Intercalibration of Bruevich’s method to determine the total alkalinity in seawater

In 2000, the Carbon Dioxide in the Ocean working group of the North Pacific Marine Science Organization (PICES) performed an international experiment on the intercalibration of the measurements of the total alkalinity in seawater using certified reference materials (CRM). Taking part in this experiment, Russian specialists presented the method by Bruevich. The results of the intercalibration showed that the alkalinity values obtained by Bruevich’s method using modern burettes, an Na₂CO₃ reactant of high purity as a standard to ascertain the acid titre, and corrections for the acid density and for the weights of the acid and seawater samples in vacuum are in agreement with the standard within ±1 μ M/kg.     

The role of pHT measurements in marine CO2-system characterizations

Spectrophotometric pH_T measurements can routinely be obtained with an imprecision on the order of ±0.0005 pH_T units. This level of imprecision is equivalent to an imprecision in total hydrogen ion concentration of ±0.1% (where pH_T=−log[H⁺]_T and [H⁺]_T≅[H⁺]+[HSO₄⁻]). At this level of precision, pH_T measurements provide an important tool in quality control assessments of other CO₂-system parameters (CO₂ fugacity, total inorganic carbon, total alkalinity). CO₂ fugacities and total alkalinities calculated using measured pH_T and total inorganic carbon, for the large data set considered in this work, have relative precisions on the order of 0.15 and 0.1%, respectively. The precision of total inorganic carbon calculated via pH_T and salinity-normalized-alkalinity is on the order of 0.02% or better. In this work, using the NOAA 1992 boreal autumn Equatorial Pacific (EqPac) dataset, it is shown that CO₂-system variables calculated via pH_T can be used to enhance both the precision and accuracy of directly measured parameters. Through the procedures described in this work significant problems were revealed in the initial version of the 1992 NOAA dataset, and the dataset was greatly improved. Additionally, in this work, we revisit CO₂-system thermodynamic consistency issues in view of changes in the pH_T values assigned to tris seawater buffers and consequential changes in the calibration of sulfonephthalein pH_T indicators. As the principal result of a +0.0047 increase in the pK of meta cresol purple, CO₂ fugacity calculations and measurements are in very good agreement for the NOAA 1992 boreal autumn EqPac dataset. We note, as well, that due to a reassignment of the titrant acid concentration used in the NOAA 1992 boreal autumn dataset, measured total alkalinities are in good agreement with total alkalinities calculated from total carbon and pH_T.     

Calculation methods and the distribution of anthropogenic variations of pH values in the Pacific Subarctic

Common approaches were used to calculate the anthropogenic CO₂, equations were proposed to characterize anthropogenic variations in pH values (ΔpH_ant) in the Pacific Subarctic. For the treated area, the temporal and spatial ΔpH_ant variability was analyzed in 1986 and 2001–2004. From the ΔpH_ant values, the influence of pH variations on iron solubility in seawater was considered.     

DIRECT DETERMINATION OP TRACE CADMIUM IN SEAWATER BY DERIVATIVE-VOLTAMMETRY

An analytical method is proposed for the direct determination of Cd in seawater by differential pulse anodic stripping voltammetry and the derivative technique with a hanging mercury drop electrode. The process of determination is quick, simple and convenient. The concentration of Cd in seawater is only determined by adjusting the acidity of seawater to pH 2 and by taking three minutes' plating time. Sensitivity of the method is about 1×10-10M, and accuracy of that satisfactory. Relative standard deviation is about 12% when the concentration of Cd in seawater is approximately 0.04 ppb. A good agreement was obtained by a standard curve and a standard addition technique respectively from determining Cd in the same seawater. Actual measurement time per sample is about 10 min.     

Dissociation of hydrogen sulphide in seawater and comparison of pH scales

The protonization constant of HS^? (K₁₂) has been determined potentiometrically (glass electrode) at atmospheric pressure in synthetic seawater in the salinity range 2.5–40‰ at 5 and 25°C and in NaCl solutions in the formal ionic strength of 0.1–0.8 M at 5 and 25°C. The difference between synthetic seawater and an NaCl solution with the same formal ionic strength can be explained in terms of the complexation of H⁺ by sulphate in seawater. These results can be used to compare the pH scales suggested by Hansson (1973c) and Bates (1975). Furthermore, comparison between the present values of K₁₂ and those of Goldhaber and Kaplan (1975) makes it possible to compare the conventional pH scale with Hansson's titration pH scale. The conditional protonization constant of HS^? in seawater of different salinities can be used to modify the Gran plots (Hansson and Jagner, 1973) for alkalinity measurements in anoxic seawater. Ion-pair formation between HS^? and Mg²⁺ or Ca²⁺ seems to be very weak.     

Comparison of the results of determination of the carbonate system and the total alkalinity of seawater according to the data obtained by using different analytic methods

On the basis of the data of hydrochemical investigations of waters of the Sevastopol Bay, we discuss the results of the analysis of the carbonate system and the total alkalinity of seawater obtained by using two methods. By using the linear method of least squares, we compare the results of the analysis of the carbonate system according to pH values and the total alkalinity and according to pH values and dissolved inorganic carbon. It is shown that, within the investigated range of pH values, Alk, and TCO₂, the mean relative deviations of values of the fugacity of CO₂ and the concentrations of dissolved CO₂ and CO₃²⁻ ions computed by using different input parameters are equal to 3.0, 1.0, and 9.6%, respectively.     

The carbonate system in hypersaline solutions: dead sea brines

Various investigators reported a decrease in pH as seawater is concentrated. A similar phenomenon was reported for Dead Sea waters which are about ten times more saline than seawater. The reasons for the low pH values of Dead Sea waters (pH 5.9–6.5), which precipitate CaCO₃, were investigated by determining the apparent dissociation constants of carbonic acid in these brines. A new method, based on alkalinity titration and least-squares fitting, was used to estimate the proton activity coefficient (γ_H+) and the first and second dissociation constants of carbonic acid (K₁′, K₂′) in natural and artificial Dead Sea waters. It was found that as the salt content increases, pK′₁ and pK′₂ values progressively decrease whereas γ_H+ sharply increase. At the highest salinity investigated (TDS = 330 gl^?1) γ_H+ pK′₁ and pK′₂ values are 24.5, 5.09 and 6.23, respectively, as compared to about 0.8, 5.9, 9.1 respectively for normal seawater (19‰ chlorinity) at the same temperature (30°C).The implication of the results of this study regarding solubility of CaCO₃ and the general behavior of the carbonate system in hypersaline solutions is discussed.     

Rapid, high-precision potentiometric titration of alkalinity in ocean and sediment pore waters

A system for rapid, high precision potentiometric determination of alkalinity in sea water and sediment pore water is presented. Two titration units were used: a 40 ml unit for seawater and a small volume unit for sediment pore water. Titration time was normally less than 10 minutes per sample, including sample exchange. With a 40 ml sample volume, the relative standard deviation of the alkalinity obtained in the laboratory was 0.05% and at sea 0.1 %. The small-volume system (0.5–1.5 ml) gave a precision of 0.07%. Five titration points, in two groups after the second equivalence point, were used to evaluate the equivalence volume. Results from equilibrium calculations and computer simulated alkalinity titrations show that it was possible to use a non-modified Gran function [(V₀ +v)^*10^(E/Z)] and still achieve good accuracy and precision.     

The use of buffers to measure the pH of seawater

The pH of seawater can be measured in the field using potentiometric and spectrophotometric methods. The use of pH standards or buffers is an important aspect of the calibration of both methods in a laboratory on a common concentration scale. The buffers can also be used to monitor the performance of pH meter and spectrophotometer during a cruise. A procedure is described for the determination of the pH of seawater, where the proton concentration is expressed as moles kg-H₂O⁻¹ using seawater buffers. The buffers are prepared in synthetic seawater in the laboratory by the methods outlined by Bates and coworkers. We have prepared four buffers (Bis, Tris, Morpholine and 2-Aminopyridine) that cover a pH range from 6.8 to 8.8. The emf values of the buffers were measured with a H₂, Pt/AgCl, Ag electrode system after their preparation and bottling for use at sea. The measured emf values were found to be in good agreement (±0.05 mV) with the original measurements of Bates and coworkers from 0 to 45°C. The measured pH of these buffers are in good agreement (±0.001 pH units) with the values calculated from the equations of Dickson on the total pH scale based on Bates et al. Studies are underway to access the long term stability of these buffers. We have also used these buffers to calibrate systems used to make potentiometric and spectrophotometric measurements of pH on seawater relative to the H₂, Pt/Ag, AgCl electrode from 5 to 45°C.     

Investigations of iron coordination and redox reactions in seawater using Fe radiometry and ion-pair solvent extraction of amphiphilic iron complexes

Iron coordination and redox reactions in synthetic and coastal seawater were investigated at nanomolar concentrations using ⁵⁹Fe radiometry and ion-pair solvent extraction of iron chelated by sulfoxine (8-hydroxyquinoline-5-sulfonate) and BPDS (bathophenanthroline disulfonate). Using sulfoxine, we determined the rate at which the monomeric Fe(III) hydroxide species present in seawater of pH 8 are complexed by the microbial siderophore deferriferrioxamine B and the synthetic chelator EDTA (ethylenediaminetetraacetic acid). Forward rate constants of 2 × 10⁶M⁻¹s⁻¹ and 20 M⁻¹s⁻¹, respectively, were obtained. The kinetics of these reactions have not been measured previously at pH values near that of seawater. Conditional equilibrium constants measured for the Fe(III)-EDTA system are consistent with published stability constants for EDTA complexes and for Fe(III) hydrolytic equilibria minus the neutral Fe(OH)₃^o species, suggesting it is not quantitatively significant near pH 8. Commercial humic acid was found to have sufficient affinity for iron to compete with Fe(III) hydrolysis in seawater, and limited evidence was obtained for an interaction with dissolved organic matter in coastal seawater.In our investigations of redox reactions using BPDS to trap Fe(II) produced in the medium, we observed enhanced photoreduction of Fe(III) by humic acid as well as reduction induced by solutes released from phytoplankton in seawater of pH 8. Although the method is sensitive enough to work at near-oceanic levels of iron, the difficulty in distinguishing Fe(II) generated by Fe(III)-BPDS interactions from Fe(II) produced by other means limits its utility. This analytical ambiguity may be generalizable to other methods which measure ferrous iron in seawater using Fe(II)-specific ligands.     

Experiments on the ocean sequestration of fossil fuel CO2: pH measurements and hydrate formation

We have carried out a series of in situ experiments to investigate the formation of a CO₂ hydrate (CO₂:5.75 H₂O) for the purpose of evaluating scenarios for ocean fossil fuel CO₂ disposal with a solid hydrate as the sequestered form. The experiments were carried out with a remotely operated vehicle in Monterey Bay at a depth of 619 m. pH measurements made in close proximity to the hydrate–seawater interface showed a wide range of values, depending upon the method of injection and the surface area of the hydrate formed. Rapid injection of liquid CO₂ into an inverted beaker to form a flocculant mass of hydrate resulted in pH initially as low as 4.5 within a few centimeters of the interface, decaying slowly over 1–2 h towards normal seawater values as dense CO₂ rich brine drained from the hydrate mass. In a second experiment, slower injection of the liquid CO₂ to produce a simple two-layer system with a near planar interface of liquid CO₂ with a thin hydrate film yielded pH values indistinguishable from the in situ ocean background level of 7.6. Both field and laboratory results now show that the dissolution rate of a mass of CO₂ hydrate in seawater is slow but finite.     

Reconstruction of pH in the Surface Seawater over the North Pacific Basin for All Seasons Using Temperature and Chlorophyll-<Emphasis Type="Italic">a</Emphasis>

We found a simple function of pH that relates to sea surface temperature (SST, K) and chlorophyll-a (Chl, µg l⁻¹) using measured surface seawater pH, SST and Chl data sets over the North Pacific: pH (total hydrogen scale at 2°C) = 0.01325 SST − 0.0253 Chl + 4.150 (R² = 0.95, p < 0.0001, n = 483). Moreover, evaluating the seasonal variation of pH based on this algorithm, we compared the measured pH with the predicted pH at the observational time series stations in subpolar and subtropical regions. The average of ΔpH (measured - predicted, n = 52) was 0.006 ± 0.022 pH. Therefore, the combination of SST and Chl can allow us to determine the spatiotemporal distribution of pH over the North Pacific. Using the climatological data sets of SST and Chl with our pH algorithms, we have described the seasonal distributions of pH at 25°C (pH₍₂₅₎) and pH in situ temperature (pH_(T)) over the North Pacific surface water.     

Effect of lower pH on settlement and development of coral,Pocillopora damicornis (Linnaeus, 1758)

The effects of pH reduction on the settlement and development of the coral Pocillopora damicornis were investigated. Three different pH treatments (pH = 7.6, 7.9, and 8.1) were used. In addition, water quality (temperature, salinity, total alkalinity) around the study site was monitored. The results showed significant differences in the settlement rates of Pocillopora damicornis larvae between pH treatments (p ≤ 0.05). A decrease in pH levels caused a strong decline in larval settlement rate. In addition, at pH 7.6 and 7.9, all larvae were unable to complete metamorphosis, and metamorphosis delay was observed. Field monitoring showed low fluctuation of all seawater parameters within 24 hours, and there was no difference between seasons. From this study, a strong negative effect of pH reduction on P. damicornis larvae was observed. Although the function of physiology is still not clearly understood, correlations are likely to exist.

     

Effect of changes in the composition of seawater on the density–salinity relationship

Over the next 5–10 years, the WOCE hydrographic program will generate reliable hydrographic data for the world oceans. The resultant conductivity salinity, temperature, and pressure data will generate calculated densities that will be used to examine mixing along constant density surfaces. Changes in the composition of deep waters due to the breakdown of plant material can effect the calculated densities at a given conductivity salinity. The increases in SiO₂, nitrate, alkalinity, and TCO₂ (or pH) can change the density of seawater as well as the conductivity. For studies of the salinity and density fields over small spatial scales, these changes will be small, but for large scale and ocean to ocean studies the differences can be significant. The density calculations based on the salinity determined from conductivity need to be adjusted for the offsets due to changes in the composition of seawater. This report describes how this correction should be made using existing information.     

pH variability off Goa (eastern Arabian Sea) and the response of sea urchin to ocean acidification scenarios

The increasing atmospheric CO₂ concentration in the last few decades has resulted in a decrease in oceanic pH. In this study, we assessed the natural variability of pH in coastal waters off Goa, eastern Arabian Sea. pH_T showed large variability (7.6–8.1) with low pH conditions during south-west monsoon (SWM), and the variability is found to be associated with upwelling rather than freshwater runoff. Considering that marine biota inhabiting dynamic coastal waters off Goa are exposed to such wide range of natural fluctuations of pH, an acidification experiment was carried out. We studied the impact of low pH on the local population of sea urchin Stomopneustes variolaris (Lamarck, 1816). Sea urchins were exposed for 210 days to three treatments of pH_T: 7.96, 7.76 and 7.46. Our results showed that S. variolaris at pH_T 7.96 and 7.76 were not affected, whereas the ones at pH_T 7.46 showed adverse effects after 120 days and 50% mortality by 210 days. However, even after exposure to low pH for 210 days, 50% organisms survived. Under low pH conditions (pH_T 7.46), the elemental composition of sea urchin spines exhibited deposition of excess Sr²⁺ as compared to Mg²⁺ ions. We conclude that although the sea urchins would be affected in future high CO₂ waters, at present they are not at risk even during the south-west monsoon when low pH waters reside on the shelf.     

Alkalinity titration in seawater: How accurately can the data be fitted by an equilibrium model?

Displaying “calculated minus observed” data for precise titrations of seawater with strong acid permits direct evaluation of important parameters and detection of systematic errors.At least two data sets from the GEOSECS (Geochemical Ocean Sections) program fit an equilibrium model (which includes carbonate, borate, sulfate, silicate, fluoride, and phosphate) within the most stringent experimental error, less than 2 μmol kg⁻¹. The effect of various parameters on the fit of calculated to observed values depends strongly on pH. Although standard potential E₀, total alkalinity A_t, total carbonate C_t, and first acidity constant of carbon dioxide pK₁ are nearly independent, and can be determined for each data set, other parameters are strongly correlated. Within such groups, all but one parameter must be determined from data other than the titration curve.Adding an acid-base pair to the theoretical model (e.g. C_x=20 μmol kg⁻¹, pK_x=6.2) produces a deviation approaching 20 μmol kg⁻¹ at constant C_t; however, adjustment of C_t by about −18 μmol kg⁻¹ to produce a good fit leaves only ± 1.5 μmol kg⁻¹ residual deviation from the reference values. Thus, at current standards of precision, an unidentified weak acid cannot be distinguished from carbonate purely on the basis of the titration curve shape.There are few full sets of numerical data published, and most show larger systematic errors (3–12 μmol l⁻¹) than the above; one well-defined source is experiments performed in unsealed vessels. Total carbonate can be explicitly obtained as a function of pH by a rearrangement of the titration curve equation; this can reveal a systematic decrease in C_t in the pH range 5–6, as a result of CO₂ gas loss from the titration vessel. Attempts to compensate for this by adjustment of A_t, C_t, or pK₁ produce deviations which mimic those produced by an additional acid-base pair.Changing from the free H⁺ scale (for which [HSO₄⁻] and [HF] are explicit terms in the alkalinity) to the seawater scale (SWS) (where those terms are part of a constant factor multiplying [H⁺]) requires modification of the titration curve equation as well as adjustment of acidity constants. Even with this change, however, omission of pH-dependent terms in [HSO₄⁻] and [HF] produces small systematic errors at low pH.Shifts in liquid junction potential also introduce small systematic errors, but are significant only at pH <3. High-pH errors due to response of the glass electrode to Na⁺ as well as H⁺ can be adequately compensated to pH 9.5 by a linear selectivity expression.     

The international at-sea intercomparison of fCO2 systems during the R/V Meteor Cruise 36/1 in the North Atlantic Ocean

The ‘International Intercomparison Exercise of fCO₂ Systems’ was carried out in 1996 during the R/V Meteor Cruise 36/1 from Bermuda/UK to Gran Canaria/Spain. Nine groups from six countries (Australia, Denmark, France, Germany, Japan, USA) participated in this exercise, bringing together 15 participants with seven underway fugacity of carbon dioxide (fCO₂) systems, one discrete fCO₂ system, and two underway pH systems, as well as systems for discrete measurement of total alkalinity and total dissolved inorganic carbon. Here, we compare surface seawater fCO₂ measured synchronously by all participating instruments. A common infrastructure (seawater and calibration gas supply), different quality checks (performance of calibration procedures for CO₂, temperature measurements) and a common procedure for calculation of final fCO₂ were provided to reduce the largest possible amount of controllable sources of error. The results show that under such conditions underway measurements of the fCO₂ in surface seawater and overlying air can be made to a high degree of agreement (±1 μatm) with a variety of possible equilibrator and system designs. Also, discrete fCO₂ measurements can be made in good agreement (±3 μatm) with underway fCO₂ data sets. However, even well-designed systems, which are operated without any obvious sign of malfunction, can show significant differences of the order of 10 μatm. Based on our results, no “best choice” for the type of the equilibrator nor specifics on its dimensions and flow rates of seawater and air can be made in regard to the achievable accuracy of the fCO₂ system. Measurements of equilibrator temperature do not seem to be made with the required accuracy resulting in significant errors in fCO₂ results. Calculation of fCO₂ from high-quality total dissolved inorganic carbon (C_T) and total alkalinity (A_T) measurements does not yield results comparable in accuracy and precision to fCO₂ measurements.     

Calibration and evaluation of a carbonate microsensor for studies of the marine inorganic carbon system

Accurate and rapid determination of inorganic carbon constituents in ocean environments is important for understanding the carbon cycle, especially in the context of ocean-acidification research. A microsensor capable of directly measuring carbonate ion (CO₃ ^2–) concentrations would be desirable. In this study, a carbonate microsensor with a polymeric liquid membrane was fabricated, and two calibration methods were used to evaluate its performance. The first method was based on continuous titration. Small increments of HCl were added to seawater or Na₂CO₃ solution to adjust the total alkalinity and pH values and thus obtain a series of carbonate concentrations. The second method used a series of discrete standards. Varying amounts of HCl or NaOH were added to separate seawater aliquots, and the CO₃ ^2– concentration of each standard was calculated from the resulting total alkalinity and total dissolved inorganic carbon. Both methods were found to be adequate for achieving accurate calibration of the CO₃ ^2– sensor, and both are suitable for field work. The discrete standards method, however, is more convenient and may provide a better linear range at low CO₃ ^2– concentrations (detection range: 2–300 μmol/kg) than the continuous titration method in seawater (detection range: 10–250 μmol/kg). This CO₃ ^2– microsensor can be used for 5–7 d and detects changes in carbonate concentration as low as 2 μmol/kg in the inorganic carbon constituents of the environments where marine calcareous organisms grow. The CO₃ ^2– microelectrode was further assessed by applying it to the measurement of pore-water CO₃ ^2– concentration profiles in a marine sediment core.     

Effects of CO<Subscript>2</Subscript> on Marine Fish: Larvae and Adults

CO₂-enriched seawater was far more toxic to eggs and larvae of a marine fish, silver seabream, Pagrus major, than HCl-acidified seawater when tested at the same seawater pH. Data on the effects of acidified seawater can therefore not be used to estimate the toxicity of CO₂, as has been done in earlier studies. Ontogenetic changes in CO₂ tolerance of two marine bony fishes (Pag. major and Japanese sillago, Sillago japonica) showed a similar, characteristic pattern: the cleavage and juvenile stages were most susceptible, whereas the preflexion and flexion stages were much more tolerant to CO₂. Adult Japanese amberjack, Seriola quinqueradiata, and bastard halibut, Paralichthys olivaceus, died within 8 and 48 h, respectively, during exposure to seawater equilibrated with 5% CO₂. Only 20% of a cartilaginous fish, starspotted smooth-hound, Mustelus manazo, died at 7% CO₂ within 72 h. Arterial pH initially decreased but completely recovered within 1-24 h for Ser. quinqueradiata and Par. olivaceus at 1 and 3% CO₂, but the recovery was slower and complete only at 1% for M. manazo. During exposure to 5% CO₂, Par. olivaceus died after arterial pH had been completely restored. Exposure to 5% CO₂ rapidly depressed the cardiac output of Ser. quinqueradiata, while 1% CO₂ had no effect. Both levels of ambient CO₂ had no effect on blood O₂ levels. We tentatively conclude that cardiac failure is important in the mechanisms by which CO₂ kills fish. High CO₂ levels near injection points during CO₂ ocean sequestration are likely to have acute deleterious effects on both larvae and adults of marine fishes. This revised version was published online in July 2006 with corrections to the Cover Date.