pKa Data Table: Values, Uses, and How to Read One
A pKa data table lists acid dissociation constants (as pKa values) for acids and bases, letting you predict protonation states, buffer ranges, and reaction behavior. You enter a compound or look up its pKa, then use the table to determine whether it will donate or accept a proton at a given pH. This article explains what pKa means, how to read a table, and how to apply the values correctly.
What Is pKa?
pKa is the negative base-10 logarithm of the acid dissociation constant (Ka):
pKa = −log�₀(Ka)
Ka measures how completely an acid donates a proton (H�) to water. A smaller pKa means a stronger acid (more dissociation); a larger pKa means a weaker acid. For example, acetic acid has a pKa of about 4.76, while hydrochloric acid has a pKa of about −7. Because the scale is logarithmic, each unit change represents a tenfold difference in acidity.
For bases, tables often list the pKa of the conjugate acid (e.g., ammonium, pKa ≈ 9.25) rather than a “pKb�. This keeps the scale consistent and lets you compare acids and bases directly.
How to Read a pKa Data Table
A typical table has columns for the compound name, the acidic proton (the one that dissociates), the pKa value, and sometimes the solvent or temperature. Here’s how to interpret the numbers:
- Lower pKa = stronger acid. A pKa of 0–2 indicates a strong acid (e.g., sulfuric acid’s first proton). A pKa of 15–50 indicates a very weak acid (e.g., alkanes, pKa ≈ 50).
- At pH = pKa, the acid is 50% deprotonated. This is the midpoint of the buffering region.
- For pH > pKa + 1, the acid is mostly deprotonated (over 90%). For pH < pKa − 1, it is mostly protonated.
- For bases, use the conjugate acid pKa. If the conjugate acid pKa is above the pH, the base is mostly protonated (charged); if below, it is mostly neutral.
Example: For acetic acid (pKa 4.76) at pH 5.76, the ratio of acetate to acetic acid is about 10:1. At pH 3.76, the ratio is about 1:10.
Common pKa Values You’ll See
Here are some frequently encountered pKa values (in water at 25°C unless noted):
- Water (H₂O) – pKa 15.74 (for the H–O bond)
- Hydronium ion (H₃O�) – pKa −1.74
- Acetic acid – 4.76
- Ammonium (NH₄�) – 9.25
- Phenol – 9.95
- Carbonic acid (H₂CO₃) – 6.35 (first), 10.33 (second)
- Phosphoric acid – 2.15, 7.20, 12.35
- Ethanol – 16.0
- Acetone – 19.3
- Methane – ~50
These values vary slightly with temperature and ionic strength, but they are adequate for most laboratory and classroom work.
How to Choose the Right pKa Table
Not all pKa tables are the same. Consider these factors:
- Solvent: pKa values are solvent-dependent. A table for water may not apply to DMSO or acetonitrile. For organic chemistry, DMSO pKa tables are common because many reactions run in non-aqueous solvents.
- Temperature: pKa changes with temperature, though the effect is small for most acids (a few hundredths per 10°C).
- Ionic strength: High salt concentrations can shift pKa by a few tenths. Tables usually report “thermodynamic� pKa at infinite dilution.
- Type of compound: Use a table that includes the functional groups you need (e.g., amino acids, drugs, buffers).
For quick work, a general table like the one in the CRC Handbook of Chemistry and Physics or a university-provided PDF is reliable. For precise measurements, consult the primary literature.
Common Mistakes When Using pKa Data
Misreading pKa tables leads to incorrect predictions. Watch out for these pitfalls:
- Confusing pKa with pH: pKa is a property of the molecule; pH is a property of the solution. They are equal only at the half-equivalence point.
- Using the wrong proton: Many molecules have multiple ionizable groups. Make sure you pick the pKa that corresponds to the proton you care about (e.g., the side chain of histidine, not the backbone).
- Forgetting the conjugate acid for bases: If you look up “ammonia pKa,� you might see 9.25 (for NH₄�), not 33 (for NH₃ as an acid). Always check which species the value refers to.
- Ignoring solvent effects: A pKa of 10 in water might be 15 in DMSO. Always match the solvent to your conditions.
- Rounding too aggressively: A difference of 0.5 in pKa changes the ratio of acid to conjugate base by a factor of ~3. Keep at least two significant figures.
Practical Applications of pKa Tables
pKa tables are essential in several fields:
- Buffer preparation: Choose a weak acid whose pKa is within ±1 of the desired pH. For pH 7.0, phosphate (pKa 7.20) is a good choice; for pH 5.0, acetate (pKa 4.76) works.
- Drug absorption: A drug’s pKa tells you whether it is ionized in the stomach (pH ~2) or intestines (pH ~7). Unionized forms cross membranes more easily.
- Protein biochemistry: Amino acid side-chain pKa values help predict charge states at physiological pH. For example, histidine’s imidazole pKa (~6.0) is near neutral pH, making it a common catalytic residue.
- Organic synthesis: pKa helps predict acid-base reactions. For example, a base with conjugate acid pKa of 15 (e.g., hydroxide, pKa of water = 15.7) will deprotonate a carboxylic acid (pKa ~5) but not an alcohol (pKa ~16).
Limits of pKa Data
pKa values are approximate and context-dependent. They assume ideal dilute solutions and a single proton transfer. In real systems, factors like intramolecular hydrogen bonding, steric effects, and solvation can shift pKa by several units. Also, polyprotic acids have multiple pKa values, and the order is usually (but not always) increasing (e.g., phosphoric acid: 2.15, 7.20, 12.35). For unusual compounds or non-standard conditions, a table may not be reliable—consider computational predictions or experimental measurement.
Frequently Asked Questions
What is the difference between pKa and pH? pH measures the acidity of a solution (the concentration of H�). pKa measures the intrinsic strength of an acid in a given solvent. They are related by the Henderson-Hasselbalch equation: pH = pKa + log([A�]/[HA]).
How do I find the pKa of a base? Look up the pKa of its conjugate acid. For example, for ammonia (NH₃), the conjugate acid is ammonium (NH₄�) with pKa 9.25. That value tells you how strongly the base holds a proton.
Why are some pKa values negative? Very strong acids, like HCl (pKa ≈ −7) or H₂SO₄ (first pKa ≈ −3), have negative pKa because they dissociate almost completely in water. The negative sign simply reflects a Ka greater than 1.
Can I use a pKa table for non-aqueous solvents? Yes, but only if the table specifies that solvent. For example, pKa values in DMSO are typically 5–10 units higher than in water for neutral acids. Always match the solvent to your reaction.
What is the Henderson-Hasselbalch equation? It is pH = pKa + log([A�]/[HA]). It lets you calculate the ratio of conjugate base to acid at any pH, assuming the acid is weak and the solution is buffered. It is most accurate when pH is within 1–2 units of pKa.
How precise are pKa values in tables? Most tables give 2–3 significant figures, which is enough for predicting protonation states and buffer ranges. For exact work (e.g., calibrating a pH meter), you need experimentally measured values under your specific conditions.
Related
Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.