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Wyznaczanie szybkości procesu wymiany jonowej w próbkach środowiskowych za pomocą pomiarów konduktometrycznych i pehametrycznych

Kłos A., Wacławek W., Wacławek M.

Chemistry-Didactics-Ecology-Metrology

2004

R. 9, NR 1-2

21--28

Wskazano na możliwości wykorzystania pomiarów konduktometrycznych I pehametrycznych do badania kinetyki procesów wymiany jonowej z udziałem protonów. Zinterpretowano przebiegi zmian mierzonych parametrów w trakcie trwania procesów zachodzących wg schematu (Mz+ = zH+). Jako przykład zaprezentowano wyniki pomiarów kinetyki interdyfuzji Donnanowskiej oraz kinetyki wymiany jonowej między roztworem a warstwą kationoaktywną porostów. Przeprowadzono analizę niepewności pomiarów.

Conductometric and pH-metric measuring are the basic methods used in chemical analysis. M. Faraday [1791- 1867] S. Arrhenius [1859-1927], F. Kohlrausch [1840-1910] and F. Haber [1868-1934] together with Z. Klemensiewicz 86-1963], who built a glass electrode for pH measuring, were among the fore-runners of these methods. Apart from generally known instances of usage, these methods are also applied to testing a number of chemical and logical processes like establishing of stability constants of complexes, studies in kinetics of diffusion processes, envelopment of bacterial cultures, in medicine and materials engineering (impedance spectroscopy) as well as in agricultural engineering. The authors are using conductometric and pH-metric measuring to study Donnan interdiffusion kinetics, to study kinetics Proton desorption from Hypogymnia physodes lichen that takes place in copper salts solutions and to analyze sorption and indicative properties of lichen. In the case of conductometric measuring application to describe the processes of ion exchange that take place according to following formulas: Mz+/MK/zH+ (where MK - cation active membrane) and Mz+ + zHR =zH+ + MR z (R- anion that is bounded in the cation active layer of lichen), it is essential that cations of metals - M, take part in the process whose mobility in solutions approximates 5x10-4 cm2 x V-1 x s-1 and more mobile hydrogen cations: 36.2x10-4 cm2 x V-1 x s-1. These differences, assuming stoichometric exchange, exert influence on the changes in electrical conductivity that are proportionate to the Nations in the concentration of aforementioned cations within the range up to 0.01 mol/dm3. The essence of the phenomenon has been presented in figures 1 and 2. The change in the participation of hydrogen ions in the solution also affects the changes in pH (fig. 3). Tables 4-6 contain formulas thanks to which, on the basis of measurements m electrical conductivity or pH of the solution, it is possible to establish the speed constant of ion exchange for reaction I, II and n order. The analysis of measurement uncertainty proved that stoichometric exchange of ions is characterized by a linear course of electrical conductivity changes dependent on the concentration of hydrogen ions k(c*) = k0+a c*, and more precisely, on the value of the direction factor: a μ (260- 290) (μS/cm) (mol/dm3)-1, depending on the kind of metal cation that takes part in the faction. Exceptions to the anticipated course of the characteristics may result from both parallel reactions, for example anion exchange and incorrect indications of the device as well. On the basic of the completed analyses, it was established that: 1. Marking of the speed in ion exchange processes, using conductometric measuring, is reasonable for the ranges of conductivity changes that are in linear dependence on molar participation of metal cations and hydrogen ions, which is diaplayed in figure 1 and 2. Additivity of electrical conductivity is retained for summary concentrations of strong electrolyte lower than 0,01 mol/dm3. 2. Comparison of solution conductivity changes in the course of the process with the changes in hydrogen ions concentration, calculated on the basic of pH changes of the solution (fig. 4) is very significant for checking the correctness of the completed analyses. It was found out that deviations from the assumed change process may result from, for example, the processes of dissolution that run parallel to ion exchange or from erroneous indications of the devices. 3. Conductometric measurements are superior to pH measurements in, which was displayed in figure 5, providing a constant absolute error resulting from the quality of the device. The parallel effect is a deteriorating resolution of measurement in hydrogen ions concentration from pH measurements that takes place with increase of solution acidity. 4. Expression of concentration in the form which includes the values of cations (c*) facilitates the study of ion exchange reaction kinetics as reaction of I or II order, independently of ion valence (fig. 6 and 7).