Carbonate total molalities, calculation

Calculation of Calcium and Carbonate Total Molalities. Because of the constancy of composition of seawater, the total ion calcium concentration in seawater can be calculated, in "open ocean seawater samples, directly from high precision salinity measurements, using the relationship (14) ... 

Acidity Enter as equivalents/kg (water). This is the total hydrogen concentration, if known initially. Generally this is only known for strong acid solutions. For example, for a 1 molal H2SO4 solution, enter 2.00. Otherwise, enter 0.0. The equations used to calculate pH for the alkalinity and acidity cases are incompatible. Thus a specification of either carbonate alkalinity or acidity requires that the other variable be assigned a value of 0.00. This will channel the calculations to the proper algorithm. 

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 of difficulties in precisely calculating the total ion activity coefficient (y) of calcium and carbonate ions in seawater, and the effects of temperature and pressure on the activity coefficients, a semi-empirical approach has been generally adopted by chemical oceanographers for calculating saturation states. This approach utilizes the apparent (stoichiometric) solubility constant (K ), which is the equilibrium ion molal (m) product. Values of K are directly determined in seawater (as ionic medium) at various temperatures, pressures and salinities. In this approach ... 

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 ... 

Although every combination of the four carbonic acid system-analyzable variables has been used, the one pair frequently used because of its relative ease from both an analytical and calculation standpoint is pH and alkalinity. When this pair is used, the equation relating total carbonate molality to pH and carbonate alkalinity is ... 

Edwards et al. (1975, 1978A, B) established a molecular-thermodynamic correlation for calculating vapor-liquid equilibria in aqueous solutions containing one or more volatile electrolytes, with special attention to the ternary systems, ammonia-carbon dioxide-water and ammonia-hydrogen sulfide-water. Their 1978 correlation was shown to give results in satisfactory agreement with the limited data then available for temperatures from 0° to 170°C (32° to 338° F) and ionic strengths of about 6 molal (equivalent to total concentrations of the electrolytes between 10 and 20 molal). 

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