Adsorption Of Ca Research Articles

Comparison of two electrokinetic methods – electroosmosis and streaming potential – to determine the zeta-potential of plane ceramic membranes

Electroosmotic flow rate and streaming potential measurements are used to characterise electrokinetic properties of plane ceramic membranes. The study is carried out at different pH, ionic strengths and electrolytes. Effects of pH and ionic strength are studied for both techniques which lead to very close values of isoelectric points. The specific adsorption of Ca 2+ cations is observed with the two methods. For identical pH and ionic strength electroosmosis gives greater zeta-potential values than those determined from streaming potential measurements. The gap between the two methods increases as the pH move of the isoelectric point and ionic strength increases. These results suggest that the location of the shearing plane depends on the electrokinetic method used.

The genesis and transformation of organo-mineral colloids in a drained peatland area

The spatial evolution of colloids from a drained peat bog to a river was studied in a karstic watershed (Jura Mountains, Switzerland) by bulk chemistry and electron microscopy. Raw waters (peat bog pore water, water from artificial drains and river water) were analyzed using ICP (Fe, Ca), Colorimetry (Fe 2+), TOC and UV (organic matter) and analytical electron microscopies (TEM-EDS, STEM-EDS, EF-TEM). Microscopic analyses correlate bulk sample results and highlight interesting features at the level of the individual colloids. In the peat, TEM-EDS shows the presence of individual globular entities made of carbon (0.4 to 1 μm), while in the river STEM-EDS reveals Fe-Ca-rich globular colloids with a core of carbon determined by EF-TEM. In parallel, bulk chemical analyses show that the ratio [Fe 2+]:[Fe] tot decreases from the peat to the river as an inverse function of pH and [O 2], while [Ca 2+] increases. We may thus postulate that Fe-Ca-C rich entities found in the river originate from the oxidation of Fe 2+ and adsorption of Ca 2+ at the surface of organic colloids, as they are transported by drains from the peat to the river.

Adsorption of metal cations by hydrous aluminium(III) or iron(III) hydroxide precipitates: enhancement by EDTA and related chelate molecules†

Stoichiometrically equivalent concentrations of ethylenediaminetetraacetate, EDTA, and of related chelating anions increase the adsorption of ca. millimolar concentrations heavy metal aqua-ions on amorphous precipitates of aluminium(III) or iron(III) hydroxide and, although higher concentrations decrease the adsorption, poly-EDTA, a polyelectrolyte containing EDTA functional groups, shows no such decrease.

Diffusivities of clavulanic acid in porous sorption systems with ion pairing

The diffusivity of clavulanic acid in different porous systems (Amberlite IRA400, activated carbon and two ion-pair adsorption systems based on Amberlite XAD4 and activated carbon as matrixes and water-soluble quaternary ammonium salts as ion pairing substances) was investigated by batch adsorption experiments. The experimental data were described by using a model based on diffusion with instantaneous adsorption and/or reaction in a homogeneous sphere and the effective diffusion coefficients were calculated under different operation conditions. The influence of sorbate concentration, temperature and competitive components on the diffusivities was discussed from the viewpoint of the ruling mechanisms. A comparison between the kinetic characteristics of the investigated systems showed that the ion-pair system XAD4-ABDA is the most advantageous for the adsorption of CA.

Molecular interactions between phospholipids and electrolytes in a monolayer of parenteral lipid emulsion

The particle size of parenteral lipid emulsions containing phosphatidylcholine decreased in solutions of electrolytes. The sizes of lipid particles in solutions of KCl or CaCl 2 decreased to 90% compared with those in water. To determine the reasons for this, the effects of electrolytes on the size of lipid particles were investigated in terms of the molecular area (limiting area). The molecular areas at the air/water interface were obtained from the surface pressures of monolayers by the Wilhelmy method. The molecular area of phospholipids which formed the interfacial membrane of a lipid emulsion decreased when electrolyte was added to the water phase. The molecular areas for monolayers of all phospholipids decreased to approximately 90% with 10 mM KCl solution. These findings suggest that the molecules of phospholipids pack closely since the repulsion between molecules weakens with increase in ionic strength. On the other hand, in the presence of CaCl 2, the areas decreased to 80% due to electrical adsorption of Ca 2+ onto the phospholipid membrane surface. Furthermore, we studied the mixed monolayers dynamically. The excess free energy of a mixed monolayer of a phosphatidylcholine and an oleic acid became negative with the addition of Ca 2+, suggesting that the mixed monolayer formed a condensed membrane. These results show that the addition of electrolytes induces membrane condensation and decreases the size of lipid emulsion.

Equilibrium and kinetic properties of a fast iminodiacetate based chelating ion exchanger and its incorporation in a FIA-ICP-AES system

The equilibrium and kinetic properties of an iminodiacetate (IDA) based chelating ion exchanger with a crosslinked agarose, Novarose™, as support has been investigated. The second and third acidity constants and some complexation constants of the ligand were determined for adsorbents with metal binding capacities of 140, 55 and 18 μmol ml −1, respectively. The adsorbent of medium capacity showed fast adsorption and desorption of Cu(II), Cd(II), Ni(II) and Ca(II) both in the batch and column mode. It was found to be about 50 times faster than Chelex-100 (50–100 mesh) in accumulation of these metal ions in the batch mode. Studies of the adsorbent in a flow system, using a 5 mm × 6 mm i.d. column, indicated quantitative accumulation of Cu(II), Cd(II), and Ni(II) at volumetric flow rates up to 110 ml min −1. Linear calibration curves with r > 0.999 and signal enhancement factors up to 1300 were obtained. Preconcentration by a FIA system connected to an ICP-AES instrument will make simultaneous measurement of ultratrace concentrations of a number of metal ions possible within reasonable cycle times due to the high flow rates which can be used with the adsorbent. Trace amounts of cadmium and copper in tap water were determined successfully at 60 ml min −1. However, copper and nickel in tap water are strongly complexed and do not accumulate quantitatively even at low flow rates. Hence a sample pretreatment is needed. Copper was completely adsorbed after UV-treatment of the sample.

Formation of ferric iron crusts in Quaternary sediments of Lake Baikal, Russia, and implications for paleoclimate

Phosphate-bearing, ferric iron and siliceous crusts ranging in age from Recent to approximately 65,000 yr B.P. are observed in sediments of Lake Baikal. In younger sediments the crusts are at the base of a spectrum of secondary iron and manganese oxides that accumulate near the sediment/water interface in the zone of positive oxidation potential beneath an oxygenated water column. In areas where the average Quaternary sedimentation rates have been slow (e.g. 0.026 mm/yr), the crusts are more common, and span a wider range of ages. No crusts have been found where the Quaternary sedimentation mode has been deltaic and rapid (0.15 mm/yr). Independent core correlation based on magnetic properties of the sediment suggests that crusts can be correlated over most of Academician Ridge, an area that is particularly sensitive to climatic events affecting the concentration of suspended sediment. These crusts may be indicative of periods of low suspended sediment concentration, which occur during sustained transitions from glacial periods of high detrital input, to interglacial periods of high diatom sedimentation. The crusts are dominated by iron-rich and siliceous amorphous mineral phases, with an FeO:SiO 2by weight of 3:1. Regardless of age or location in the lake the Fe phase always includes Ca, P and Mn. Extensive microprobe data for these four elements recast as normalized elemental weight percent reveal linear trends of Ca:P and Fe:P. With increasing P, Ca also increases such that the two elements maintain a linear relationship passing very close to the origin and with a mean molar Ca:P = 0.3 (too low for well-characterized apatite). Conversely, with increasing P, Fe decreases (mean molar Fe:P = 3.4). There is no correlation between Mn and P. Molar Fe:P ratios for vivianite (an Fe(II) phosphate mineral observed in sediments closely below some crusts) are clustered around a stoichiometric composition. The covariant increase in Ca:P and the corresponding decrease in Fe:P may be explained by: (1) coupled adsorption of aqueous Ca and P by a colloidal ferric hydrous oxide; (2) loss of Fe from a Ca-P-Fe phase; or (3) oxidation of vivianite to a metastable mineral phase that gradually loses Ca and gains Fe. The first explanation is favored, because there is no petrographic evidence for either the existence of an originating Ca-P-Fe phase, or, for the oxidation of vivianite. Further, it is suggested that by continually equalizing surface charge, Ca allows more phosphate to be adsorbed leading to thicker crusts and longer preservation after burial.

Geochemistry, mineralogy, and geochemical mass balance on major elements in two peat bog profiles (Jura Mountains, Switzerland)

The mineralogical and chemical composition of peats from two Sphagnum bogs in the Franches Montagnes region (Jura Mountains, Switzerland) were compared. The peats in the top 102 cm at Etang de la Gruère (EGr) represent 2110 years of peat formation compared with 1730 years for the first 84 cm at Tourbière de Genevez (TGe). Scandium was used as a conservative tracer to distinguish between the primary sources of major elements to the bogs: atmospheric deposition of soil-derived aerosols (EGr and TGe); physical incorporation of elements along with mineral matter from the underlying sediments (TGe); and adsorption or complexation by the peat following diffusion from groundwater (TGe). These results support previous findings which showed that the EGr core (102 cm) is exclusively ombrogenic compared with TGe where the peats below approx. 30 cm become predominantly minerogenic. The modern rates of Sc accumulation (past 100 years) are similar in the two cores: 39 μg m −2 a −1 at EGr versus 52 μg m −2 a −1 at TGe. However, comparison with deeper sections of the EGr core reveals that the present-day rates of atmospheric Sc deposition are nearly 3 times greater than the long-term average rate, reflecting the higher concentrations of soil-derived atmospheric aerosols today. The rates of Sc accumulation in the TGe core are slightly higher than at EGr; this reflects the incorporation of mineral matter from the underlying sediments In the deeper, minerogenic peats at TGe, the long term rates of Fe and Ca accumulation are 5 and 7 times higher, respectively, than in the EGr core. These fluxes greatly exceed the difference in Sc flux, and cannot be explained by the differences in amount of mineral matter in the peats. These enrichments require an independent explanation, and are most likely the result of adsorption and/or complexation of cationic Ca and Fe species diffusing from deeper layers in the profile. The mineralogical composition of the ash is mainly quartz (60–90%), with feldspar (5–15%) and muscovite (5–15%), although various other minerals (5–10%) are commonly present. Biogenic Si represents another important fraction of ash and is abundant (30–70%) at discrete depths. The vertical profiles showed no significant changes in mineralogy with depth: assuming a constant composition of source materials, the lack of progressive mineralogical change suggests that the fine-grained silicates supplied by soil dust have not been measurably weathered during the past two millennia. A number of peaks in Fe/Sc are found in both cores above peaks in ash content and density. These appear to correspond with temporal fluctuations in wet/dry conditions: accumulation of Fe-oxides during dry periods and leaching during wetter conditions.

Effects of crystallinity of goethite: I. Preparation and properties of goethites of differing crystallinity

SummaryWe prepared five samples of pure goethite and three samples of aluminous goethite with mole fractions of aluminium ranging from 0·05 to 0·20. One of the samples of pure goethite was repeatedly heated in an autoclave at 150°C. We studied the samples using transmission electron microscopy and XRD and we measured the surface area (BET), rate of dissolution of the samples in acid, the surface charge in four concentrations of sodium nitrate from pH 4–10, and the sorption of calcium and chloride from a dilute solution of calcium chloride. Surface areas ranged from 18 to 132 m2 g−1. The BET adsorption‐desorption isotherms had hysteresis loops which increased as the surface area increased. These showed that there were pores present with diameters smaller than 2 nm and that the number of pores increased as the surface area increased. All samples of goethite showed typical X‐ray diffraction patterns but with peak heights decreasing, and width increasing, as surface area increased. Mean coherence lengths, as determined by XRD line broadening, were smaller than those determined by transmission electron microscopy. This was because each crystal was composed of several domains, and the XRD data reflect the size of the domains. Heating one of the goethite samples in an autoclave caused most of the domains to coalesce and slightly decreased the surface area. As the surface area of the eight goethite samples increased, the mean coherence length in the direction of the a, b and c axes decreased. The better crystallized samples dissolved more slowly in acid and the dissolution curves were sigmoid because the acid was able to penetrate between the domains allowing access to a larger surface area and consequently accelerated dissolution. The points of zero charge increased with increasing surface area of the goethite. The charge carried by the goethite at pH values distant from the point of zero charge increased as the surface area increased. This may indicate movement of protons or hydroxyl ions into pores or other defects in the crystal. The effects of pH and of salt concentration on surface charge were closely described by the variable‐charge/variable‐potential model. This model also closely described adsorption of Ca2+ and Cl− from calcium chloride solutions requiring only the allocation of constants for these ions. The substitution of aluminium for iron did not affect the charge properties of goethite.

Calcium Effects on Quantity-Intensity Relationships and Plant Availability of Ammonium

Competition among cations for exchange sites in soil may influence NH 4 concentrations in soil solution and consequently affect NH 4 availability to plants. Laboratory, greenhouse, and field experiments were conducted to determine whether Ca additions enhance NH 4 availability, thereby increasing the potential for NH 4 absorption by plants. A Shano silt loam soil (coarse-silty, mixed, mesic Andic Mollic Camborthid) was equilibrated with NH 4 solutions ranging in concentration from 0 to 70 mg N L -1 with either distilled water or a 400 mg Ca L -1 matrix. The Ca matrix resulted in 20 mg (167%) more NH 4 -N L -1 in solution than the distilled water matrix at an exchangeable NH 4 concentration of 100 mg N kg -1 soil. Calcium also reduced the potential maximum quantity of NH 4 adsorbed by the soil as well as the soil buffer capacity for NH 4 . Combining Ca with NH 4 fertilizers (1 :2 Ca/N molar ratio) increased NH 4 concentrations in the soil solution in subsequent greenhouse and field experiments. The influence of Ca on soil solution NH 4 concentrations is attributed to preferential adsorption of Ca and the displacement of NH 4 from soil exchange sites. Mechanistic model predictions of NH 4 uptake were 22% higher for rice (Oryza sativa L.) and 7.5% higher for corn (Zea mays L.) when Ca was combined with NH 4 , as a result of an increase in soil solution NH 4 concentrations and the effective diffusion coefficient for NH 4 . These experimental results and model simulations indicate that in NH 4 -fertilized soils, the application of Ca will increase NH 4 availability and absorption by plants.

Competitive adsorption of potassium and calcium on manganese oxide as studied with two ion‐selective electrodes

AbstractCompetitive adsorption of K and Ca in a Mn oxide (birnessite) system was investigated using a double indicator electrode technique. The data showed that Ca was more competitive than K for adsorption sites on the Mn oxide. The preference of the oxide for Ca over K was partially attributed to the specific adsorption of Ca on the oxide. The extent of the preference did not consistently increase as the amount of K and Ca was continuously added to the system, possibly due to a decrease in the availability of the specific adsorption sites as these sites were increasingly occupied by the added Ca. The pK‐0.5pCa value for the system significantly increased when the solution to solid ratio was above 16 (mL g−1). Raising pH caused an increase in the solution K/Ca equivalent activity ratio and a linear decrease in the pK‐0.5pCa value, though the influence of the pH on the K/Ca equivalent activity ratio was not substantial until the pH was above 5.23. These results demonstrate an unique interaction of K and Ca on the surface of the Mn oxide, which cannot be solely interpreted by the classical ratio law. This study is of significance in the chemistry of K in soils which have properties of variable charge and specific adsorption of the competitive cations.

Competitive adsorption of Ca 2+ and local anesthetics on lipid membrane surfaces

Adsorption fo tertriary amine local anesthetics and Ca 2+ onto lipid membranes having various negative surface charge densities was studied by measuring lipid vesicle electrophoretic mobility. As the surface charge density of the membrane was reduced, the adsorption of the local anesthetics dominated that of the divalent cation. For a relatively high negatively charged membrane, the adsorption of both local anesthetic and Ca 2+ became comparable and competitive. It is deduced that the major factor for the adsorption of local anesthetic onto lipid membranes is due to simple physical partitioning between aqueous and membrane phases, and not due to ionic type of binding as seen for divalent cations with membranes. However, the adsorption of anesthetics is influenced by the surface potential of membranes which is in turn related to the surface concentration of local anesthetics near the membrane. The amounts of competitive adsorption of divalent cations and local anesthetics are analyzed with respect to their bulk concentrations and various surface charge densities of the membranes. With the results of the above studies, a possible interpretation for the interaction site as well as the mode of adsorption of local anesthetics onto axon membranes is made in relation to divalent cation concentrations in the bulk phases.

Cytotoxicity of synthetic barium hydroxyapatite

Barium hydroxyapatite (Ba10(PO4)6(OH)2, Ba-HAp) was synthesized by a wet method using Ba(OH)2.8H2O and (NH4)2HPO4 as starting materials. The Ba-HAp obtained had a Ba/P atomic ratio of 1.76 and contained CO3 groups. The Ba-HAp was sintered at 1073 K for 12 hours. The sintered Ba-HAp had a three point bending strength of 29 MPa and Young's modulus of 27 GPa. Cytotoxicity of the sintered bodies and particles was tested using L-cells. The sintered Ba-HAp showed no cytotoxicity, and the cells were closely in contact with the surfaces of sintered Ba-HAp. Morphological observation of the cell around the Ba-HAp particles also showed no cytotoxicity. However, cell growth was inhibited by Ca adsorption on the Ba-HAp particles. These results suggested that the Ba-HAp had no cytotoxicity and can be applied as a bioactive X-ray opaque material.

INTERACTIVE MECHANISMS OF ANION ADSORPTION WITH CALCIUM LEACHING AND EXCHANGE

Anions can be adsorbed to soil colloids by specific (inner-sphere complexation) and nonspecific adsorption (outer-sphere complexation and diffuse ion association). The mechanism for anion adsorption determines whether and by what reaction anions in soil solution will enhance metal adsorption. The objectives of this study were (i) to determine the influence of the accompanying anion on Ca leaching and cation exchange in a sandy, kaolinitic Ultisol, (ii) to describe possible mechanisms for anion effects on Ca adsorption based on diffuse double-layer theory, and (iii) to discuss the practical implications of these mechanisms on soil fertility management. The Ca forms used in this study were calcium chloride, calcium nitrate, calcium sulfate, calcium phosphate, and calcium carbonate. Undisturbed soil cores from a Plinthic Kandiudult were collected, and a Ca solution (5, 500, and 1500 mg L−1) was applied to soil columns. The effluent was collected from each column, and cations (Ca2+, Mg2+, K+) in solution were measured. Exchangeable bases and effective cation exchange capacity (CEC) were measured in soil from soil cores after effluent collection. Sulfate and phosphate retarded Ca break-through, compared with chloride and nitrate. Specific adsorption of sulfate and phosphate in the inner-sphere increased the net negative surface charge of the soil and, thus, increased Ca adsorption. Higher influent concentration increased the extent of ion pairing (CaSO4°), and consequently, enhanced Ca adsorption directly. Calcium breakthrough was most gradual in the carbonate effluent as a result of ion pairing and increased pH dependent charge leading to both direct and indirect enhancement of Ca adsorption. The Mg peaks for phosphate and carbonate were broader and shorter than those peaks for nitrate, chloride, and sulfate. The choice of the accompanying anion in Ca fertilizer materials could be critical in controlling the leaching of Ca from the soil surface and optimizing the exchange and subsequent leaching of other cations.

Effect of H-Bonding Compounds on the Adsorption of Ca2+ to Lipid Membranes in the Gel State

The binding of Ca2+ to dipalmitoylphosphatidylcholine (DPPC) bilayers in the gel state is studied by determination of the spectroscopical features of merocyanine 540. The procedure is based on the measurement of the effect produced by the extent of the binding on the membrane surface. In the range between 0 and 5 mM Ca2+ a peak at 450 nm is obtained which disappears with the inclusion of cholesterol in a 1:1 ratio. The maximum of absorbance at 450 nm is obtained at around 2.5 mM, which is coincident with the critical concentration of Ca2+ promoting a drastic transition in the interbilayer distance as reported in the literature. Glycerol, sucrose, glucose, and ribose decrease the intensity of the peak corresponding to the number of OH groups present in each compound. For 0.5 mM Ca2+ , the 450 nm absorbance intensity shows a maximum at 30°C, which is absent in the presence of cholesterol. Based on the results in a previous paper (Bernik, D. L., and Disalvo, E. A., Biochim. Biophys. Acta 1146, 169, 1993) which demonstrated that the absorbance at 450 nm indicates the presence of Pβ phase in dimyristoylphatidylcholine (DMPC) bilayers, the magnitude of the 450 nm peak as a function of Ca2+ was taken as an indication that Ca2+ adsorption induces similar surface properties on DPPC membranes in the gel state. This action appears to be counteracted by hydrogen bonding compounds included in the membrane or in the aqueous phase.

低精米歩合の白米の溶解に及ぼすカルシウムイオンの役割

Effects of calcium ion, which is contained richly in well water used in brewing sake, was studied on the dissolution of rice grains. The adsorption of Ca2+on low-refined rice (Yamadanishiki, 35%) in washing and steeping processes was examined by using model solutions containing Ca2+alone (0-50 mg/l) or in combination with Mg2+and Na+. Results indicated that Ca2+was well adsorbed on rice during washing and steeping. Ca2+-adsorbed steamed rice was solubilized by a-amylase more rapidly than the rice adsorbed other ions, and the solution of Ca2+-adsorbed steamed rice was further acc elerated by Ca2+in brewing water.

Electrosurface properties of lysophosphatidylcholine foam films: Effect of pH and Ca 2+

The electrosurface properties of microscopic foam films stabilized with the zwitterionic soluble phospholipid palmitoyl-glycero-3-phosphocholine (lyso PC) are studied by measuring the effect of pH on the equilibrium film thickness at a constant ionic strength (NaCl + HCl) of the solutions for different lyso PC concentrations. It is found that a decrease in the pH has a strong effect on the thickness of the films. At low lyso PC concentration, an isolectric point (pH*) corresponding to pH 3.6 is found at which all the films rupture. In the case of higher lyso PC concentration, a critical pH (pH cr) value of 4.1 is found at which a silver-to-Newtonian black film transition occurs. In both cases, metastable zones are outlined where in the first case occasional rupture of the films is observed, while in the second case, Newtonian black films are formed. Using the equations of the Derjaguin—Landau—Verwey—Overbeek (DLVO) theory, the diffuse electric layer potential and surface charge density are computed. The results obtained corroborate the assumption that the surface charge and potential at the air/film interfaces originate from the preferential adsorption of OH − ions. Experimental data obtained with CaCl 2 added to the lyso PC solutions are in accordance with this mechanism of the formation of electrostatic interactions in the case of films stabilized with unchanged substances. It is shown that the specific adsorption of the positive Ca 2+ ions leads, at low CaCl 2 concentrations, to a decrease in the initially negative surface charge and the diffuse electric layer potential, and to a recharging of the film interfaces at higher CaCl 2 concentrations.

Adsorption of Ca 2+ and La 3+ to bilayer membranes: Measurement of the adsorption enthalpy and binding constant with titration calorimetry

The adsorption of Ca 2+ and La 3+ ions to the surface of lipid bilayer membranes was studied with high sensitivity titration calorimetry. Ca 2+ adsorbs to mixed phosphatidylcholine/phosphatidylglycerol membranes with a reaction enthalpy of Δ H ≈ 0.1–0.2 kcal/mol. La 3+ binds to sonified phosphatidylcholine vesicles with a reaction enthalpy of Δ H ≈ + 1.8 kcal/mol. Adsorption of La 3+ to phosphatidylcholine bilayers imparts a net positive charge to the membrane surface which makes the binding of further La 3+ increasingly more difficult. From the decreasing amplitudes in the calorimetric titration experiment a La 3+ adorption constant of K ≈ (4.1 ± 1.1)· 10 3 M −1 was evaluated. Electrostatic effects were corrected for by means of the Gouy-Chapman theory. The adsorption constant of Ca 2+ was determined previously as K ≈ 10–20 M −1 using the same binding model. Since the reaction enthalpies of Ca 2+ and La 3+ adsorption are endothermie, the adsorption of both metal ions to the membrane surface is driven by a distinct change in entropy.

Positively charged submicron emulsions — a new type of colloidal drug carrier

A positively charged submicron emulsion was prepared and stabilized by forming a mixed film comprising phospholipids, poloxamer and stearylamine at the o/w interface of the oil droplets. This was confirmed concomitantly by the selective adsorption of the thiocyanate anion and the lack of adsorption of Ca 2+ onto the emulsified oil droplets of the actual emulsions. The incorporation of various drugs in the new type of submicron emulsions altered neither the nature of the surface charge nor the mean droplet size. These results clearly demonstrated that the positively charged submicron emulsion can be used as a new type of colloidal drug carrier.

Ca chemisorption on Cu(100) in the submonolayer regime: Metastable and stable adsorption phases.

Ca adsorption on Cu(100) has been studied by high-resolution electron energy-loss spectroscopy, work-function measurements, and low-energy electron diffraction as a function of coverage, up to one monolayer, at 110 and 300 K. At each coverage a condensed phase is formed at 110 K that irreversibly transforms into a dispersed phase upon heating. The 300-K data suggest that Ca occupies substitutional sites at this temperature. A comparison with the chemisorption of alkali metals is presented.