Václav Vacek

A critical review and experimental verification of the correct use of the dynamic method for the determination of oxygen transfer in aerated agitated vessels to water, electrolyte solutions and viscous liquids

Abstract Inconsistencies are frequently observed in data for the volumetric mass transfer coefficient for oxygen kla obtained by various dynamic methods. The reasons for this phenomenon are critically reviewed. On the basis of our own experimental data for water, solutions of electrolytes, glycerol and carboxy-methylcellulose, the various dynamic methods are compared with the result that only one variant gives correct results over a wide range of power dissipated per unit volume. In this variant, pure oxygen is introduced as a step input to a batch freed from any dissolved gas by e.g. vacuum desorption. All the data obtained by this method agree with those measured by the steady state sulphite method and conform to a simple relation of the form kla ∼ en1. The decrease in the slope n1 of a log(kla) vs. log e plot, or even a decrease in kla with increasing e at higher e values, as reported by a number of researchers, is merely a result of incorrect techniques used for kla measurement.

Analysis of differences in kℓa values determined by steady-state and dynamic methods in stirred tanks

Abstract Comparison is made between kla values determined by the dynamic method in several systems (water, Na2O4, KCl and NaCl solutions, and glucose solutions containing the enzymes catalase and glucose oxidase) in a mechanically agitated bubble tank. The dynamic method was performed either by effecting an inlet gas interchange while maintaining constant aeration and agitation of the batch (procedure A) or with a simultaneous start-up of aeration and agitation (procedure B). The types of gas interchange used were N2→ O2, O2 → N2, N2 → air, air → N2, O → air and O → O2. The kla values calculated on the basis of different models differ systematically. The kla values obtained have been compared with those resulting from steady-state methods. Procedure B performed with the interchange O → O2 yields kla values which agree with those obtained by the steady-state method. It is shown that the previously reported decrease of kl with increasing power input dissipation in the liquid was a result of misinterpretation of kla data.

An experimental study of oxygen probe linearity and transient characteristics in the high oxygen concentration range

Abstract A systematic study was made of the linearity and transient characteristics of oxygen probes with large-surface cathodes ( d K = 0.35–7.0 mm) to elucidate the effects of experimental conditions. The following effects were investigated: electrolyte pH and layer thickness; addition of catalase to electrolyte; membrane material and thickness; cathode material; and the direction and level of concentration changes. The experimental results were so organized as to provide a synoptical pattern of information which would lend itself, as an aid, to the efforts of those seeking oxygen probes of more desirable linearity and transient characteristics.

Enthalpy of crystallization of lithium chloride from aqueous solution at 25°C

The enthalpies of crystallization of LiCl and LiCl·H2O from aqueous solutions at 25°C are reported as measured by a calorimetric method and derived from the previously published concentration dependence of the enthalpy of solution data. The results are compared with those obtained from the concentration dependence of activity coefficients and from the temperature dependence of solubilities.

Heats of crystallization of potassium chloride and magnesium chloride hexahydrate from aqueous solutions at 298.15 K. A critical review

Abstract Published data on the heats of crystallization from aqueous solutions of potassium chloride and magnesium chloride hexahydrate are critically reviewed and compared with those evaluated from solubility and activity data. The recommended values of the heats of crystallization are −13.8±0.1 kJ mole−1 for KCl and −15.8−0.3+2.8 kJ mole−1 for MgCl2·6 H2O. A test of the mutual consistency of data for dissolution heats, temperature dependence of solubility and concentration dependence of activity and/or osmotic coefficient is a side-issue of the review.

The effects of the accumulation of cathodic reaction products in clark oxygen probes on probe linearity, signal overshooting, tailing and hysteresis of transient characteristics for step changes in the concentration of oxygen

Abstract The paper gives a summary of published observations on the tailing and hysteresis of transient characteristics for steps in the concentration of oxygen and of concepts which have been used to explain the observations. A new model of tailing is developed in terms fo the accumulation of cathodic reaction products in the electrolyte. Published observations on the tailing of transient characteristics are discussed in terms of the proposed model, taking into account a previously derived model of tailing due to lateral diffusion of oxygen to the cathode. It is shown that the hysteresis of tailing responses, signal overshooting and non-linear behaviour are connected with the accumulation effects.

Integral heat of dissolution of potassium chlorate in water at 298.15 K

Abstract The heat of dissolution of potassium chlorate in water at 298.15 K has been measured on an LKB 8700-1 calorimeter in the concentration range 0.063–0.659 m. The concentration dependence of the measured data was fitted by an empirical equation ΔHm (kJ mole−1) = 41.3538 + 1.8626m 1 2 − 6.4300m which was derived from our and Andauer—Lange data. The heat of crystallization calculated from this dependence was ΔHcryst. = 34.7 ± 0.5 kJ mole−1, which agrees with data calculated for potassium chlorate from solubility and activity data.

Enthalpies of solution and of crystallization of lithium nitrate and of lithium nitrate trihydrate in water at 25°C

The enthalpies of crystallization of LiNO3 and LiNO3−3H2O from aqueous solutions at 25°C, measured by a calorimetric method and determined from the previously published data on the concentration dependence of the enthalpy of solution, are reported. The results are compared with the values obtained from the concentration dependences of the activity coefficients and from the temperature dependences of the solubilities. The enthalpy of solution at infinite dilution and the enthalpy of hydration are given.

Concentration dependence of activity coefficients at saturation and at 298.15 K evaluated from heat of crystallization and solubility data

Abstract For inorganic electrolytes of AgNO 3 , CsNO 3 , KNO 3 , LiNO 3 ·3H 2 O, NaNO 3 , NH 4 NO 3 , RbNO 3 , TlNO 3 , KClO 3 , K 2 CrO 4 , K 2 Cr 2 O 7 , K 2 SO 4 and K 2 S 2 O 8 , the concentration dependence of activity coefficients at saturation is evaluated from heats of crystallization and from the temperature dependence of their solubilities. Calculated values are compared with those deduced from measured data. The degree of their mutual consistency differs considerably from case to case.

The effect of an external magnetic field on the solubility of oxygen in water

Abstract This work describes observations on the effect of magnetic fields on the equilibrium solubility of pure oxygen in water at 25°C. It was found that neither a weak rotating magnetic field nor a uniform static magnetic field (up to 10 kG) changed the solubility of oxygen in water within experimental accuracy (1%), in contrast to previously reported data. This supports recent findings on the size of effects due to weak and moderate magnetic fields on the physico-chemical properties of pure water and/or water solutions.