Carbonic acid system

To see how the concentrations of species present in a solution vary with pH, we take the carbonic acid system as an example. Consider the following proton transfer equilibria ... 

He S, Morse JW (1993) The carbonic acid system and calcite solubility in aqueous Na-K-Ca-Mg-Cl-S04 solutions from 0 to 90°C. Geochim Cosmochim Acta 57 3533-3555... 

By analogy with the carbonic acid system, OCS hydrates to form H2O.OCS in aqueous solution, and the bicarbonate and carbonate like anions OCS.OH and OCS.O2 known collectively as monothiocarbonates (MTC). The entire carbonate analog system is shown in Figure 1, and it should be clear that MTC species are likely to serve as intermediates in both overall hydrolysis channels CL). Several pieces of information point to a lack of reversibility to OCS, or, schematically, to... 

The relative proportions of the different carbonic acid system species can be calculated using equilibrium constants. If thermodynamic constants are used, activities must be employed instead of concentrations. The activity of the ith dissolved species (a,) is related to its concentration (mj) by an activity coefficient... 

Based on these considerations, it is possible to write the expressions for the carbonic acid system thermodynamic equilibrium constants (Kj). 

We now introduce two other important relationships in the carbonic acid system, total C02 (XC02) and alkalinity (A). These quantities are important because they are usually determined by chemical analysis. If any two of the four analytic parameters pH, alkalinity, Pco2 and total C02 are known, the other two parameters can be calculated as well as the various components of the C02 -carbonic acid system. 

Alkalinity refers to the acid neutralization capacity of the solution in moles of protons, generally referred to as "equivalents", per unit volume. For our purposes, it can be divided into the alkalinity associated with the carbonic acid system, carbonate alkalinity (Ac), and the alkalinity associated with all other solution components. 

It can also be computed as ZCO2 was from an+ and Pc02> by using the carbonic acid system relationships ... 

Figure 1.1. A Bjerrum diagram for the relative proportions of carbonic acid system chemical species as a function of pH, for the case where all activity coefficients are equal to 1. pK s are values at 25°C.

Figure 1.3. Variation of the log of carbonic acid system constants with temperature. (After Plummer and Busenberg, 1982.)...

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... 

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. 

Jacobsen and Langmuir (1974) determined a value for pKSp (25°C) for calcite of 8.42 0.01, whereas Berner s (1976) value was 8.45 0.01. Berner also determined the pKSp for aragonite at 25°C to be 8.28 0.03. An aspect of particular interest of both studies was that to obtain internal consistency for the carbonic acid system or constant values for the solubility products over the range of conditions studied, it was necessary to neglect ion pair formation. The potentially important ion pairs that could have formed in the experimental solutions are CaHCC>3+ and CaC03°. The former is by far the most important species, and a vast body of previous literature supported its existence (see Plummer and Busenberg, 1982, for summary). 

In all of the examples given, a temperature of 25°C and 1 atm total pressure are assumed. The task will be to calculate the concentration of all of the carbonic acid system components, pH, At, CO2, Pcc>2< Ca2+ concentration, ionic strength, and the appropriate ion activity coefficients. The values of the thermodynamic constants used in these calculations were calculated from appropriate equations in Chapter 1. Concentrations are on the molal scale, and results for each case are summarized in Table 2.4. 

Because At is known (0) and cannot change upon addition of only CO2, the equation for At can be readily written for the addition of CO2. Using the equations for the equilibrium relations in the carbonic acid system, it is possible to write an equation for alkalinity in terms of Pc02 (known) and mH+ (our unknown). 

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