It is instructive to plot 1/𝑘 ! against pH or the log of the chloride ion concentration (as appropriate for the case). This is shown in Figure . As all cases were prepared from HgCl2, there was always a background Cl -concentration arising from the 5 ppm HgCl2. For this reason, the pH = 7 case has been treated as a (low level) reference for the acid, base and chloride cases.