H2PO4 Acid or Base: 7 Facts Beginners Should Know!

In this article, we will understand various facts about H2PO4 acid or base.

H2PO4 acid or base is a very frequent question. So the answer to that it is an acid (weakly acidic). Its IUPAC name is dihydrogen phosphate.

Is H2PO4 acid or base, if yes is it a strong acid?

H2PO4 is an inorganic type of anion which is monovalent. It contains of phosphoric acid where in one out of three groups of OH have been deprotonated.

It’s molecular weight is around 96.98 g/mol. So H2PO4 is a comparatively weak acid and not a strong acid. We can differentiate or categorize an acid is weak or strong acid based on the fact that, whether it has the potential to undergo complete (100%) dissociation or it cannot dissociate completely to the extent of 100%.

h2po4 acid or base
h2po4 acid or base

Image credit: Wikipedia

When dihydrogen phosphate is dissolved in a solvent like water or any other aqueous type of solvent, it will not dissociate completely. Meaning it will dissociate partially. Which leads to the production of less number of ions of hydrogen.

How is H2PO4 a weak acid?

We know that a weak acid will not dissociate to full extent when it is dissolved in solvent (aqueous solvent). It has been observed that a weak acids conjugate base will be a weak base.

Also, the pH of the weak kind of acids is much higher than the pH of a strong acid (at equal concentration). Due to less dissociation, there is tendency that less number of protons are given out. So we can say an acid which is weak will have a pKa value that will be greater than the pKa value of strong acid. Meaning a weak acid will have a higher pKa value.

Talking about the conductivity of weak acids, they are not at all good conductors of electricity. Taking into consideration the rate, weak acids will show slow rate in comparison to strong acids. And the dissociation constant (Ka) is observed to have a lesser value.

So now after looking at the characteristics of a weak acid, we can conclude that dihydrogen phosphate has similar characteristics as mentioned above. Hence it is a weak type of acid.

Is H2PO4 a lewis acid?

In order to predict a substance is lewis acid or not, we must know the important characteristics of it.

The species (chemical) should necessarily possess orbitals that are empty and is capable of accepting pairs of electrons from the bases (lewis).

So H2PO4 acts as an acid when dissolved in water by producing ions (hydronium) and the HPO4 will be the conjugate base of it.

Is H2PO4 Arrhenius acid or base and why?

Let’s understand the Arrhenius acid base concept. A compound will be Arrhenius acid if it possesses atoms of hydrogen and has the potential to give away those atoms (in the solution like aqueous).

In short Arrhenius acid will be the substances which will increase the concentration of the ions of H+. Or we can say that when this compounds are added to an aqueous solvent like water, it will increase the concentration of the ions of H+. So an acid which has the ability to ionize almost completely when dissolved in an aqueous solution like that of water.

And this leads to an increase in the concentration of ions of H+. This will be a strong kind of Arrhenius acid. The acid that does not have the ability to dissociate almost completely, but dissociates partially when dissolved in an aqueous solution like water will be the weak acid.

Arrhenius concept of bases

Now coming to the Arrhenius base, it is a substance that on dissolving in an aqueous solution like that of water will give out OH ions.

So the concentration of OH will be increased. So based on the ability to give away OH ions we can categorize substances into strong and weak bases. If it can ionize completely and give away OH ions it is a strong base and if it can ionize only partially and give OH ions it is a weak base.

We can see from the structure of H2PO4 that it possesses 2 atoms of hydrogen, so it has the potential to give away those hydrogen atoms. Hence H2PO4 will be an Arrhenius acid.

Is H2PO4 a bronsted acid?

So according to the theory, a bronsted acid will be any substance that can give away protons or ions of hydrogen.

So the proton is accepted or donated to the conjugate base of that substance that is being considered for the study. So the strength of the bronsted acid depends on the dissociation constant of the substance under study.

The higher the value of the dissociation constant (Ka), the stronger will be the acid. H2PO4 is a bronsted acid.

Why is H2PO4 a bronsted acid?

As we have seen in the earlier sections that a substance with the ability to give away proton is a bronsted acid.

We have also seen that H2PO4 possesses hydrogen atoms and it has the potential to donate them. Hence H2PO4 is a bronsted acid.

What is the conjugate pair of H2PO4 and why?

As we know a H2PO4 is a bronsted acid and hence it will donate its proton. So when H2PO4 gives away its hydrogen ion it forms HPO42-.

Thus acting as an acid. But in the case when H2PO4 will accept a hydrogen ion, it will lead to the formation of H3PO4. So here it acts as a base. So when H2PO4 behaves as an acid it’s conjugate base will be HPO42-.


To wrap the post, we can conclude that H2PO4 is a weak acid. Behaves like Arrhenius acid as it has the potential to give away its proton and has the tendency to increase the concentration of H+ ions when dissolved in aqueous solution like that of water.

Sania Jakati

This is Sania Jakati from Goa. I am an aspiring chemist pursuing my post graduation in organic chemistry. I believe education is the key element that moulds you into a great human being both mentally and physically. I'm glad to be a member of scintillating branch of chemistry and will try my best to contribute whatever I can from my side and Lambdageeks is the best platform where I can share as well as gain knowledge at the same time. LinkedIn-https://www.linkedin.com/in/sania-jakati-0954101bb

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