Relative Dissolution Rates Research Articles

The characteristics of the potentiodynamic potential/current profiles obtained with the Ni/0.5N H 2SO 4 interface. A contribution to the mechanism of the electrode process

Electrochemical data derived from different simple and complex potentiodynamic techniques for the Ni/0.5 N H 2SO 4 interface furnish a new insight about the activation and initial stage passivation of nickel. The anodic dissolution of nickel involves a metal surface which is either partially or completely covered by species such as NiOH, [NiOH.Ni(OH) 2] or [Ni(OH) 2.NiOOH]. The chemical dissolution of the various species in the acid electrolyte occurs at different rates. The relative dissolution rates can be estimated from the data derived from the complex triangular potential perturbations. The change of the E/ I profile during cycling is explained through a complex reaction sequence which is in agreement with recently postulated reaction schemes and reported optical data.

Physical Characterization of Erythromycin: Anhydrate, Monohydrate, and Dihydrate Crystalline Solids

Hot-stage microscopy, thermoanalytical methods, and X-ray powder diffraction were used to demonstrate that crystalline erythromycin dihydrate converta to the crystalline anhydrate via a noncrystalline intermediate. X-ray powder diffraction, IR spectral, thermogravimetric, and differential thermal analyses were used to characterize the monohydrate material. The flow interrupt technique, a procedure recently developed to deal with low surface area samples, was employed successfully in obtaining isotherms and specific surface areas for the monohydrate and anhydrate. The relative dissolution rates of the various hydrates were determined in an aqueous solution (0.01 M phosphate buffer, pH 7.5) at 37°. The results showed a significant difference in the dissolution rate of the dihydrate compared to the monohydrate and anhydrate.

Recording pH Method of Characterizing Composition and Monitoring Dissolution Profile of an Anhydride–Acid Copolymer and Its Salt Derivatives

A sensitive potentiometric monitoring method was developed that permits the continuous measurement of the dissolution profiles of methyl vinyl ether-maleic anhydride-acid copolymers and salt derivatives. Three distinct rate periods were observed in the dissolution of the anhydride copolymer. The relative dissolution rate of the anhydride copolymer, expressed as percent anhydride dissolved, was independent of sample weight over the weight range studied. The acid form of the copolymer showed only one dissolution rate period, with dissolution being very rapid. The rapid initial pH decrease observed during the first stage of dissolution for a series of anhydride-acid copolymer powder samples correlated closely with the anhydride-acid ratio, permitting chemical characterization of the copolymer functionality simultaneously with the analysis of dissolution profiles. Similarly, the extent of copolymer alkaline salt conversion was inversely proportional to the initial maximum pH increase observed during the first stage of dissolution of these salts. Mechanisms of dissolution of copolymer powder materials are discussed and compared to the dissolution of compressed disks and films reported previously.

A Mathematical Model of Evaporation and Dissolution From Oil Spills on Ice, Land, Water and Under Ice

Abstract Mathematical models are presented which quantify the processes of evaporation and dissolution of components of crude oil in three situations: a spill on water, a spill on ice, and a spill under ice cover in which the oil lies between the water and ice phases. Constant spill area is assumed. The evaporation flux is calculated using a mass transfer coefficient based on windspeed and spill dimensions. The dissolution flux can be calculated from two models, a mass transfer coefficient approach and an eddy diffusivity approach involving the integration of a set of partial differential equations in depth and time. The selection of model parameters is discussed. For the three physical situations, using a synthetic crude oil, results are presented giving the relative rates of evaporation and dissolution and the aqueous phase concentration of selected hydrocarbons. The implications of the results for clean-up technology and aquatic toxicity are discussed, particularly with regard to spills under ice.

Kinetic models for the setting of plaster of paris

AbstractA general rate equation is derived for processes such as the hydration of calcium sulphate hemihydrate in which the position of the rate maximum depends on the relative rates of dissolution and precipitation. Allowance is made for these processes to depend on powers of under or super‐saturation other than unity and for limitation of the precipitation rate by crystal‐crystal contact.The equations of Ridge and of Coombe et al. have the rate maximum fixed at 60 and 28 % respectively due to inadequate derivation, however they are both readily fitted by the general equation.

Chemical Analysis of the Liquid within a Propagating Stress Corrosion Crack in 70:30 Brass Immersed in Concentrated Ammonium Hydroxide

Abstract Studies were made of the liquid within a stress corrosion crack in 70:30 brass (CDA Alloy No. 260) propagating in 15N ammonium hydroxide containing 8 g/l dissolved copper. The liquid in the crack was isolated for study by removing the sample from the solution, immersing the sample in liquid nitrogen to freeze the liquid within the crack, fracturing the sample, and removing the liquid once it had thawed. Spectrophotometric analysis of the liquid indicated a copper concentration of 14.5 g/l and a zinc concentration of 11.5 g/l after one hour exposure and a crack length of 5 mm. The relative dissolution rates of copper and zinc were 0.57 compared to the 2.3 ratio of these elements in the alloy. The total corrosion rate in the crack was estimated to be 4 × 10−6 g/cm2/min. The corrosion rate of a bulk sample under identical experimental conditions was 5.4 × 10−6 g/cm2/min. The pH within the crack showed no measurable change during crack propagation. The Pourbaix diagram was calculated for pure copper ...

Dissolution Rates under Sink Conditions

Equations are given for the dissolution of a drug into an aqueous phase overlayered with a lipid phase, in which back transfer from the lipid phase is assumed possible. Consideration of both the presence and absence of back transfer, assuming in each that the amount of drug in the aqueous phase either approaches saturation or remains negligible, results in four equations for following the amount of drug in the lipid phase versus time. Three of the four equations are identical in form, predicting that after a brief lag time, a plot of the amount of drug in the lipid phase versus time will become linear. However, only in the instance for which no back transfer occurs and for which the buildup of drug in the aqueous phase is negligible (Case IV) is the slope of the line equal to the zero-order dissolution-rate constant, k0. In the other cases, the slope represents a fraction of k0, depending upon the relative rates of dissolution, partitioning, and back transfer. Thus, it becomes important to be completely certain that the conditions under which dissolution studies into a „perfect sink”︁ are carried out do actually meet the requirements of Case IV.

Aggressive Ion Accessibility and the Morphology of Corrosion Pits

Corrosion pits were formed under potentiostatic conditions on {100}, {111}, and {110} surfaces of nickel in solution by adding Cl− ions at passive potentials. The morphologies of the pits obtained were analyzed using a model based on the atomistic mechanics of dissolution. It is found that a simulation, which removed atoms one by one according to the principle that atoms more accessible to the aggressive ions are removed more easily, approximates the morphologies of pits obtained experimentally. Observed morphological changes with the reaction rate (dependent on the degree of passivation, the potential of chloride addition and the acidity of the solution) could also be accounted for by appropriate modification of the simulation procedure. The simulation also is shown to predict the morphology of corrosion pits in body‐centered cubic metals reported by other workers. It is proposed that the state of close‐packing of a crystal surface is not the only factor to be considered in determining the relative dissolution rates of crystallographic surfaces when aggressive ions are present.

Morphology of Crystallographic Etch Pits in Iron

It is shown that the development of low-index crystallographic facets in pits produced in the surface of polycrystalline iron is controlled primarily by the relative dissolution rates of different crystallographic orientations. The successive shapes of dissolving iron whiskers are used to predict the observed etch-pit shapes using a topographical theory of crystal dissolution.

Physical-Chemical Evaluation of 3-(3-Hydroxy-3-methylbutylamino)-5-methyl-as, Triazino [5,6-b] Indole (SK&F 30097)

X-ray diffraction, IR spectroscopy, and differential scanning calorimetry data are presented for the identification of two polymorphic forms and a hydrate of SK&F 30097. The relative rates of dissolution and the solubilities of the various forms were determined in artificial gastric fluid, water, and 50% ethanol solution. No appreciable difference in the dissolution rate was detected for the respective forms in artificial gastric fluid; however, Form II had a more rapid dissolution rate in the 50% ethanol solution. Dissolution studies in water indicated the formation of a hydrate. Protective colloids were shown to have an effect on the rate of hydrate formation. The change in particle-size distribution of Forms I and II in electrolyte suspension was investigated using the Coulter counter. The relative rate of crystal growth for Form I in the presence of protective colloids was determined. The data presented indicate that methylcellulose slows down the rate of crystal growth of Form I significantly.

Dissolution Behavior of Polymorphs of Chloramphenicol Palmitate and Mefenamic Acid

Relative rates of dissolution and solubilities of three polymorphic forms of chloramphenicol palmitate in 35% tertiary butanol-water mixture and two polymorphs of mefenamic acid in dodecyl alcohol were measured. The thermodynamic relationships involving the transition of the metastable polymorphs to the stable one were examined. The heats of solution, enthalpy, entropy, free energy differences, and transition temperatures were calculated. The relative stability of the polymorphs in the dissolving media is shown. The significance of free energy differences between the polymorphs and their absorptivity as reflected by blood levels in humans is discussed.

Dissolution Behavior and Solubility of Anhydrous and Trihydrate Forms of Ampicillin

Anhydrous ampicillin and ampicillin trihydrate were compared for solubility and relative rates of dissolution in distilled water at temperatures ranging from 7.5 to 50°. Differences were noted in the physical-chemical properties of these two forms of ampicillin. The thermodynamic properties of these compounds have been experimentally evaluated. The properties noted for the two forms of the antibiotic are consistent with the observed differences in the biological utilization of the two forms after oral administration to laboratory animals and human subjects.

On the slope of etch pits

It is pointed out, by means of a literature review, that etch pits formed at the sites of singular defects in an otherwise slowly dissolving surface are usually shallow, composed of faces misoriented from that surface by only a few degrees. An etch pit study of the (0001) zinc surface dissolved in alcoholic hydrochloric acid solutions supports the theory that the slopes of etch pits are controlled by the dissolution kinetics of the crystal. Pits widen at a rate independent of the type of defect attacked, but the slopes are dictated by the relative rates of dissolution at the defect sites.

Dissolution Kinetics of Certain Crystalline Forms of Prednisolone

The activity, dissolution rate, and crystal behavior of three crystalline forms of prednisolone, each exhibiting distinctly different properties, have been investigated. By determining the relative dissolution rates of the crystal forms under different agitation conditions, it was found that dissolution could be described by consecutive processes involving a reaction at the interface and transport away from the interface. The data suggested that these processes pose a double barrier to dissolution under the experimental conditions.

Dissolution Behavior of Crystalline Solvated and Nonsolvated Forms of Some Pharmaceuticals

Relative rates of dissolution of several crystalline steroids, xanthines, and other solid drugs have been measured to determine the effect of solvate formation on this property. Equations have been derived relating the solubility products and diffusional constants to rates of solution of organic solvates. The results suggest that the tendency of many drugs to form such adducts provide pharmaceutical investigators a powerful tool in effecting rapid dissolution of highly insoluble substances.

Investigation of Drug Release from Solids II. : Theoretical and Experimental Study of Influences of Bases and Buffers on Rates of Dissolution of Acidic Solids

A theory is presented which describes the rate of dissolution of solids in basic and buffered media. The formulation is based on the Nernst-Brunner film model assuming a solution diffusion controlled process. Useful expressions are obtained which give relative dissolution rates as functions of the primary variables. Results of calculations predict that for a solid weak acid dissolving in an aqueous basic solution, the rates of dissolution become essentially independent of the strength of the base above certain values. In the region where it is independent of the base strength the rate is a linear function of the product of the base concentration and the base diffusion coefficient. The theoretical conclusions are satisfactorily substantiated by experiments on the rates of dissolution of benzoic acid in basic media of varying strength and diffusivities. The following bases were used as additives: sodium hydroxide, sodium bicarbonate, sodium tetraborate, disodium phosphate, sodium acetate, and ethanolamine. Studies were also carried out with added swamping electrolyte. It was found that the rates of dissolution in varying concentrations of sodium chloride in neutral solutions can be largely explained by the salting out effects of the electrolyte.