Dissociation Constants Of Carbonic Acid Research Articles

The influence of a permanent magnetic field on the crystallization of calcium carbonate from carbonate aqueous solutions

The effect of a permanent magnetic field on the crystallization of calcium carbonate from carbonate aqueous solutions was studied. Based on the equations for the first- and second-order dissociation constants of carbonic acid, the stability constants of NaCO3- and NaHCO30 complexes, the mass balance and electroneutrality equations, the values of the system components and the ratio between the components of the carbonate subsystem were determined, and it was established that the main species in experimental conditions are СО32-, NaСО3- and HСО3- and the main components of the carbonate subsystem are СО32- (86.7%) and HСО3- (13.19%). Studies have shown that with a significant excess of CO32- over HCO3- ions in the solution at temperatures of 18-20˚C, calcite and vaterite are most likely formed. During the crystallization of calcium carbonate from an aqueous carbonate solution in a magnetic field of 125-250 mT, the size of the crystallites increases significantly compared to the absence of a magnetic field.

Preliminary assessment of carbonic acid dissociation constants: Insights from observations in China's east coastal oceans

Based on observations in China's east coastal oceans, we conducted a preliminary assessment of 16 sets of carbonic acid dissociation constants (K1* and K2*) by comparing spectrophotometrically measured pH values at 25 °C with those calculated from total alkalinity and dissolved inorganic carbon. We obtained that K1* and K2* often performed differently within different salinity ranges, and that the constants of Millero et al. (2002) (M02) demonstrated the best performance for the salinity range of 24–35. In contrast, the often recommended constants of Mehrbach et al. (1973) refit by Dickson and Millero (1987) (DM87-M) and Lucker et al. (2000) (L00) would underestimate pH at salinities of 24–30. This was mainly associated with the higher product of K1* and K2* by DM87-M and L00 than by M02 at this salinity range. Also, we found almost no differences between pH values calculated with DM87-M and L00.

Determining freshwater pCO2 based on geochemical calculation and modelling using PHREEQC

Fossil fuel combustion results in rising atmospheric carbon dioxide (CO2), which is known to impact the global climate and the oceans. Latest insights indicate that rising atmospheric CO2 levels also affect CO2 partial pressure (pCO2) in freshwaters, where pCO2 is controlled by a multitude of parameters. However, up to date there is no standardized method, which allows the determination of current and past freshwater pCO2 levels. Ideally methods should incorporate numerous hydrogeochemical and -physical factors to reflect the interplay of all interacting components and their effect on pCO2. We here describe the application of the geochemical program PHREEQC. This freeware serves as an easy method enabling a plausible and comprehensive analysis of pCO2 for field, laboratory, and especially long-term data. We present the use of the different input parameters of a laboratory- and a field long-term monitoring dataset including dissociation constants of carbonic acid measured as total inorganic carbon (TIC) and total CO2 concentration (TCO2) or total alkalinity (TA), together with hydrogeochemical and -physical parameters. Based on current literature and our analyses PHREEQC appears a solid strategy to determine freshwater pCO2 that can moreover be used for long-term datasets.•Comprehensive analysis of pCO2 for field, laboratory, and long-term data.•PHREEQC is not dependent on just one sampling method or parameter scheme.•PHREEQC includes testing the plausibility of a water analysis and enables the assessment of the quality of the laboratory analysis, as well as automatic calculation of all relevant aquatic complexes.

Insights from GO-SHIP hydrography data into the thermodynamic consistency of CO2 system measurements in seawater

Due to advances in technology, routine seawater pH measurements of excellent repeatability are becoming increasingly common for studying the ocean CO2 system. However, the accuracy of pH measurements has come into question due to a widespread observation, from a large number of carefully calibrated state-of-the-art CO2 measurements on various cruises, of there being a significant pH-dependent discrepancy between pH that was measured spectrophotometrically and pH calculated from concurrent measurements of total dissolved inorganic carbon (CT) and total alkalinity (AT), using a thermodynamic model of seawater acid-base systems. From an analysis of four recent GO-SHIP repeat hydrography datasets, we show that a combination of small systematic errors in the dissociation constants of carbonic acid (K1 and K2), the total boron-salinity ratio, and in CT and AT measurements are likely responsible for some, but not all of the observed pH-dependent discrepancy. The residual discrepancy can only be fully accounted for if there exists a small, but meaningful amount (~4 μmol kg–1) of an unidentified and typically neglected contribution to measured AT, likely from organic bases, that is widespread in the open ocean. A combination of these errors could achieve consistency between measured and calculated pH, without requiring that any of the shipboard measurements be significantly in error. Future research should focus on establishing the existence of organic alkalinity in the open ocean and constraining the uncertainty in both CO2 measurements and in the constants used in CO2 calculations.

The stoichiometric dissociation constants of carbonic acid in seawater brines from 298 to 267 K

The stoichiometric dissociation constants of carbonic acid (K1C∗ and K2C∗) were determined by measurement of all four measurable parameters of the carbonate system (total alkalinity, total dissolved inorganic carbon, pH on the total proton scale, and CO2 fugacity) in natural seawater and seawater-derived brines, with a major ion composition equivalent to that of Reference Seawater, to practical salinity (SP) 100 and from 25 °C to the freezing point of these solutions and −6 °C temperature minimum. These values, reported in the total proton scale, provide the first such determinations at below-zero temperatures and for SP > 50. The temperature (T, in Kelvin) and SP dependence of the current pK1C∗ and pK2C∗ (as negative common logarithms) within the salinity and temperature ranges of this study (33 ≤ SP ≤ 100, −6 °C ≤ t ≤ 25 °C) is described by the following best-fit equations: pK1C∗ = −176.48 + 6.14528 SP0.5−0.127714 SP + 7.396×10−5SP2 + (9914.37 − 622.886 SP0.5 + 29.714 SP) T−1 + (26.05129 − 0.666812 SP0.5) lnT (σ = 0.011, n = 62), and pK2C∗ = −323.52692 + 27.557655 SP0.5 + 0.154922 SP − 2.48396×10−4SP2 + (14763.287−1014.819 SP0.5−14.35223 SP) T−1 + (50.385807−4.4630415 SP0.5) lnT (σ = 0.020, n = 62). These functions are suitable for application to investigations of the carbonate system of internal sea ice brines with a conservative major ion composition relative to that of Reference Seawater and within the temperature and salinity ranges of this study.

Spatial variability in surface-water <i>p</i>CO<sub>2</sub> and gas exchange in the world's largest semi-enclosed estuarine system: St. Lawrence Estuary (Canada)

Abstract. The incomplete spatial coverage of CO2 partial pressure (pCO2) measurements across estuary types represents a significant knowledge gap in current regional- and global-scale estimates of estuarine CO2 emissions. Given the limited research on CO2 dynamics in large estuaries and bay systems, as well as the sources of error in the calculation of pCO2 (carbonic acid dissociation constants, organic alkalinity), estimates of air–sea CO2 fluxes in estuaries are subject to large uncertainties. The Estuary and Gulf of St. Lawrence (EGSL) at the lower limit of the subarctic region in eastern Canada is the world's largest estuarine system, and is characterized by an exceptional richness in environmental diversity. It is among the world's most intensively studied estuaries, yet there are no published data on its surface-water pCO2 distribution. To fill this data gap, a comprehensive dataset was compiled from direct and indirect measurements of carbonate system parameters in the surface waters of the EGSL during the spring or summer of 2003–2016. The calculated surface-water pCO2 ranged from 435 to 765 µatm in the shallow partially mixed upper estuary, 139–578 µatm in the deep stratified lower estuary, and 207–478 µatm along the Laurentian Channel in the Gulf of St. Lawrence. Overall, at the time of sampling, the St. Lawrence Estuary served as a very weak source of CO2 to the atmosphere, with an area-averaged CO2 degassing flux of 0.98 to 2.02 mmol C m−2 d−1 (0.36 to 0.74 mol C m−2 yr−1). A preliminary analysis revealed that respiration (upper estuary), photosynthesis (lower estuary), and temperature (Gulf of St. Lawrence) controlled the spatial variability in surface-water pCO2. Whereas we used the dissociation constants of Cai and Wang (1998) to calculate estuarine pCO2, formulations recommended for best practices in open ocean environments may underestimate pCO2 at low salinities, while those of Millero (2010) may result in overestimates.

The marine carbonate system of the Arctic Ocean: Assessment of internal consistency and sampling considerations, summer 2010

The Arctic Ocean has experienced tremendous changes in recent years. To evaluate temporal and spatial variations of the marine carbonate system, a rigorous evaluation of the quality and internal consistency of Arctic field data and the applicability of existing thermodynamic constants to Arctic conditions is needed. Low Arctic temperatures fall outside the range of conditions used to experimentally determine these constants. Using data collected during the Chinese National Arctic Research Expedition (CHINARE) cruise of summer 2010, we compared underway measurements of the partial pressure of carbon dioxide (pCO2) to pCO2 values calculated using measurements of dissolved inorganic carbon (DIC) and total alkalinity (TAlk) and seven sets of dissociation constants. For waters sampled outside areas of active sea ice melt, calculated and measured pCO2 values agreed best when the calculations incorporated the carbonic acid dissociation constants of Mehrbach et al. (1973) (as refit by Dickson and Millero (1987)) (mean difference of 1.5μatm±standard deviation of 5.7μatm) or Lueker et al. (2000) (2.3±5.4μatm). Differences between calculated and measured pCO2 values were related to temperature and salinity and were, for all sets of constants, increasing with increasing T and S over a temperature range of −1.5 to 10.5°C and a salinity range of 25.8 to 33.1. In the relatively warm Bering Sea, calculated pCO2 was higher than measured pCO2, but in the colder Canada Basin, calculated values were lower than measured values. This pattern indicates that calculations of pCO2 in very cold waters may underestimate pCO2, with an uncertainty of ~5μatm in accuracy. In areas of active sea ice melt (ice cover <35%), large differences between calculated and measured pCO2 values occurred. We explored possible explanations for these large differences and concluded that dissolution of CaCO3 precipitates from sea ice in samples is the most likely cause. Further research including a comparison of filtered and unfiltered samples is needed to resolve this issue. Many processes influence the marine carbonate system of the Arctic Ocean, and further assessment of their relative roles is needed.

Over-determination of the carbonate system in natural sea-ice brine and assessment of carbonic acid dissociation constants under low temperature, high salinity conditions

By over-determining the carbonate system in natural sea-ice brine samples, we have confirmed that stoichiometric equilibrium constants for the dissociation of carbonic acid in seawater (K1⁎, K2⁎) derived for general oceanic conditions may not be readily extrapolated to sea-ice brine systems. Dissolved inorganic carbon, total alkalinity, pH, and pCO2 measurements in sea-ice brine have allowed us to rigorously examine the validity of K1⁎ and K2⁎ at salinities as high as 82 and in-situ temperatures as low as −4.8°C. The use of seawater-derived constants yields average offsets between calculated and measured values ranging from 10% to 43% for brine salinities of 38–82. Future high-accuracy determination of sea-ice brine carbonate system components will require the specific determination of carbonate system equilibrium constants under appropriate temperature and salinity conditions. Furthermore, the community is in need of a means to evaluate the accuracy of carbonate system measurements in natural, high-salinity brine samples using the presently available analytical methods.

Carbamate Stability Measurements in Amine/CO2/Water Systems with Nuclear Magnetic Resonance (NMR) Spectroscopy

Abstract NMR experiments were carried out for 30 mass % aqueous solutions of monoethanolmine (MEA), ethylenediamine (EDA) and piperazine (PZ) at 25 °C. To distinguish between the amine/protonated amine and bicarbonate/carbonate pairs, an earlier method developed for primary amine (2-amino-2-methyl-1-propanol) was implemented. The amine protonation constant, the dissociation constant of carbonic acid from literature, and pH measurements for different ionic strengths, were all employed and the data were correlated with ionic strength.

Carbon budgets for two Portuguese estuaries: implications for the management and conservation of coastal waters

Carbon budgets for two Portuguese estuaries: implications for the management and conservation of coastal waters

Experimental study on carbamate formation in the AMP–CO2–H2O system at different temperatures

Carbamate formation in the 30wt% of 2-amino-2-methyl-1-propanol (AMP) system at different CO2 loadings and temperatures was studied via nuclear magnetic resonance (NMR) spectroscopy. The results indicate that the main species in this system are AMP/AMPH+, AMPCO2− and HCO3−/CO32−. The carbamate was also observed at low loadings and the apparent carbamate stability constant was estimated based on the experimental concentrations of the species from NMR analysis. Carbamate formation was found to have weak temperature dependence in the range tested (25–45°C). To distinguish within the AMP/AMPH+ and HCO3−/CO32− pairs, the amine protonation constant, the dissociation constant of carbonic acid from literature, and pH measurements for different ionic strengths were all employed. All the data were correlated with ionic strength and temperature.The accuracy of the carbamate stability constant determined from the concentration measurements will depend on pKa, ionic strength (I) used to calculate the speciation and on the uncertainty in the species concentration determinations from NMR, particularly for the carbamate species at low CO2 loadings and high temperature.

Measurements of Dissociation Constants of Carbonic Acid in Synthetic Seawater by Means of a Cell Without Liquid Junction

Using the “total hydrogen ion concentration scale”, the first, K 1, and second, K 2, stoichiometric dissociation constants of carbonic acid have been determined in synthetic seawater for temperature and salinity ranges of 0–30 °C and 1.5–40, respectively. The values of the K 1 have been determined potentiometrically by means of a cell without liquid junction, composed using a pH-glass electrode and silver–silver chloride electrode. This cell was equilibrated with CO2 at 1 atm total pressure. The same cell was used for the determination of K 2. Dissolved inorganic carbon was measured by the coulometric method, total alkalinity was known from the preparation of the synthetic seawater, and pH measurements provided the data required for the calculations of K 2. Estimated precision is about ±0.003 pK and uncertainty less than 0.01 pK for K 1 and twice that for K 2. Our K 1 data agree with “best” published data within ±0.01 for log10 K 1 in the 30–40 salinity range obtained for natural seawater, but become progressively higher as the salinity decreases. Our results for the second stoichiometric dissociation constants agree within ±0.015 for log10 K 2 in the 30–40 salinity range with available “best” published data for natural seawater, but become progressively lower as the salinity decreases. Since constants obtained in this paper are reliable for the high salinity range (25–40) and strive to the thermodynamic constants at 0 salinity, they are recommended for study of carbonate system in estuaries.

Monte Carlo simulation of the dissociation constants of CO2 in 0 to 1 molal sodium chloride between 0 and 25 °C

Stoichiometric dissociation constants of the carbon dioxide system in NaCl solution between 0 and 1mol and 0 to 25°C were estimated by Monte Carlo (MC) simulations, and compared with Pitzer calculations and experimental measurements. The MC simulations used experimentally determined dielectric constants of water at different temperatures, and optimal agreement with the experimental data and Pitzer calculations was achieved by adjusting the ionic radii. This simple procedure resulted in effective ionic radii which were further used to simulate the activity coefficients of salt mixtures. The first and second stoichiometric dissociation constants of carbonic acid in NaCl solution (pK1⁎ and pK2⁎) were estimated from the MC-derived activity coefficients of mixed salts in NaCl. The MC results are in good agreement with the experimental data as well as with the Pitzer calculations. This study shows that Monte Carlo simulations in the temperature and ionic strength range relevant to seawater can provide pK values of the same quality as Pitzer calculations, and constitutes the first step in developing a temperature-dependent MC model for seawater. While MC calculations require greater computing resources, the number of parameters derived by fitting to thermodynamic data is substantially smaller than for Pitzer calculations.

New Approaches in Modeling Carbonate Alkalinity and Total Alkalinity

Alkaline measurements (carbonate alkalinity and total alkalinity) are of great importance in analyses of ocean, marine, lake and river waters, and other samples. Carbonate alkalinity (CA) refers to the cases in which only bicarbonate and carbonate species are considered. Total alkalinity (Alk) implies additional species having acid-base properties. Biochemical processes give rise to the organic alkalinity (OA) in natural waters. The value of these topics is obvious if we refer to the plethora of publications in this field. As a matter of fact, the approaches towards Alk provide ambiguous results and those for CA arouse some reservations as well. Having this in mind, a new approach for alkalinity determination, presented in recent review paper published in this journal (Crit. Rev. Anal. Chem. 2011, 41, 151), was developed in a more systematic and complete way, and new acronyms, TAL for total alkalinity and CAM for carbonate alkalinity, were introduced in that article. The methods proposed there and here are based on potentiometric pH titration, as the most common method applied for studying acid-base properties of humic substances (humic and fulvic acids). The parameters involved with CAM are resolved with the use of an Excel spreadsheet for the resolution of linear equations, whereas the parameters of a nonlinear regression equation referring to the TAL method are obtainable according to a more advanced, iterative computer program. A compact approach to CAM, based on the modified Gran II method, is carried out, but in a manner quite different and more thorough than the ones presented hitherto. Except for equivalence volumes, the CAM method enables evaluating the activity coefficient of hydrogen ions and values for (hybrid and concentration) dissociation constants of carbonic acid and ionic product of water. The possibilities and limitations of this method are tested by using a simulation procedure. The results obtained from simulated titrations provide the reference level related to error-free V and pH values at the points of titration curve ph = ph(V), ph = – log h, h – activity of hydrogen ions. The TAL model with Simms constants involved allows “homogenizing” the treatment of mono- and polyprotic acids and bases, especially those of undefined composition, i.e., carbonate species, fulvic acids, and their soluble complexes with metal ions. The TAL model may also be extended to the option where an approach based on a pre-dose addition is applied. pH and acid-base equilibria are very important topics and some aspects of them have been treated in this journal. Alkalinity determinations are of value in anaerobic digesters and anaerobic wastewater treatment, mine drainage and associated water, coastal seawater, marine aquaculture, estuarine water, and food and beverages, to point out a few recent examples.

ESIEX-Electrical Swing Ion Exchange a Process Coupling Ion Exchange, Carbonic Acid Elution, and Electrochemical Regeneration

A process which can be used for purification of a peptide is presented. This process is called ESIEX: Electrical Swing Ion Exchange. The principle of this process is based on a three steps cyclic mode, which avoids using pH buffers or generating wastes. The experimental set-up consists of an electrodialysis apparatus in which the cells are filled with anion exchange resins. Carbon dioxide dissolved in water is used as “green eluent.” Peptide is obtained pure. Assuming that equilibria in solution play a major role on the global separation factor compared to affinities for the support, peptides which can be fixed in a step but also eluted in another step should satisfy with a criterion: . The peptide dissociation constant must be between the two dissociation constants of carbonic acid, that is, 6.35 and 10.35 (at 298 K).

In situmeasurements confirm the seasonal dominance of benthic algae over phytoplankton in nearshore primary production of a large lake

Summary1. Despite the recognition of its importance, benthic primary production is seldom reported, especially for large lakes. We measuredin situbenthic net primary production by monitoring flux in dissolved inorganic carbon (DIC) concentration in benthic incubation chambers, based on continuous measurements of CO2(aq)flux, alkalinity, and the temperature‐dependent dissociation constants of carbonic acid (K1andK2). This methodology has the advantages of monitoring net primary production directly as change in carbon, maintaining continuous water recirculation, and having sufficient precision to detect change in DIC over short (i.e. 15 min) incubations, even in alkaline waters.2. Benthic primary production onCladophora‐dominated rocky substrata in western Lake Ontario was measured biweekly. Maximum biomass‐specific net photosynthetic rates were highest in the spring (2.39 mgC g Dry Mass−1 h−1), decreased to negative rates by early summer (−0.76 mgC g DM−1 h−1), and exhibited a regrowth in late summer (1.98 mgC g DM−1 h−1).3. ACladophoragrowth model (CGM), previously validated to predictCladophorabiomass accrual in Lake Ontario, successfully simulated the seasonality and magnitude of biomass‐specific primary production during the first cohort ofCladophoragrowth. Averaged over this growing season (May–Aug), mean areal net benthic production at the estimated depth of peak biomass (2 m) was 405 mg C m−2 d−1.4. We measured planktonic primary production in proximity to the benthic study and constructed a depth‐resolved model of planktonic production. Using the CGM, benthic primary production was compared with planktonic primary production for the period May–Aug. Net benthic production from the shoreline to the 12 m contour (1–2 km offshore) equalled planktonic production. Closer to shore, benthic primary production exceeded planktonic primary production. Failure to account for benthic primary production, at least during abundantCladophoragrowth, will lead to large underestimates in carbon and nutrient flows in the nearshore zone of this Great Lake.

Determination of carbonate ion concentration and inner sphere carbonate ion pairs in seawater by ultraviolet spectrophotometric titration

We describe a novel method for determination of carbonate ion concentration in seawater by acidimetric titration with UV detection. Because CO 3 2− absorbs light at wavelengths of less than ~ 250 nm, it is feasible to titrate most carbonate-containing natural waters with acid and observe an increase in %Transmittance. The observed signal is proportional to the concentration of carbonate ion in the original sample. Present technology permits a theoretical precision in the determination of [CO 3 2−] in natural seawater background of ~ 0.5% (at 10 cm pathlength, 200 μM CO 3 2−, ± 0.0001 AU). The procedure has been tested at 1 and 10 cm pathlengths using single and multipoint titration methods, respectively. Results using natural seawater test solutions indicate a resolution in [CO 3 2−] of 3.6% in a standard 1 cm cuvette using a very simple manual method, and 0.7% using a custom-built 10 cm closed titration cell. Estimates of the relative distribution of CO 3 2− between inner and outer sphere complexes with Mg 2+ and Na + were also determined and the equilibrium constants agree with published values. This method provides a new tool for studies of several fundamental aspects CO 2 chemistry, including the second dissociation constant of carbonic acid, CO 3 2− ion pairing, and can be used to directly measure the distribution of carbonate ion in seawater and many other types of natural waters.

Spectrophotometric determination of pH in seawater off Taiaroa Head, Otago, New Zealand: Full-spectrum modelling and prediction of pCO 2 levels

Accurate measurement of seawater pH has long been sought by marine chemists (for example: [Dickson, A.G. 1993a. The measurement of sea water pH. Marine Chemistry, 44, 131–142, Dickson, A.G. 1993b. pH buffers for sea water media based on the total hydrogen ion concentration scale. Deep-Sea Research, 40, 107–118; Zhang, 1996; Tapp, M., Hunter, K.A., Currie, K. and Macaskill, B. 2000. Apparatus of continuous-flow underway spectrophotometric measurement of surface water pH. Marine Chemistry 72(2–4), 193–202; Friis, K., Koetzinger, A., Wallace, D.W.R. 2004. Spectrophotometric pH measurement in the ocean: Requirements, design and testing of an autonomous charge-coupled device detector system. Limnology and Oceanography: Methods 2, 126–136]. Recently, such attempts have taken on greater significance as anthropogenic carbon dioxide emissions may create rapidly changing oceanic pH. Spectrophotometric techniques have been accepted generally as the best for determination of seawater pH. Here we report a new technique using thymol blue as the indicator dye and fitting the entire spectrum from 400 to 900 nm rather than measuring the absorbance values at only two or three points in the spectrum. This full-spectrum modelling enables a reduction in signal to noise over other techniques. In the laboratory, we find with seawater samples a pH precision increase of five-fold “within” a sample and seven-fold “between” samples when comparing the full spectrum to the three-point method of analysis [Zhang, H., Byrne, R.H. 1996. Spectrophotometric pH measurements of surface seawater at in-situ conditions: absorbance and protonation behaviour of thymol blue. Marine Chemistry 52, 17–25]. We then use this analysis technique for a series of transects between July 2005 and March 2006 east of Taiaroa Head, New Zealand. The pH values determined at sea using the full-spectrum modelling technique along with independently measured alkalinity ( A T) are used to determine the total inorganic carbon concentration ( C T). C T in turn is used with A T to calculate pCO 2 levels in surface seawater. Comparing pCO 2 levels from independently measured pCO 2 acquired in parallel with pH, we show an average internal consistency agreement of 1.15% ( σ = 0.23%) when using the Mehrbach et al. [Mehrbach, C., Culberson, C.H., Hawley, J.E., Pykowicz, R.M. 1973. Measurements of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure. Limnology and Oceanography 18, 897–907] (as refit by Dickson and Millero[Dickson, A.G., Millero, F.J. 1987. A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Research 34, 1733–1743] carbonate dissociation constants. The pCO 2 values predicted from pH are very close to the measured values of pCO 2. We indicate that while both pH and pCO 2 are of great interest in surface seawater, it may be possible to measure only pH and alkalinity to obtain an excellent predictor of pCO 2 levels. The instrument we use to measure pH combined with the full-spectrum modelling show that regular, real-time analysis of the carbonate system in surface seawater on a small boat (14 m) can be precise and consistent.

Dissociation constants of carbonic acid in seawater as a function of salinity and temperature

Potentiometric measurements of the stoichiometric constants on the seawater pH scale for the dissociation of carbonic acid in seawater ( K 1 ⁎ = [H +][HCO 3 −]/[CO 2] and K 2 ⁎ = [H +][CO 3 2−]/[HCO 3 −]) have been made as a function of salinity (1 to 50) and temperature (0 to 50 °C). The results have been fitted to the equations ( T/ K) p K i − p K i 0 = A i + B i / T + C i ln ⁢ T . The values of p K i 0 in pure water are taken from the early work of Harned and Davis (1943) and Harned and Scholes (1941) p K 1 0 = − 126.34048 + 6320.813 / T + 19.568224 ⁢ ln ⁢ T p K 2 0 = − 90.18333 + 5143.692 / T + 14.613358 ⁢ ln ⁢ T . The value of the adjustable parameters A i , B i and C i for p K 1 ⁎ are given by ( σ = 0.0054 and N = 466) A 1 = 13.4191 S 0.5 + 0.0331 ⁢ S − 5.33 ⁢ E − 05 S 2 B 1 = − 530.123 S 0.5 − 6.103 ⁢ S C 1 = − 2.06950 S 0.5 . For p K 2 ⁎ the parameters are given by ( σ = 0.011 and N = 458) A 2 = 21.0894 S 0.5 + 0.1248 ⁢ S − 3.687 ⁢ E − 04 S 2 B 2 = − 772.483 S 0.5 − 20.051 ⁢ S C 2 = − 3.3336 S 0.5 . The values of p K 1 ⁎ and p K 2 ⁎ determined in this study are in good agreement with the seawater (SW) measurements of Mehrbach et al. (1973) and Mojica-Prieto and Millero (2002) from S = 15 to 45 and 0 to 40 °C. The values of p K 1 ⁎ near S = 35 are also in reasonable agreement with the measurements in artificial seawater (ASW) of Goyet and Poisson (1989) and Roy et al. (1993) from 0 to 35 °C. The values of p K 2 ⁎ in real seawater, however, do not agree with the measurement made in artificial seawater at temperatures above 5 °C. Calculations of p K 1 ⁎ and p K 2 ⁎ near 25 °C using an ionic interaction model (Millero and Roy, 1997) suggest that the p K 2 ⁎ results in SW are more reliable than in ASW. The equations from this study should be valid from S = 0 to 50 and t = 0 to 50 °C for most estuarine and marine waters (check values at S = 35 and t = 25 °C are p K 1 ⁎ = 5.8401 and p K 2 ⁎ = 8.9636).

The effect of sea-ice growth on air–sea CO2 flux in a tank experiment

In order to clarify the CO2 exchange between the seawater and the overlying air during the sea-ice formation, we have carried out tank experiments in a low-temperature room. CO2 concentration above the sea-ice began to increase since the beginning of the sea-ice formation, and increased at larger rates with time and the decrease in air temperature. This increase of CO2 concentration in air was mainly caused by the increase in dissolved inorganic carbon concentration in the brine of the upper part of sea-ice, changes in CO2 solubility and dissociation constants of carbonic acid. The CO2 flux increased logarithmically with time, and reached a level of 2 × 10-4 to 5 × 10-4 g-C m-2 hr-1 at 50 mm ice thickness. We found that the CO2 flux was correlated well with the salinity and negatively with the volume of the brine in the upper part of the sea-ice. These suggested the larger role of the difference in partial pressure of CO2 between brine and air as compared to that of competitive change in the brine volume. Present results suggest the necessity to examine the CO2 exchange between the seawater and air in seasonal sea-ice areas.