It is to be expected that 𝑘 ! will depend on a number of factors, in particular, the local chemical environment and temperature. The presence of other species, although they may not have any obvious direct involvement in the reaction, may give rise to various forms of interactions, for example crowding, shielding, attraction or repulsion, which can influence the likelihood of the reaction (1) occurring and so will affect 𝑘 ! . Likewise, the presence of other nearby entities of X and S may also give rise to interactions and so influence whether a close contact leads to a reaction. So, 𝑥 and 𝑠 may affect the overall reaction rate not just through the product term 𝑥𝑠, but also through 𝑘 ! . However, it is assumed that 𝑘 ! does not change significantly during the course of the reactions considered here and consequently 𝑘 ! can be regarded as approximately constant. This constant may, however, have different values under different chemical conditions, such as widely different pH and NaCl concentrations as are investigated in the study.