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Amphiprotic Species

What are Amphiprotic Species?

• Amphiprotic species are substances that act as both an acid and a base due to being able to both accept a proton and donate a proton

• All amphiprotic species are amphoteric (meaning they can act as either an acid or a base)

• These species can be both molecules or ions!

Note: Although all amphiprotic substances are amphoteric, not all amphoteric substances are amphiprotic! (This is because amphoteric substances do not need to be able to donate/accept protons in order to show acid-base behavior)

Examples of Amphiprotic Species:

• $H_{2}O$

• $H_{2}PO_{4}$-

• $H_{2}PO_{3}$-

Difference Between Amphiprotic And Amphoteric

Amphoteric is a term used to describe a chemical species which can act as either an acid or a base.

Now we need to consider how acids and bases are defined. The Brønsted-Lowry theory defines them in terms of hydrogen atoms (commonly called protons in acid-base reactions):

• Acids donate protons

• Bases accept protons

By the definition of Lewis acid base theory:

• Acids accept electrons

• Bases donate electrons

Amphoteric species then:

• Accept and donate electrons

• Accept and donate protons

There is a category of amphoteric species called amphiprotic species. Amphiprotic species can be both, a proton donor and a proton acceptor. This means that a species that is amphiprotic is a Brønsted-Lowry acid and base.

Amphiprotic Examples

Water ($H_2O$) is a very common solvent and an amphiprotic species. If it gains a hydrogen atoms (proton), it becomes a positive hydronium ion. If it loses a proton, it becomes a negative hydroxide ion.

But when does an amphiprotic species donate or accept a proton?

Often times, it will depend on the reaction conditions, other species present, and whether it is an acid or base. A water molecule donates protons when a base is present, like ammonia ($NH_3$). An acid, like hydrochloric acid ($HCl$) will donate a proton to water. Some species have known pKa values which indicate specific pHs where the amphiprotic species will accept or donate protons.

Examples of pH dependent amphiprotic species are amino acids. Amino acids are compounds which are used by living organisms in proteins, enzymes, biosynthesis, and more. Amino acids have two mains parts which can accept or donate protons:

• Amino ($-NH_2$)

• Carboxyl ($-COOH$)

At physiological pH (pH=7), or the pH a living organism is at, the zwitterion form is present. A zwitterion has both a negative and positive charge.

Amphiprotic Species

(Video) 8.1 Amphiprotic species (SL)

By Mike Sugiyama Jones

According to the Bronsted-Lowry theory on acids and bases, acids are proton donors and bases are proton acceptors. Amphiprotic species are molecules and ions that can act as both an acid and a base under this theory.

This video will explain more about what the term means, as well as give examples of different amphiprotic species. It also shows the chemical reactions of these species to help better understand how an they work.