The Carbonic Acid System in Seawater

One major concept applicable to problems dealing with the behavior of carbonic acid and carbonate minerals in seawater is the idea of a constant ionic media . This concept is based on the general observation that the salt in seawater is close to constant in composition, i.e., the ratios of the major ions are the same from place to place in the ocean. Seawater in evaporative lagoons, pores of marine sediments, and near river mouths can be exceptions to this constancy. Consequently, the major ion composition of seawater can generally be determined from salinity. It has been possible, therefore, to develop equations in which the influences of seawater compositional changes on carbonate equilibria can be 

In theory, it should be possible to deal with all carbonate geochemistry in seawater simply by knowing what the appropriate activity coefficients are and how salinity, temperature, and pressure affect them. In practice, we are only now beginning to approach the treatment of activity coefficients under this varying set of conditions with sufficient accuracy to be useful for most problems of interest. That is why apparent and stoichiometric equilibrium constants, which do not involve the use of activity coefficients, have been in widespread use in the study of marine carbonate chemistry for over 20 years. The stoichiometric constants, usually designated as K. involve only the use of concentrations, whereas expressions for apparent equilibrium constants contain both concentrations and aH+ derived from apparent pH . These constants are usually designated as K Examples of these different types of constants are  

It should be noted that in seawater the molinity concentration scale (moles kg-1 of seawater) is often used, and care must be taken to make certain that stoichiometric and apparent constants are on the same concentration scale as the measured values. The ratios of thermodynamic constants to their apparent or stoichiometric constant are activity coefficients, for example  

It is appropriate at this point to discuss the apparent pH, which results from the sad fact that electrodes do not truly measure hydrogen ion activity. Influences such as the surface chemistry of the glass electrode and liquid junction potential between the reference electrode filling solution and seawater contribute to this complexity (see for example Bates, 1973). Also, commonly used NBS buffer standards have a much lower ionic strength than seawater, which further complicates the problem. One way in which this last problem has been attacked is to make up buffered artificial seawater solutions and very carefully determine the relation between measurements and actual hydrogen ion activities or concentrations. The most widely accepted approach is based on the work of Hansson (1973). pH values measured in seawater on his scale are generally close to 0.15 pH units lower than those based on NBS standards. These two different pH scales also demand their own sets of apparent constants. It is now clear that for very precise work in seawater the Hansson approach is best. 

As a practical matter, pH values of seawater are generally measured at 25°C and 1 atmosphere total pressure. To apply these pH measurements to in situ conditions it is necessary to correct for pressure and temperature changes. For salinities between 30 and 40, temperatures of 0 to 40°C, and pH=7.6 to 8.2, the temperature correction on the NBS scale is (Millero, 1979)  

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In this chapter, we introduced the reader to some basic principles of solution chemistry with emphasis on the C02-carbonate acid system. An array of equations necessary for making calculations in this system was developed, which emphasized the relationships between concentrations and activity and the bridging concept of activity coefficients. Because most carbonate sediments and rocks are initially deposited in the marine environment and are bathed by seawater or modified seawater solutions for some or much of their history, the carbonic acid system in seawater was discussed in more detail. An example calculation for seawater saturation state was provided to illustrate how such calculations are made, and to prepare the reader, in particular, for material in Chapter 4. We now investigate the relationships between solutions and sedimentary carbonate minerals in Chapters 2 and 3. 

In these days following the plate tectonic revolution in natural science, there has been an increased propensity for specialization among scientists. This trend is apparent in the field of study of the geochemistry of sedimentary carbonates. Chemical oceanographers deal with the chemistry of the carbonic acid system in seawater. Some marine geologists and geochemists concern themselves with the relationship between factors controlling the lysocline and carbonate compensation... 

Millero, F. J. (1979) The Thermodynamics of the Carbonic Acid System in Seawater, Geochim. Cosmochim. Acta 43 1651-1661. 

The distribution of CO2 and the associated carbonic acid system species in the upper ocean (here loosely defined as waters above the thermocline and generally only a few hundred meters in depth) is primarily controlled by the exchange of CO2 across the air-sea interface, biological activity, and circulation of the ocean, mainly through vertical mixing processes. Other factors, such as the temperature and salinity of the water, can also contribute to variations by influencing the solubility of CO2 in seawater and the equilibrium constants of the carbonic acid system. 

The calculation of the total ion molal carbonate ion concentration is more complex than for calcium because it is part of the carbonic acid system. The following reactions take place between CO and carbonic acid in seawater ... 

The equations and methods given in this chapter can be used to calculate the distribution of carbonic acid system components and the saturation state of a solution with respect to a carbonate mineral under varying temperature, pressure, and composition. To illustrate the type of changes that occur, a calculation has been done for seawater, and the results summarized for nine different cases in Table 1.12. Case 1 is used as a reference typical of surface, subtropical, Atlantic seawater in equilibrium with the atmosphere. In all other cases the salinity and total... 

It should be kept in mind that, in spite of these major variations in the CO2-carbonic acid system, virtually all surface seawater is supersaturated with respect to calcite and aragonite. However, variations in the composition of surface waters can have a major influence on the depth at which deep seawater becomes undersaturated with respect to these minerals. The CO2 content of the water is the primary factor controlling its initial saturation state. The productivity and temperature of surface seawater also play major roles, in determining the types and amounts of biogenic carbonates that are produced. Later it will be shown that there is a definite relation between the saturation state of deep seawater, the rain rate of biogenic material and the accumulation of calcium carbonate in deep sea sediments. 

As previously mentioned, the primary processes responsible for variations in the deep sea C02-carbonic acid system are oxidative degradation of organic matter, dissolution of calcium carbonate, the chemistry of source waters and oceanic circulation patterns. Temperature and salinity variations in deep seawaters are small and of secondary importance compared to the major variations in pressure with depth. Our primary interest is in how these processes influence the saturation state of seawater and, consequently, the accumulation of CaC03 in deep sea sediments. Variations of alkalinity in deep sea waters are relatively small and contribute little to differences in the saturation state of deep seawater. 

Another practical consideration when dealing with the seawater carbonic acid system is that in addition to carbonate alkalinity, H and OH , a number of other components can contribute to the total alkalinity (TA). The seawater constituent that is usually most important is boric acid. Under most conditions, boric acid contributes — 0.1 mmol alkalinity it is usually taken into consideration when making calculations. Nutrient compounds, such as ammonium, phosphate, and silica, whose concentrations in seawater are highly variable, can also influence alkalinity. They must be taken into account for very precise work. In anoxic pore waters a number of compounds, such as hydrogen sulfide and dissolved organic matter, can be significant contributors to alkalinity (e.g., see Berner et al, 1970). 

In either case, the production of hydroxyl ions results in an increase in pH for the electrolyte adjacent to the metal surface. In other terms, an increase in OH is equivalent to a corresponding reduction in acidity or H+ ion concentration. This situation causes the production of a pH profile in the diffuse layer, where the equilibrium reactions can be quite different from those in the bulk seawater conditions. Temperature, relative electrolyte velocity, and electrolyte composition will all influence this pH profile. There is both analytical and experimental evidence that such a pH increase exists as a consequence of the application of a cathodic current. In seawater, pH is controlled by the carbon dioxide system described in Eqs. (2.18) through (2.20) ... 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

Further evidence for the addition of H2S to carbon-carbon double bonds very early in sediments, and further insights into reaction mechanisms, have been reported by Vairavamurthy and Mopper in 1987 and 1989 (109.110). They identified 3-mercaptopropionic acid (3-MPA) as a major thiol in anoxic intertidal marine sediment and demonstrated that the thiol formation could occur by the reaction of HS with acrylic acid in sediment water and seawater at ambient temperature The formation of 3-MPA was hypothesized to occur by a Michael addition mechanism whereby the nucleophile HS adds to the activated double bond in the a,/3-unsaturated carbonyl system ... 

Measurements of a number of acids have been made in seawater, and equations are available to determine the elfect of salinity, temperature, and pressure on the dissociation constants (Millero, 2001). The most widely studied acids in seawater are those related to the carbonate system. Most of these studies have been made in artihcial seawater that contains the major ionic components (Na, Mg +, Ca +, K+, Cr, SO, F ). It was thought that constants determined in this artificial seawater could be used in real seawater. Although this is the case for most acid-base systems, as will be discussed below, this is not the case for the carbonate system (Mojica and Millero, 2002). 

Example 7.8. Calcite in Seawater Compare the composition of a CaC03(s) (calcite)-C02-H20 seawater model system, made by adding calcite to pure H2O containing the seawater electrolytes (but incipiently no Ca and no carbonates and, for simplicity, no borate) and by equilibrating this solution at 25°C and 1 atm total pressure with the atmosphere (pcoi = 3.55 X 10 atm), with the composition of a real surface seawater whose carbonate alkalinity, Ca(II) concentration, and pH have been determined as 2.4 x 10 eq liter", 1.06 X 10 M, and 8.2, respectively. Estimate the extent of oversaturation of this seawater with respect to calcite. The solubility of calcite at 25 °C is taken as "K q = [Ca/-] [CO3 ] = 5.94 X 10 , where [Ca ] and [CO37] are the concentration of total soluble Ca(II) ([Ca ] plus concentration of Ca complexes with medium ions) and of total soluble carbonate ([CO "] and concentration of carbonate complexes with medium ions), respectively. The other constants needed, Henry s law constant and the acidity constant of H2CO, are taken as ... 

Flow-injection analysis (FIA) is a technique for automating chemical analyses. The principles of FIA are reviewed here. Methods for applying FIA to the anayses of nitrate, nitrite, phosphate, silicate, and total amino acids in seawater are examined. Analyses of other nutrients, metals, and carbonate system components are also discussed. Various techniques to eliminate the refractive index effect are reviewed. Finally, several examples of the application of FIA to oceanographic problems are presented. 

One of the most important components of the chemical perspective of oceanography is the carbonate system, primarily because it controls the acidity of seawater and acts as a governor for the carbon cycle. Within the mix of adds and bases in the Earth-surface environment, the carbonate system is the primary buffer for the aridity of water, which determines the reactivity of most chemical compoimds and solids. The carbonate system of the ocean plays a key role in controlling the pressure of carbon dioxide in the atmosphere, which helps to regulate the temperature of the planet. The formation rate of the most prevalent authigenic mineral in the environment, CaCOs, is also the major sink for dissolved carbon in the long-term global carbon balance. 

These simple equations and ideas provide the basis for describing the carbonate system in terms of the/coj, DIG, pH, and alkalinity of seawater. We will build up a plot similar to that in Fig. 4.1 for the important acids and bases in seawater. These are listed along with their concentrations and apparent equihbrium constants in Table 4.1. It will then be demonstrated how the constraint of charge balance (called alkalinity) determines the pH of seawater. 

The acid-hase pair with the second highest concentration and a pK near the pH of seawater is horic acid (Table 4.1). The carbonate system and boric acid turn out to be by far the most important contributors to the acid-base chemistiy of seawater, but they contrast greatly in their reactivity in the ocean carbon is involved in all metabohc processes and varies in concentration from place to place, whereas borate is conservative and maintains a constant ratio to salinity. The equih-brium reaction and total boron, Bj, equations are ... 

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