Polyprotic Acid Calculator

Calculate pH and speciation for diprotic and triprotic acids. Handle carbonic acid, phosphoric acid, amino acids, and EDTA with multiple pKa values step by step.

🔬 Acid-Base📐 Stepwise: H2A to HA^- to A^2-🧪 Chemistry
Initial acid concentration C (mol/L)
First Ka1 value
Second Ka2 value
Solution pH (known or to calculate step 1)
Please enter valid values.

Formula & Reference

VariableSymbolFormulaUnits
Polyprotic Acid Calculator—Stepwise: H2A to HA^- to A^2-pH

Step-by-Step Examples

Example 1
Carbonic Acid

0.100 M H2CO3. Ka1=4.3x10^-7, Ka2=4.7x10^-11.

  • Step 1: x1 = sqrt(4.3x10^-7 x 0.100) ~ 2.07x10^-4 M
  • pH1 = 3.68 (dominated by first dissociation)
  • At pH 3.68: H2CO3 >> HCO3- >> CO3^2-
  • H2CO3 ~ 99.95%, HCO3- ~ 0.05%, CO3^2- ~ 10^-13%
✓ pH = 3.68 for 0.1 M H2CO3
Example 2
Phosphoric Acid

0.100 M H3PO4. Ka1=7.1x10^-3, Ka2=6.3x10^-8, Ka3=4.5x10^-13.

  • First dissociation: x^2/(0.100-x) = 7.1x10^-3
  • Quadratic: x = 0.0238 M, pH = 1.62
  • Second and third contribute negligibly to [H+]
✓ pH = 1.62 for 0.1 M H3PO4
Example 3
Speciation at Given pH

H2CO3 at pH 7.0. Ka1=4.3x10^-7, Ka2=4.7x10^-11, C=0.001 M.

  • Denominator = H^2 + Ka1*H + Ka1*Ka2 at [H+]=10^-7
  • HCO3- = C*Ka1*H/denominator = dominant species
  • At pH 7: ~96% HCO3-, ~3% H2CO3, ~0.047% CO3^2-
✓ HCO3- is dominant at blood/ocean pH

Real-World Applications

̊
Ocean Chemistry
CO2 + H2O = H2CO3 (Ka1) = HCO3- (Ka2) = CO3^2-. Ocean pH determines CO3^2- concentration for shell formation.
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Phosphate Buffers
Phosphate exists as H3PO4, H2PO4-, HPO4^2-, PO4^3- depending on pH. Biological systems at pH 7.4 use H2PO4-/HPO4^2-.
🧬
Amino Acids
Zwitterion form: amino acids have pKa1 (~2) for COOH and pKa2 (~9.5) for NH3+. At isoelectric point: net charge zero.
💧
Water Treatment
Carbonate equilibria control water pH, corrosion of pipes, and scaling. Managed by adjusting pH/alkalinity.

Common Mistakes to Avoid

⚠️
Ka1 >> Ka2 >> Ka3 usually

Each successive dissociation is much weaker. First H+ dissociates most easily; subsequent ones face stronger electrostatic repulsion.

⚠️
pH mainly from first dissociation

Usually pH calculated from Ka1 only. Ka2 and Ka3 contribute negligibly to [H+] since Ka1 >> Ka2.

⚠️
Speciation requires knowing all Ka values

To find fraction as each species at given pH: use full distribution diagram formulas with all Ka values.

Frequently Asked Questions

What is a polyprotic acid? ▾
Acid that can donate more than one proton per formula unit. H2SO4 (diprotic), H3PO4 (triprotic), EDTA (tetraprotic).
What is speciation? ▾
Distribution of a substance among its different charge/protonation states as a function of pH. Shown in species distribution (alpha) diagrams.
What is the isoelectric point? ▾
pH where amphoteric species (amino acid, ampholyte) has zero net charge. For diprotic: pI = (pKa1 + pKa2)/2.
How does blood maintain carbonate equilibrium? ▾
H2CO3/HCO3- buffer (pKa=6.1) controlled by lungs (CO2 exhalation). Blood pH 7.4: [HCO3-]/[H2CO3] = 10^(7.4-6.1) = 20:1.
What is EDTA speciation? ▾
EDTA is hexaprotic (H6Y^2+) with pKa values 0, 1.5, 2.0, 2.7, 6.1, 10.2. At pH 10: mostly Y^4- which forms strongest metal complexes.

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Formula Explorer connections

Interpretation: This relationship connects hydrogen-ion activity, dissociation, conjugate ratios or titration stoichiometry to acid–base behavior. Assumption: Concentration approximates activity mainly in dilute solutions. Temperature, ionic strength, acid strength and equilibrium approximations affect accuracy.

Strong Acid pH Calculator →Titration Calculator →Titration Endpoint & Indicator Calculator →Chemistry Formula Explorer →