Variation Of Dissolution Rate Research Articles

Pharmaceutical and Clinical Assessment of Multi-source Tegretol Sold in Egypt

across generic Narrow therapeutic index (NTI) medications compared to brands. However, little attention has been given to possible inequality within the same brand due to transportation and storage conditions, production site, manufacturing condition, and product specification requested by different countries. Market surveys has shown that Egypt is one of the countries suffering from this problem, which consequently augments the need for comparing and evaluating brand products marketed under the same brand name, manufactured in different countries under its own licensing trademark™ and sold in Egypt. This work studies the drug carbamazepine (CBZ) available in Egypt as Tegretol® tablets used as an oral first-line anti-epileptic drug (AED). Objectives: The present work investigate and evaluate the pharmaceutical quality and clinical efficacy of Tegretol®, obtained from Egypt and Saudi Arabia and being marketed and sold in Egypt from 2020 to 21. Methods: In-vitro quality testing included potency and uniformity of content tests performed according to USP 2019 monograph. Dissolution rates were also carried by adopting USP dissolution test for the drug. Studies of stability were adjusted at 25°C/65% RH, 45°C/75% RH and 10°C/15% RH. The clinical efficacy was studied by evaluating the pharmacokinetics parameters and blood sodium level during six months of therapy. Results: Variability between multisource Tegretol® brand products were ensured. Potency results and uniformity of content revealed statistically significant difference between Tegretol® sources. Also, variations in dissolution rates were recognized in both Tegretol® sources when dissolved in HCL/KCL, Acetate and Phosphate Buffer dissolution media. Dissimilarities were obtained for hardness and friability testing. Furthermore, a pharmacokinetically and pharmacodynamically inequivalence was ensured. Hence, hyponatremia was more prominent in patients receiving Saudi Arabian Tegretol® compared to patients taking Egyptian Tegretol®. Conclusion: Interchangeability between multi-source Tegretol brand products should be limited.

The contribution of hydrodynamic processes to calcite dissolution rates and rate spectra

Recent measurements on the dissolution rate of nano- and micron-scale rough calcite surfaces have shown lateral variations in dissolution rate, which can be quantified using rate spectra. This study uses numerical simulations to investigates the hydrodynamic processes during such experiments to explore whether hydrodynamic effects can explain the observed dissolution rate spectra. For this purpose, we simulated the dissolution processes of nano- and micron-scale rough calcite surfaces in COMSOL Multiphysics. We imposed surface topographies and local reaction rates measured using Vertical Scanning Interferometry (VSI), and implemented the same flow rate (i.e., 6 × 10−8 m3 s−1), solution chemistry (pH 8.8, alkalinity 4.4 meq/kg-H2O and pCO2 10−3.48 bar) and flow-cell geometry as those used in the experiment. We have compared the simulated rate spectra against the experimentally measured values at a calcite surface having the same surface topography and reactive-flow conditions.Simulations using a single dissolution rate for the rough calcite surface did not produce similarly wide dissolution rate spectra like those observed experimentally. Our results have shown that only by explicitly incorporating the rate spectra in the model the simulated and the measured rate spectra would match. Sensitivity analyses by varying chemical composition and flow velocity were performed to examine the effects of these parameters on the calculated rate spectra. This study concludes that for the reactive-flow regimes where dissolution rate spectra are observed experimentally, the chemical heterogeneity, topography of the crystal surface and the resulting heterogeneity in the free energy landscape at the surface play a major role in controlling the dissolution rate spectra. With the injection of more acidic (pH 2) solutions at higher velocities (i.e., 0.04 m s−1), we observed an increase in the hydrodynamics-induced rate variability at microscopically rough surfaces.

The influence of different antimony (Sb) compounds and ageing on bioavailability and fractionation of antimony in two dissimilar soils.

Assessing the bioavailability of various Sb substances plays a crucial role in human health and the ecological risk assessment of contaminated soils. However, fate, behaviour and bioavailability of different Sb compounds in soils are insufficiently known. Therefore, in this present study, the effects of soil properties and ageing on bioavailability of four different Sb compounds (C8H4K2O12Sb2, Sb2S3, Sb2O3 and Sb2O3 nanoparticles) were evaluated during 120 days ageing time. A black soil (BS) with approximately 12% organic matter (OM) and a red soil (RS) with less than 1% OM were amended with 1000mg Sb kg-1 of different Sb compounds and subjected to single extractions with distilled (DI) water, 2M HNO3, Simplified Bioaccessibility Extraction Test (SBET) and a modified Community Bureau of Reference (BCR) sequential extraction method. The results revealed that there are substantial variations in dissolution rate of various Sb sources, depending upon soil type and Sb compound. The amounts of DI water extractability of Sb during the incubation time varied between <1% and 2%, whereas HNO3 extractable fractions and Sb bioaccessibility at the end of ageing time ranged between about 1%-3% and <1%-9% of the total Sb, with maximum bioaccessibility observed in BS contaminated with C8H4K2O12Sb2. The residual and labile fractions accounted for 77-93% and 0.1-4% of the total Sb, respectively, indicating that Sb is mostly associated with recalcitrant fractions of the soils. The results of single and sequential extraction studies revealed that source of Sb, ageing time and soil properties can greatly affect the bioavailability of Sb in soils. The findings of this research provide a deeper understanding of the potential risks associated with Sb compounds and highlights the role of site-specific considerations for improving the robustness of toxicity guidelines and long-term management of Sb contaminated sites.

Morphology, mechanical properties, and biodegradability of all‐cellulose composite films from oil palm empty fruit bunch

AbstractPalm oil plantations are very important in that they supply vegetable oil globally. However, increased production of palm oil prompts the accumulation of large lignocelluloses residues in the form of oil palm empty fruit bunch (OPEFB). This study explored the advantages of using OPEFB in the production of all‐cellulose composite (ACC) films. The isolation process of the raw OPEFB fiber was carried out using a chemical process to extract OPEFB nanocellulose. ACC films from OPEFB and microcrystalline cellulose (MCC) were prepared using a dimethylacetamide (DMAC) and lithium chloride solvent system whereby the partially dissolved cellulose was transformed into the matrix phase surrounding the remaining nondissolved fiber. ACC films with 1% (wt/vol) OPEFB and 3% (wt/vol) MCC were prepared and the effects of chemical treatment of the OPEFB cellulose on the mechanical properties, crystalline structure, morphology, moisture absorption and soil biodegradability of the ACC film were investigated. The tensile strength of the ACC film was tremendously improved by chemical treatment. For instance, when acetic acid was used to treat the nanocellulose, the resultant film showed 279% increment in tensile strength value. However, formic acid‐treated films demonstrate greater moisture uptake and soil biodegradation rate. The findings could be related to the alterations of hydroxyl group composition in the nanocellulose and variation in dissolution rate of the nanocellulose during chemical treatment.

Preservation conditions of CLIPPIR diamonds in the earth’s mantle in a heterogeneous metal–sulphide–silicate medium (experimental modeling)

The genesis of CLIPPIR diamonds (Cullinan–like, large, inclusion–poor, pure, irregular, and resorbed) have attracted much interest due to their possible crystallization from metal melt in deep horizons of the earth’s mantle. These diamonds usually show a pronounced resorption and irregular morphology. The present paper reports new experimental data on the dissolution of diamond crystals at high P–T parameters in Fe–S melt containing large amounts of silicate components (5–20 wt%). The experiments were performed using a split–sphere multi–anvil apparatus (BARS) at a pressure of 4 GPa and a temperature of 1450 °C. The samples consisted of natural diamond crystals placed in mixtures of Fe, S, and kimberlite. Wide variations in dissolution rates of diamond crystals were obtained. The absence of diamond dissolution in a heterogeneous medium indicates that the amount of solid silicate phases present in metal melt plays a role in the preservation of diamonds. This study demonstrated how diamonds can be stored in natural environments due to the heterogeneity of the medium composition which could insulate diamonds from the metal–sulphide melt. The obtained results improve our understanding of processes that lead to preservation of CLIPPIR diamonds in the deep mantle.

Variability in content and dissolution profiles of MDMA tablets collected in the UK between 2001 and 2018 - A potential risk to users?

3,4-Methylenedioxymethamphetamine (MDMA, Ecstasy) tablets are widely used recreationally, and not only vary in appearance, but also in MDMA content. Recently, the prevalence of high-content tablets is of concern to public health authorities. To compare UK data with other countries, we evaluated MDMA content of 412 tablets collected from the UK, 2001-2018, and investigated within-batch content variability for a sub-set of these samples. In addition, we investigated dissolution profiles of tablets using pharmaceutical industry-standard dissolution experiments on 247 tablets. All analyses were carried out using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Our data supported other studies, in that recent samples (2016-2018) tend to have higher MDMA content compared to earlier years. In 2018, the median MDMA content exceeded 100mg free-base for the first time. Dramatic within-batch content variability (up to 136mg difference) was also demonstrated. Statistical evaluation of dissolution profiles at 15-minutes allowed tablets to be categorized as fast-, intermediate-, or slow-releasing, but no tablet characteristics correlated with dissolution classification. Hence, there would be no way of users knowing a priori whether a tablet is more likely to be fast or slow-releasing. Further, within-batch variation in dissolution rate was observed. Rapid assessment of MDMA content alone provides important data for harm reduction, but does not account for variability in (a) the remainder of tablets in a batch, or (b) MDMA dissolution profiles. Clinical manifestations of MDMA toxicity, especially for high-content, slow-releasing tablets, may be delayed or prolonged, and there is a significant risk of users re-dosing if absorption is delayed.

On the variation of dissolution rates at the orthoclase (0 0 1) surface with pH and temperature

The pH- and temperature-dependent variations of the dissolution rates of orthoclase (001) surfaces in acidic aqueous solutions were examined in situ by using X-ray reflectivity and compared to density functional theory results. A phenomenological analysis of these data using the conventional relation Rd(pH,T)=pH−α exp(−ΔEapp/kT), with an apparent activation energy of ΔEapp∼65kJ/mol, requires a variable pH-dependent order of dissolution having a value of α∼1 for pH⩾2.5, and α∼0.37 for pH⩽2.5. These data also can be described by a two-term Arrhenius model characterized by activation energies, ΔEA and ΔEB, with prefactors in the form of Langmuir adsorption isotherms controlled by the proton adsorption enthalpies, δE. This analysis reveals that dissolution at the primary reactive site (site A) for pH>0.5 has an apparent activation energy of ΔEA=67±3kJ/mol and a proton adsorption enthalpy of δEA=−13.0±0.5kJ/mol. A secondary reactive site (site B), which dominates dissolution at pH<0.5, is described by the sum of these two energies, ΔEB+δEB=66±3kJ/mol. Density functional theory calculations provide insight into activation energies of water reactions with the protonated sites as a function of site coordination. These results suggest that the dominant reactive site for pH>0.5 is the bridging oxygen between Al and Si sites, while the dominant reactive for pH<0.5 is identified as the bridging oxygen between Si surface sites. The superposition of these two reactions leads to the change in the apparent order of reaction near pH 2. Comparison to kinetic theory reveals that the pre-exponential factor for dissolution reaction at site A is well-reproduced by the proton impingement rate, suggesting that the sticking coefficient is effectively unity.

Importance of surface structure on dissolution of fluorite: Implications for surface dynamics and dissolution rates

Dissolution rates are usually calculated as a function of surface area, which is assumed to remain constant ignoring the changes occurring on the surface during dissolution. Here we present a study of how topography of natural fluorite surfaces with different orientation changes during up to 3200h of dissolution. Results are analyzed in terms of changes in surface area, surface reactivity and dissolution rates.All surfaces studied present fast changes in topography during the initial 200h of dissolution. The controlling factors that cause the development of topography are the stability of the step edges forming the initial surface and its inclination to the closest stable planes, which are specific for each surface orientation. During an initial dissolution regime dissolution rates decrease significantly, even though the total surface area increases. During a second dissolution regime, some surfaces continue to present significant changes in topography, while for others the topography tends to remain approximately constant. The observed variation of dissolution rates are attributed to a decrease of the density of step edges on the surface and the continuous increase in exposure of more stable surfaces. Calculations of dissolution rates, which assume that dissolution rates are directly proportional to surface area, are not valid for the type of surfaces studied. Instead, to develop accurate kinetic dissolution models and more realistic stochastic dissolution simulations the surface reactivity, determined by the relative stability of the planes and type of edges that constitute a surface needs to be considered. Significant differences between dissolution rates calculated based on surface area alone, and based on surface reactivity are expected for materials with the fluorite structure.

Development and physicochemical characterization of saquinavir mesylate solid dispersions using Gelucire 44/14 or PEG 4000 as carrier

Solid dispersions of saquinavir mesylate containing either Gelucire® 44/14 or poly(ethylene glycol) (PEG) 4000, or mixtures of each carrier with Tween 80 or polyvinyl pyrrolidone (PVP) K30 were prepared in order to enhance the drug dissolution rate. These systems were prepared by the melting method and characterized by X-ray powder diffraction, microscopical techniques, and Raman spectroscopy aiming to establish a relationship between physicochemical and dissolution properties under different cooling conditions. Modifications in degree of crystalline order/disorder over time were observed in preparations with both carriers. Overall, formulations cooled and stored at -20 °C showed less variation in dissolution rates than those at 25 °C. Although Tween 80 has enhanced the known self-emulsifying properties of Gelucire® 44/14, its combination with PEG 4000 displayed miscibility problems. The addition of PVP K30 was not the most effective approach in enhancing the dissolution in early steps; however, the drug dissolution was stable after 7 days of storage at 25 °C. The combination of PEG 4000 and PVP K30 maintained the dissolution properties for 60 and 90 days at 25 °C/95% relative humidity and 40 °C/75% (f₂ values >50), respectively.

The dissolution behavior of individual powder particles

The formation of lumps is a common problem during powder reconstitution in food industry. If more knowledge regarding the phenomena controlling the process could be obtained, the process of industrial powder recombination could be optimized. This demands investigation of the process at three different levels: the scale of individual powder particles, the scale of powder beds, and the scale of industrial recombination. If knowledge from all three scales could be combined, a model of the entire process could be constructed. In this study, we present a new flow-cell-based method for investigating the dissolution rate of individual powder particles in controlled liquid flow. The flow cell allows for alteration of the flow velocity and only subjects the investigated powder particles to mild stress before the analysis. The dissolution process in the cell is recorded, and data are obtained by image analysis. Sodium caseinate has been used as a model substance in these experiments. The results show that the particle dissolution rate is a function of the flow velocity of the liquid, and the results correspond to earlier established methods. The investigation of individual particles shows a variation in dissolution rate between particles resulting from the diverse particle morphology from an industrial batch of powder. The results indicate that the dissolution interface concentration of dissolving sodium caseinate particles corresponds well to the overlap concentration of the specie.

Atomic emission spectroelectrochemistry applied to dealloying phenomena: I. The formation and dissolution of residual copper films on stainless steel

The rates of elemental dissolution were measured for a 304 stainless steel containing 0.19% Cu in real time during linear sweep voltammetry experiments using the atomic emission spectroelectrochemistry (AESEC) method. The results demonstrate that Fe, Cr, Ni, Mn, and Mo dissolve simultaneously leaving a residual copper film that inhibits the steel dissolution. The formation and dissolution of the copper film leads to the appearance of two peaks in the anodic dissolution transient, one due to inhibition of steel by residual copper and the other due to the formation of the passive film. The addition of NaCl to the electrolyte results in a marked increase in the intensity of the second dissolution peak, while hardly affecting the first peak. This is interpreted in terms of the lowering of copper dissolution potential by chloride ions due to the stabilization of CuCl 2 −. A simple phenomenological kinetic model is used to simulate the variation of dissolution rate with potential.

Does reactive surface area depend on grain size? Results from pH 3, 25 °C far-from-equilibrium flow-through dissolution experiments on anorthite and biotite

Laboratory determined mineral weathering rates need to be normalised to allow their extrapolation to natural systems. The principle normalisation terms used in the literature are mass, and geometric- and BET specific surface area (SSA). The purpose of this study was to determine how dissolution rates normalised to these terms vary with grain size. Different size fractions of anorthite and biotite ranging from 180–150 to 20–10 μm were dissolved in pH 3, HCl at 25 °C in flow through reactors under far from equilibrium conditions. Steady state dissolution rates after 5376 h (anorthite) and 4992 h (biotite) were calculated from Si concentrations and were normalised to initial- and final- mass and geometric-, geometric edge- (biotite), and BET SSA. For anorthite, rates normalised to initial- and final-BET SSA ranged from 0.33 to 2.77 × 10 −10 mol feldspar m −2 s −1, rates normalised to initial- and final-geometric SSA ranged from 5.74 to 8.88 × 10 −10 mol feldspar m −2 s −1 and rates normalised to initial- and final-mass ranged from 0.11 to 1.65 mol feldspar g −1 s −1. For biotite, rates normalised to initial- and final-BET SSA ranged from 1.02 to 2.03 × 10 −12 mol biotite m −2 s −1, rates normalised to initial- and final-geometric SSA ranged from 3.26 to 16.21 × 10 −12 mol biotite m −2 s −1, rates normalised to initial- and final-geometric edge SSA ranged from 59.46 to 111.32 × 10 −12 mol biotite m −2 s −1 and rates normalised to initial- and final-mass ranged from 0.81 to 6.93 × 10 −12 mol biotite g −1 s −1. For all normalising terms rates varied significantly ( p ⩽ 0.05) with grain size. The normalising terms which gave least variation in dissolution rate between grain sizes for anorthite were initial BET SSA and initial- and final-geometric SSA. This is consistent with: (1) dissolution being dominated by the slower dissolving but area dominant non-etched surfaces of the grains and, (2) the walls of etch pits and other dissolution features being relatively unreactive. These steady state normalised dissolution rates are likely to be constant with time. Normalisation to final BET SSA did not give constant ratios across grain size due to a non-uniform distribution of dissolution features. After dissolution coarser grains had a greater density of dissolution features with BET-measurable but unreactive wall surface area than the finer grains. The normalising term which gave the least variation in dissolution rates between grain sizes for biotite was initial BET SSA. Initial- and final-geometric edge SSA and final BET SSA gave the next least varied rates. The basal surfaces dissolved sufficiently rapidly to influence bulk dissolution rate and prevent geometric edge SSA normalised dissolution rates showing the least variation. Simple modelling indicated that biotite grain edges dissolved 71–132 times faster than basal surfaces. In this experiment, initial BET SSA best integrated the different areas and reactivities of the edge and basal surfaces of biotite. Steady state dissolution rates are likely to vary with time as dissolution alters the ratio of edge to basal surface area. Therefore they would be more properly termed pseudo-steady state rates, only appearing constant because the time period over which they were measured (1512 h) was less than the time period over which they would change significantly.

Line Edge Roughness in Chemically Amplified Resist: Speculation, Simulation and Application

Computer simulations and direct measurements were carried out to explore the effect of adding base quencher on the line edge roughness (LER) of photoresist images. The effect of variation of the polymer sequence distribution and hydrophilic units was also studied. The variation of surface roughness at different developed film thicknesses was measured by atomic force microscopy (AFM) for each polymer. It was concluded that the variation of dissolution rate (DR) and the distribution of acid and base are important parameters that must be considered in order to solve the LER problem for future lithography generation.

Technique for Monitoring the Etching Rate of Alumina

AbstractThe effect of chemical and thermal treatments on the grains and grain boundaries of polycrystalline μ-Al2O3has been examined using a combination of microscopy techniques. Commercially available alumina samples (Lucalox™) were chemically etched in phosphoric acid at 200°C in increments of 15 min. Thermal treatments were carried out at 1650°C before chemical treatments. Using maps obtained by visible-light microscopy (VLM) as a guide, the same regions were re-examined using atomic force microscopy (AFM) after subsequent treatments. Variations in the dissolution rates of different grains and grain boundaries could then be studied using AFM. The geometry of the grain-boundary grooves was compared after thermal and chemical treatments. Electron backscattered diffraction (EBSD) patterns recorded in the scanning electron microscope (SEM) were used to obtain crystallographic orientations of the grains which enabled variations in dissolution rates between grains to be correlated to orientation.

Variation of crystal dissolution rate based on a dissolution stepwave model.

A formulation based on defect-generated dissolution stepwaves of the variation of dissolution rate with the degree of undersaturation is validated by near-atomic-scale observations of surfaces, Monte Carlo simulations, and experimental bulk dissolution rates. The dissolution stepwaves emanating from etch pits provide a train of steps similar to those of a spiral but with different behavior. Their role in accounting for the bulk dissolution rate of crystals provides a conceptual framework for mineral dissolution far from equilibrium. Furthermore, the law extends research to conditions closer to equilibrium and predicts a nonlinear decrease in the rate of dissolution as equilibrium is approached, which has implications for understanding artificial and natural processes involving solid-fluid reactions.

An experimental study of calcite and limestone dissolution rates as a function of pH from −1 to 3 and temperature from 25 to 80°C

Dissolution rates of single calcite crystals, limestones, and compressed calcite powders were determined from sample weight loss using free-drift rotating disk techniques. Experiments were performed in aqueous HCl solutions over the bulk solution pH range −1 to 3, and at temperatures of 25°, 50°, and 80°C. Corresponding rates of the three different sample types are identical within experimental uncertainty. Interpretation of these data using equations reported by Gregory and Riddiford [Gregory, D.P., Riddiford, A.C., 1956. Transport to the surface of a rotating disc. J. Chem. Soc. London 3, 3756–3764] yields apparent rate constants and H + diffusion coefficients. The logarithms of overall calcite dissolution rates ( r) obtained at constant disk rotation speed are inversely proportional to the bulk solution pH, consistent with r= k 2′ a H +,b , where k 2′ stands for an apparent rate constant and a H +,b designates the hydrogen ion activity in the bulk solution. This variation of dissolution rates with pH is consistent with corresponding rates reported in the literature and the calcite dissolution mechanism reported by Wollast [Wollast, R., 1990. Rate and mechanism of dissolution of carbonates in the system CaCO 3–MgCO 3. In: Stumm, W. (Ed.), Aquatic Chemical Kinetics. Wiley, pp. 431–445]. Apparent rate constants for a disk rotation speed of 340 rpm increase from 0.07±0.02 to 0.25±0.02 mol m −2 s −1 in response to increasing temperature from 25° to 80°C. H + diffusion coefficients increase from (2.9 to 9.2)×10 −9 m 2 s −1 over this temperature range with an apparent activation energy of 19 kJ mol −1.

THE DISSOLUTION RATE OF Ca(OH)2 IN AQUEOUS SOLUTIONS

The dissolution rate of reagent grade Ca(OH)2 in aqueous solutions has been determined by means of a spinning disc method. The dissolution rate was found to be independent of disc velocity at a disc Reynold's Number above 9×l03. The dissolution rate is important for environmental applications such as flue gas desulfurization using Ca(OH)2 in wet scrubbers and spray dryer reactors, and for pH control. The lime discs were prepared under high pressure in a cylinder and determined to have similar thermogravimetric response to temperature as that of the parent lime powder. The discs were tested in a pH slat device where the dissolution rate was correlated to acid consumption required to maintain a preset value of pH. Limestone powder was used to verify the experimental system as the results could be compared with published data. Lime dissolution rate was measured as a function of disc velocity, ω (rpm), solution pH and temperature (K). The variation of dissolution rate with temperature was found to follow an Arrhenius type expression, and the activation energy of dissolution was found to be 15,188J/mol, Also, dissolution rate increased exponentially with decreased values of pH over the pH range of 6.5 — 3.0. A correlation which has been used to describe limestone dissolution rate was also found to express lime dissolution for disc Reynolds’ numbers < 104. This correlation has the form $\ (Ns) = -17.68 - 280(1/7”) - 0.105(pH) + 0.458 ln(w).

Mass transfer between solid and liquid in a gas-stirred vessel

Mass transfer between solid and bulk liquid in an axisymmetric gas-stirred water model of a metallurgical reactor has been investigated both experimentally and theoretically. To this end, mass transfer rates from benzoic acid compacts submerged in an aqueous gas bubble driven system were measured via a weight loss technique. In conjunction with the weight loss measurements, liquid velocity and turbulence kinetic energy distributions in the bath were also mapped via laser doppler velocimetry (LDV). From the detailed LDV measurements, relevant dimensionless groups such as \(\operatorname{Re} _{loc,r} \left( { = \frac{{d\rho \sqrt {u^2 + v^2 } }}{\mu }} \right)\) and \(\operatorname{Re} _t \left( { = \frac{{d\rho \hat u}}{\mu }} \right)\) were estimated. Experimental measurements indicated that flow parameters varied from one location to another within the system. The corresponding variation in dissolution rates was, however, less pronounced. Such a trend was observed for all three gas flow rates studied. It was found that experimentally measured dissolution rates can be correlated with the measured flow and turbulence parameters (viz., √u2+v2 and u) in terms of a previously reported dimensionless correlation, viz., Sh=0.73 (Reloc,r)0.25 (Ret)0.32 (Sc)0.33. Parallel to flow measurements, a two-phase turbulent flow model was also applied to numerically compute the distributions of mean and fluctuating velocity components in the vessel. Embodying the predicted velocity components in the aforementioned correlation, mass transfer rates were recalculated. A comparison between the two sets of Sherwood numbers (estimated on the basis of the experimentally measured and theoretically predicted flow fields) suggests that solid-liquid mass transfer rates in a gas-stirred vessel can be predicted reasonably well via an axisymmetric, steady-state, two-dimensional turbulent flow model.

Microtextural controls of weathering of perthitic alkali feldspars

The relationship between the microtexture and dissolution behaviour of fresh, HF acid-etched and naturally weathered alkali feldspar phenocrysts from the Lower Devonian Shap granite has been investigated by SEM and TEM. A novel resin impregnation technique has revealed the three dimensional shape and interconnectivity of etch pits beneath the weathered crystal surface. Further electron microscope work suggests that Shap phenocrysts are representative of the alkali feldspar in the protolith of many soils. Fresh and unweathered Shap feldspars have a complex microtexture, comprising areas of pristine cryptoperthite and lamellar microperthite cross-cut by volumes of microporous altered feldspar or “patch perthites.” Cryptoperthites are made up of <75 nm wide albite exsolution lamellae (platelets) in tweed orthoclase, whereas lamellar microperthites contain >75 nm wide albite films. The platelets are coherent, but albite films have numerous edge dislocations along their interface with orthoclase; (001) and (010) cleavage surfaces intersect ∼2–3 edge dislocations/μm 2. In three dimensions, these edge dislocations form an orthogonal net in the “Murchison plane” of easy fracture, close to (601). Patch perthites are irregular, semicoherent to incoherent intergrowths of albite and irregular microcline subgrains, with ∼0.65-0.70 sub-μm to μm-sized pores/μm 2. Microporous patch perthites form by dissolution-reprecipitation reactions with magmatic or hydrothermal fluids and pores are present before the alkali feldspars enter the weathering regime. Dissolution of Shap feldspars during natural weathering and laboratory acid etching is controlled by their microtexture, especially by dislocations and exsolution lamellae. The core and strain energy associated with dislocation outcrops on (001) and (010) cleavage surfaces promotes rapid dissolution at those sites and formation of nn-sized etch pits after <30 s of laboratory etching with HF acid vapour. With progressive HF etching, crystallographically controlled differences in the reactivity of etch pit walls cause them to expand more rapidly into orthoclase than albite. Naturally weathered feldspars were collected from the glacial erratic boulders, fine gravels surrounding exposed granite surfaces, and from peat soil overlying the granite. During natural weathering, etch pits on microperthites enlarge almost exclusively by dissolution of albite and resin casts demonstrate that they can penetrate ≥15 μm below the cleavage surface, forming an interconnecting, ladder-like grid of submicrometer wide channels in the Murchison plane. Coherent albite platelets and volumes of albite between dislocations in films dissolve uniformly, but faster than orthoclase. This is probably because the albite lamellae have significant elastic coherency strain, but this is much less, per unit volume of albite, than the core and strain energy associated with edge dislocations. Patch perthites etch in HF vapour and weather rapidly in nature to produce a honeycomb-like texture of interconnecting nanometer- to micrometer-sized pits, which nucleate at preexisting micropores or incoherent subgrain boundaries. The size and density of etch pits on microperthite surfaces, which is determined by rates of growth and coalescence, may be a useful progress variable for natural and experimental dissolution. All alkali feldspars are highly heterogeneous materials whose chemical composition and microtexture can vary on a submicrometer scale. These microtextures are critical variables with regard to the origin of surface roughness of fresh and weathered grains, the controls on absolute dissolution rates and why they commonly change over time, the nonlinear variation of dissolution rate with grain size, the ratio of alkali ions released into solution, and disparities between laboratory dissolution rates and those observed in the field.

Effect of Aging on the Dissolution Rate of Nalidixic Acid Tablets

AbstractInvestigation of the dissolution rate profiles of nalidixic acid tablets of three commercial brands was carried out. Using the U.S.P. paddle method, significant inter-brand variations in dissolution rates were found and the tablets did not pass the U.S.P. dissolution test. The dissolution of the tablets was also found to be adversely affected on aging. The observed differences in dissolution rates of the tablets examined were unrelated to their disintegration times. An attempt was made to improve the dissolution rate of nalidixic acid tablets through hydrophilization of nalidixic acid powder and use of tablet excipients with high aqueous solubility were found to yield tablets of good physical qualities which were unaffected on aging.