pKb and Kb Calculator

This pKb calculator converts between the base dissociation constant, Kb, and its logarithmic form, pKb, the base analogue of pKa. Choose which you have and enter the value; the calculator returns the other and the matching pKa of the conjugate acid, using the relation that pKa plus pKb equals 14 at 25 degrees Celsius. A lower pKb means a stronger base. These constants let you compare bases, work with conjugate acid base pairs and design buffers. The formula, a worked example and the assumptions are below.

Matching pKa (pKa + pKb = 14)

Base strength

The formula

pKb = minus the base ten logarithm of Kb, and Kb = 10 to the power minus pKb. For a conjugate acid base pair in water at 25 degrees Celsius, pKa + pKb = 14.

Worked example

Ammonia has Kb about 1.8 times 10 to the minus 5, so pKb is about 4.74, and the conjugate acid (ammonium) has pKa about 14 minus 4.74 = 9.26. Enter 1.8e-5 with Kb selected to confirm.

Frequently asked questions

What is pKb?

The negative base ten logarithm of the base dissociation constant Kb. A lower pKb means a stronger base.

How do pKa and pKb relate?

For a conjugate acid base pair in water at 25 degrees Celsius, pKa plus pKb equals 14.

What is Kb?

The equilibrium constant for a base accepting a proton from water; larger Kb means a stronger base.

Who this calculator is for

This calculator is for chemistry students, teachers and lab workers.

What this calculator assumes

You enter values in the units shown.
Standard conditions unless stated.
Results are rounded for display.

Formula and sources

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