The solution is an exponential function with 𝐶 ∞,𝑖 as the upper limit of the soluble ions. After fitting the function to the data, the value of 𝐶 ∞,𝑖 can therefore not exceed the values in Table . Solid components cannot react because they also have a diffusive character. Diffusion within the particles is not considered in this work. As a result, 𝐶 ∞,𝑖 can be lower than the value after fitting the equation to the data. 𝐶 0,𝑖 corresponds to the concentration at the beginning of the leaching process and k corresponds to the reaction rate. Equation ( ) can be used for both alkaline ions calcium and magnesium: