Amphiprotic anions are negative ions that can undergo both acid and base hydrolysis. Here we’ll look at some of these ions and show you how to find which hydrolysis will predominate for a particular ion.

It is important to be able to recognize amphiprotic anions. Here is how we do it.

Firstly, the formulas for amphiprotic anions always start with “H”. It may be just H, or H two. Formula starts with “H”

The second thing is amphiprotic anions always have a negative charge. It could be just negative 1 or negative 2. Formula starts with “H” Has a negative charge

You will find amphiprotic anions on both sides of the acid table. This is because they can undergo both acid hydrolysis and base hydrolysis.

The name for an amphiprotic anion is found using the one on the (click) left side of the table. For example, this ion is called the hydrogen oxalate ion.

The names of amphiprotic anions (click) start with the word “hydrogen”

Or (click) dihydrogen,

As in the dihydrogen citrate ion, H2 C6 H5 O7 minus

Notice HSO3 minus (click) is called the hydrogen sulphite ion.

HSO3 minus is also found on the ion table.

It can be called either (click) hydrogen sulphite like it is called on the acid table,

Or it can also be called bisulphite

These other amphiprotic anions found on the ion table can also be found on both sides of the acid table. You will find it easier to locate them as you use them more.

Now that we know how to identify ions as amphiprotic, lets take a closer look at how these can undergo both acid and base hydrolyis.

We’ll use the dihydrogen phosphate ion, H2PO4 minus, as an example.

Because its on the left side of the acid table, we know it can undergo acid hydrolysis.

When H2PO4 minus undergoes acid hydrolysis, it plays the role of an acid Acid Hydrolysis of H 2 PO 4 – acid

And water plays the role of a base Acid Hydrolysis of H 2 PO 4 – acidbase

So a proton is transferred from the H2PO4 minus ion (click) to the water molecule Acid Hydrolysis of H 2 PO 4 – H+H+ acidbase

When the water gains a proton it forms (click) its conjugate acid, a hydronium ion, H3O plus Acid Hydrolysis of H 2 PO 4 – H+H+ acidbase

And when H2PO4 minus loses a proton (click) it forms its conjugate base HPO4 2 minus. Acid Hydrolysis of H 2 PO 4 – acidbase H+H+

So this is the equation for the acid hydrolysis of the dihydrogen phosphate ion, H2PO4 minus. Acid Hydrolysis of H 2 PO 4 –

Looking on the acid table, we see that H2PO4 minus (click) is also the right side. This means it can also undergo base hydrolysis

When H2PO4 minus undergoes base hydrolysis, it plays the role of a base Base Hydrolysis of H 2 PO 4 – base

And water plays the role of an acid. Base Hydrolysis of H 2 PO 4 – baseacid

So a proton is transferred from a water molecule to the H2PO4 minus ion. Base Hydrolysis of H 2 PO 4 – baseacid H+H+

When a water molecule loses a proton (click), it forms its conjugate base, a hydroxide ion, OH minus Base Hydrolysis of H 2 PO 4 – baseacid H+H+

And when H2PO4 minus gains a proton (click) it forms its conjugate acid H3PO4, phosphoric acid. Base Hydrolysis of H 2 PO 4 – baseacid H+H+

So this is the equation for the base hydrolysis of H2PO4 minus. Base Hydrolysis of H 2 PO 4 – baseacid

In order to find out whether an amphiprotic anion makes a solution acidic or basic, we have to determine which is predominant, acid hydrolysis or base hydrolysis.

So we have the acid hydrolysis of dihydrogen phosphate, which produces hydronium ions in solution, Acid Hydrolysis of H 2 PO 4 – baseacid

And we have the base hydrolysis of dihydrogen phosphate, which produces hydroxide ions in solution. Both of these reactions take place when dihydrogen phosphate is in solution. Base Hydrolysis of H 2 PO 4 – baseacid Acid Hydrolysis of H 2 PO 4 – baseacid

Remember that if the hydronium ion concentration in a solution is greater than the hydroxide ion concentration, If: then the solution is Acidic then the solution is Basic then the solution is Neutral

Then the solution is acidic If: then the solution is Acidic

if the hydroxide ion concentration in a solution is greater than the hydronium ion concentration, If: then the solution is Acidic then the solution is Basic then the solution is Neutral

Then the solution is basic If: then the solution is Acidic then the solution is Basic then the solution is Neutral

And if the hydroxide ion concentration is equal to the hydronium ion concentration, If: then the solution is Acidic then the solution is Basic then the solution is Neutral

Then the solution is neutral. If: then the solution is Acidic then the solution is Basic then the solution is Neutral

So if the acid hydrolysis of an amphiprotic anion occurs to a greater extent, or is predominant over base hydrolysis, (click) more hydronium is formed than hydroxide. then the solution is Acidic then the solution is Basic then the solution is Neutral If the acid hydrolysis of an amphiprotic anion is predominant then more hydronium is produced than hydroxide

And the solution will be acidic. and the solution is Acidic then the solution is Basic then the solution is Neutral If the acid hydrolysis of an amphiprotic anion is predominant then more hydronium is produced than hydroxide

If the base hydrolysis of an amphiprotic anion is predominant, then (click) more hydroxide is formed than hydronium and the solution is Acidic then the solution is Basic then the solution is Neutral If the base hydrolysis of an amphiprotic anion is predominant then more hydroxide is produced than hydronium

And the solution will be basic. and the solution is Acidic and the solution is Basic then the solution is Neutral If the base hydrolysis of an amphiprotic anion is predominant then more hydroxide is produced than hydronium

The extent of acid hydrolysis is determined by (click) the value of the Ka for dihydrogen phosphate. The Ka expression is shown here. Acid Hydrolysis of H 2 PO 4 – Value

The concentration of hydronium is in the numerator, so the greater the Ka value the more hydronium ion there is. Acid Hydrolysis of H 2 PO 4 –

The extent of base hydrolysis is determined by (click) the value of the Kb for dihydrogen phosphate. The Kb expression is shown here. Base Hydrolysis of H 2 PO 4 – Acid Hydrolysis of H 2 PO 4 – Value

The concentration of hydroxide is in the numerator, so the greater the Kb value the more hydroxide ion there is. Base Hydrolysis of H 2 PO 4 – Acid Hydrolysis of H 2 PO 4 –

We can summarize by saying, if the Ka for an amphiprotic anion is greater than the Kb for the same ion,

Then acid hydrolysis will predominate

And the concentration of H3O plus will be greater than the concentration of OH minus.

Now, if Kb for an amphiprotic anion is greater than the Ka for the same ion,

Then base hydrolysis will predominate

And the concentration of OH minus will be greater than the concentration of H3O plus.

Its important to remember these two statements. If Ka is larger than Kb, then acid hydrolysis predominates and if Kb is larger than Ka, then base hydrolysis predominates

The value of Ka for dihydrogen phosphate is found by (click) looking it up on the LEFT side of the table, (click) then reading its value directly from the right side of the row.

we’ll make a note of it up here

In order to find the value of Kb for H2PO4 minus, we must locate H2PO4 minus (click) up here on the RIGHT side of the table. To find Kb, we must treat this ion as a base, so we look for it on the right side.

So we want to find the value of Kb for H2PO4 minus. We want to find the value of K b for this ion.

The conjugate acid of H2PO4 minus is H3PO4, phosphoric acid The conjugate acid of H 2 PO 4 – is H 3 PO 4

So the Ka of the conjugate acid of H2PO4 minus is 7.5 × 10 –3. The K a for the conjugate acid of H 2 PO 4 – is 7.5 ×10 –3

Remember, the formula for the Kb of a species such as H2PO4 minus The K a for the conjugate acid of H 2 PO 4 – is 7.5 ×10 –3

Is Kb equals Kw divided by the Ka of its conjugate acid. The K a for the conjugate acid of H 2 PO 4 – is 7.5 ×10 –3

Which is Kw divided by the Ka of H3PO4, phosphoric acid. The K a for the conjugate acid of H 2 PO 4 – is 7.5 ×10 –3

Kw is 1 times 10 –14. The K a for the conjugate acid of H 2 PO 4 – is 7.5 ×10 –3

And the Ka of the conjugate acid H3PO4 The K a for the conjugate acid of H 2 PO 4 – is 7.5 ×10 –3

Is 7.5 ×10 –3 The K a for the conjugate acid of H 2 PO 4 – is 7.5 ×10 –3

So the value for Kb, is 1.0 ×10 –14 divided by 7.5 ×10 –3 The K a for the conjugate acid of H 2 PO 4 – is 7.5 ×10 –3

Which comes out to 1.3 ×10 –12 The K a for the conjugate acid of H 2 PO 4 – is 7.5 ×10 –3

So we’ll make a note of that up here.

We had said that if Ka of H2PO4 minus is greater than its Kb,

Then acid hydrolysis predominates

And if Kb of H2PO4 minus is greater than its Ka

Then base hydrolysis predominates.

6.2 × 10 –8 is greater than 1.3 × 10 –12, >

So the Ka of H2PO4 is larger than its Kb >

Therefore, acid hydrolysis predominates. >

And the equation for the acid hydrolysis of H2PO4 minus is > Acid Hydrolysis of H 2 PO 4 –

Is H2PO4 minus > Acid Hydrolysis of H 2 PO 4 –

Plus H2O > Acid Hydrolysis of H 2 PO 4 –

Gives H3O plus > Acid Hydrolysis of H 2 PO 4 –

Plus HPO4 2 minus > Acid Hydrolysis of H 2 PO 4 –

Because acid hydrolysis predominates > Acid Hydrolysis of H 2 PO 4 –

We can say that the equation for predominant hydrolysis of H2PO4 minus is (click) this equation, H2PO4 minus acting as an acid in its reaction with water, an forming a hydronium ion. > Acid Hydrolysis of H 2 PO 4 – Predominant Hydrolysis of H 2 PO 4 –

To review, an Amphiprotic anions such as dihydrogen phosphate can act both as and acid (click) and as a base.

Undergoing both acid hydrolysis (click) to form a hydronium ion and base hydrolysis to form (click) a hydroxide ion Base Hydrolysis of H 2 PO 4 – baseacid Acid Hydrolysis of H 2 PO 4 – baseacid

The extent of acid hydrolysis is (click) reflected by the value of its Ka Base Hydrolysis of H 2 PO 4 – baseacid Acid Hydrolysis of H 2 PO 4 – baseacid Extent of acid hydrolysis

And The extent of base hydrolysis is (click) reflected by the value of its Kb Base Hydrolysis of H 2 PO 4 – baseacid Acid Hydrolysis of H 2 PO 4 – baseacid Extent of acid hydrolysis Extent of base hydrolysis

Once we know the values of Ka and Kb for an amphiprotic anion, we compare them. If (click) Ka for the ion is larger than Kb (click) acid hydrolysis will predominate and more hydronium will be produced than hydroxide.

And if (click) Kb for the ion is larger than Ka (click) base hydrolysis will predominate forming more hydroxide than hydronium.

In the example we used, H2PO4 minus, it’s Ka value (click) 6.2 × 10 –8 is larger than its Kb value, 1.3 × 10 –12, so (click) Acid hydrolysis is predominant. >

And the equation for the predominant hydrolysis of dihydrogen phosphate is (click) H2PO4 minus plus water gives H3O plus, plus HPO4 2 minus. > baseacid