Hydrochemistry: Basic Concepts and Exercises by Worch Eckhard

Author:Worch, Eckhard [Worch, Eckhard]
Language: eng
Format: epub
Publisher: De Gruyter
Published: 2015-05-07T16:00:00+00:00

With γ1, we can calculate the activity coefficient of the Tillmans equation:

Now, we can insert all data into the Tillmans equation to find c(CO2):

A graphical representation of the Tillmans equation is shown in Figure 9.1. The curve represents the equilibrium states, here calculated for the condition 2c(Ca2+) = 0 and under the simplifying assumption fT = 1. In the area above the curve, we find all water compositions (with respect to CO2 and ) where the CO2 concentration is higher than in the state of equilibrium. Such waters can reach the equilibrium state by dissolving calcium carbonate (calcite) as can be seen from Equation (9.4) (reaction from left to right). They are therefore referred to as carbonate-dissolving or calcite-dissolving waters. Waters with compositions that fall into the area below the equilibrium curve contain lower CO2 concentrations in comparison to the equilibrium. They can reach the equilibrium state by precipitating calcium carbonate (Equation (9.4), reaction from right to left). Such waters are referred to as carbonate-precipitating or calcite-precipitating waters.

The lines in the sketch represent water compositions with equal pH. The respective linear equations can be derived from Equation (9.8):

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