1. For the sample in which the pH was not adjusted (neutral sample, pH = 6.99), 𝑟 % was taken to be a model parameter to be estimated. Its estimated value was found to be 𝑟 % ≈ 1, implying that the sorbent and aqueous HgCl2 are close to stoichiometric balance (Fig. ). Changing 𝑟 % by as little as ±2.5%, produced noticeable deviations from the experimental data. Since all other experimental cases used the same amounts of HgCl2 and sorbent, it was supposed that 𝑟 % would be unchanged, and it was given the fixed value 𝑟 % = 1 for these cases also.