Phosphoric acid or ortho-phosphoric acid is a phosphorous-containing triprotic inorganic acid with the chemical formula H3PO4.
The commercial method of preparation of phosphoric acid is the wet process in which sulfuric acid gets added to phosphate rock in a chain of well-stirred reactors.
It appears as a colorless, odorless syrupy liquid or transparent crystalline solid. It has an important role as a human metabolite, detergent additives, fertilizer, sanitizing agent, and solvent; however, can severely damage skin and eyes too.
In this article, we will discuss some of the most searched questions related to Phosphoric acid-like
• Nature of H3PO4 (acid or base)
• Nature of strength (strong or weak)
• Conjugate pair of H3PO4
• Acidity or Basicity of H3PO4
So, is H3PO4 an acid or base? H3PO4 is a weak acid because it does not dissociate completely in its aqueous solution or water. The strength of acidity of the compound depends on the number of hydrogen ions in the solution after its dissociation and H3PO4 releases only a few hydrogen ions in the solution which makes it a weak acid.
Strong acids have a lower pKa value (below zero). However, the pKa value of H3PO4 is around 2.14, again indicating it to be a weak acid with a lower ability to donate a proton in an aqueous solution.
|Name of compound
Why do H3PO4 act as an acid?
Two major theories in chemistry that define acids and bases are-
• Arrhenius theory
• Bronsted-Lowry theory
|It defines an acid as an H+ ions donor in a solution
|It defines an acid as a proton donor to form the conjugate base
It explains the reaction between acids and bases that yield water and salts
It fails to explain how compounds like HNO2 lacking hydroxide ion forms basic solution in a water solution
It explains the acidic or basic nature of both neutral and ionic species in an aqueous as well as non-aqueous medium.
It fails to explain how compounds like BF3 and AlCl3 show acidic properties without hydrogen
Let’s understand the nature of H3PO4 (acidic or basic) with the help of the above theories
Arrhenius theory for acid:
When H3PO4 is dissolved in water, it breaks down into two constituent ions, H2PO4‾ and H+. So, according to Arrhenius’s theory, it acts as an acid.
H3PO4 (aq) ⇌ H+ (aq) + H2PO4‾(aq)
Bronsted-Lowry theory for acid:
When H3PO4 is dissolved in water, it donates one proton to the H2O to form a hydronium ion and itself becomes a conjugate base, H2PO4‾.
So, according to Bronsted-Lowry, it is an acid.
H3PO4 + H2O —-> H3O+ + H2PO4−
Is Phosphoric (H3PO4) a Strong Acid or Weak Acid?
|Strong acid ionizes completely in an aqueous solution
|Weak acid ionizes or dissociates partially in a solution.
|Releases good amount of hydrogen ions when dissolved in water
|Releases few hydrogen ions only when dissolved in water
|Strong tendency to remove proton to other species to form a weak conjugate base
HA + H20 → H3O+ + A– (weak conjugate base)
|Form strong conjugate base and quickly takes up the released proton from the solution
HA + H20 ⇌ H3O+ + A– (strong conjugate base)
|Strong acids have higher degrees of dissociation (α)
|Weak acids have lower degrees of dissociation (α)
|A strong acid has a high value for acid dissociation constant (Ka)
|Weak acids have a low value for acid dissociation constant (Ka)
|Strong acids have a low value for logarithmic constant (pKa)
|Weak acids have a high value for logarithmic constant (pKa)
|For Example, HCl, HNO3, H2SO4
|For Example, H2S03, CH3COOH, H3PO4
Now is H3PO4 a weak or strong acid?
H3PO4 is triprotic acid (having three hydrogen ions or acidic protons) and it donates its three hydrogen ions sequentially.
The dissociation of H3PO4 in aqueous solution is as follows:
H3PO4 + H2O ↔ H3O+ + H2PO4− [pKa = 2.14]
H2PO4− + H2O ↔ H3O+ + HPO42− [pKa = 7.20]
HPO42− + H2O ↔ H3O+ + PO43− [pKa = 12.37]
First step: In this step, H3PO4 donates its one acidic proton to water and form H2PO4-
Second step: In this step, H2PO4− donates the second acidic proton to water to form HPO42− but H2PO4− is a weaker acid than H3PO4 (Weak acid has higher pKa value) so it is more difficult for H2PO4− to remove proton in aqueous solution.
Third step: In this step, HPO42− donates the second acidic proton to water to form PO43−.
Again, HPO42− is a weaker acid than H2PO4− and H3PO4 (indicating a higher pKa value of 12.37) so it becomes more difficult for HPO42− to remove the third acetic proton in an aqueous solution.
As the order of acidity decreases with each subsequent dissociation, the removal of acidic hydrogen becomes more difficult.
As a result, H3PO4 releases few hydrogen ions.
Moreover, all the above reactions are reversible (indicated by ↔).
It means that the reaction will undergo both forward and backward reactions.
After the dissociation of H3PO4 in an aqueous solution, H2PO4− reacts again with hydrogen ions to form H3PO4 because H2PO4− is less stable.
Similarly other unstable compounds like HPO42− and PO43− also react with hydrogen ions to make H3PO4 again.
In this way, most of the moles of H3PO4 remain un-dissociated and it is considered a weak acid.
Conjugate Base of H3PO4
When an acid donates its acidic proton to another species in a reaction, it forms its corresponding conjugate base.
Similarly, when a base takes an acidic proton from another species in a reaction, it forms its corresponding conjugate acid.
According to the conjugate acid-base pair, the nature of the compound and its corresponding pair is always opposite in nature.
A strong acid always forms a weak conjugate base.
A strong base always forms a weak conjugate acid.
A weak acid always forms a strong conjugate base.
A weak base always forms a strong conjugate acid.
In the case of H3PO4, it forms H2PO4- (dihydrogen phosphate anion) as its conjugate base after removing an acidic proton.
Moreover, H3PO4 is a weak acid so according to the conjugate acid-base pair, its corresponding pair, H2PO4− is a strong conjugate base.
Why is H3PO4 a weaker acid than H3PO3?
H3PO3 is a stronger acid than H3PO4 because of-
Experimentally calculated pKa values: pKa value of H3PO3 (1.3) is lower than H3PO4 (2.1) indicating the former to be a stronger acid.
The lower the pKa value, the higher is the acidic character of that compound.
• P-OH bond: The more the P-OH bond, the lower the acidic strength of the oxyacid.
So, H3PO4 is a weaker acid than H3PO3
• Polarity: H3PO3 is more polar than H3PO4 hence, the O-H bond is weaker in the former causing the easy removal of H+ ion.
As a result, H3PO3 is more acidic than H3PO4.
BTW, you must also read out the article on lewis structure of H3PO4.
• Stability of conjugate base: The more stable the conjugate base of an acid, the higher the acidic character.
The conjugate base of H3PO4 is H2PO4− and of H3PO3 is H2PO3- and according to resonance structures, the former conjugate base is more stable.
Hence, H3PO3 is a stronger acid than H3PO4.
Basicity of H3PO4
Basicity is defined as the number of acidic hydrogen atoms present in an acid.
When the Hydrogen atom is bonded to a strongly electronegative atom like fluorine, oxygen, then only it is considered to be acidic.
In H3PO4, there are three O-H bonds or three acidic hydrogen atoms hence, it is a tribasic acid.
Properties of H3PO4
• Acidic property
H3PO4 has three acidic hydrogen with dissociation constants pKa1 = 2.14, pKa2 = 7.20, and pKa3 = 12.37.
• Reducing property
The oxidation state of Phosphorus in H3PO4 is +5 which cannot be oxidized further.
So, H3PO4 cannot be a reducing agent. It is a poor oxidizing agent.
H3PO4 can dimerize like carboxylic acids via dehydration reaction.
Uses of H3PO4
• It is used as a phosphate fertilizer.
• It is used as a sanitizing agent, dispersing agent, electrolyte, etc.
• It is used as teeth whiteners or mouth washing liquids.
• It is used as an additive and flavoring agent in animal or poultry feed.
• It is used as a rust remover from the surfaces of various metals like steel, iron, etc.
• It is used in cosmetics as a pH modifier or pH controller.
• It is used in food and beverages to acidify them.
• It is used in compound semiconductor processing.
Phosphoric acid is an IUPAC name for H3PO4. It contains four Oxygen atoms, one Phosphorus atom, and three Hydrogen atoms. It is present in our bones and teeth and acts as a metabolite.
Is H3PO4 an acid or base? Phosphoric acid is a weak oxyacid with three replaceable hydrogen atoms. Its pKa value is 2.14. It donates its acidic hydrogen in an aqueous solution to form its conjugate base, H2PO4-.
Is H3PO4 a strong or weak acid? It follows three sequential deprotonation steps to release all three acidic hydrogens but even after dissociation, split ions reform with each other and make Phosphoric acid again.
Hence, it remains un-dissociated in an aqueous solution and is considered a weak acid.
The basicity of H3PO4 is three (3).
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