Consider the polyprotic species H_4A^+ Leftrightarrow H_3A^- Leftrightarrow H_2A

Question

Consider the polyprotic species H_4A^+ Leftrightarrow H_3A^- Leftrightarrow H_2A^- Leftrightarrow HA^2- Leftrightarrow A^3- How many amine and how many carboxyolate groups are attached to this species? Amines_____ Carboxylates________ For the following, assume pK_a 1-4 = 2, 4, 7 and 11 respectively. What is the pH at the isoelectric point? At what pH is [H_4A^+] = [H_3A]? At what pH is [H_4A^+] = [H_2A]? Which is the principle species at pH = 5? Which is the second principle species and ratio to principle at pH = 5? What is the principle species at pH = 10? Which is the second principle species and ratio to principle at pH = 3? At which pH range(s) would this be a useful buffer?

Explanation / Answer

For the polyprotic species H4A+

Number of amine group attached to speices = 1

Number of carboxylate attached to species = 3

For the given species with pKa1 = 2 ; pKa2 = 4 ; pKa3 = 7 ; pKa4 = 11

pH at isoelectric point = (2+7)/2 = 4.5

pH at which [H4A+] = [H3A] = 2

pH at which [H4A+] = [H2A-] = (2+4)/2 = 3

Principle species at pH 5 = H3A

Second principle species at pH 5 = H2A-

Principle species at pH 10 = HA^2-

Second principle species at pH 3 = H3A

ratio to principle at pH 3,

3 = 4 + log(H3A/[H4A+)

ratio to principle species [H3A]/[H4A+] = 0.1

pH range(s) where this is a buffer = 2 to 2.9 ; 3.1 to5.4 ; 5.6 to 8.9

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