Polyprotic Acid — Definition, Formula, Properties & Uses | AllChemicals | AllChemicals

Q: What is the formula for Polyprotic Acid?

The formula or notation for Polyprotic Acid is: Polyprotic acid: H₃PO₄ ⇌ H₂PO₄⁻ + H⁺ (Ka1); H₂PO₄⁻ ⇌ HPO₄²⁻ + H⁺ (Ka2)

Formula / Notation	Polyprotic acid: H₃PO₄ ⇌ H₂PO₄⁻ + H⁺ (Ka1); H₂PO₄⁻ ⇌ HPO₄²⁻ + H⁺ (Ka2)
Also Known As	Polyprotic acid, diprotic acid, triprotic acid, multiprotic acid

What is Polyprotic Acid?

An acid that can donate more than one proton per molecule. Sulfuric acid (H₂SO₄) is diprotic; phosphoric acid (H₃PO₄) is triprotic. Each ionization step has its own Ka value (Ka1 > Ka2 > Ka3). Successive proton losses become progressively more difficult.

Formula & Notation

Polyprotic acid: H₃PO₄ ⇌ H₂PO₄⁻ + H⁺ (Ka1); H₂PO₄⁻ ⇌ HPO₄²⁻ + H⁺ (Ka2)

Other Names / Synonyms: Polyprotic acid, diprotic acid, triprotic acid, multiprotic acid

Properties & Characteristics

A polyprotic acid can donate more than one proton per molecule. Diprotic acids (H₂SO₄, H₂CO₃, H₂S) have two Ka values; triprotic acids (H₃PO₄, H₃AsO₄, H₃Cit) have three. Successive Ka values decrease: Ka1 >> Ka2 >> Ka3 (each successive proton removed from an increasingly negative anion). H₂SO₄ is unusual: Ka1 = very large (strong), Ka2 = 1.2 × 10⁻² (moderate weak). At intermediate pH values, buffer regions occur between each pair of dissociation steps.

Uses & Applications

Polyprotic acids are used in: multi-stage buffer systems (phosphate buffer with three pKa values: 2.1, 7.2, 12.4; useful across wide pH range), analytical titrations (polyanions show multiple equivalence points allowing simultaneous determination), biochemistry (phosphate buffers for biological systems, citrate buffers for food applications), and industrial processes (phosphoric acid in food acidulants, semiconductor cleaning).

Safety Information

Polyprotic strong acids (H₂SO₄, H₃PO₄) are corrosive at all concentrations. Concentrated H₂SO₄ is also a dehydrating agent and reacts violently with organic matter. H₃PO₄ at high concentrations forms a highly viscous syrup that is difficult to work with safely. Always add acid to water, never water to acid.

Always consult the SDS/MSDS before handling any chemical. This information is for educational purposes only.

Key Facts

Term Polyprotic Acid

Formula Polyprotic acid: H₃PO₄ ⇌ H₂PO₄⁻ + H⁺ (Ka1); H₂PO₄⁻ ⇌ HPO₄²⁻ + H⁺ (Ka2)

Index All "P" terms

Synonyms Polyprotic acid, diprotic acid, triprotic acid, multiprotic acid

Frequently Asked Questions

What is Polyprotic Acid?

An acid that can donate more than one proton per molecule. Sulfuric acid (H₂SO₄) is diprotic; phosphoric acid (H₃PO₄) is triprotic. Each ionization step has its own Ka value (Ka1 > Ka2 > Ka3). Successive proton losses become progressively more difficult.

What is Polyprotic Acid used for?

Is Polyprotic Acid hazardous?

What is the formula for Polyprotic Acid?

More "P" Terms

P-type Semiconductor Pairing Pairing Energy Palladium Pd Parachor [P] = M(γ)^(1/4) / (ρ_L - ρ_V) Paramagnetic χ_m > 0; unpaired electrons; μ = √(n(n+2)) BM (n = unpaired electrons) Paramagnetism Partial Ionic Character % ionic = (1 - e^(-Δχ²/4)) × 100% Partial Pressure Partial Pressure p_i = x_i × P_total; P_total = Σp_i Particulate Matter Parts Per Million ppm = mg/L (water) or mg/kg; = (mass solute/mass solution) × 10⁶

View all "P" terms →