Polyprotic and monoprotic acids.
Acids that can donate just one proton are known as monoprotic acids while those acids that can donate more than one proton are known as polyprotic acids.
Let's see some examples.
Some monoprotic acids are shown on the right.
The proton that is donated is shaded in blue.
Two categories of polyprotic acids exist, diprotic and triprotic
Diprotic acids are capable of donating two protons while triprotic acids can donate up to three protons.
Some diprotic acids are shown on the right.

 

 
Some triprotic acids are shown on the right

 

 

A polyprotic acid ionizes in successive stages.

The first ionization stage of sulfuric acid is shown on the right.

 

 

The second stage of ionization now takes place and is shown on the right.

 

 
Notice the use of the uneven double arrows. Sulfuric acid is a strong acid so the first ionization stage proceeds to completion, as shown by the single arrow. That is nearly all the molecules of sulfuric acid will react in stage one of ionization

HSO4-, however, is a weak acid and will only partially ionize in water. This is represented by the uneven arrows. As a polyprotic acid loses protons in successive ionization reaction the negative charge on the acid ion increases making it harder for the remaining protons to be donated, hence making the acid weaker at each successive ionization stage.

1. Phosphoric acid is a triprotic acid. Consider the reaction of phosphoric acid with water.
Write a balanced chemical equation, with states and arrows to represent the:

- first ionization stage .
Solution

- second ionization stage
Solution

- third ionization stage
Solution

 

2. Consider the following species, HSO4- , SO4-2 and H2SO4.
i. Identify the amphiprotic species, base and acid.
ii. Identify two acid/base conjugate pairs
iii. Place the three species on the spectrum below.

strong acid ------------------------------------------- strong base

Solution

3. Write balanced equations, with states and arrows, for the successive ionization stages of H3AsO4 in water

Solution