Formation Of Dicalcium Phosphate Dihydrate Research Articles

Effects of pullulan on the biomechanical and anti-collapse properties of dicalcium phosphate dihydrate bone cement.

In this work, a modified dicalcium phosphate dihydrate (DCPD) bone cement with unique biodegradable ability in a calcium phosphate cement system was prepared by the hydration reaction of monocalcium phosphate monohydrate and calcium oxide and integration with pullulan (Pul), a non-toxic, biocompatible, viscous, and water-soluble polysaccharide that has been successfully used to improve defects in DCPD bone cement, especially its rapid solidification, fragile mechanical properties, and easy collapse. The effect of different contents of Pul on the structure and properties of DCPD were also studied in detail. The modified cement was characterised by X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, ultraviolet-visible absorption, X-ray photoelectron spectroscopy analysis, and rheological property measurements. The results indicated that Pul promoted the hydration formation of DCPD, and interface bonding occurred between Pul and DCPD. With increasing content of Pul, the setting time of the DCPD bone cement increased from 2.6 min to 42.3 min, the compressive strength increased from 0 MPa to 20.4 MPa, and the anti-collapse ability also improved owing to the strong interface bonding, implying that the DCPD bone cement improved by Pul has better potential for application in the field of non-loading bone regenerative medicine compared to unmodified DCPD bone cement.

Influence of adding Pluronic F127 in the electrolyte on the morphology, structure and biofilm adhesion of calcium phosphate coatings

In this work, we have investigated the addition of Pluronic F127 (PF127) into the electrolyte solution on the structure, morphology and antibacterial growth of calcium phosphate (CaP) coatings synthesized by an electrochemical method on Ti substrate. Different parameters of the deposition process were investigated, which were temperature, time and PF127 wt%. The coatings were characterized by X-ray diffraction (XRD), gravimetric analysis, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In vitro antimicrobial assays against S. aureus were performed. XRD analysis confirmed the formation of dicalcium phosphate dihydrate (DCPD) or anhydrous dicalcium phosphate (DCPA) and octacalcium phosphate (OCP). The structure and morphology depended mainly on PF127 wt% and deposition temperature in lower deposition time. Interestingly, structure and morphology were not influenced by adding PF127 at higher deposition time using higher PF127 concentrations. The morphology by SEM changed from plate-like to fiber by varying the PF127 concentration as well as blocks at higher temperatures. Moreover, all coatings were crystalline, uniform and crack-free. Antibacterial assays showed that biofilm formation was prevented when a coating of PF127 is deposited on the surface. Bacterial growth depended on the phase and morphology of washed coatings. Thus, the structure, morphology and consequently bacterial growth resistance can be tailored by the use of PF127 in the electrochemical processing at shorter deposition times.

Formation and transformation of calcium phosphate phases under biologically relevant conditions: Experiments and modelling

The experimental data on calcium phosphates formation were collected in dilute solution at constant pH (7.40) and temperature (37.0 °C) at different levels of ionic strength (IS). The evolution of the solid phase formation is described in detail using a thermodynamic-kinetic model. The thermodynamic model takes into account all relevant chemical species as well as Posner’s clusters; the kinetic model, based on the discretized population balance approach, accounts for the solid formation from solution. The experimental data are consistent with an initial formation of dicalcium phosphate dihydrate (DCPD, brushite), which dominates the nucleation rate, and its rapid transformation into octacalcium phosphate (OCP) or hydroxyapatite (HA), which dominates the growth rate. Depending on the experimental conditions and, including the influence of the IS level, OCP may be further transformed into apatite. The classical nucleation theory is able to describe the experimental results very well and the solid phase growth is limited by the diffusion of Ca2+ ions. The precipitation pathway described by a complete thermodynamic-kinetic model is expected to contribute to the understating of the in vivo osteogenesis. Statement of SignificanceThe formation mechanism of calcium phosphates under biomimetic conditions is unraveled. The formation pathway is mathematically described based on a thermodynamic-kinetic model in which (i) the nucleation stages (primary and secondary) are dominated by the formation of dicalcium phosphate dihydrate (DCPD) and (ii) the fast growth stage is limited by the diffusion of Ca2+ ions under the driving force of octacalcium phosphate (OCP), or hydroxyapatite (HA), solubility. The obtained solid phase seems correlated to the activity coefficient of phosphate ions, thus to the ionic strength and local phosphate speciation. The model, being able to highlight the details of the precipitation pathway, is expected to contribute to the understanding of the apatitic phase formation in the biomineralization-biodemineralization processes under in-vivo conditions.

Tailoring the structure of biphasic calcium phosphate via synthesis procedure

Nano calcium phosphate ceramics (CaPC) were synthesized using simple co-precipitation method at different preparation conditions. The selected Ca/P ratio with a variation of pH value lead to formation of dicalcium phosphate dihydrate (DCPD) at pH 5 and 6 while, hydroxyapatite (HAP) nano particles were formed at pH 9 and 12 at room temperature. The crystallite size was in the range of 15–55 nm depending on the obtained crystalline phase. The study displayed variation of decomposition depending on the annealing temperature. The significant note is the different transformation trend of each phase depending on the starting pH value. The HRTEM illustrated that the DCPD phase was formed as fibers with diameter around 4–6 nm, while HAP was formed in rod shape. The aspect ratio decreased from 6.6 at pH 9 to 4 at pH 12 which refer to the great influence of pH value on the morphology of calcium phosphates.

Pulse electrodeposition of hydroxyapatite/chitosan coatings on titanium substrate for dental implant

In this work, calcium phosphate/chitosan coatings were produced by the pulse electrodeposition method with variation of the deposition cycle and contents of chitosan solution. To investigate the morphology of the crystals and the electrochemical properties of hydroxyapatite/chitosan coating, a uniform hydroxyapatite coating was applied on the Ti-6Al-4 V substrate using the pulse voltage electrodeposition method. The coating formed in the as-deposited condition was identified as dicalcium phosphate dihydrate (DCPD) (brushite), which was converted to hydroxyapatite (HA, Ca10(PO4)6(OH)2) after immersion in 1 M NaOH at 80 °C for 2 h. The XRD patterns confirmed the formation of DCPD and HA. During electrodeposition, the H2PO4 − ion was reduced and the reaction between Ca2+ ions and the reduced phosphate ions induced the formation of DCPD, which was converted to HA following treatment in NaOH. The coatings were composed of 5 to 20% chitosan by volume. Homogeneous nano-size deposits were formed under the controlled deposition period of 15 cycles and chitosan content of 15 v%. The composite coatings showed significant improvement in corrosion resistance compared to the bare Ti-6Al-4 V substrate.

Preparation of micro/nano-fibrous brushite coating on titanium via chemical conversion for biomedical applications

Calcium phosphate coatings have been applied on the surface of Ti implants to realize better osseointegration. The formation of dicalcium phosphate dihydrate (CaHPO4·2H2O), mineralogically named brushite on pure Ti substrate has been investigated via chemical conversion method. Coating composition and microstructure have been investigated by X-ray diffractometer, Fourier transform infrared spectrometer and field emission scanning electron microscope. The results reveal that the coatings are composed of high crystalline brushite with minor scholzite (CaZn2(PO4)2·2H2O). A micro/nano-scaled fibrous morphology can be produced in the acidic chemical conversion bath with pH 5.00. The surface of the fibrous brushite coating exhibits high hydrophilicity and corrosion resistance in the simulated body fluid. The osteoblast cells grow and spread actively on the coated samples and the proliferation numbers and alkaline phosphate activities of the cells improve significantly compared to the uncoated Ti. It is suggested that the micro/nano-fibrous brushite coating can be a potential approach to improve the osteoinductivity and osteoconductivity of Ti implant, due to its similarity in morphology and dimension to inorganic components of biological hard tissues, and favorable responses to the osteoblasts.

Phase transformations in a human tooth tissue at the initial stage of caries.

The aim of the paper is to study phase transformations in solid tissues of the human teeth during the development of fissure caries by Raman and fluorescence microspectroscopy. The study of the areas with fissure caries confirmed the assumption of the formation of a weak interaction between phosphate apatite enamel and organic acids (products of microorganisms). The experimental results obtained with by Raman microspectroscopy showed the formation of dicalcium phosphate dihydrate - CaHPO4-2H2O in the area of mural demineralization of carious fissure. A comparative analysis of structural and spectroscopic data for the intact and carious enamel shows that emergence of a more soluble phase - carbonate-substituted hydroxyapatite - is typical for the initial stage of caries. It is shown that microareas of dental hard tissues in the carious fissure due to an emerging misorientation of apatite crystals have a higher fluorescence yield than the area of the intact enamel. These areas can be easily detected even prior to a deep demineralization (white spot stage) for the case of irreversibly changed organomineral complex and intensive removal of the mineral component.

Formation of dicalcium phosphate dihydrate on magnesium alloy by micro-arc oxidation coupled with hydrothermal treatment

A layer containing dicalcium phosphate dihydrate (DCPD) and β-Ca3(PO4)2 was prepared on magnesium alloy by hydrothermal treatment of micro-arc oxide (MAO) layer. The biocorrosion resistance of the oxide layers before and after hydrothermal treatment was analyzed by anodic polarization and electrochemical impedance spectroscopy (EIS) in Hank’s solution. The prepared MAO layers consisted mainly of MgO and MgAl2O4, and Ca and P inside the oxide layers existed with amorphous phase. Hydrothermal treatments not only made the amorphous Ca and P change into DCPD and β-Ca3(PO4)2 crystals, but also improved the biocorrosion resistance of magnesium alloys, especially the pitting corrosion resistance.

Mimicking the Initial Development of Calcium Urolithiasis by Screening Calcium Oxalate and Calcium Phosphate Phases in Various Urinelike Solutions, Time Points, and pH Values at 37 °C

Crystalluria may not always lead to urolithiasis. To understand how crystals of <4 μm size were retained in the beginning for the formation of a clinically symptomatic stone, we had devised a screening experiment of calcium oxalate and phosphate phases systematically in various urinelike solutions at t = 30 s and 30 min and pH = 6, 7, and 8 at 37 °C. Five key lessons were learned: (1) A simple direct association between hypercalciuria and calcium oxalate dihydrate stones in the literature might have only told part of the story until the conditions that produce calcium oxalate dihydrate in vivo are better understood. (2) Magnesium ions could minimize the formation of calcium oxalate dihydrate crystals and induce and stabilize the formation of dicalcium phosphate dihydrate or brushite. (3) A new role of citrate in inhibiting the formation of dicalcium phosphate dihydrate was discovered. (4) The fast-growing amorphous calcium phosphate due to a slight pH imbalance and variation was thought to serve as a “glu...

3D powder printed calcium phosphate implants for reconstruction of cranial and maxillofacial defects

The aim of this study was to investigate the processing and the possible use of3Dpowder printed calcium phosphate implants for the reconstruction of cranial and maxillofacial defects. The fabrication of the implants was carried out with a commercial 3D powder printing system. Diluted phosphoric acid was printed onto tricalcium phosphate powder, leading to the formation of dicalcium phosphate dihydrate (Brushite). Hydrothermal conversion of the brushite matrices led to the formation of dicalcium phosphate anhydrous (Monetite). Bony defects were generated using a human cadaver skull. The implants were computer-aided designed (CAD) using a mirror imaging procedure following computed tomography of the skull. Specific implants were manufactured by the 3D powder printing rapid prototyping technique. The processing chain from data acquisition to printing of the implants proved to be practical and uncomplicated. The individual implants showed a high degree of accuracy of fit. Mechanical and physical investigations revealed suitable characteristics. 3D powder printing of calcium phosphate cement material provides a promising new method for the manufacturing of biodegradable synthetic patient-specific craniofacial implants.

31P Solid‐State NMR study of the chemical setting process of a dual‐paste injectable brushite cements

The composition and evolution of a brushite-type calcium phosphate cement was investigated by Solid-State NMR and X-ray during the setting process. The cement is obtained by mixing beta-tricalcium phosphate [Ca(3)(PO(4))(2), beta-TCP] and monocalcium phosphate monohydrate [Ca(H(2)PO(4))(2).H(2)O, MCPM] in presence of water, with formation of dicalcium phosphate dihydrate or brushite [CaHPO(2).2H(2)O, DCPD]. Analysis of the initial beta-TCP paste has shown the presence of beta-calcium pyrophosphate [Ca(2)P(2)O(7), beta-CPy] and that of the initial MCPM a mixture of MCPM and dicalcium phosphate [CaHPO(4), DCP]. Follow-up of the chemical composition by (31)P Solid-State NMR enables to show that the chemical setting process appeared to reach an end after 20 min. The constant composition observed at the end of the process was similarly determined.

Cytocompatibility of brushite and monetite cell culture scaffolds made by three-dimensional powder printing

This study investigated the cytocompatibility of low-temperature direct 3-D printed calcium phosphate scaffolds in vitro. The fabrication of the scaffolds was performed with a commercial 3-D powder printing system. Diluted phosphoric acid was printed into tricalcium phosphate powder, leading to the formation of dicalcium phosphate dihydrate (brushite). Hydrothermal conversion of the brushite matrices led to the formation of dicalcium phosphate anhydrous (monetite). The biocompatibility was investigated using the osteoblastic cell line MC3T3-E1. Cell viability and the expression of alkaline phosphatase served as parameters. The culture medium was analyzed for pH value, concentration of free calcium and phosphate ions and osteocalcin. Both types of scaffolds showed a considerable increase of cell proliferation and viability; the monetite matrices were a little inferior compared with the brushite ones. The activity of alkaline phosphatase showed a similar pattern. Optical and electron microscopy revealed an obvious cell growth on the surface of both materials. Analysis of the culture medium showed minor alterations of pH value within the physiological range. The concentrations of free calcium and phosphate ions were obviously different among brushite and monetite cultures, due to their different solubility. The content of osteocalcin of the culture medium was reduced by the printed scaffolds due to adsorption. We conclude that the powder printed brushite and monetite matrices have a suitable biocompatibility for their use as cell culture scaffolds. Both materials enable osteoblastic cells in vitro to proliferate and differentiate due to the expression of typical osteoblastic markers.

Precipitation of Dicalcium Phosphate Dihydrate in the Presence of Organic Acids

AbstractInhibition of dicalcium phosphate dihydrate (DCPD) precipitation rate by organic acids may play an important role in controlling phosphate activity and availability in P‐fertilized soils. The precipitation rate of DCPD was measured at pH 5.7 and 25 °C in the absence and presence of organic acids common to soil solutions using a seeded crystal growth method. Forward rate constants (kf) were determined from an integrated form of the second‐order rate law expression: rate = kfS[Ca2+][HPO2‐4]γ22, where brackets represent concentrations, S is the surface area (m2 L−1), and γ2 is the divalent ion activity coefficient calculated from the Davies equation. The average kf (L2 mol−1 m−2 s−1) for DCPD precipitation without organic acids was 9.7 ± 0.5. Additions of 0.26 to 2.06 mM C as humic acid and 1.1 to 3.3 mM C as fulvic acid resulted in induction periods lasting 2.0 to 632 and 1.5 to 10 min, respectively, prior to subsequent precipitation of DCPD. This indicates that DCPD precipitation was able to overgrow adsorbed humic and fulvic acids. Rates of DCPD precipitation were determined in the presence of humic, fulvic, citric, and tannic acids at concentrations ranging from 0.26 to 9.0 mM total soluble C (CTS). The highest levels of humic, fulvic, citric, and tannic acids added were 2.05, 7.8, 8.0, and 9.0 mM CTS, which resulted in decreased kf values of 1.1, 0.1, 0.6, and 2.1 L2 mol−1 m−2 s−1, respectively. Precipitation was inhibited by adsorption of these organic acids onto DCPD surfaces blocking sites acting as nuclei for new crystal growth. Adsorption characteristics of these organic acids were related to their functional‐group content and size. The ability of DCPD to precipitate on seed crystals with adsorbed C may explain the kinetically favored formation of DCPD vs. more thermodynamically stable octacalcium phosphate (OCP) and hydroxyapatite (HAP), where precipitation rates are more strongly inhibited by soluble C.

Kinetic and thermodynamic aspects of enamel demineralization.

A model for subsurface lesion formation in dental enamel is presented that takes into account the known calcium phosphate chemistry and the transport processes occurring within the enamel and across the enamel surface-solution interface. The model, illustrated through the use of solubility diagrams (‘potential plots’), explains the presence of a relatively intact enamel layer overlying a zone of demineralization as a result of reprecipitation processes occurring within the enamel surface layer as dissolved ions, from beneath the surface, diffuse into the enamel surface and across the enamel surface-solution interface. Two conditions must be fulfilled, however, in order to form the intact layer. First that the rate at which ions diffuse across the enamel surface-solution interface does not exceed the rate of the envisioned reprecipitation processes. This rate is effectively controlled by the level of saturation of the demineralization media with respect to enamel mineral. The second requirement is that appropriate conditions are present within the enamel surface which support the reprecipitation processes. A working hypothesis is presented which states that demineralization solutions which support the formation of dicalcium phosphate dihydrate within the enamel surface will foster the formation of subsurface lesions under the appropriate kinetic conditions. The proposed model is noted to be consistent with experimental observations.

Effect of Pyrophosphate on Phosphate Sorption and Ammonia Volatilization by Calcareous Soils Treated with Ammonium Phosphates

AbstractTechniques are needed to simultaneously reduce phosphorus (P) precipitation and ammonia (NH3) volatilization from MAP and DAP (monoammonium‐ and diammonium phosphate). The purpose of this study was to investigate the inhibitory effect of sodium pyrophosphate (NaPP) on P precipitation and NH3 volatilization in a pure calcium carbonate (CaCO3) system and in two calcareous soils from Egypt.In the CaCO3 system, additions of NaPP‐P equivalent to 4 and 8% of MAP‐P and DAP‐P, respectively, reduced the amounts of P sorption and NH3 volatilization. Solution ammoniacal‐N was higher with PP than without it for both MAP and DAP. Pyrophosphate acts as a nucleation and/or crystal growth inhibitor for dicalcium phosphate dihydrate (DCPD), thereby maintaining higher solution P concentrations. Because NaPP inhibits the formation of DCPD, the pH of the solution was lowered by its addition, which, in turn, reduced NH3 volatilization. Pyrophosphate did not affect NH3 volatilization from NH4Cl in the CaCO3 system since no insoluble reaction products were formed. The solution pH values were about the same for NH4Cl with or without NaPP in the CaCO3 system. The reaction of DAP with CaCO3 did not directly affect NH3 volatilization but indirectly increased solution pH which, in turn, controlled NH3 volatilization.In the soil system the inhibitory effect of NaPP was relatively more effective in reducing P sorption and NH3 volatilization from MAP than from DAP, and its effect lasted longer in the soil with less CaCO3 content than in the other soil. The effect of NaPP increased when the rate of PP‐P was increased from 2 to 4% of MAP‐P, but no further reduction in NH3 volatilization was noted between 4 and 8% on either soil. The inhibitory effect of PP on P sorption from MAP, however, did increase with an increase in PP‐P from 4 to 8%. Sodium pyrophosphate reduced the P sorption and NH3 volatilization from DAP more efficiently if applied in solution form or applied to the dried soil surface followed by the addition of water than if NaPP was applied to the premoistened soil surface. This is probably related to the dissolved PP concentration in the soil solution. In summary the results obtained with the soil system are in agreement with the results obtained with the pure CaCO3 system.

Effect of Small Amounts of Pyrophosphate on Orthophosphate Sorption by Calcium Carbonate and Calcareous Soils1

In calcareous soils, sorption of orthophosphate (OP) by CaCO3 in soils often results in the formation of water-insoluble calcium (Ca) phosphate compounds that may reduce the available phosphorus (P) levels in soils for plant uptake. There is a need to develop techniques that will reduce P precipitation from water-soluble P fertilizers, thereby improving their fertilizer use efficiencies by crops on calcareous soils. Addition of small amounts of sodium pyrophosphate (NaPP) to monocalcium phosphate (MCP) may inhibit the formation of dicalcium phosphate dihydrate (DCPD) in calcareous soils and hence enhance the fertilizer P efficiency. The purpose of this study was to investigate: (i) the mechanism of the inhibitory effect of pyrophosphate (PP), and (ii) factors influencing the effect. The inhibitory effect of PP on OP sorption by CaCO3 was studied using chemical-reagent CaCO3 and four calcareous soils from Egypt under the condition of either continuous shaking or incubation. In the CaCO3 system, the effect depended on the amount of CaCO3, OP concentration, and reaction time. The inhibitory effect increased as the P ratio of PP/OP increased from 1:100 to 1:25. Under the condition of continuous shaking, PP did not act as a crystal growth inhibitor for DCPD in the presence of excess CaCO3, but rather reacted with CaCO3 surface which retarded the formation of DCPD nuclei on the CaCO1 surface and hence temporarily retarded the sorption of OP. Some of the materials from the incubated MCP-CaCO3-H2O system and some related soil studies were examined with the scanning electron microscope (SEM) to observe the physical changes resulting from the inhibitory effect of PP on OP sorption. The major reaction products of MCP in the CaCO3 system or in the calcareous soils with or without NaPP were identified as DCPD. However, the DCPD crystals formed in the presence of NaPP were much smaller than those formed in the absence of NaPP. This suggests that PP appears to act as a crystal growth inhibitor for DCPD when DCPD precipitates as discrete crystals from solution rather than precipitating on the surface of CaCO3. The smaller crystal size of DCPD formed in the presence of NaPP also may explain why more water-soluble P was extracted from the DCPD in the soils treated with NaPP than without, even after the inhibitory effect had disappeared.

The Growth of Calcium Phosphates on Hydroxyapatite Crystals. The Effect of Fluoride and Phosphonate

The nature of the calcium phosphate phase which precipitates on hydroxyapatite seed crystals can be controlled by varying the HAP seed concentration. At low seed concentrations, dicalcium phosphate dihydrate (DCPD) is formed while at high seed levels a more basic phase precipitates. It has been found that fluoride ion increases the percentage of basic phase that crystallizes while ethylidenediphosphonic acid encourages the formation of DCPD. This behavior is explained by the competition between the nucleation of DCPD and the crystal growth on sites already present on the HAP seeds.

The seeded growth of calcium phosphates. The effect of solid/solution ratio in controlling the nature of the growth phase

The nature of the calcium phosphate phases which grow on hydroxyapatite seed crystals when these are added to stable supersaturated solutions of calcium phosphate is shown to be markedly influenced by the supersaturation and pH. At a supersaturation close to the physiological level, the addition of relatively low concentrations of inoculating seed (24 mg HAP/liter, pH = 5.6, 37°C) results in the exclusive formation of dicalcium phosphate dihydrate. At higher seed concentrations of about 230 mg/liter this phase is absent and a more basic calcium phosphate is formed. The dependence of the growth phase on the solid/solution ratio is explained by the competition between the development of the more basic calcium phosphate on the growth sites of the seed crystals and the surface nucleation of dicalcium phosphate dihydrate. The rate of nucleation of DCPD is markedly dependent on the degree of supersaturation with respect to DCPD, decreasing rapidly as the supersaturation decreases. The marked dependence of growth phase on the concentration of seed crystals is highly significant and may explain many of the discrepancies in the literature dealing with calcium phosphate mineralization. Furthermore, the technique may be used in general crystallization processes as a method for controlling the nature of the growth phase.

The mechanism of biological mineralization

The control of supersaturation and the nucleation and growth of crystals in calcium phosphate systems are important in relation to the physiological deposition of bone and tooth. Other calcium salts such as the carbonate and oxalate hydrates are significant components of pathological mineral deposits. The use of a highly reproducible seeded growth technique has enabled kinetic studies to be made of the crystal growth of these minerals. Under conditions of relatively high supersaturation, secondary nucleation may be induced upon the surface of the seed crystals. In the case of the calcium phosphates, temperature, supersaturation, surface concentration, pH, ionic strength and presence of foreign ions are very important in determining the nature of the phase which grows upon the added seed crystals. Kinetic considerations are of overriding importance in determining the course of the reactions. It is not possible to predict the phase which forms purely on the basis of thermodynamic solubility data. Thus, in solutions appreciably supersaturated with respect to both dicalcium phosphate dihydrate (DCPD) and hydroxyapatite (HAP), the addition of low concentrations of HAP seed results in the exclusive formation of DCPD whereas this phase is absent when the seed concentration is increased. The kinetic results for calcium oxalate and phosphates are discussed in terms of the important problems relating to tooth mineralization and the origin and growth of renal calculi.

ON THE POSSIBILITY OF CHARACTERIZING CALCIUM PHOSPHATES IN CALCAREOUS SOILS BY ISOTOPIC EXCHANGE

SummaryCharacterization of calcium phosphates depended upon the nature and the amount of phosphate used to react with reagent‐grade CaCO3. Formation of octa‐calcium phosphate (OCP) was inferred from the solubility equilibria after reacting CaCO3 with KH2PO4 solutions. Isotopic exchange measurements confirmed the presence of OCP, when the amount of P retained exceeded 44 μrnoles/g CaCO3. The determined surface‐Ca to surface‐P molar ratios were close to the theoretical Ca/P ratio of 1.33 in OCP. As P retained on CaCO3 decreased the surface Ca/P ratio markedly increased because of interference from surface Ca of the CaCO3. When CaCO3 was reacted with monocalcium phosphate (MCP), solubility equilibria indicated the formation of dicalcium phosphate dihydrate (DCPD). Isotopic exchange measurements, however, showed an average Ca/P ratio of only 0.375. This value corresponds to the composition of the metastable triple point solution (MTPS) formed on MCP dissolution rather than to the Ca/P ratio in DCPD.MCP application decreased the measured surface‐Ca (exchangeable Ca) either for soil or Ca‐resin, because of blocking of the exchange sites by the MCP reaction products and, consequently, a lower rate of isotopic exchange. Surface phosphorus of the two investigated calcareous soils proved to be proportional to the lowering in pH initiated by MCP application. Characterization of MCP reaction products in calcareous soils may thus prove infeasible, in view of the unexpected reduction in surface‐Ca and the pH dependency of surface P.