Neutralisationstitration?

Let’s look at the titration curve in greater detail. For this I calculated it with c=0.11 mol/l, pK1=1.83 and pK2=9.13.

You see not only the titration curve (black), but also its first wear (white) and the species distribution, i.e. the relative proportions of the different protonation stages:

• Red is the cation CH3–CH(C6H5)N+H3–COOH. This is the completely protonated form which was weighed into the titration flask (c=0.11 mol/l). Because the thing is a medium-strong acid, it is dissociated to a good part (31%), and the pH is correspondingly low to 1.47.
• Violet is the Zwitterion CH3–CH(C6H5)N+H3–CO2 ̄. Up to the first equivalence point (V=5.5 ml, pH 5.51) more and more of it is formed, and on the way to the second equivalence point (V=11 ml, pH=11.04) it is then consumed again.
• Blue is the anion CH3–CH(C6H5)NH2–CO2 ̄. It forms between the first and second equivalence points, and then it is practically the only species in the solution.

Between the first and second equivalence points, there is a buffer region in which the pH digs around in the vicinity of the pK2 and does not change greatly when adding base; the ring at V=8.25 exactly in the middle between the two equivalence points marks the buffer point, at the pH=pK2 (in each case 50% of tweener ion and anion are present). The requested pH=8.8 is not far from the pK2 (in the titration curve at V=7.25 ml) and is therefore still in the range of powder (grob pK2±1).

There is also a buffer point in the first half of the titration, but it is not so good to see; it is not exactly half the consumption at the equivalence point, but rather somewhat earlier (V=1.98 ml), because the phenylanaline is already markedly dissociated in the starting solution and the 1:1 point is therefore already reached before the halfway mark.

We can also look at the bufferability of phenylalanine in a somewhat different plot. For this, I draw the species distribution of a 1 mol/l solution in the whole pH range, from 0 to 14. The black curve indicates how much base you have to admit to exactly this pH, and the yellow shows the buffer capacity of the solution.

The proportion of buffer capacity resulting from phenylalanine is indicated orange. You see two maxima exactly around the pKa values. The buffer peak at an approximate pH ≅9 also covers pH=8.8; at pH=8.8 we get a pH of 0.5 units per mole of acid or base added; that is not much smaller than the 0.58 at the maximum at pH=pK2=9.13. Phenylalanine is therefore well suited as a graft in this pH range.