Kinetics of CO2 reactions in seawater

Although most of the reactions between carbonate species in seawater are nearly instantaneous, the hydration of CO2 

The mechanisms for this reaction are discussed in the chapter on kinetics (Chapter 9). It is a combination of first- and second-order reactions, which is not solvable anal5dically because of the nonlinear terms following the rate constants koH and kcoj.r- The rate constants were determined in the laboratory by choosing the experimental conditions in which one of the two mechanisms predominated. pH values of natural waters, however, often fall in the range 8-10, in which the reaction with both water and OH can be important. To determine the life time of CO2 as a function of pH, one must derive the solution to the reaction rate equation. This is facilitated by employing the DIG and carbonate alkalinity, Ac, (Eqs. (4.15) and (4.26)) to eliminate the concentration of bicarbonate [HCOJ], in the CO2 reaction rate equation. This substitution results in an expression 

This is an approximation because the OH concentration does change during the reaction, but since the change is not very great the equation is adequate to illustrate the importance of the two reaction mechanisms. Equation (4.31) is the solution for a reversible reaction that begins with an initial concentration of [C02]° and progresses toward an equilibrium value of [C02]° + BjA. The value represented in A is the reciprocal of the residence time of CO2 with respect to chemical reaction and incorporates both mechanisms of reaction. 

The reaction rate constants have been determined as a function of temperature and salinity by Johnson (1982). Values in Table 4.6 are calculated from the best-fit equations for his experiments. After a 

The values are from the equation which best fit the data of Johnson (1982). His values for koH Kw are reinterpreted as indicated in Emerson (1995). The equilibrium constants necessary to calculate the reverse rate constants are also tabulated. Where two values are presented in column 1 the first is for fresh water (I = 0) and the second is for seawater. The exponential notation in column 1 indicates the order of magnitude the variable is multiplied by to equal the number in the table. (For example, x 10 in column 1 means 3.44 x 10 was multiplied by 1 x 10 before tabulating it as 3.44 in column 2.) 

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