Determination of urinary organic acids by HPLC

Summary A high-performance liquid chromatographic method for the simultaneous determination of urinary organic acids and creatinine for following the metabolism of aromatic solvents is reported. After extraction of acidified, filtered urine with diethylether followed by evaporation, the dried residue is dissolved in mobile phase. Hydroxybenzoic acid is used as internal standard. A column of Nucleosil C₁₈ is used with a precolumn of the same material. The mobile phase is acetonitrilephosphate buffer, pH 3.3 (1783). For determination of creatinine the sample is simply diluted 10-fold and the eluate monitored at 215 nm (UV). This technique gives highly reproducible results and is simple, reliable and useful for biological monitoring.     

Optimization of hydrolase efficiency in organic solvents

The application of hydrolases in organic solvents for synthetic purposes is a procedure routinely adopted in organic chemistry, especially for the preparation of chiral building blocks. Numerous studies have shed light on several aspects of the mechanism of hydrolase action in low-water environments. Procedures suitable to improve the catalytic efficiency of enzymes and productivity of the synthetic processes have been reported. These fundamental and applied investigations have made hydrolase-catalyzed reactions in organic solvents of industrial interest. In this article we describe and discuss various approaches adopted to optimize the performance of hydrolases in organic media, with special emphasis on the formulation of the biocatalysts which, under proper conditions, can display an activity equal to that displayed in aqueous buffers.     

Ionization constants for deuterium oxide in deuterium oxide-organic mixtures

A potentiometric method using a glass electrode has been applied to determination of ionization constants for deuterium oxide (D₂O) in binary mixtures of D₂O with dioxane, tetrahydrofuran, acetone, dimethylsulfoxide, CH₃CH₂OD, and CH₃OD at 25°C. The results are compared with values of ionization constants for H₂O obtained previously in the corresponding H₂O-organic mixtures, and the isotope effect is shown to be small. Further calculations with the D₂O results show that the first five solvents mentioned above are neither appreciably acidic nor basic in D₂O solution, but that CH₃OD shows slightly acidic behavior (pK_a=16.0±0.3).     

Role of solvent in the compatibility of a vinylchloride–vinylacetate–maleic acid terpolymer and nitrocellulose blends

The compatibility of cast films of a vinylchloride–vinylacetate–maleic acid terpolymer (VMCH) and nitrocellulose (NC) blends is influenced by solvents. Transparent films of VMCH/NC blends are obtained when cast from solvents such as tetrahydrofuran or cyclohexanone, whereas hazy films are obtained when cast from solvents such as acetone or ethylacetate. Visible spectroscopy and phase morphology were used to analyse the compatibility–incompatibility of the blend. Differential scanning calorimetry (DSC) studies demonstrate that the transparent film is compatible, but the hazy film is incompatible. Fourier transform infra-red (FTIR) studies establish that a greater interaction is observed between the polymer pair in case of the compatible blend than in the case of the incompatible blend. A solvent dependency of blend compatibility is reflected in this study. The conformational state of the polymers in solution, which is responsible for the compatibility phenomena, may depend on the donor number and/or Taft-β value of the solvent. The greater the donor number and/or the Taft-β value, the higher may be the level of interaction between the solvent and the polymer molecules, which in turn may give a compatible blend after removal of the solvent.     

A view and a review of the melting of alkali metal halide crystals

Summary A representational model, proposed to account for the physical changes that accompany the melting of alkali halides, was described in Part 1 [1]. The liquid is portrayed as undergoing continual dynamic structural reorganization of its constituent ions between individual small domains, zones of various regular, crystal-type arrays. These alternative arrangements are stabilized by the enthalpy of melting, which, in liquids, relaxes the restriction for solids that only the single, most stable, crystal structure can be present. The dynamic character of the melt accounts for its fluid character and the loss of long-range order [1, 2]. This model is extended here to consider the phase diagrams of binary, common ion, alkali halide mixtures comprehensively reviewed in [3]. Factors determining whether each of these yields a eutectic, or a solid solution, on cooling are discussed and several trends in the 70-phase diagrams are identified. Eutectic formation, involving maintenance of the liquid state below the melting points of the pure components, is ascribed to the participation, in an extended dynamic equilibrium, of additional domains having the regular structures characteristic of double salts. The known crystalline double binary halides [3], Li/Cs or Rb/F, Cl, Br or I, melt at temperatures well below those of the simpler pure component salts. It is concluded that the set/liq model for melting, proposed in [1, 2], accounts for some important properties of the phase diagrams presented in [3].     

Making the liquid-phase catalytic hydrodechlorination of pce over Pd/C safer

Summary The hydrodechlorination of tetrachloroethylene in methanol (MeOH) over palladium catalysts on activated carbon (Pd/C) at 20°C and atmospheric pressure was rendered safer by using ethanol (EtOH), isopropanol (IPA) or water-alcohol mixtures as solvents. Adding water to MeOH was better than using EtOH or IPA since it resulted in a faster reaction and better stability of Pd/C, which could also be completely reactivated.     

Investigation of eutectic transformation of Zn−Al alloy by DTA

The paper discusses the investigation of crystallization of metals and alloys by differential thermal analysis (DTA). It was assumed that this method allows determination of the mechanism and kinetics of volumetric crystallization underiso conditions (e.g. anisothermal) on the basis of the parameters of the equation expressing an integral form of the DTA curve. From DTA, a course of eutectic transformation was determined for a technical Zn?Al alloy containing 4wt% Al. Investigations were carried out under continuous cooling at various rates and the kinetics parameters were determined with the KEKAM equation:-ln(1-x)=kl _n     

Intramolecular and liquid structure of 2,2,2-trifluoroethanol by X-ray diffraction

Intramolecular and liquid structure of 2,2,2-trifluoroethanol (TFE) have been investigated by x-ray diffraction at 25°C. The structural parameters for the skeleton of the molecules in the liquid phase are similar to those in the gas phase. The conformers of TFE molecules in the liquid phase are discussed. The O...O distance at about 284 pm and additional F...O one at about 302 pm were found to be characteristic for the first neighbor interactions. Various models (dimers and trimers) have been examined for analyzing the first neighbor structure. The liquid structure was explained in terms of small clusters consisting of two to three molecules rather than of a more extended polymeric network.Central Research Institute for Chemistry of the Hungarian Academy of Sciences, Budapest, P. O. Box 17, H-1525, Hungary.     

Thallium-205 NMR spectroscopy. Solvation of the thallous ion in dilute aqueous-amide and amide-amide binary solvent systems of formamide,N-methylformamide,andN-ethylformamide

Solvation of the thallous ion in dilute solutions of six binary solvent systems (formamide/water,N-methylformamide/water,N-ethylformamide/water, formamide/N-methylformamide, formamide/N-ethylformamide, andN-methylformamide/N-ethylformamide) was studied with²⁰⁵Tl NMR spectroscopy. An attempt was made to separate solvation effects related to the electrondonating ability (Lewis basicity) of the solvents from effects resulting from structural changes in the solvation sphere. Structural effects were found to be greatest in theN-methylformamide/water system and least in theN-methylformamide/formamide system.