Arne E. Nielsen

Electrolyte crystal growth mechanisms

Abstract When very soluble electrolytes crystallize from aqueous solution they usually grow by a linear rate law - the rate being proportional to the difference between the concentration and the solubility. The rate is controlled by the transport of the ions in the bulk solution or by the adsorption process. Sparingly soluble electrolytes mostly grow by a parabolic or exponential rate law at small or moderate supersaturations, and may change to transport control at larger concentrations. Both with parabolic and with exponential rate laws the rates can be accounted for in terms of the classical theories of surface spirals caused by screw dislocation, and surface nucleation, respectively, when it is assumed that the rate-determining molecular mechanism is the integration of the cations into crystal lattice positions at kinks in the surface steps (whether caused by dislocations or surface nucleation). The rate of integration at a kink is of the order of one thousandth of the rate of dehydration of the cation. The factor 10 -3 is interpreted as due to the activation energy of diffusion, as an ion, in order to integrate, must dehydrate and make a diffusional jump at the same time. The theory is confirmed by application to 20 electrolytes following the parabolic rate law, and two electrolytes following the exponential law.

Solubility of amorphous calcium carbonate

Abstract At high supersaturation in water, Ca 2+ and CO 2- 3 spontaneously form a rather unstable precursor consisting of spherical particles of calcium carbonate, 50–400 nm in diameter. X-ray diffraction indicated that the substance is amorphous. The solubility was measured at different temperatures by titration with water immediately after the precipitate was formed. The solubility product is 5.9, 4.0, 2.5, and 1.5×10 -7 M 2 at 10, 25, 40 and 55°C, respectively.

Interfacial tensions electrolyte crystal-aqueous solution, from nucleation data

Abstract Homogeneous nucleation of sparingly soluble electrolytes was observed by means of a rapid flow technique. When the experimental values of the interfacial tensions are plotted as a function of the decadic logarithm of the solubilities the points are close to a straight line of slope, -0.02 J/m 2 . The intersection with the log solubility axis is at 1.6≙40 mol/1. These results support the theory that the hydration of ions and the wetting of the surface of an electrolyte crystal are very similar phenomena that can be described by the same interfacial tension.

Electrolyte crystal growth kinetics

The growth rate of an electrolyte crystal in a supersaturated aqueous solution depends on the concentrations of the constituent ions. These concentrations may be varied independently when indifferent ions are present. For a binary electrolyte AαBβ the dependence of the rate on the concentrations of the ions A and B may be presented as isograms for constant growth rate in a diagram with the coordinates [A] and [B], or similar. The growth rate is controlled by the transport of the ions A and B through the solution by convection and diffusion, or by a process in the crystal surface, or by a combination hereof. The following surface controlled kinetics have been indentified: linear, parabolic, and exponential kinetics. They are related, respectively, to the adsorption process, to the integration of ions in screw dislocation centred surface spiral steps, and to the formation and growth of surface nuclei. Growth rates are reported for the following electrolytes: AgCl, CaF2, SrF2, Ag2C2O4, BaSO4, CaCO3, and CaC2O4·H2O.

Vaterite growth and dissolution in aqueous solution I. Kinetics of crystal growth

The kinetics of precipitation of vaterite (CaCO3) from aqueous solution between about 10 and 45°C and at ionic strengths from 15mM to 315mM was determined by recording pH as a function of time and analysing these data by a BASIC computer program, correcting for complexes and using activity coefficients. The particles were spherulites with diameters less than 5 μm, and the growth kinetics parabolic, with the rate constants (dr/dt)/(S-1)2 equal to 0.180, 0.468, 0.560, 1.250 and 1.860 nm/s at 11.4, 21.7, 25.0, 33.8 and 42.5°C, respectively, and 0.560, 0.588, 0.594 and 0.610 nm/s at ionic strengths 15mM, 65mM, 115mM and 315mM, respectively. The change with temperature corresponds to the activation energy, 57.1 kJ/mol. It is shown that the rate constants found are in quantitative agreement with those calculated from the theory of growth of electrolyte crystals from aqueous solution. This means that the rate-determining mechanism is the release of water molecules from the calcium ions as they “jump” into lattice positions.

Mortality and life-table in Down's syndrome.

ABSTRACT: Øster, J., Mikkelsen, M. and Nielsen, A. (Department of Paediatrics, Randers Centralsygehus, Randers, John F. Kennedy Institute, Glostrup, and the Institute for Human Genetics, University of Copenhagen, Denmark). Mortality and life‐table in Downs syndrome. Acta Paediatr Scand, 64:322, 1975.–The causes of death in 130 patients with Downs Syndrome and mortality rates from a material of 524 patients were tabulated; a life‐table for the ages over 5 years was constructed. An overall death rate of 5–7 times the general population rate was found. No sex difference was observed. The excess mortality was especially high for heart disease and respiratory disease. Also infectious diseases, others than pneumonia and tuberculosis, showed high mortality rates.

GROWING PAINS: A Clinical Investigation of a School Population

Only relatively few authors have expressed opinions and even fewer have undertaken investigations concerning this form of pain in the limbs which has been termed growing pains by both laymen and the medical profession from time immemorial (3). Only three works are available in the international literature which illustrate growing pains directly (2, 5, 7). This feature, in association with the differential diagnostic significance of the condition and its presumed frequent incidence, appears to justify a review of the relevant literature available (9) and an investigation of the incidence and clinical findings among school children. The object of the present paper is to account for the latter. By means of ordinary clinical investigation including radiography and laboratory investigations, it has not hitherto proved possible to reveal any recognizable etiology and, in particular, any evidence of rheumatic infection. In order to establish relatively stable diagnostic directives, we have established the following criteria from the international literature (1, 8, 9) and from personal experience: The symptoms consist of intermittent and frequently quite incapacitating pain localized deeply in the arms and/or legs in children and young people. The pain is not articular. Occasionally, it is accompanied by sensations of restlessness but never by tenderness, redness or local swelling. Similarly, pain in the legs is not provoked by walking and the gait is always normal. The pain disappears in the morning.

Homogeneous nucleation of droplets and interfacial tension in the liquid system methanol-water-tribromomethane

Abstract Unstable homogeneous solutions within the miscibility gap of the ternary liquid system methanol-water-tribromomethane were made by rapid mixing of stable solutions. Characteristic times (induction periods) for the appearance of turbidity were measured at 25 °C. These induction periods are of the order 0.1 to 10 ms, depending on the composition of the unstable solution. For induction periods shorter than about 1 ms the measured data can be explained by assuming homogeneous nucleation of droplets followed by diffusion controlled growth. The ration between the interfacial tension calculated from the nucleation data and the bulk interfacial tension determined by the classical drop weight method is less than one, and approaches unity when the composition of the unstable system approaches the critical composition, i.e., when the interfacial tension approaches zero, and the critical nuclei become infinitely large.

Vaterite growth and dissolution in aqueous solution II. Kinetics of dissolution

Abstract The kinetics of dissolution of vaterite (CaCO 3 ) crystals in aqueous potassium chloride solutions were studied at temperatures between 15 and 45°C and ionic strengths between 50 and 200 mol/L. The vaterite preparate used consisted of spherulites of vaterite crystals with initial radii of 2.7 to 4.6 μm. The progress of the dissolution was followed by recording pH as a function of time. From pH the concentration of the dissolved matter was calculated, and from this we found the particle size, r , and the dissolution rate, - d r /d t , where r = average particle radius and t = time. Straight lines were obtained by plotting - r (d r /d t ) as a function of undersaturation, c s - c , which may be explained by diffusional rate control. Both the values of the apparent diffusion coefficients determined and their dependence on temperature (activation energy = 24.38 kJ/mol) support the assumption that the rate-determining process for the dissolution is the diffusion of the dydrated calcium and carbonate ions away from the crystal surface into the bulk solution.

NUCHAL NAEVI AND INTERSCAPULAR TELANGIECTASES: Incidence in Danish School Children

Naevi in the fovea nuchae and telangiectases in the superior interscapular region have been found with great frequency in 2,171 school children aged 6‐17 years during a systematic search for these phenomena. The incidence of nuchal naevi was 46.2 per cent in girls, 35.1 per cent in boys. The incidence of telangiectasis was 39.9 per cent in girls and 32.3 per cent in boys.