We observe a significant change in the behaviour of the roots of β when C 1 = C 2 . Indeed, as shown in figure .a, the triple point disappears and is split in two, even if there still exists a 1-and a 3-roots regime, with a transition point for a given value of ∆pK c a that appears dependent upon the ratio r = C 2 /C 1 (see figure .b) but not on the magnitude of the concentrations. Here, as r < 1, C 2 varies below C 1 . In that case, the single root regime corresponds to a maximum that is closer to pK a1 than to pK a2 and not anymore equal to pK a . However, it logically goes to 4 when ∆pK a → 0. In the three roots regime, the "middle root" is above pK a and goes to pK a as ∆pK a increases beyond ∆pK c a . The situation is perfectly symmetrical when