Journal of the University of Chemical Technology and Metallurgy, XXXVIII, 2003, Book 1
DIELECTRIC PROPERTIES OF Ba0.5Sr(0.5-x)MgxTiO3 CERAMICS OBTAINED BY THE EROXOMETHOD
V. ParvanovaIR INVESTIGATIONS OF MULLITE, SINTHESIZED BY SOL - GEL TECHNOLOGY
Y. Dimitriev, B. Samuneva, I. - M. Salvado, E. Kashchieva, A. BachvarovaQUANTITATIVE DETERMINATION OF METHYL TERTIARY-BUTYL ETHER IN GASOLINE
BY GAS CHROMATOGRAPHY
A. Pavlova, R. IvanovaPOLAROGRAPHIC INVESTIGATIONS OF HEAVY METALS IN NAOH MELTS
Tz. TzvetkoffCORROSION OF GOLD, SILVER, PLATINUM, RHODIUM, PALLADIUM AND IRIDIUM IN ALKALI BASE MELTS
Tz. TzvetkoffDESIGN OF ELECTROMEDIATED DETECTION OF MAIN COMPOUNDS INVOLVED IN FOOD AND ENVIRONMENTAL CONTROL
C. Bala*, L. Rotariu
DIELECTRIC PROPERTIES OF Ba0.5Sr(0.5-x)MgxTiO3 CERAMICS OBTAINED BY THE PEROXOMETHOD
V. Parvanova
|
University of Chemical Technology and Metallurgy, 8 Kl. Ohridski, 1756 Sofia,
Bulgaria E-mail: vparvanova@hotmail.com |
Received 4 February 2003 Accepted 24 March 2003 |
The system Ba0.5Sr(0.5-x)MgxTiO3 (x = 0.0, 0.1, 0.2, 0.3, 0.4 mol at.) is obtained by the peroxometod. The temperature dependence of the permittivity ( r) and dielectric losses (tan ) at frequency 1kHz from the calcination temperatures of ceramics Tcal = 1250, 1300, 1400 C are studied. Ceramic material with compound Ba0.5Sr0.1Mg0.4TiO3 calcinated at 1400 C have TK r 0 for the temperature interval - 40 to 140 C, r = 870, and low dielectric losses tan = 17.10-4. Those values make it suitable for production of thermal stability capacitors as well as capacitors with low dielectric losses.
Keywords: Titanates, Ba0.5Sr(0.5-x)MgxTiO3 , dielectric properties
|
University of Chemical Technology
and Metallurgy, 8 Kl. Ohridski, 1756 Sofia, Bulgaria |
Received 18 December 2002 Accepted 11 February 2003 |
Gels with composition 40 mol % SiO2 : 60 mol % Al2O3 (stoichiometrical mullite) and 70 mol % SiO2 : 30 mol % Al2O3 (nonstoichiometrical mullite) were obtained in the Al2O3 - SiO2 system. As precursors were used Al(NO3)3.9H2O and TEOS [Si(OC2H5)4 - thetraetilortosilicate]. It was confirmed, that the transition of the gel to gel glass (thermal treatment up to 9000C) is connected with the structural transformation in the amorphous condition of the AlOn polyhedra. The heat treated at 16000C leads to the formation of polycrystalline mullite, which is characterize by IR and XRD.
Keywords: IR spectra, mullite, gel, sol - gel.
QUANTITATIVE DETERMINATION OF METHYL
TERTIARY-BUTYL ETHER IN GASOLINE
BY GAS CHROMATOGRAPHY
A. Pavlova, R. Ivanova
|
Analytical Department, Refining and
Petrochemistry Institute, |
Received 16 March 2003 Accepted 22 March 2003 |
Gas chromatographic (GC) method for routine determination of the content of methyl t-butyl ether (MTBE) in gasoline is described. Quantitative analysis is performed using an internal standard method. It has been demonstrated that the developed GC/FID method has several advantages including simplicity of handling, quickness and highly precise and accurate measurement of MTBE content in gasoline. The analysis time is 26 min using selective capillary column and equipment.
Keywords: MTBE, gasoline, capillary gas chromatography, flame ionization detector.
POLAROGRAPHIC INVESTIGATIONS OF HEAVY
METALS IN NAOH MELTS
Tz. Tzvetkof
|
University of Chemical Technology
and Metallurgy, 8 Kl. Ohridski, 1756 Sofia, Bulgaria |
Received 3 April 2003 Accepted 18 April 2003 |
Polarigraphic investigations of Sb2O3, PbO and Bi2O3 in NaOH melt in the temperature range of 350-450°C were performed by means of Pt RDE.
It was found out that there was a proportional correlation between the diffusion current and the depolarizator concentration (first and second polarographic waves) for all of the oxides studied.
The presence of Sb2O3 in the NaOH melt involves residual water formation from complex ions, which explains the abnormal variation of the first two peaks heights and the appearance of the third peak. Probably Sb4+ and Sb5+ complex ions maxima are more electronegative than H+ and Na+ of the supporting melt.
The first peak in the derivative polarogram of PbO corresponds to a two electron process while the second one probably indicates reduction of polymer type complexes.
The first maximum in the polarogram of Bi2O3 could be attributed to fourth order polarographic peaks. It is determined by suboxide formation. The second one results from Bi suboxide reduction after an intermediate reaction at the cathode - electrolyte interface.
It was ascertained that the polarographic waves of Bi2O3 and PbO are in good agreement with the Hejrovski-Ilkovoch equation. The higher slope values for Sb2O3 could be attributed to an irreversible simple and complex ion reduction.
The apparent activation energy of the diffusion controlled process for Bi2O3 and PbO is 25,34 KJmol-1 and 41,31 KJmol-1, correspondingly. The considerably high value for PbO suggests existence of large polymerized molecules.
The fairly low diffusion coefficient ( = 7,2.10-6 cm2s-1) of the system Bi2O3 - NaOH indicates that there is considerable diffusion hindrance and concentration polarization.
Keywords: polarography, ion melts, oxides, heavy metals.
CORROSION OF GOLD, SILVER, PLATINUM, RHODIUM, PALLADIUM AND IRIDIUM IN ALKALI BASE MELTS
Tz. Tzvetkoff
University of Chemical Technology and Metallurgy, 8 Kl. Ohridski, 1756 Sofia, Bulgaria
E-mail: tzvetkof@uctm.eduReceived 3 April 2003
Accepted 18 April 2003
In our former investigations some preliminary experiments connected with the determination of corrosion resistance of some noble metals in alkali melts were reported.
It was important to continue these studies in order to find the most resistant material for experimental crucible preparation. The present investigations were carried out with Au, Ag, Pt, Rh, Pd, and Ir, of 99,9% purity. The greater part of the experiments was done with fine silver crucibles. The latter were placed in a cryptolic based furnace corrosion equipment. The atmosphere above the melt consisted of medicinal oxygen (99,9%), pure nitrogen (99,9%) or a mixture of both gases (10% O2).
All experimental results lead to the conclusion that the corrosion resistance of the investigated metals in alkali melts decreases along the line Ag, Pt, Au, Rh, Pd and Ir and that KOH is a more aggressive medium than NaOH.
The experiments show also that the corrosion rate of the metals studied is strongly influenced by the atmosphere above the melt. Most probably the peroxide presence in the melt, which in turn, depends on oxygen and water vapour partial pressure, is crucial. The importance of Na2O2 content in the melt is obvious also from the fact that the addition of Na2O2 to NaOH melt in nitrogen atmosphere accelerates drastically the corrosion rate.
It is necessary to note that no inhibition effect was observed with Ag, which shows that there is not a considerable peroxide influence on Ag corrosion. It is suggested that the corrosion process in this case followed another mechanism.
It could be also concluded from the results obtained that water free melts were less aggressive in most cases. This could be due to the fact that oxy- instead of hydrooxy-complexes were formed in the alkali melts studied.
Keywords: corrosion, ionic melts, gold, silver, platinum, rhodium, palladium, iridium.
DESIGN OF ELECTROMEDIATED DETECTION OF MAIN COMPOUNDS INVOLVED IN FOOD AND ENVIRONMENTAL CONTROL
C. Bala*, L. Rotariu
Laboratory of Quality Control and Process Monitoring, Department of Analytical Chemistry,
Faculty of Chemistry, University of Bucharest, 90 Sos. Panduri, 762351 Bucharest-5, Romania;
E-mail: cbala@rnc.roReceived 3 April 2003
Accepted 18 April 2003
Different strategies have been compared to produce amperometric biosensors for the detection of organic compounds, important in the quality control of food and environment. Amperometric detectors based on mediator-modified electrodes represent quite a popular approach, as they could provide sensitive detection of NADH in a potential window where electrochemical interferences are minimal. Electrochemical biosensors with Meldola Blue and TCNQ (7,7,8,8,-tetracyanoquinodimethane) based on NAD+- dehydrogenases and on the enzymatic inhibition of acetylcholine esterase developed by our group is presented.
Numerous ways to immobilize both mediators on the electrodes have been explored, in order to produce amperometric sensors with good operational stability, in order to reduce costs and to provide more environmentally friendly analytical approaches.
Amperometric and pulse techniques were used both for developing biosensors based on chemically modified electrodes.
Keywords: biosensor, mediators, food and environmental analysis.
