Showing papers on "Phosphotungstic acid published in 1994"

Mixed phosphotungstic acid/titanium oxide gels and thin solid xerogel films with electrochromic-ionic conductive properties

Abstract: Electrochromic-ion conductive gels (σ = 1.7 × 10−4 S/cm) and thin solid films composed of phosphotungstic acid (PWA) incorporated in titanium oxide xerogel (PWA:Ti = 0.07) were made via the sol-gel route by the dip-coating technique. The electrochromism of the as-deposited films was studied with the help of cyclic voltammetric (CV) measurements in HClO4 electrolyte, in situ ultraviolet-visible spectroelectrochemical measurements and ex situ near-grazing incidence angle (NGIA) reflection spectroscopy. It was demonstrated that the colour change from transparent to blue is about 40% efficient and is accompanied by effective inserted charges up to 35 mC/cm2, giving rise to HxPW12y+O403− species with x = 4.7 and y = 5.6. Ex situ NGIA Fourier transform infrared reflection measurements revealed that proton injection decreases the intensity of the longitudinal optical (LO) modes corresponding to the intra νW-Oc-W and inter νW-Ob-W vibrations of the Keggin's ions, while the νW-Od modes remain unaffected. An intensity decrease of the LO modes of TiO vibrations was also detected in the ex situ NGIA reflection spectra of cathodically coloured pure Ti xerogel films. It was found that the proton insertion process is accompanied by the hydroxylation of titanate ions, while no such effect was noted for PWA/Ti and Ti xerogel films aged in water and the corresponding electrolytes without the applied potential. The electrochromic-ionic conductive properties of mixed PWA/Ti gels were demonstrated in a semi-liquid electrochromic cell.

Potentiometric microtitration of vitamin B1 with phosphotungstic acid by using a phosphotungstate-sensitive electrode and its application to the analysis of pharmaceutical preparations

Abstract: A method for the determination of micro-amounts of vitamin B1 by direct potentiometric titration in aqueous medium with a standard phosphotungstic acid solution is described. A graphite electrode coated with a PVC matrix membrane containing the benzyldimethyltetradecylammonium–phosphotungstate ion association complex as electroactive material and dioctyl phthalate as plasticizer was constructed and used in conjunction with a Ag–AgCl double-junction reference electrode to detect the end-point of the titration. This method gave very satisfactory results in the range 1–4 mg of vitamin B1, also in comparison with other methods; the recoveries were 100.00 ± 0.25% and the mean relative standard deviation was 0.22%. The method was successfully applied to the analysis of pharmaceutical preparations.

Comparison of two precipitation methods for determination of phosphotungstic acid soluble nitrogen in cheese

Abstract: Two methods were used for preparation of cheese extracts containing small peptides and amino acids. Comparison of soluble nitrogen values points on systematic differences are probably attributable to varying molecular weight cut-off levels.

Sequential treatment by phosphotungstic acid and uranyl acetate enhances the adherence of lipid membranes and membrane proteins to hydrophobic EM grids

Abstract: A simple procedure for screening by electron microscopic observations of conditions for the reconstitution of membrane proteins into lipid bilayers is described. This procedure consists of a 5-10 s treatment of electron microscopic grids, to which the sample has already been applied, with 1% phosphotungstic acid before proceeding with final staining in uranyl acetate. The method substantially enhances the adherence of lipid membranes and membrane protein particles to hydrophobic collodion/carbon grids.