Browsing by Author "Nedeltcheva T."

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    Influence of the alkaline electrolyte bath on the oxygen content of REBa2Cu3OY (RE = Y, Gd) superconducting ceramics
    (2013-12-17) Georgieva S.; Stoyanova-Ivanova A.; Nedeltcheva T.
    The oxygen stability of the REBa2Cu3Oy (RE = Y, Gd; y = 6,5+δ) superconducting ceramics previously immersed in strong alkaline electrolyte bath have been studied. The samples were placed in solution of 7M KOH (pH 13) for 96 h and then the oxygen content was determined by spectrophotometry and XRD-spectrometry. The results obtained by the both methods are identical in the limit of random errors. Consequently the electrolyte treatment of the samples does not influence the oxygen content. The critical temperature and the structure of the ceramics do not change also and these superconducting ceramics can be used in the electrotechnical devices.
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    Spectrophotometric method for rapid estimation of the oxygen stoichiometry in YBCO superconducting samples
    (2013-03-01) Georgieva S.; Nedeltcheva T.; Vladimirova L.; Stoyanova-Ivanova A.
    Detailed investigations on the possibility for determination of the oxygen stoichiometry in YBa2Cu3Oy superconducting samples by measuring the absorbance of the colored Co(III)-EDTA and Cu(II)-EDTA complexes in one solution have been performed. The absorptivity of the complexes at pH 2.5 and 4.9 as well as Beer's law have been studied. The content of the solution for the sample dissolving has been established. The stoichiometry of the oxidation-reduction reaction between Cu(III) from the sample and Co(II) in the solution has been proved. The possibility to dissolve the sample in air has been tested. As a result a simple and rapid spectrophotometric method for the determination of the δ non-stoichiometric oxygen coefficient in YBa 2Cu3O6.5+δ samples has been developed without precise measurement of the sample mass. The standard deviation of the δ- and y-coefficient results is 0.017. The reproducibility of the y-values in relative units (s r ) is 0.2%. The accuracy of the results has been confirmed by the method, which is based on the absorbance measurement of the I 3 - -starch compound. © 2012 Versita Warsaw and Springer-Verlag Wien.
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    Spectrophotometric study of competitive complexation equilibria involving overlapped spectral responding species: Determination of the stability constant of bismuth-pyrophosphate complex
    (2012-12-01) Nedeltcheva T.; Surleva A.; Nikolova L.; Borissova R.; Georgieva S.
    Spectrophotometric study of competitive complex formation equilibria involving overlapped spectral responding species applying a simple and versatile algorithm was carried out. The algorithm involves multivariable regression for calculation of equilibrium concentrations from multiwavelength data and mass action law for the stability constant calculation. The used regression functions are part of common statistical software. Stability constants and complex stoichiometry of competing equilibria were simultaneously determined. The species concentration profiles at several spectral overlapping and α-coefficient of competing reaction were obtained. Non-absorbing bismuth - pyrophosphate (PPh) system was studied as a competitive reaction of bismuth - 4-(2-Pyridylazo) resorcinol (PAR) complex. The formation of Bi-PPh complex with 1:1 stoichiometry was proved in the studied concentration region (C Bi = 1×10 -5 mol L -1; C PPh = 5×10 -6 - 1×10 -4 mol L -1). The stability constant of the complex at pH 1 and μ= 1.0 have been determined: logβ = 4.2±0.2. © Versita Sp. z o.o.
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    Study Of The Potential Of Silver/Sulfide Ion Selective Electrode In Solutions Of Silver And Thiosulfate Ions When The Thiosulfate Is In Excess
    (2016-01-01) Vladimirova L.; Nedeltcheva T.
    A dependence of the potential of silver ion-selective electrode with silver sulfide membrane versus the analytical concentrations of silver ions and thiosulphate is derived theoretically. Equations, valid at a ratio between the molar concentrations of thiosulfate and silver ions at least 20 times and a concentration of thiosulfate from 0.001 to 0.2 mol L-1 are found theoretically and proven experimentally.