Solubility Product Of Calcite Research Articles

Chemical denudation dynamic based on hydrochemical measurements in the area of Drienovecká jaskyňa Cave basin (Slovak karst, Slovakia)

Denudation is a fundamental process for the development of karst relief. It has different intensity in different geological and climatic condition and beside this many other partial factors influence the intensity and dynamic of the process during the year. In karst systems, there is often a positive correlation between water partial pressure of CO2 and denudation. The Drienovecká jaskyňa Cave spring represents typical active fluviokarstic spring and its catchment area is represented by Wetterstein and Waxenec limestones, and Drienovec conglomerates. This catchment study area is located at the contact of the conti-nental and oceanic climate, in terms of altitude at the border between the lowland and moun-tain climate. During three hydrological years we measured main karst water parameters that helped us bring first results on chemical denudation in the catchment area. The water from the spring is of calcium-magnesium-bicarbonate type. Hydrogeochemical analyses in-dicate that it is mostly water from an open hydrogeological structure. Water of the cave spring is oversaturated with respect to calcite, which is documented by the saturation index of calcite, whose values are higher than the solubility product of calcite. During the studied period, the dynamic of the chemical denudation rate is clearly observable with peaks in spring months in year 2014 and 2016. The average value of denudation rate for the entire period is 21.06 mg Ca2+. Factors that influence chemical denudation include, for example, type of carbonate rock, CO2 partial pressure, water and air temperature, chemical compo-sition of the aqueous solution, activity of soil organisms, vegetation and others. Water from melted snow can also play a significant role in changes in water chemistry and thus chemical denudation. URL: https://www.gcass.science.upjs.sk/

Solubility product constants for natural dolomite (0–200 °C) through a groundwater-based approach using the USGS produced water database

The calculation of a reliable temperature dependent dolomite solubility product constant (K_sp−dol) has been the subject of much research over the last 70 years. This study evaluates log₁₀(^aCa²⁺/^aMg²⁺) values using PHREEQC (Pitzer approach) for a screened subset (n = 11,480) of formation waters in the U.S. Geological Survey National Produced Waters Geochemical Database V2 (PWGD), an extensive inventory of 165,960 formational waters from a range of sedimentary lithologies in North America up to 6.6 km depth (Blondes and others, 2016). Through extensive ground truthing against datasets sourced from Texas Gulf Coast basin and the Mississippi Salt Dome basin we establish both the geochemical data from the PWGD and a new geothermal model of the US that is used to determine temperatures at-formation-depth to be reliable data sources. The vast majority (90%) of PWGD samples have log₁₀(^aCa²⁺/^aMg²⁺)-temperature values that are interpreted to be indicative of calcite-dolomite equilibrium and buffering by the bulk mineral solubilities. Using statistical models with different parameterisations (different Maier-Kelly formulas, mixed-effects models with various random effects and linear models) log₁₀(^aCa²⁺/^aMg²⁺) values are regressed against the estimated at-formation-depth temperatures to determine K_sp−dol between 0 and 200 °C. This process relies on the well constrained calcite solubility product constant (K_sp−cal). Local effects that modify log₁₀(^aCa²⁺/^aMg²⁺) values are evaluated through the addition of random effects to the mixed model which both improves the statistical reliability of the K_sp−dol model and enables the determination of K_sp−dol values for local dolomite phases. The nature of these local effects is open to interpretation, but we suggest the primary influence on log₁₀(^aCa²⁺/^aMg²⁺) values is the stoichiometry of the dolomite phase systematically modifying log₁₀(^aCa²⁺/^aMg²⁺) values. We discount the influence on log₁₀(^aCa²⁺/^aMg²⁺) values of dolomite order, the solution ionic strength, equilibration with anhydrite and chlorite group minerals, illitization of smectite and albitization of feldspar. For the dolomite solubility equation; the mixed-effects model (model J23) chosen as most representative yields a pK_sp−dol (log₁₀K_sp−dol); We determine pK_sp°−dol to be −17.27 ± 0.35 (25 °C, 1 atm) which is close to prior estimates, including the most recent experimental value reported by Bénézeth and others, 2018 (pKs_p°−dol = −17.19 ± 0.3) validating the groundwater regression analysis approach of this study.

Experimental determination of calcite solubility and the stability of aqueous Ca– and Na–carbonate and –bicarbonate complexes at 100–160 °C and 1–50 bar pCO2 using in situ pH measurements

The solubility of calcite was measured at 100, 120, 140 and 160 °C at 1–50 bar pCO2 in 10−3–0.1 mol·kg−1 NaCl solutions using a new experimental setup involving in situ pH measurements with high-temperature solid-contact H-selective glass and two types of reference electrodes: (i) Ag/AgCl in 3.5 M KCl, saturated AgCl placed in a Teflon extensible container with liquid junction, and (ii) solid-contact high-T Na-selective glass electrode in the cell without liquid junction. The stability constants of NaHCO3° and NaCO3− aqueous complexes formation were determined in NaCl-Na2CO3/NaHCO3 solutions in CO2-free media and under 10 bar pCO2 from 100 to 160 °C. These values allowed calculation of the pH of the calibration solution in the system NaCl-CO2-H2O used in the cell without liquid junction with Na+-selective electrode as a reference. This highly stable, low-cost electrode system can be recommended for routine pH measurements at 4 < pH < 10 in sodium-bearing solutions up to 160 °C and the critical point of CO2. The values of the stability constants of CaCO3° and CaHCO3+ aqueous complexes and calcite solubility product were generated at 100, 120, 140 and 160 °C allowing a description of the solubility of calcite in a wide range of pH and pCO2.

The Role of Solution Chemistry in Calcite Dissolution

Chemical speciation calculations using Pitzer equations are used to assess the potential errors associated with the calcite dissolution kinetics reported by Truesdale (2015). While these errors are shown to be small, agreement between the measured saturation concentration of calcium and the thermodynamic solubility product of calcite is less good. Truesdale has derived an exponential equation for the increase in calcium concentration with time and has proposed that the observed long tails of slow dissolution constitute a second, slower exponential decay. The exponential equation assumes that the back reaction has an order of one half in both calcium and carbonate, orders that do not represent a consensus in the calcite dissolution literature. An alternative exponential function, based on a back reaction that is first order in calcium and carbonate, is derived.

Calcite and gypsum solubility products in water‐saturated salt‐affected soil samples at 25°C and at least up to 14 dS m−1

Calcite and gypsum are salts of major ions characterized by poor solubility compared with other salts that may precipitate in soils. Knowledge of calcite and gypsum solubility products in water‐saturated soil samples substantially contributes to a better assessment of processes involved in soil salinity. The new SALSOLCHEMIS code for chemical equilibrium assessment was parameterized with published analytical data for aqueous synthetic calcite and gypsum‐saturated solutions. Once parameterized, SALSOLCHEMIS was applied to calculations of the ionic activity products of calcium carbonate and calcium sulphate in 133 water‐saturated soil samples from an irrigated salt‐affected agricultural area in a semi‐arid Mediterranean climate. During parameterization, sufficiently constant values for the ionic activity products of calcium carbonate and calcium sulphate were obtained only when the following were used in SALSOLCHEMIS: (i) the equations of Sposito &amp; Traina for the free ion activity coefficient calculation, (ii) the assumption of the non‐existence of the Ca (HCO 3)+ and CaCO3o ion pairs and (iii) a paradigm of total ion activity coefficients. The value of 4.62 can be assumed to be a reliable gypsum solubility product (pKs) in simple aqueous and soil solutions, while a value of 8.43 can only be assumed as a reliable calcite solubility product (pKs) in simple aqueous solutions. The saturated pastes and saturation extracts were found to be calcite over‐saturated, with the former significantly being less so (p IAP = 8.29) than the latter (p IAP = 8.22). The calcite over‐saturation of saturated pastes increased with the soil organic matter content. Nevertheless, the inhibition of calcite precipitation is caused by the soluble organic matter from a dissolved organic carbon threshold value that lies between 7 and 12 mm. The hypothesis of thermodynamic equilibrium is more adequate for the saturated pastes than for the saturation extracts.

Defluoridation of brackish northern Sahara groundwater — Activity product calculations in order to optimize pretreatment before reverse osmosis

High fluoride concentrations in water cause problem in terms of public health. Owing to its efficiency, reverse osmosis (RO) was considered for water defluoridation. However, this technique is not selective. The recovery by means of brackish water RO system is therefore limited by salt precipitation. A computer calculation program was developed to determine the activity products a Ca 2 + . a CO 3 2 − , a Ca 2 + . a SO 4 2 − and a Ca 2 + . a F − 2 by taking into account the influence of all present free and associated species (ion pairs). These activity products are then compared with the solubility product of calcite (CaCO 3), gypsum (CaSO 4,2H 2O) and fluorite (CaF 2). The results allowed a theoretical estimation of the brackish water RO recovery which can be achieved without scaling. From the calco-carbonic equilibrium curve, the Legrand–Poirier diagram allowed to optimize the amount of softening substance (sodium carbonate and soda mixtures) to be added to obtain water which can be subsequently concentrated by reverse osmosis. An experimental softening pretreatment of Algerian brackish water was considered according to the optimal calculated softening amount of 15 mmol L − 1 of sodium, added in the form of sodium carbonate and soda (16%). The pretreatment allowed not only an almost total removal of the calcium but also a partial elimination of magnesium and fluoride.

Influence of pancreatic ducts on saturation of juice with calcium carbonate in dogs.

In several species, bicarbonate and calcium concentrations of pancreatic juice are known to vary during the different phases of pancreatic secretion. The effects of these variations on the saturation of juice with calcium carbonate, a critical factor for the formation of pancreatic stones, are not known. In this work, we studied the saturation degree of pancreatic juice with calcium carbonate in six unanesthetized beagle dogs equipped with Thomas cannulae during basal secretion and after bolus injections of cerulein (30 ng/kg) or secretin (0.25 units/kg). In the different samples of pure pancreatic juice, pH, PCO2, bicarbonate, and proteins were measured by standard methods. Total calcium (CaT) and ionized calcium (Ca2+) were determined using calcium-specific electrodes. Saturation with calcium carbonate was calculated by reference to the solubility product of calcite at 37 degrees C. Almost all the samples were found to be supersaturated with calcium carbonate but large variations of the saturation index were observed. In basal samples, obtained during periods of low secretion rate, the mean saturation index (3.35 +/- 3.01) was significantly lower than under secretion (12.10 +/- 5.14) or cerulein (18.01 +/- 8.42). This low basal saturation index, in spite of a high Ca2+ content, was explained by a low bicarbonate concentration (37.6 +/- 18.9 mmol/liter) and a high PCO2 (13.4 +/- 7.5 kPa). In contrast, in juice obtained after hormonal stimulation, PCO2 (4.8 +/- 1.6 kPa) was similar to plasma PCO2 (5.5 +/- 1.2 kPa).(ABSTRACT TRUNCATED AT 250 WORDS)

The dissolution of calcite in seawater from 40° to 90°C at atmospheric pressure and 35‰ salinity

Calcite solubility experiments in 35‰ salinity seawater were carried out from 40° to 90°C at 1 atm total pressure. An open-system technique with constant CO 2 partial pressure was used. Since calcite solubility in seawater has been reported to be reversible, equilibrium was approached by undersaturation only. A fit to our own experimental data and to low-temperature data from the literature was computed, resulting in the following equation for the apparent solubility product of calcite, K′ c: log K ′ c=1.6968−1096.69/T−1.4699·10 −×T where K′ c is expressed in (mol/kg sw) 2; and T is the absolute temperature from 273.15 to 363.15 K. The standard deviation of this line is ±0.037 log unit (±90% K′ c). The mean apparent enthalpy variation between 40° and 90°C was calculated by means of the log K′ c( T) equation. After a close examination of the data from the literature, a new experimental value of the temperature coefficient, ∂K′ c/ ∂T, was assessed at 25°C. Using the thermodynamic and apparent solubility products, experimental mean activity coefficients, γ ± c, were computed for CaCO 3 and a temperature-dependent equation was derived. A comparison between solubility products in seawater and NaCl solutions confirms that, as far as calcite dissolution is concerned, seawater and NaCl solutions do not exhibit the same chemical behavior. Calcite is far more soluble in standard seawater than in NaCl solutions up to 6 m.

Whitening of Brown-Shelled Eggs: Mineral Composition of Uterine Fluid and Rate of Protoporphyrin Deposition

Changes in the mineral composition of uterine fluid during shell formation and the rates of color appearance and porphyrin deposition on the shell were measured in two subpopulations of brown egg-laying hens with familial histories of low or high incidences of shell whitening. Increases in shell weight and shell breaking strength were correlated with, and proportional to, time spent by the egg in the uterus and were similar in both subpopulations.Shell reflectance decreased and the amount of porphyrin deposited increased linearly 20 to 24 h after oviposition of the preceding egg. Porphyrin deposition was slightly higher at the 23-h stage in the high whitening population but similar amounts of porphyrin were deposited on the shell during the final stage of shell formation in both groups. The coating on the shell responsible for whitening was deposited during the hour prior to oviposition.Uterine fluid pH, pCO2, bicarbonate, and ionized Ca concentrations changed during shell formation but these changes were not related to the incidence of whitening. A milieu supersaturated with calcite solubility product was observed whatever the stage of shell formation. Inorganic phosphorus was not detectable in the uterine fluid whatever the stage of shell formation. The soluble phosphorus fraction of uterine decreased 22 h after oviposition and phosphorus deposition on the shell increased. At the end of egg formation uterine fluid could not be collected. It was concluded that shell whitening was associated with changes in the kinetics of porphyrin deposition rather than with changes in the amount of porphyrin deposited or modifications of uterine fluid composition.

Calculations of ion activities in soil solutions from humic horizons

Concentrations of the main components and activities of Ca 2+, Na +, Cl −, and NO 3 − were determined in soil solutions displaced from chernozem samples (A 1 horizon). To calculate ion activity in these solutions, it was necessary to consider formation of Ca 2+, Mg 2 and Na + complexes with organic anions. If 1 : 1-composition of these complexes was assumed, dependence of their stability constants on the ionic strengths was found; this allowed a 17% average error for Ca 2+ activity and 8% for Na + activity in the calculations. As followed from the computations, 68–80% of the Ca 2+ and Mg 2+ were included in ion pairs and complexes with 91–97% of these amounts bound with organic anions. The values for the calcite solubility product were close to those for pure saltwater systems.

The CaCO3 -CO2 -H2 O system in soils

Equations used to develop the CO2-Ca-pH relationships in calcareous soils are reviewed. The equation P CO 1 ( Ca ) = ( H ) 2 K c , is used to draw a three-dimensional surface and to derive three partial differential equations to illustrate the relationships between CO2 partial pressure, Ca activity and pH. Kc is a combination of Henry's Law constant, the first and second dissociation constants for carbonic acid and the calcite solubility product. The three dimensional CO2-Ca-pH surface illustrates how the three parameters relate to each other under ideal conditions. The partial differential equations are presented to illustrate how changes in one parameter affect the other two. The CO2-Ca-pH surface provides a graphical method for introducing the idea of three component equilibria, while the partial differential equations provide a mathematical representation of these interactions for those with chemical thermodynamics or strong mathematic or modeling backgrounds. Deviations from this ideal model in natural systems are discussed for those who wish to extend the discussion to natural systems.

Apparent calcite supersaturation at the ocean surface: Why the present solubility product of pure calcite in seawater does not predict the correct solubility of the salt in nature

It is generally accepted that seawaters near the ocean surface are supersaturated with calcite and that increasing atmospheric CO 2 will not overcome this condition until late in the 21st century. These expectations are based on comparisons of the ion activity products (IAPs) of various seawaters to the solubility product ( K) of seawater saturated with pure calcite at 1 atm and a given temperature. It has been shown in recent papers, however, that calcites in contact with natural seawater in the surface oceans are not pure, but are magnesian calcite compositions. As a result, the presently accepted values of the solubility product of calcite in seawater cannot be used to obtain a correct, or even a reasonable estimate of the saturation state of sea water relative to either pure or magnesian calcite. Data are presented demonstrating that use of the currently accepted solubility product of calcite in seawater to determine seawater's relative calcite saturation leads to gross overestimates of its extent.

Calcite supersaturation and precipitation kinetics in the Lower Colorado River, All‐American Canal and East Highline Canal

In situ pH determinations and analysis of major ions in solution indicated that the lower Colorado River is supersaturated with respect to calcite throughout the entire daily cycle, in both winter and summer. Although the ion activity product was 4 to 6 times greater than the calcite solubility product, there was no detectable precipitation. Chemical analyses of water samples taken along 350 km of the river and canals from Parker Dam to the Salton Sea also revealed no evidence of calcium carbonate precipitation despite the inflow of saline and highly supersaturated irrigation return flows. Laboratory kinetic studies indicated that calcite crystal growth rates with Colorado River water are about 30% of the rate for pure Ca‐HCO3 waters and about 70% of that for synthetic Colorado River water. Calcite precipitation by crystal growth in the river is limited by the combination of short residence times and unavailability of reactive calcite. Critical supersaturation levels necessary for heterogeneous nucleation do not occur; a high suspended load limits algal photosynthesis and thus prevents large decreases in daytime H2CO3 levels.

The solubility of calcite — probably containing magnesium — in seawater

The apparent solubility product of calcite was measured by saturometry as a function of temperature and salinity. Simplified equations for the carbonic-acid dissociation constants of Mehrbach et al., 1973 (Limnol. Oceanogr., 18: 897–907) have been derived from their experimental data and used to calculate apparent solubility product, K′ sp, K′ sp at 25°C and 35‰ salinity, was found to be K′ sp = 4.70 × 10 −7( mol 2 kg seawater −2) An equation was fitted to the experimental data, resulting in pK′ sp = 6.5795 − 3.7159 × 10 5( TS) + 0.91056( T/ S) − 22.110(1.0/ S) The mean activity coefficients, γ± CaCO 3 , were calculated at various temperatures and salinities, using the thermodynamic solubility product of Jacobson and Langmuir, 1974 (Geochim. Cosmochim. Acta, 38: 301–318) and the apparent solubility products quoted in their paper. The change in K′ sp at each salinity, as a function of temperature, was used to calculate the apparent enthalpy of dissociation for calcite, ΔH′, and the extrapolated value of ΔH 0 was in good agreement with that of Jacobson and Langmuir. Finally, this work was used to calculate saturation profiles for oceanic stations and as a basis for comment of the accuracy of in-situ saturometry, as well as the applicability of in-situ K′ sp pressure corrections.

The dissolution kinetics of biogenic calcium carbonates in seawater

Dissolution rate as a function of degree of undersaturation was measured on shells of individual species of coccoliths and foraminifera, various size fractions of sediment from the Ontong-Java Plateau and the Rio Grande Rise, a collection of large pteropods, and on synthetic calcite and aragonite powder. Results of the study indicate that all biogenic and synthetic calcium carbonate follows the rate law R% = k%(1 − Ω) n where Ω  [Ca 2+][CO 3 2−]/K' sp and K' sp is the apparent solubility product of calcite or aragonitic seawater. In the case of all calcite samples, n c = 4.5, while for aragonitic samples n a = 4.2. The ‘rate constant’, k%, varies widely between samples and in many cases is inversely correlated with grain size. However, the individual species of coccoliths, E. huxleyi and C. neohelis, which were cultured in the laboratory appear not to follow this rule, with dissolution rates an order to magnitude lower than expected.

The thermodynamics of the carbonate system in seawater

The apparent constants ( K' i ) for the ionization of carbonic acid in seawater at various salinities ( S,%.) have been fit to equations of the form ln K' i = ln K i + A iS 1 2 + B iS where K i is the thermodynamic ionization constant in water, A i , and B i are adjustable parameters. The temperature dependence (TK) of K i , A i and B i were of the form, a 0 + a 1/ T + a 3 ln T. Equations of similar forms have been used to analyze the ionization constants for water and boric acid and the solubility product of calcite in seawater. The effect of pressure on the apparent constants ( K p i K o i ) have been fit to equations of the form ln ( K p i K o i ) = − (ΔVP + 0.5 ΔK P 2)/RT where the volume (ΔV) and compressibility (ΔK) changes are polynomial functions of temperature. The equations generated for various açids in seawater have been used to examine the carbonate system in seawater. Equations relating the NBS and Tris pH scales have been derived as well as equations of pH as a function of temperature and pressure. The equations from Hansson (1972, Ph.D. Thesis, University of Göteborg, Sweden) and Mehrbach et al. (1973, Limnol. Oceanogr. 18, 897–907) have been used to examine the components of the carbonate system. At a fixed total alkalinity and total carbon dioxide, differences of ±0.01 m-equiv kg −1 in HCO − 3 and CO 2− 3 were found; however, the [CO 2] and P co2 are nearly the same. The contribution of borate ion, B(OH) − 4 determined from the equations of Hansson (1972, Ph.D. Thesis, University of Göteborg, Sweden) and Lyman (1957, Ph.D. Thesis, University of California, Los Angeles) differ by ±0.01 m-equiv kg −1 for waters with the same salinity and temperature.

The solubility of calcite in seawater at atmospheric pressure and 35%permil; salinity

The apparent (stoichiometric) solubility product of calcite in artificial seawater of salinity 35‰ was measured by a saturometer technique. The value of the apparent solubility product was found to be (4·59 ± 0·05) × 10 −7 moles/(kilogram of seawater) 2 at 25°C with a temperature coefficient of −0·0108 × 10 −7/°C between 2 and 25°C. These values are significantly smaller than those found by MacIntyre (1965) and other workers. The effect of these results on the saturation of the oceans with respect to calcite is examined.

The variation of dissolved constituents with discharge in some Norfolk rivers

Weekly samples were collected during 1970 from the rivers Yare, Tud, Tas and Wensum in Norfolk and analysed for dissolved sodium, potassium, calcium, magnesium, silicon, chloride, bicarbonate, nitrate, phosphate and sulphate. The relationship between the concentration and discharge was examined by regression analysis. Magnesium, bicarbonate and, for the Yare and Wensum, phosphorus were diluted by increases in discharge. Nitrate and sulphate concentrations were positively correlated with discharge, possibly due to these constituents being derived from the leaching of the upper part of the soil profile. Potassium concentration increased at the beginning of most storm hydrographs but, like sodium, was diluted on other occasions. Large increases in nitrate, sulphate, potassium, sodium, chloride and silicon occurred in November, with accumulated weathered products being flushed from the soil, as the summer moisture deficit was replenished. Silicon was insensitive to changes in discharge during the first months of 1970. During the spring and summer it was depleted by diatoms. Calcium concentration was also uncorrelated with discharge, possibly due to being derived from the solution of calcium carbonates and calcium sulphate. The rivers were supersaturated calcium carbonate, when compared with the solubility product of calcite and showed no signs of equilibrating.

On Some Aspects of the Chemical Evolution of Cave Waters

The evolution of cave waters can be divided naturally into three stages: a stage of carbonation in the soil zone, a stage of solution of calcite and/or dolomite, and a stage of equilibration with cave air. Much of the necessary physical-chemical data for the interpretation of the chemistry of cave waters is available: the solubility of in water is well known, as is the solubility of calcite and aragonite in pure water and in solutions containing dissolved . Only the solubility of dolomite is still in doubt. Water samples from Indian Echo Cave and Carpenter Cave in Pennsylvania and from the Luray Caverns in Virginia were analyzed, and their composition was used to interpret their chemical evolution. The waters in each cave must have absorbed a large amount of on their passage through the soil zone. Even so, the calcium and magnesium content of some of the water at Luray is so large that impossibly high pressures would have had to prevail in the soil zone if the solubility data of Yanat'eva (1954) and of Garrels, Thompson, and Siever (1960) for dolomite were correct. We suggest that the solubility product of dolomite is probably near , about two orders of magnitude higher than the value proposed by Garrels, Thompson, and Siever (1960). During the precipitation of from cave waters, the activity product has been found to exceed the solubility product of calcite by as much as a factor of seven. In bodies of standing water from which no is precipitating the activity product is very nearly equal to the accepted value of the solubility product of calcite. Precipitation of takes place essentially entirely by escape of from cave water to the cave air. This is demonstrated convincingly by the observed constancy of the magnesium concentration in these waters during precipitation. At Luray nearly all of the water is supersaturated with respect to dolomite. However, no dolomite has been observed to precipitate in the cave even where the activity product is in excess of . The strontium content of precipitated from cave water depends on the mineralogy of the precipitate and on the ratio of the concentration of strontium to the concentration of calcium in the parent water. The strontium content of our Luray aragonites is always greater than the strontium content of the Luray calcites, and the absolute value of the strontium concentration can be related satisfactorily to the strontium-calcium ratio in the parent water via the distribution coefficients and determined in the laboratory. The strontium content of the cave waters at Luray is determined by the amount of dolomite dissolved by the carbonated rain water, by the strontium content of the dissolved dolomite, and by the amount and mineralogy of the deposited during equilibration with cave air.