Calcium Speciation Research Articles

Thermochemical Transformation of Calcium during Biomass Burning and the Effects on Postfire Aqueous Dissolution of Macronutrients.

Calcium is commonly the most abundant element in fire residues and its speciation largely determines the geochemical properties of fire residues and their effects on postfire soil chemistry. To explore the effects of biomass composition and fire conditions on ash Ca speciation, this study characterizes the speciation of Ca in charcoal and ash samples that were derived from different plant compartments and thermal conditions, using Ca K-edge X-ray absorption near edge spectroscopy. Results showed that biomass contains abundant organic Ca complexes, which were mineralized into fairchildite and calcite after heating at 450 to 600 °C and then CaO, as temperature increased to 750 °C. Apatite could be an abundant Ca species in fire residues if the Ca/P molar ratio of the biomass is small (<2). The mineralization of organic Ca to the identified Ca minerals during burning was negligibly affected by the oxygen level. Calcium speciation in prescribed fire residues resembled that of biomass ash burned at 550 °C with similar Ca/P molar ratios. Batch experiments showed that macronutrients (Ca, Mg, K, and P) were differentially released, as a result of different solubility of minerals in ashes and reprecipitation of minerals. The aqueous solubility of Ca, Mg, and P decreased as pH increased from 5 to 9, while K showed no pH dependency and was almost completely soluble. Results from this study improve our understanding of the chemistry of fire residues and their geochemical behaviors, which can help evaluate the impact of fire on postfire soil properties and macronutrient cycling.

Identifying true collectorless flotation of chalcopyrite in calcium chloride solutions

Given the increasing use of seawater containing primary ions (e.g. Na+ and Cl-) and secondary ions (e.g. Ca2+ and Mg2+) in flotation plants to counteract the global freshwater scarcity, more and more studies have been performed to determine the impacts of seawater on mineral flotation behaviors. From a new and practical niche, this study aimed to determine the natural floatability of chalcopyrite in the presence of Ca2+ as a secondary ion by considering the alterations of surface hydrophobicity from both the aspects of surface oxidation and metal ion precipitation. Through true collectorless flotation, Cyclic Voltammetry analyses, calcium speciation and Cryogenic X-ray Photoelectron Spectroscopy measurements, this study identified that the true flotation recovery of chalcopyrite was much lower in CaCl2 solutions than in NaCl solutions even with the same ionic strengths. While the presence of NaCl enhanced chalcopyrite surface oxidation to produce hydrophobic and hydrophilic products, the presence of CaCl2 inhibited chalcopyrite surface oxidation through the formation of CaCO3 precipitate which not only served as a passive layer but also being hydrophilic itself. As a result, the surface hydrophobicity on chalcopyrite was determined by the amount of oxidation products in NaCl solutions, but it was determined by the amount of CaCO3 precipitate in CaCl2 solutions. Despite the larger amount of CaCO3 precipitate in CaCl2 solutions with ionic strengths of 0.7 and 1.2 M, compared to deionized water and CaCl2 solution with an ionic strength of 0.1, the true flotation recovery of chalcopyrite was higher. This was attributed to much smaller air bubbles generated in CaCl2 solutions with high ionic strengths of 0.7 and 1.2 M, promoting bubble-particle collision and hence the true flotation of chalcopyrite.

Calcium speciation and coordination environment in intracellular amorphous calcium carbonate (ACC) formed by cyanobacteria

An increasing number of bacteria has been shown to form intracellular amorphous calcium carbonates (ACC), as previously observed for biomineralizing eukaryotes. Yet, the short-range order of these bacterial intracellular ACC remains largely unexplored. Because ACC in bacteria is easily lost upon sample preparation, the use of in situ techniques is required to probe ACC within intact cells at the atomic scale. Here, we show the application of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies to study the calcium (Ca) speciation and coordination environment in intact cells of diverse ACC-forming cyanobacteria. Our results show that XANES was effective in detecting ACC in intact cells. Between 60 and 85% of the total Ca in ACC-forming cyanobacteria strains is contained in the ACC phase and the rest in a single phase or a mixture of amorphous phases, possibly Ca complexed by organic molecules. Moreover, the short-range order of cyanobacterial ACC was different from eukaryotic ACC. This short-range order was similar to monohydrocalcite. These findings shed light on the fate of cyanobacterial ACC and the role of ACC-forming cyanobacteria in the Ca biogeochemical cycle.

Changes in solubility and chemical speciation of potassium, calcium, and silicon in wood ash by acid treatment

ABSTRACT This study investigated the relationship between the solubility and the chemical speciation of plant nutritional elements (K, Ca, and Si) in wood ash treated with a dilute solution of sulfuric acid or phosphoric acid. The diluted acid solutions were added to fly ash (FA) collected from a power plant, and the untreated and acid-treated FAs were analyzed for their contents of K, Ca, and Si. The crystalline phases of the untreated and acid-treated FAs were identified using X-ray diffraction (XRD), and the Si species were determined using X-ray absorption near-edge structure (XANES) spectroscopy. Sulfuric acid and phosphoric acid treatments did not affect the solubility of K in FA. Water soluble and citrate soluble K in FA were decreased by the addition of these acids. The water soluble Ca content in FA treated with phosphoric acid did not show a significant change compared to that in the untreated FA, but it increased up to 65 times in FA treated with sulfuric acid. It is suggested that CaCO3, which was the main Ca species in the untreated FA, was transformed into syngenite and gypsum by the sulfuric acid treatment. Hydrochloric acid soluble Si in the acid-treated FA decreased compared to that in the untreated FA. In contrast, the NaOH-soluble Si fraction, a method for selectively extracting silica gel, increased especially in the phosphoric acid-treated FA. The results of XANES analysis showed that the FA samples treated with 10–30% sulfuric acid or 10–30% phosphoric acid had a lower degree of Si polymerization than the untreated FA, which corresponded generally to the increased Si solubility in FAs. However, FA treated with 30% sulfuric acid had lower soluble Si despite a lower degree of polymerization than FA, which was attributed to a lower pH. These results suggest that the change in solubility of Si in the acid-treated FA was influenced by the degree of polymerization and the decrease in pH. From the viewpoint of a fertilizer use of FA, sulfuric acid-treated ash, which consists of both water soluble Ca and citrate soluble Ca, is suitable for Ca fertilizer, and phosphoric acid-treated ash, which has high NaOH-soluble Si, is suitable for Si fertilizer

Increasing calcium phosphate aqueous solubility and spontaneous supersaturation combining citrate and gluconate with perspectives for functional foods

Uptake of calcium from food depends on solubility of calcium salts in the intestines, and precipitation of calcium phosphates decreases bioaccessibility of food calcium. Citrate as a high affinity complex binder for calcium was found spontaneously to create strongly supersaturated solutions by rapid dissolution of calcium hydrogen phosphate characterized by short lag phases for precipitation. Gluconate with weaker affinity for calcium binding showed longer lag phases for precipitation from supersaturated solutions. For citrate/gluconate combinations, the highest degree of supersaturation with longest lag phases for precipitation were found by trial-and-error experiments for a citrate/gluconate ratio of 1:10 for dissolution of calcium hydrogen phosphate resulting in supersaturation factors around three and without precipitation for more than a month. The aim of the present study was to provide a physicochemical explanation of this robust supersaturation. Calcium speciation based on electrochemical calcium activity measurement identified a low [Ca2+]·[HCitr2−] product as critical for supersaturation.

Portlandite solubility and Ca2+ activity in presence of gluconate and hexitols

The current paper investigates the impact of gluconate, D-sorbitol, d-mannitol and D-galactitol on calcium speciation at high pH values by i) solubility measurements of portlandite (Ca(OH)2) and ii) potentiometric titration measurements of calcium salt solutions. Thermodynamic modelling was used to fit the chemical activities of Ca2+ and OH− ions and thus to determine the strength and kind of the different Ca-organic-hydroxide complexes. The strength of complex formation with Ca2+ decreases in the order gluconate ≫ sorbitol > mannitol > galactitol, which follows the same order as sorption on portlandite. Heteropolynuclear gluconate complexes with calcium and hydroxide dominate the Ca-speciation in the presence of portlandite, while for sorbitol ternary CaSorbOH+ complexes were dominant under alkaline conditions. We expect that these results will help in better understanding the influence of gluconate and hexitols on the hydration of alite and Portland cement.

The Role of Redox on Bridgmanite Crystal Chemistry and Calcium Speciation in the Lower Mantle

AbstractThe amount of ferric iron Fe3+ in the lower mantle is largely unknown and may be influenced by the disproportionation reaction of ferrous iron Fe2+ into metallic Fe and Fe3+ triggered by the formation of bridgmanite. Recent work has shown that Fe3+ has a strong effect on the density and seismic wave speeds of bridgmanite and the incorporation of impurities such as aluminum. In order to further investigate the effects of ferric iron on mineral behavior at lower mantle conditions, we conducted laser‐heated diamond‐anvil cell (LHDAC) experiments on two sets of samples nearly identical in composition (an aluminum‐rich pyroxenite glass) except for the Fe3+ content; with one sample with more Fe3+ (“oxidized”: Fe3+/ΣFe ~ 55%) and the other with less Fe3+ (“reduced”: Fe3+/ΣFe ~ 11%). We heated the samples to lower mantle conditions, and the resulting assemblages were drastically different between the two sets of samples. For the reduced composition, we observed a multiphase assemblage dominated by bridgmanite and calcium perovskite. In contrast, the oxidized material yielded a single phase of Ca‐bearing bridgmanite. These Al‐rich pyroxenite samples show a difference in density and seismic velocities for these two redox states, where the reduced assemblage is denser than the oxidized assemblage by ~1.5% at the bottom of the lower mantle and slower (bulk sound speed) by ~2%. Thus, heterogeneities of Fe3+ content may lead to density and seismic wave speed heterogeneities in Earth's lower mantle.

Determination of thermodynamic parameters for complexation of calcium and magnesium with chondroitin sulfate isomers using isothermal titration calorimetry: Implications for calcium kidney-stone research

Chondroitin sulfate (CS) occurs in human urine. It has several potential binding sites for calcium and as such may play an inhibitory role in calcium oxalate and calcium phosphate (kidney stone disease by reducing the supersaturation (SS) and crystallization of these salts. Urinary magnesium is also a role player in determining speciation in stone forming processes. This study was undertaken to determine the thermodynamic parameters for binding of the disaccharide unit of two different CS isomers with calcium and magnesium. These included the binding constant K. Experiments were performed using an isothermal titration calorimeter (ITC) at 3 different pH levels in the physiological range in human urine. Data showed that interactions between the CS isomers and calcium and magnesium occur via one binding site, thought to be sulfate, and that log K values are 1.17–1.93 and 1.77–1.80 for these two metals respectively. Binding was significantly stronger in Mg-CS than in Ca-CS complexes and was found to be dependent on pH in the latter but not in the former. Furthermore, binding in Ca-CS complexes was dependent on the location of the sulfate binding site. This was not the case in the Mg-CS complexes. Interactions were shown to be entropy driven and enthalpy unfavourable. These findings can be used in computational modeling studies to predict the effects of the calcium and magnesium CS complexes on the speciation of calcium and the SS of calcium salts in real urine samples.

Depth-dependent Calcium Speciation in Individual Aerosol Particles by Combination of Fluorescence Yield and Conversion Electron Yield XAFS Using X-ray Microbeam

Particle surfaces of mineral aerosols are important reaction sites in the atmosphere. Previous studies suggested that calcite in aerosols, which are emitted during dust periods, neutralizes sulfuri...

Recovery of copper from PVC multiwire cable waste by steam gasification

Screened multiwire, PVC insulated tinned copper cable was gasified with steam at high temperature (HTSG) under atmospheric pressure for recovery of cooper. Gases from the process were additionally equilibrated at 850°C on the bed of calcined clay granules and more than 98% of C+H content in the cable was transformed to non-condensing species. Granules prepared from local clay were generally resistant for chlorination, there was also almost no deposition of metals, Cu and Sn, on the catalytic bed. It was found that 28% of chlorine reacted to form CaCl2, 71% was retained in aqueous condensate and only 0.6% was absorbed in alkaline scrubber. More than 99% of calcium existed in the process solid residue as a mixture of calcium chloride and calcium oxide/hydroxide. PVC and other hydrocarbon constituents were completely removed from the cable sample. Copper was preserved in original form and volatilization of copper species appeared insignificant. Tin was alloying with copper and its volatilization was less than 1%. Fractionation and speciation of metals, chlorine and calcium were discussed on the basis of equilibrium model calculated with HSC Chemistry software. High temperature steam gasification prevents direct use of the air and steam/water is in the process simultaneously gaseous carrier and reagent, which may be recycled together with hydrocarbon condensates.

Soil Calcium Speciation at Different Geomorphological Positions in the Yaji Karst Experimental Site in Guilin, China

Abstract: Limestone soil in karst areas is the product of weathering and carbonate rock dissolution in tropical and subtropical regions. Limestone soil in karst regions has a Ca-rich alkaline geochemical environment, but the characteristics of calcium speciation in limestone soil remain unclear. Here, to study changes in calcium speciation in soil across seasons in a karst area, different geomorphological positions and soil samples were collected from the Yaji karst experimental site. Using European Community Bureau of Reference (BCR) methods, we analyzed and discuss soil calcium speciation and seasonal migration characteristics. We found that total soil Ca content was 2.80–11.75 g kg-1, with an average of 5.25±0.68 g kg-1 (mean±SE). The order of content of each Ca speciation is extractable-Ca>reducible-Ca>residual-Ca>oxidizable-Ca. In addition to oxidizable-Ca, other Ca species had a positive correlation with total calcium. The calcium content and various forms of calcium content decreased gradually with...

Alteration of fossil-bearing shale (Autun, France; Permian), part II: Monitoring artificial and natural ageing by combined use of S and Ca K-edge XANES analysis, Rock-Eval pyrolysis and FTIR analysis

Fossil-bearing shale specimens that include sulfides in their compositions are chemically reactive and sometimes also mechanically fragile. This decay is often related to iron sulfate efflorescence resulting from the oxidation of sulfide compounds. The processes underlying these degradations are poorly known, thus impeding the elaboration of curative or preventive treatments. The present contribution aims to identify the origin of museum specimen alterations. It focuses on the Flouest collection housed at the Muséum National d’Histoire Naturelle (MNHN, Paris, France) and originating from the Autun Basin (Saône-et-Loire, France, Permian). To evaluate the alteration of MNHN specimens, it appeared necessary to compare their composition with that of unaltered shale so as to identify chemical changes occurring during ageing. Therefore, new material was collected in the Autun Basin, among others on the locality of Muse that corresponds to the same lithostratigraphic unit as that of the MNHN specimens. This material was, if necessary, artificially aged. The first part of this work, presented elsewhere, deals with the use of X-ray diffraction and Mössbauer spectroscopy for characterizing iron reactivity and speciation. It leads to the conclusion that the reactivity of iron in the shale matrix was limited and could not account for the large efflorescence of iron (II) sulfate occurring nearby the fossil. The second part presented here focuses on the use of S K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy for characterizing sulfur speciation and reactivity. Measurements were performed on the shale matrix and on thin layers of maceral found in the proximity of damaged areas. As sulfur may be found in association with calcium or organic matter, complementary techniques were implemented, such as FTIR spectroscopy, Rock-Eval pyrolysis (characterization of organic matter content) and Ca K-edge XANES (analysis of calcium speciation) spectroscopy. It was shown that sulfur is mainly related to thioether, sulfoxide, iron sulfide, and sulfates whereas calcium is mainly bound to carboxylate, carbonate and/or sulfate groups. FTIR analysis of the macerals confirmed the presence of vitrinite on damaged MNHN specimens. The low oxygen content of new shale samples determined by Rock-Eval pyrolysis indicates that the organic matter is well preserved, despite the fact that samples come from outcrop surface. In the newly collected material, sulfur is mainly related to organic sulfides (thioether) with a minor occurrence of iron sulfide. In the shale fraction of damaged MNHN specimens, sulfur is mostly oxidized into a mixture of iron and calcium sulfate. However, in the vitrinite layers of the same specimens, a large proportion of sulfur corresponds to organic sulfides. Also the oxidation of sulfur does not occur homogeneously but preferentially in the shale fraction, probably because this latter is porous whereas vitrinite is not. Artificial ageing of new shale material showed that the oxidation of organic sulfides could be reproduced at 90 °C, 80% of relative humidity. However, the obtained efflorescence almost exclusively corresponds to calcium sulfate whereas iron (II) sulfates are mostly observed on MNHN specimens. The new material collected on site is probably to be questioned, and future studies will have to select new samples with fossil remains. This will be the object of the third part of this work.

The effect of pH on calcium and phosphorus distribution between micellar and serum phase after enrichment of skim milk with calcium d-lactobionate

Speciation is important for good bioavailability of calcium from food and beverages. Effective calcium enrichment of dairy products for prevention of osteoporosis accordingly depends on the calcium compound added. Calcium d-lactobionate (50, 30 or 3 mM) was added to reconstituted skim milk with pH adjusted to 6.6, 6.0, 5.7 and 5.4 at 25 °C in order to investigate the distribution of calcium and phosphorus between micellar and serum phase and the speciation of calcium in the serum phase for enriched milk. The calcium from added calcium d-lactobionate was found to distribute between the micellar and serum phase at pH 6.6–6.0, while at pH 5.7–5.4, the added calcium remained in the serum phase. The concentration of phosphorus after enrichment was increased in the micellar phase and decreased in the serum phase at pH 6.6–6.0, while at pH 5.7–5.4, phosphorus remained constant in both phases. A decrease of total protein content and caseins in milk serum phase was observed after milk enrichment at all pH values as documented by protein determination and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Moreover, concentration of α-lactalbumin remained unchanged, while concentration of β-lactoglobulin decreased after addition of calcium d-lactobionate at all investigated pH values. Bioavailability of calcium from enriched dairy products depends on transfer to the micelles, and pH control is accordingly concluded to be critical for the use of calcium d-lactobionate for enrichment. The pH value needs to be taken into account when new calcium-enriched dairy products are designed to fulfil the primary role of having a product with high nutritional value and good calcium bioavailability.

Calcium nutrition. Bioavailability and fortification

Both transcellular, vitamin-D dependent and the quantitatively more important paracellular, nonsaturable calcium absorption depend on the presence of free calcium ions or on the presence of reactive precursors for free calcium ions in the intestines. Complex binding of calcium preventing precipitation of calcium salts under the neutral to slightly alkaline conditions of the intestines accordingly determines calcium absorbability. The presence of suitable ligands for calcium complexation is important already when the partly digested food is transferred from the acidic stomach to the neutral intestines. The competitive complex binding and precipitation of calcium decisive for subsequent absorption in the intestines and further for bioavailability is accordingly influenced by the calcium speciation in the stomach resulting from food digestion. Calcium salt solubilities relative to calcium binding strength in complexes are the thermodynamic factors controlling the level of free calcium ions matching the free calcium level allowing facile absorption. However, kinetic factors like rates of precipitation and rates of complex formation and of complex dissociation seem of equal importance, especially when the same anion precipitates calcium and forms complexes with calcium as for hydroxycarboxylates like lactate. Spontaneous supersaturation resulting from isothermal dissolution overshooting during digestion or from gradually increasing pH is suggested to increase calcium mobility and bioavailability and needs to be incorporated in current theories for calcium bioavailability. Understanding the dynamic aspects of calcium speciation throughout digestion, and not only in the intestines, will also allow for better calcium fortification strategies to ensure optimized calcium bioavailability.

Modeling precipitate-dominant clogging for landfill leachate with NICA-Donnan theory

Bioclogging of leachate drains is ubiquitous in landfills for municipal solid wastes. Formation of calcium precipitates and biofilms in pore space is the principal reason for clogging. But the calcium speciation in leachte rich in dissolved organic matters (DOM) remains to be uncovered. In spite of its complexity, NICA-Donnan model has been used to compute the speciation of metals and the binding capacities of humic substances. This study applies NICA-Donnan theory into the simulation of calcium speciation during the formation of precipitate-dominant clogging in leachate drainage aggregates for the first time. The consideration of DOC–Ca complexation gives reasonable explanation to the speciation of calcium, which is viewed as oversaturated, in leachate with concentrated DOM. The modeling results for calcium speciation are in good agreement with a large collection of experimental observations, suggesting that NICA-Donnan theory could be used in the modelings of reactive transport and clogging of landfill leachate collection systems.

Calcium content and speciation in alkaline-cooked corn studied by synchrotron Ca K-edge X-ray absorption spectroscopy

Using Ca K-edge X-ray absorption spectroscopy the calcium uptake and species in alkaline corn have been studied and compared with corn starch and crystalline cellulose as main compounds in corn endosperm and pericarp. XANES data showed that calcium binds preferably to hemicelluloses in the pericarp. The main calcium species found by EXAFS in cellulose and starch is calcium carbonate although some protein-calcium species could be formed within the starch granules. From X-ray diffraction and microscopy techniques, pericarp dissolution, mainly at the amorphous hemicellulose fraction was inferred. The hemicellulose dissolution during steeping increases the apparent pericarp crystallinity, while the relative calcium content determined by XANES absorption edges reduces. XANES and EXAFS spectroscopies showed to be a valuable tool in order to determine the calcium species and to make a semiquantitative determination of calcium in the different nixtamalized corn fractions.

Pilot-scale ex situ electrokinetic restoration of saline greenhouse soil

This study was conducted to investigate the feasibility of using an ex situ electrokinetic system for the restoration of saline greenhouse soil. An experiment was conducted in a pilot-scale reactor (1 × 1 × 0.25 m) during 14 days. Chloride accumulated in the top layer through pore-water evaporation. There was no removal of the nitrate in the top layer because of the continuous upward transport of nitrate accompanying the evaporation of soil moisture. However, the removal of sulfate was uniform across all the layers. In the case of cations, a significant amount (64.4%) of potassium was removed, and there was no removal of the magnesium in the top layer. The electrokinetic treatment changed the speciation of calcium in the soil, resulting in an increase in the amount of exchangeable calcium. Furthermore, nitrate was transported toward the anode and reduced to nitrogen gas as the result of an electrochemical reaction at the surface of the anode. Sulfate formed a very stable and insoluble precipitate with calcium—calcium sulfate (gypsum). Ex situ electrokinetic system removed salts from nitrate and sulfate-accumulated saline soil. However, the overall removal efficiency was not high enough because of relatively short operation time.

A theoretical and experimental analysis of calcium speciation and precipitation in dairy ultrafiltration permeate

Dairy ultrafiltration permeate is a fluid that is often discarded from the manufacturing process. However, downstream processes such as nanofiltration can be used to separate the valuable lactose and calcium from the monovalent salts, facilitating recovery of a valuable by-product. Previous work has shown that the nanofiltration membrane can suffer from significant fouling due to calcium precipitation. In this work, a model describing the speciation and fouling potential of calcium salts in skim milk ultrafiltration permeate is presented. Results are compared with experimentally determined calcium concentrations in both laboratory prepared solutions and permeate obtained directly from a dairy factory. The model is able to fit the experimental data well for solutions below the saturation concentration for calcium salts, but is less able to fit systems where precipitation of such salts is possible as the extent of the metastable salt region is not readily predicted.

Identification and analysis of calcium speciation in red blood cells by ultrasonic-dialysis capillary electrophoresis inductively coupled plasma optical emission spectrometry

Ultrasonic dialysis and capillary electrophoresis inductively coupled plasma optical emission spectrometry were developed for the identification and determination of calcium species in human red blood cells (HRBC). Separation was optimized at 20 kV with a 40 mmol L−1 pH 7.40 Tris-HCl buffer. The free Ca2+ concentration in HRBC cytoplasm was found to be 112 nmol L−1. The recovery for Ca2+ spiked cytoplasm samples ranged from 96.0–103%. Eight primary calcium-containing species (concentrations 1.21–21.8 mg L−1) were found in HRBC. One species had a molecular weight 100 kDa. The calcium species with the greatest mobility was not free Ca2+, but a calcium-erythrocyte membrane species of 50–100 kDa. This method is sensitive and more selective than other common methods for free Ca2+ determination in HRBC.

Determination of Calcium with Ion‐Selective Electrode in Black Liquor from a Kraft Pulping Process

AbstractA solid‐contact Ca2+‐selective electrode was used to study speciation of calcium in black liquor. During extended time of operation the standard potential was found to change, but the sensitivity of the electrode, and the selectivity coefficients remained unchanged. Laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) measurements showed sodium sorption to the electrode membrane during the measurement in black liquor. Linearization of the complexometric titration curves were used to determine the concentrations of Ca2+, CaOH+ as well as weak and strong calcium complexes. With inductively coupled plasma optical emission spectrometry (ICP‐OES), similar total calcium concentrations were obtained as by the EDTA titration.