pKa Values Table: How to Read and Use It

This page explains what a pKa values table is, how to read one, and how to use pKa values to predict acid strength and chemical behavior. You enter a compound’s pKa value or look it up in the table below, and the calculator converts between pKa, Ka, and percent ionization for a given pH.

Formula

The pKa is defined as the negative base-10 logarithm of the acid dissociation constant (Ka):

pKa = −log�₀(Ka)

Conversely, Ka = 10^(−pKa). The calculator also estimates percent ionization for a weak acid in water using the Henderson–Hasselbalch approximation:

% ionization = 100 / (1 + 10^(pH − pKa))

Here, pKa is the acid’s dissociation constant (unitless), pH is the solution’s acidity (unitless), and Ka is the acid dissociation constant (mol/L). The percent ionization formula assumes a monoprotic acid and that the acid concentration is much larger than Ka.

Worked Example

Using the default values: pKa = 4.75 (acetic acid) and pH = 7.

1. Ka = 10^(−4.75) = 1.78 × 10��
2. Percent ionized = 100 / (1 + 10^(7 − 4.75)) = 100 / (1 + 10^2.25) = 100 / (1 + 177.8) = 100 / 178.8 = 0.56%

So at pH 7, acetic acid is only 0.56% ionized. The calculator shows Ka = 0.000018 and percent ionized = 0.56%.

What a pKa Values Table Shows

A pKa table lists acids and their pKa values, typically ranging from about −10 (very strong acids like HI) to 50 (very weak acids like alkanes). The lower the pKa, the stronger the acid—meaning it more readily donates a proton (H�). Common tables include:

• Strong acids: HCl (pKa ≈ −7), Hâ‚‚SOâ‚„ (pKaâ‚� ≈ −3), HNO₃ (pKa ≈ −1.3)
• Carboxylic acids: acetic acid (pKa = 4.75), formic acid (pKa = 3.75)
• Water: pKa = 15.7 (for Hâ‚‚O acting as an acid)
• Amines: ammonium (pKa ≈ 9.25), anilinium (pKa ≈ 4.6)
• Alcohols: ethanol (pKa ≈ 16), phenol (pKa ≈ 9.95)
• Alkanes: methane (pKa ≈ 50)

Tables often include conjugate bases, so you can quickly see which base can deprotonate which acid. For example, hydroxide (pKa of water = 15.7) can deprotonate an alcohol (pKa ≈ 16) but not an alkane (pKa ≈ 50).

How to Choose a pKa Table

Not all pKa tables are the same. Some list values in water, others in DMSO or the gas phase. When choosing a table:

• Match the solvent to your reaction—most biochemistry uses water, while organic synthesis often uses DMSO.
• Check if the table gives pKa for the acid or the conjugate acid (e.g., for amines, the pKa is for the ammonium ion).
• Look for temperature—pKa changes slightly with temperature, usually given at 25°C.
• Prefer tables that cite original sources if you need high precision.

For most general chemistry and biochemistry, a table with aqueous pKa values at 25°C is sufficient.

Common Mistakes and Misconceptions

One common error is confusing pKa with pH. pKa is a property of a specific acid, while pH is a property of a solution. Another mistake is using pKa values for polyprotic acids without noting which proton is being removed—citric acid has three pKa values (3.13, 4.76, 6.40), each for a different dissociation step.

A frequent misconception is that a lower pKa always means a stronger acid in any solvent. While true in water, the order can change in other solvents. Also, pKa is temperature-dependent; values at 25°C are standard but not universal.

Limits of pKa Tables

pKa values are equilibrium constants, so they apply to dilute solutions at a given temperature. They do not predict reaction kinetics—a strong acid may react slowly if the proton transfer is sterically hindered. Also, pKa tables usually assume ideal solutions; at high concentrations, activity coefficients matter.

For polyprotic acids, each pKa corresponds to a separate equilibrium, and the species present at a given pH can be calculated using the Hill equation or a speciation diagram. The simple percent ionization formula here only works for monoprotic acids.

Practical Uses of pKa Values

Chemists and biologists use pKa to:

• Predict the direction of acid-base reactions (the acid with higher pKa donates a proton to the conjugate base of a lower pKa).
• Design buffer solutions—buffers work best at pH close to the pKa of the weak acid.
• Estimate drug absorption—a drug crosses membranes better when un-ionized, which depends on pH and pKa.
• Determine the charge state of amino acids and proteins at a given pH.
• Choose a base for deprotonation in organic synthesis (e.g., LDA has a conjugate acid pKa ≈ 36, so it can deprotonate ketones with pKa ≈ 20).

For example, the pKa of the ammonium group in glycine is 9.60, so at physiological pH 7.4, it is mostly protonated (NH₃�), while the carboxyl group (pKa 2.34) is deprotonated (COO�).

Frequently Asked Questions

What is the difference between pKa and pH?

pKa is a constant that measures how readily an acid donates a proton, independent of concentration. pH measures the hydrogen ion concentration in a solution. A low pKa means a strong acid; a low pH means an acidic solution.

How do I convert pKa to Ka?

Use Ka = 10^(−pKa). For example, if pKa = 4.75, Ka = 10^(−4.75) ≈ 1.78 × 10��.

Why are strong acids like HCl given negative pKa values?

Strong acids have very large Ka values, so their pKa is negative. HCl has a pKa around −7, meaning it dissociates almost completely in water. Negative pKa values are simply a consequence of the logarithmic scale.

Can I use a pKa table for bases?

Yes, but you need to look at the pKa of the conjugate acid. For a base B, the relevant pKa is for BH�. For example, ammonia has pKa ≈ 9.25 for NH₄�, so NH₃ is a weak base.

Why does pKa vary with solvent?

pKa depends on the ability of the solvent to stabilize the charged species formed after dissociation. Water stabilizes ions well, so acids appear stronger in water than in nonpolar solvents. Tables often provide values in DMSO for organic reactions.

What is the pKa of water?

The pKa of water is 15.7, but this is rarely used directly. For practical purposes, water is amphoteric—it can act as both an acid and a base. The autoionization constant of water (Kw) is 10�¹� at 25°C.

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in pka | pKa value | - | 4.75 | -10
in ph | pH of solution | - | 7 | 0
out ka | Ka (acid dissociation constant) | - | 10^(-pka) | 6
out percent_ion | Percent ionized (for monoprotic acid) | % | 100 / (1 + 10^(ph - pka)) | 2