Rotating Disk Method Research Articles

Metal ion adsorption—incorporation mechanism in the Zn(II)-bearing ferrite formation reaction in aqueous solution

Zn(II)-bearing ferrite films were accumulated on a glass plate (5 × 10 mm) in an aqueous system by the “rotating-disk method” (RD method) at pH 7–8 and at 65°C. In the RD method, the Zn(II)-ferrite films grow by the repetition of Zn(II) and Fe(II) ion adsorption reactions and the oxidation reaction of the adsorbed Fe(II) ion. Analysis of the Zn(II) content in the Zn(II)-ferrite films in terms of the adsorption equilibrium equations on the surface of the ferrite films led to the conclusion that it is determined by the adsorption equilibria of Zn(II) and Fe(II) ions on the surface of the ferrite film and that the mole ratio of Zn(II) to Fe(II) ions adsorbed is equal to that of the Zn(II) ion to iron ions incorporated into the ferrite film (adsorption-incorporation mechanism). Also, supported by the results from Co(II)-bearing ferrite film formation by the RD method, the analysis showed that an increase or decrease in the mole ratio of the metal ion [Zn(II) or Co(II)] to iron ions in the ferrite films with an increase in the reaction pH could be attributed to the first hydrolysis constants and the contribution of the nonhydrolysis-adsorption reactions of the metal ions and Fe(II) ions.

Kinetics of non-congruent dissolution of E-glass fiber in saturated calcium hydroxide solution

The kinetics of non-congruent dissolution of long E-glass fiber in saturated calcium hydroxide solution has been investigated at 25°C in a closed reactor adopting the rotating disc method for the sample holder. The fiber dissolution rate (α), measured from the relative loss weight of the fiber after selective dissolution of the attack layer in acetic acid, is a linear function of time, what implies an interfacial dissolution process. However, the kinetics does not stay regular for a long time due to local detachment of the attack layer. The comparison between α = f (t) and α′ = f (t) curves, where α′ is the formation rate of the attack layer, shows that the formation of the layer does not only imply the deposit of a bad-crystalline hydrated calcium silicate but the nucleation and growth of solid calcium hydroxide and calcium carbonate resulting from the transfert, to the interface, of ions coming from the liquid phase. This phenomenon was precedently observed by some other authors in the case of glass-fiber reinforced cement matrix.

A preformulation study on the in vitro dissolution characteristics of the organophosphorus poisoning antidote HI-6

The in vitro dissolution of the new organophosphorus antidote HI-6 has been investigated by using the rotating disc technique. By applying centrically rotating discs, the diffusion controlled dissolution rates at different laminar flow rates, pH and temperatures have been obtained. From the well-known Levich relationship, a diffusion coefficient was obtained. The enthalpy of activation of the dissolution rate indicates that the ability of HI-6 to dissolve is very high. This is also supported by the value of the initial mass transfer from solid to aqueous phase i.e. the intrinsic rate of dissolution, which was found to be 7.4 mg·cm −2· −1. This rate constant was obtained by using a modified rotating disc method. The intrinsic dissolution rate of HI-6 was, however, much higher than expected from solubility data. The crystal form of HI-6 seems to change when equilibrated with water as indicated by scanning electron microscopy and thermal analysis, which thus might explain the observed deviation from the general solubility — dissolution rate relationship.

Kinetics of Aqueous Polysulfide Solutions: Part III . Investigation of Homogeneous and Electrode Kinetics by the Rotating Disk Method

The steady‐state current density‐overpotential relationships for cobalt and platinum rotating disk electrodes in aqueous polysulfide electrolyte (1.3M total S2−, 1M S0, and ) were determined over a temperature range of 25°–77°C. A model developed to take into account the rate of the homogeneous dissociation of to as well as a finite heterogeneous electrochemical reaction rate yielded parameters consistent with the results of previous transient experiments (in the low overpotential region). At higher overpotentials in the cathodic region a second cathodic reaction was evident. This second cathodic reaction was less prominent when the current densities were lower (as on Pt electrodes) or when the concentrations were higher (as temperature increased). The addition of 13 mole percent dimethylformamide (DMF) to the aqueous polysulfide solution was found to increase significantly the current density, at a given temperature and overpotential, by increasing the equilibrium concentration of . In the range of overpotentials studied, additions of DMF also caused the first cathodic reaction wave to become much more prominent. Although independent quantitative data are lacking for the DMF‐water mixtures, the use of reasonable values for and allows the quantitative prediction of the observed cathodic behavior.

Structurally heterogeneous electrode films of acrylamide/vinylpyridine gels: Dynamics of ferricyanide mediated electrooxidation of ascorbic acid and their dependence on the electrode film structure and composition

A novel electrode coating material was synthesized by bulk, radical copolymerization of acrylamide, bisacrylamide and vinylpyridine in aqueous solutions. Highly swelled, porous gels were obtained. Sections of the copolymeric gels, a fraction of a micrometer in thickness, were cut with a microtome and deposited on glassy carbon disk electrodes. Electrochemical measurements involving chronocoulometry, cyclic voltammetry and rotating disk methods were used to characterize these electrode coatings in terms of their physical structure and ion-exchange properties. After ion-exchange incorporation of ferricyanide ions, mediated electrooxidation of ascorbic acid was investigated in an acidic electrolyte. The kinetics of the processes involved in this electrocatalysis were analyzed according to the Savéant-Andrieux theory. All electrocatalytic systems behaved according to the R + S case. Detailed analysis of the electrocatalysis data allowed us to relate the key parameters determining the system's performance to the structure and the composition of the gel films. Self-segregation of vinylpyridine groups during polymerization of a gel in acidic solution was revealed and related to its superior ion-exchange properties and the high catalytic activity of the bound ferricyanide ions.

Studies of the limiting currents associated with several oxidation reactions mediated by tris(2,2′-bipyridyl)iron(III) incorporated in nafion® films on carbon electrodes

The kinetics of the oxidation of ferrocyanide, catechol and tiron at carbon electrodes covered with Nafion films containing tris(2,2′-bipyridyl)iron(III) were studied by rotating-disc methods. Under conditions in which these reactions were confined to the film-solution interface, limiting currents were observed that were independent of both the substrate concentration and the electrode rotation rate. This saturated behaviour was interpreted in terms of the rate of electron transfer within the precursor complex formed between the dissolved substrate and the limited number of active mediator sites accessible on the coating surface.

The diffusion coefficient of cobalt(II) and bisulphate in moderately acidic sulphate solutions

A value for the diffusion coefficient of cobalt(II) in 0.5 M sodium sulphate solutions (pH 2–6) equal to (6.4 ± 0.3) × 10 −10 m 2 s −1 has been determined by the rotating disc method. The precision of the diffusion coefficient measurements was found to decrease significantly with decreasing pH. This effect has been attributed to increases in both the background current and hydrogen evolution, which causes noise in the limiting currents. The background current is due to the discharge of protons and bisulphate ions. Since proton and bisulphate diffusion in a sulphate solution are thought to be coupled by a rapid equilibrium, it is only possible to determine a joint diffusion coefficient for the two species. From a series of such measurements, a value of (18 ± 2) × 10 −10 m 2 s −1 was determined by extrapolation for the (hypothetical) isolated HSO − 4 species.

Determination of Mass Diffusivity of Simple Sugars in Water by the Rotating Disk Method

ABSTRACTThe rotating disk method is shown to be an accurate method for determining the mass diffusivity of solid food components in liquid solutions. The diffusivities of sucrose and glucose in water were determined to be 0.50 and 0.66 (m2/sec × 109) at 25°C with activation energies of 36.3 and 31.6 kJ/g‐mol, respectively.

Ferrite Plating by Means of Thin Film of Reaction Solution; "Thin Liquid¿ Film Method"

A new method is proposed for ferrite plating to overcome the disadvantages with the conventional ferrite plating based on the rotating disk method and spraying method. The ferrite film is grown in a stainless steel reaction cell, in which the reaction solution forms a "thin (~ 50 μm) liquid film" between a substrate and a cell wall. By intermittently introducing an oxidizing solution, which contains air as an oxidizing reagent, the metal ions (Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , Co <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , etc) in the "liquid thin film" react into crystalline spinel film on the surface of the substrate. In the new method, oxygen concentration and the flow rate of the solution are much easier to control than in the previous method, and the reaction cell is much simpler too.

The dissolution/precipitation kinetics of calcium carbonate: An assessment of various kinetic equations using a rotating disk method

The dissolution/precipitation kinetics of calcium carbonate has been studied using a rotating disk technique. A crystal of Iceland Spar was allowed to dissolve in water until equilibrium was established and the solution thus saturated with CaCO 3. The Ca 2+ concentration was monitored throughout the period of dissolution using a calcium ion selective electrode. In this way various kinetic equations proposed by different workers to describe the dissolution/precipitation process have been tested. In particular it is found that the equation of Sjoberg, rate S = k S {K sp 1 2 − [ Ca 2+] 1 2 [ CO 3 2−] 1 2 } , where K sp is the solubility product, and the equation of Plummer, rate P = k P 3 − k P 4[Ca 2+][HCO 3 −], both give excellent fits to the observed data. In contrast, the equations of Davies and Jones and of Dorange and Guetchidjan are unsatisfactory.

The dissolution of iceland spar crystals: The effect of surface morphology

The effect of surface morphology on the dissolution rate of Iceland Spar crystals has been examined by a rotating disk method. A cleaved (100) surface was found to be initially unreactive even in 10 −3 M HCl. Subsequently dissolution develops and surface roughness increases until a steady state is reached. An equation describing the transition is presented. Dissolution experiments in the pH range 6.0–7.0 with deliberately roughened surfaces and with surfaces misoriented at a known angle to the (100) plane are described and the dissolution rates correlated with the surface morphology as revealed by electron microscopy.

Kinetics of dissolution of synthetic scheelite by an alkaline EDTA leach solution

The dissolution rate of synthetic scheelite in an alkaline EDTA leach solution under atmospheric pressure has been determined by use of a rotating disc method. The effects of disc rotation speed, temperature, reagent concentration and pH of leach solution have been investigated. The dissolution rate was proportional to the square root of the disc rotation speed. The apparent activation energy of dissolution of synthetic scheelite was 22 kJ mol −1 within the experimental temperature region of 24 to 99°C. A first order dependence of dissolution rate on EDTA concentration was observed. It has been shown from the experimental results that the dissolution reaction was controlled by the mass transfer in the aqueous solution.

Defect concentration measurements in solids using a lyoluminescence method

The dependence of alkali halide crystal lyoluminescence on crystal lattice defect concentration has been studied using the rotating-disk method. It is shown that lyoluminescence intensity changes in proportion to changes of F-center concentration in a crystal. It is also demonstrated that by gradually dissolving the crystal in the form of a disk, while rotating it with a constant angular rate around its axis of symmetry, it is possible to determine the defect distribution over the crystal depth with a resolution of the order of 1 μm according to measurements of lyoluminescence intensity.

Program for evaluating drug dissolution kinetics in preformulation.

The in vitro dissolution kinetics of various drugs were studied at different experimental conditions using both a centrically rotating disc method and a modified excentrically rotating disc method. The combination of these two methods provides a preformulation program that includes many important pharmaceutical and biopharmaceutical parameters related to drug dissolution. The influence of the hydrodynamic conditions on the dissolution rate was demonstrated and the intrinsic dissolution tendency was obtained. Further, the acid dissociation constant, diffusion coefficient and enthalpy of dissolution can be calculated from data obtained from the rotating disc experiments. The relationship between dissolution rate and aqueous solubility of each drug tested is also considered.

Réaction de la sphalérite en milieu acide chloruré non oxydant. I. - Influence des paramètres physico-chimiques liés à la solution

Dissolution kinetics of a pure sphalerite have been studied in concentrated non-oxidising chloride acid medium by the rotating disk method. The reaction rate, uncontrolled by difftision, is slow and constant. The reaction is influenced by the following factors : temperature, concentrations of H+, CI-, H2S, ZnT (total dissolved zinc). The specific rate of solubilisation, expressed in mole.cm-2.s-1, can be described by the equation : r = 8.2 . 10-3 . aH+ . aCl- . (ZnT)-1/2 . p H2S-1 . exp [-7500/T] This heterogeneous surface reaction being a chemical equilibrium, the reaction mechanism proposed for the interpretation of experimental data assumes, on the one hand, a competitive adsorption between active species at solid surface and, on the other hand, a chemical equilibrium between solid and adsorbed phase as rate controlling step.

Studies of some characteristics of molecular dissolution kinetics from rotating discs

Abstract The maximum mass transfer from solid to liquid phase i.e. the intrinsic rate of dissolution has been shown to be experimentally accessible with good precision using a generalized rotating disc method in combination with extrapolation to infinite speed of revolution and/or infinite distance from the disc to the center of rotation. The method has with very good result been applied to the determination of dissolution rates from non-disintegrating discs of sulfamethizole and alaproclate hydrochloride in water. In the present paper assumptions for the theory of such experiments are reviewed and compared with numerical values for a number of characteristic kinetic parameters applying simple mathematical models. This comparison supports the assumptions made. The modified rotating disc method described in this paper could therefore be recommended as part of a preformulation program for new solid drug compounds.

Preparation and in Vitro Evaluation of Salts of an Antihypertensive agent to Obtain Slow Release

The hydrochloride, acetate, tartrate, sulfate, p-hydroxy- benzoate, ebonate, napsylate, 3-hydroxy naphthoate and methacrylic acid-methacrylate copolymer salts of 9-[2-(indol-3-yl)ethyl]-1-oxa-3-oxo-4, 9-diazaspiro[5,5] undecane (I) were prepared. In vitro dissolution rates for the pure salts and the capsule formulation were determined using rotating disk method and USP method 2, respectively, in 0.1 N HCL, water and 0.05 M phosphate buffer pH 7.5. The intrinsic dissolution rates in all media showed dramatic differences among various salts corresponding to the differences in their solubilities in water. The in vitro dissolution rates from capsules in neutral media showed marked differences among salts, however, there were no significant differences in dissolution rates from capsules when 0.1 N HCL was used as the medium. The data indicated that the ebonate, 3-hydroxy naphthoate and napsylate salts when formulated in an enteric coated dosage form would provide a slow release of I.

The dissolution kinetics of Iceland spar single crystals

The dissolution of Iceland spar has been studied using a rotating disk method. This allows measurements to be made under conditions of well-defined mass transport and with single crystals prepared with surfaces having reproducible characteristics. The dissolution reaction was studied in the pH range 3.0–6.2 and with varying partial pressures of carbon dioxide. In general the flux of dissolving Ca 2+ is given by the equation j Ca2+/mole cm −2 sec −1 = k 1[H +] + k 2 + k 3[H 2CO 3], where we find k 2 to be 2.1 × 10 −9 mole cm −2 sec −1 at 25°C (in citrate buffers) and k 1 and k 3 are given by the rate of transport of H + and H 2CO 3, respectively, to the disk surface. In the presence of weak acids such as acetic acid which can rapidly dissociate on the rotating disk time scale then [H +] should be replaced by [weak acid] in the above equation and the diffusion coefficient used in calculating k 1 should be that of the weak acid.

Importance of Viscosity in the Dissolution Rate of Cholesterol in Monooctanoin Solutions

Several factors affecting the dissolution rate of cholesterol in monooctanoin were investigated. This solvent is used clinically for dissolution of residual cholesterol gallstones in the bile duct after cholecystectomy. The effect of added water on dissolution rate, measured using the static-or rotating-disk methods, was not consistent with the previously measured solubility. The discrepancy was found to be due to the decreasing viscosity of the solvent as water was added. Addition of cholesterol, however, increased the viscosity of monooctanoin. The viscosity effect on dissolution rate was investigated further by addition of polymers (povidone and poloxamer 237) which increased solvent viscosity. Dissolution rate was proportional to viscosity to the –0.4 power with these polymers. An equation was derived which predicts that dissolution rate should be proportional to viscosity to the −2/3 power. The predicted exponent was very close to reported experimental values for benzoic acid, but the dissolution rate/viscosity relationship for cholesterol in aqueous monooctanoin was nonlinear with apparent exponents of −0.65 to −2.3. Although the Arrhenius activation energies for viscosity (3.79 kcal/mol) and dissolution rate constant (3.66 kcal/mol) were almost equal for benzoic acid, a nonlinear relationship was again observed for cholesterol in aqueous monooctanoin with approximate Ea values of 5.6–10 kcal/mol. The strong influence of viscosity on dissolution rate in this system is attributed to the viscosity-increasing effect of cholesterol in the diffusion layer. The increased viscosity at higher cholesterol concentrations reduces the diffusion coefficient of cholesterol and causes the dissolution rate to be slower even though solubility may have been higher. The data suggest that it may be possible to increase cholesterol gallstone dissolution by addition of water to monooctanoin.

On the determination of true dissolution rate parameters from rotating disc experiments

A vertical rotating disc method has been used to further investigate the dissolution process. By varying the initial drug concentration in the receptor solution, the dissolution rate could be resolved into both the dissolution step and the re-entry step. Sulfamethizole, alaproclate-HCl and FLA 731 with very different aqueous solubilities served as model substances. Dissolution rates at infinite rotating speed were obtained at each initial concentration by using a previously described extrapolation procedure. Results are in good agreement with equilibrium solubility data and previously determined 'intrinsic' rates of dissolution.