Polyprotic Acid

polyprotic acid possesses more than 2 hydronium ions (H+). 

Polyprotic Acids

Our understanding of diprotic acids and bases can be used to understand polyprotic acids and bases. 

Concept: The most common type of polyprotic acid is the triprotic acid, which contains 3 acidic hydronium ions. 


Concept: Triprotic bases contain 3 Kb values, so they would have 3 equilibrium equations. 


Concept: As a result of these equations for polyprotic acids and bases the relationship between Ka and Kb can be established. 


Example: Determine the pH of 0.300 M sodium hydrogen phosphate, Na2HPO4. Phosphoric acid, H3PO4, contains Ka1 = 7.5 x 10-3, Ka2 = 6.2 x 10-8 and Ka3 = 4.2 x 10-13


Example: Determine the pH of 0.300 M citric acid, H3C6H5O7 it possesses Ka1 = 7.4 x 10-4, Ka2 = 1.7 x 10-5 and Ka3 = 4.0 x 10-7


Polyprotic Acid Additional Practice Problems

Phosphorous acid dissociates to form dihydrogen phosphate and hydronium ion. Phosphorous acid has Ka of 5.0 x 10-2

Write the equation dissociation for phosphorous acid. Express your answer as a chemical equation. Identify all of the phases in your answer. 

What is the expression constant dissociation for phosphorous acid? 

Watch Solution

Phosphoric acid is a triprotic acid (Ka1 = 6.9 x 10^-3, Ka2 = 6.2x 10^-8, and Ka3 = 4.8 x10^-13). To find the pH of a buffer composed of H2PO4^- (aq) and HPO4^2- (aq), which pKa value would you use in the Henderson-Hasselbalch equation?

Calculate the pH of a buffer solution obtained by dissolving 13.0 g of KH2PO4(s) and 26.0 g of Na2HPO4(S) in water and then diluting to 1.00 L

Watch Solution

Cysteine is an amino acid with R=-CH2SH, which ionizes according to:

The three Pka's of cysteine are: pKA'=1.71, pKA''=8.33, and pKA=10.78

If one prepares 0.40 M solution of cysteine and the pH is adjusted to pH=8.75, calculate the molar concentrations, [Cys] and [Cys-], in the resulting solution.

Watch Solution

Identify the triprotic acid.


B) H3PO4

C) H2CO3


E) H2SO4

Watch Solution

Farmers who raise cotton once used arsenic acid, H3AsO4 , as a defoliant at harvest time. Arsenic acid is a polyprotic acid with K1 = 2.5 x 10-4 , K 2 = 5.6 x 10-8 , and K3 = 3 x 10-13 . What is the pH of a 0.500 M solution of arsenic acid?

A) 0.85

B) 1.95

C) 3.90

D) 4.51

Watch Solution

Consider a solution that is 0.10 M in a weak triprotic acid which is represented by the general formula H3A with ionization constants K1 = 1.0 × 10−3 , K2 = 1.0 × 10−8 , and K3 = 1.0 × 10−12. What is the pH of the solution?

1. 2.15

2. 2.25 

3. 2.02

4. 2.35

5. 2.54

Watch Solution

Determine the pH of a 0.03 M solution of NaH 2PO4? Assume H3PO4 has a pKa1 of 2.1 and a pKa2 of 7.2 and a pKa3 of 12.7. 

1. 4.36

2. 7.11

3. 4.65

4. 7.40

5. 9.95

6. 1.81

Watch Solution

Identify the weak triprotic acid.


B) H3PO3

C) H3PO4

D) H2SO4

E) H2SO3

Watch Solution

For the polyprotic acid H3PO4 (phosphoric acid), which of the following expressions would correspond to Ka3?

1. H3PO4 ⇌ PO43− + 3 H+

2. PO43− + 3 H+ ⇌ H3PO4

3. HPO42− ⇌ PO43− + H+

4. H2PO4 ⇌ HPO42− + H+

5. H3PO4 ⇌ H2PO4 + H+


Watch Solution

Phosphoric acid (H3PO4) is characterized by three reactions for stepwise dissociation. Arrange the following species in the order of decreasing concentration in an aqueous solution of H3PO4. Clearly indicate the order of concentration change. JUSTIFY your answer.

H3PO4          H 2PO4-          HPO 42-          PO 43-          H 3O+



Watch Solution