Can I solve for different variables?

Yes! This calculator supports reverse solving. You can solve for: Acid [HA], Conjugate base [A⁻], Kₐ, Logba, pKₐ, pH.

Henderson Hasselbalch

Solve henderson hasselbalch relationships quickly using core chemistry formulas.

Formula shown5 inputs definedUpdated Nov 2025By Automated Chemistry GeneratorFree, no sign-up

Quick Summary

Use this when you need:

Design buffer recipes by balancing conjugate base and acid amounts
Estimate how adding conjugate base shifts pH from pKa
Teach why buffers lose capacity when [A-]/[HA] drifts too far from 1

You’ll need:Conjugate base [A⁻], Kₐ, Logba, pKₐ, and 1 more

You’ll get:Acid [HA]

How this is calculated

Formula:

pH = pKa + log10([A-] / [HA])

In plain language:

Taking the negative log of the acid dissociation expression gives the Henderson–Hasselbalch form, so adding log10 of base-to-acid ratio to pKa yields the buffer pH.

The formula is always visible so you can verify the math and explain your numbers to anyone who asks.

What these terms mean

Acid [HA]

Acid [HA] used in the calculation.

Conjugate base [A⁻]

Conjugate base [A⁻] used in the calculation.

Kₐ

Kₐ used in the calculation.

Logba

Logba used in the calculation.

pKₐ

pKₐ used in the calculation.

pH

pH used in the calculation.

When to use this calculator

Design buffer recipes by balancing conjugate base and acid amounts
Estimate how adding conjugate base shifts pH from pKa
Teach why buffers lose capacity when [A-]/[HA] drifts too far from 1

Last updated: November 19, 2025

Henderson-Hasselbalch calculator explained

The Henderson-Hasselbalch equation links pH to p $K_a$ and the ratio of conjugate base to acid: $pH = pK_a + \\log_{10}([A^-]/[HA])$ . This calculator takes your acid concentration {acid}, conjugate base concentration {conjugate_base}, and acid dissociation constant {k_a} to output $pH$ {ph}, $pK_a$ {logk}, and the log ratio {logba}.

Use it to design buffer recipes, anticipate how dilution or dilution of one component will shift pH, or teach why buffers work best near their $pK_a$ values.

How the conversion works

Starting from the acid dissociation equilibrium $K_a = \\frac{[A^-][H^+]}{[HA]}$ , taking log $_{10}$ and rearranging gives:

pH = pK_a + \\log_{10}\\left(\\frac{[A^-]}{[HA]}\\right)

The calculator computes $pK_a = -\\log_{10} K_a$ then adds the base-to-acid log term to get $pH$ . Solving for ratios is straightforward:

\\frac{[A^-]}{[HA]} = 10^{(pH - pK_a)}

Units and conversions

Quantity	Units	Notes
$[A^-]$ , $[HA]$	mol/L or any consistent concentration	Ratios, not absolute units, drive the math.
$K_a$	unitless (activity-based)	Enter the acid dissociation constant; the tool converts to $pK_a$ .
$pH$	dimensionless	Output of the calculator.
$pK_a$	dimensionless	Negative log of $K_a$ .

Worked examples

Acetate buffer at pH 5.0
Acetic acid has $pK_a = 4.76$ . Solve for the required ratio:

\\log_{10}\\left(\\frac{[A^-]}{[HA]}\\right) = 5.00 - 4.76 = 0.24

\\frac{[A^-]}{[HA]} = 10^{0.24} = 1.74

Therefore 1.74 moles of acetate for every mole of acetic acid yields pH 5.0.

Physiological phosphate buffer
Second dissociation of phosphoric acid: $pK_a2 = 7.21$ . Target pH 7.40.

\\frac{[HPO_4^{2-}]}{[H_2PO_4^-]} = 10^{(7.40 - 7.21)} = 1.55

Mixing 1.55 parts dibasic sodium phosphate with 1 part monobasic salt delivers a pH 7.4 buffer at room temperature.

Tips and pitfalls

The equation assumes $[A^-]$ and $[HA]$ greatly exceed $[H^+]$ ; buffers more than 1 pH unit away from $pK_a$ violate that assumption.
Ionic strength and temperature shift $pK_a$ , so adjust $K_a$ for the actual operating conditions.
For polyprotic systems, choose the $pK_a$ that corresponds to the relevant equilibrium (e.g., $pK_a2$ for phosphate near neutral pH).
Combine this calculator with the buffer capacity tool to ensure the final formulation resists acid or base additions.

References and further reading

Frequently Asked Questions

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Calculator info

Version:: v1.0.0
Last reviewed:: Nov 15, 2025
Sensitivity:: low
References:: 1

Last updated: November 19, 2025

Henderson-Hasselbalch calculator explained

Use it to design buffer recipes, anticipate how dilution or dilution of one component will shift pH, or teach why buffers work best near their $pK_a$ values.

How the conversion works

Starting from the acid dissociation equilibrium $K_a = \\frac{[A^-][H^+]}{[HA]}$ , taking log $_{10}$ and rearranging gives:

pH = pK_a + \\log_{10}\\left(\\frac{[A^-]}{[HA]}\\right)

The calculator computes $pK_a = -\\log_{10} K_a$ then adds the base-to-acid log term to get $pH$ . Solving for ratios is straightforward:

\\frac{[A^-]}{[HA]} = 10^{(pH - pK_a)}

Units and conversions

Quantity	Units	Notes
$[A^-]$ , $[HA]$	mol/L or any consistent concentration	Ratios, not absolute units, drive the math.
$K_a$	unitless (activity-based)	Enter the acid dissociation constant; the tool converts to $pK_a$ .
$pH$	dimensionless	Output of the calculator.
$pK_a$	dimensionless	Negative log of $K_a$ .

Worked examples

Acetate buffer at pH 5.0
Acetic acid has $pK_a = 4.76$ . Solve for the required ratio:

\\log_{10}\\left(\\frac{[A^-]}{[HA]}\\right) = 5.00 - 4.76 = 0.24

\\frac{[A^-]}{[HA]} = 10^{0.24} = 1.74

Therefore 1.74 moles of acetate for every mole of acetic acid yields pH 5.0.

Physiological phosphate buffer
Second dissociation of phosphoric acid: $pK_a2 = 7.21$ . Target pH 7.40.

\\frac{[HPO_4^{2-}]}{[H_2PO_4^-]} = 10^{(7.40 - 7.21)} = 1.55

Mixing 1.55 parts dibasic sodium phosphate with 1 part monobasic salt delivers a pH 7.4 buffer at room temperature.

Tips and pitfalls

The equation assumes $[A^-]$ and $[HA]$ greatly exceed $[H^+]$ ; buffers more than 1 pH unit away from $pK_a$ violate that assumption.
Ionic strength and temperature shift $pK_a$ , so adjust $K_a$ for the actual operating conditions.
For polyprotic systems, choose the $pK_a$ that corresponds to the relevant equilibrium (e.g., $pK_a2$ for phosphate near neutral pH).
Combine this calculator with the buffer capacity tool to ensure the final formulation resists acid or base additions.

Quick Summary

Calculator Tool

How this is calculated

What these terms mean

Acid [HA]

Conjugate base [A⁻]

Kₐ

Logba

pKₐ

pH

When to use this calculator

Henderson-Hasselbalch calculator explained

How the conversion works

Units and conversions

Worked examples

Tips and pitfalls

References and further reading

Frequently Asked Questions

Related Calculators

pKa

Hydrogen ion concentration

Beer-Lambert law

Molarity

Mole

Neutralization

Was this calculator helpful?

Found an error?

Calculator info

Henderson-Hasselbalch calculator explained

How the conversion works

Units and conversions

Worked examples

Tips and pitfalls

References and further reading

Frequently Asked Questions

Related Calculators

pKa

Hydrogen ion concentration

Beer-Lambert law

Molarity

Mole

Neutralization

Was this calculator helpful?

Found an error?

Calculator info