to the dehydrochlorination of PVC to form a polyene and the second step is attributed to the decomposition of the ELNR and the polyene. Introduction of 20 and 50 mol% of epoxy group into the liquid NR is found to enhance the thermal stability of PVC. Probable mechanisms of degradation have been suggested on the basis of the kinetic analysis of the degradation studies. It is found that the mechanism is influenced by the epoxy content of the blend system. Activation energy for the degradation and the entropy change have also been reported.