Can amphoteric substances with very small (or negative) Δp K difference exist and what properties would their water solutions exhibit?

For calculating the buffer capacity of ampholytes an application of the stepwise and parallel dissociation models is considered. It is demonstrated that the scheme with stepwise dissociation possesses a “nonadditive” sum, i.e. , the resulting buffer capacity of the system exceeds the sum of the separate contributions of the two ionogenic groups. On the other hand, the scheme of parallel dissociation is free from this disadvantage, and thus does not impose any restriction on the Δp K parameter.