Question

A peptide H3P+ can lose protons according to:
H3P+ ⇌ H+ + H2P0    pKa1 = 2.4

H2P0 ⇌ H+ + HP-      pKa2 = 4.7

HP- ⇌ H+ + P-2          pKa3 = 9.8

Sketch a fractional speciation diagram for this peptide, labeling axes and significant points.

Answer 1

Fractional speciation diagram is also known as α plot.

Speciation is the relative concentration of each of the related species in a solution.

α value = molar concentration of species x / total molar concentration of all species.

As we can see the α value will always be half i.e 0.5 as we are dividing one species by two and all are one moles in the given reaction.

α = 1H3P+ /1H+ + 1H2PO = ½ = 0.5 ( no units as it is a fraction)

We have pKa of all three reactions, we can safely assume that we have triprotic peptide.