pKa Calculator
Convert between Ka and pKa for acids.
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How it works
The pKa is the standard way chemists express the strength of an acid, and this calculator converts between the acid dissociation constant Ka and its logarithmic form pKa in either direction. The dissociation constant Ka measures how completely an acid gives up its proton in water, but its values span many orders of magnitude, from around ten for very strong acids down to tiny fractions like 0.000000000001 for very weak ones. Taking the negative base-ten logarithm turns these unwieldy numbers into a compact, readable scale, just as pH does for hydrogen ion concentration. A lower pKa means a stronger acid that dissociates more readily, while a higher pKa means a weaker acid. The pKa is central to predicting the direction of acid-base reactions, choosing buffers, and understanding how drugs ionise and are absorbed in the body, since a molecule's charge at a given pH depends on the relationship between that pH and its pKa. Students of chemistry and biochemistry use pKa constantly, as do pharmacologists and formulation scientists. Enter a Ka value to find its pKa, or a pKa to recover the Ka, and the calculator shows both.
pKa = −log₁₀(Ka); Ka = 10^(−pKa)
The pKa is the negative base-ten logarithm of the acid dissociation constant Ka, which compresses the enormous range of Ka values into a convenient scale. A small Ka, meaning a weak acid that barely dissociates, gives a large pKa, and a large Ka, meaning a strong acid, gives a small or even negative pKa. Converting back, Ka equals ten raised to the negative pKa, recovering the original constant from its logarithm.
Source:Chemistry (acid dissociation constant)
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Frequently asked questions
What is pKa?
The pKa is the negative base-ten logarithm of an acid's dissociation constant Ka. It provides a convenient scale for acid strength: the lower the pKa, the stronger the acid. Because Ka values range across many orders of magnitude, taking the logarithm turns them into manageable numbers, typically between about minus ten and fifty, that are far easier to compare and remember.
How are Ka and pKa related?
The pKa equals the negative base-ten logarithm of Ka, and conversely Ka equals ten raised to the power of the negative pKa. This means a tenfold change in Ka corresponds to a change of one unit in pKa. The two express exactly the same information about acid strength, with pKa simply being the more compact, logarithmic form of the dissociation constant.
Does a lower pKa mean a stronger acid?
Yes. A lower pKa indicates a stronger acid that dissociates more completely, releasing its proton readily. Strong acids can have pKa values near zero or even negative, while weak acids like acetic acid have pKa values around five and very weak acids have much higher values. Comparing pKa values is a quick way to rank acids by strength.
Why is pKa important for buffers and drugs?
A buffer works best when the desired pH is close to the pKa of its weak acid, so knowing pKa lets you choose the right buffer system. In pharmacology, a drug's pKa determines how much of it is charged or neutral at body pH, which strongly affects its solubility, absorption, and distribution. The relationship between pH and pKa governs the ionisation state of any weak acid or base.
How is pKa different from pH?
The pH measures the actual acidity of a particular solution, reflecting its hydrogen ion concentration, and it changes as you dilute or mix solutions. The pKa is a fixed property of a specific acid, describing its intrinsic tendency to donate a proton, and it does not change with concentration. Both use a negative logarithmic scale, but they describe different things.