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pKa to pH calculator

Submit the values of the Substance type, pKa, acid, and salt concentration in the pKa to pH calculator to measure the pKa and pH values.

Change Ring:

Buffer Atom:

Ka:

Base Salt Concentration:

M ▾

Concentration:

M ▾

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pKa to pH Calculator

This calculator measures the pH of a buffer solution using its pKa and concentrations. Buffer solutions can be acidic or basic and maintain a stable pH.

What is a Buffer Solution?

A buffer is a water-based solution containing a weak acid and its conjugate base. It resists changes in [H⁺] or pH when small amounts of acid or base are added. Buffers are used in pharmaceuticals, preservatives, electroplating, printing, and laboratory applications.

Types of Buffer Solutions

1. Acidic Buffer Solution

Prepared by mixing a weak acid with its conjugate salt. Maintains pH < 7.

Example: Acetic acid (CH₃COOH) + sodium acetate (CH₃COONa), pH ≈ 4.74

pH < 7
Composed of weak acid + salt

2. Basic Buffer Solution

Prepared by mixing a weak base with its conjugate salt. Maintains pH > 7.

Example: Ammonium hydroxide (NH₄OH) + ammonium salt, pH ≈ 9.25

pH > 7
Composed of weak base + salt

Calculating pH of a Buffer Solution

1. Acidic Buffer (Weak Acid + Salt)

Weak acid HA partially ionizes:

HA + H₂O ⇌ H⁺ + A⁻

Acid dissociation constant:

Ka = [H⁺][A⁻]/[HA]

Henderson-Hasselbalch equation:

pH = pKa + log([Salt]/[Acid])

[Salt] = concentration of conjugate base [A⁻]
[Acid] = concentration of weak acid [HA]

2. Basic Buffer (Weak Base + Salt)

Equilibrium:

B + H₂O ⇌ BH⁺ + OH⁻

Base dissociation constant:

pOH = pKb + log([Salt]/[Base])

Then:

pH = 14 - pOH

Buffer Solution Calculations Table

Buffer Solution	pKa (25°C)
Sulfonate	1.8
Aspartic acid	2.1, 3.9, 9.8
Phosphate	2.1, 7.2, 12.3
Chloroacetate	2.9
Citrate	3.1, 4.8, 6.4
Gluconic acid	3.6
Lactate	3.9
Acetate	4.8
Carbonate	6.4
Ammonia	9.2