Acetic Acid pKa: Values, Table and Buffer Guide

The acid dissociation constant (pKa) of acetic acid in aqueous solution at 25 °C is 4.756 (commonly cited as 4.76), corresponding to an acid ionization constant (Ka) of 1.75 × 10⁻⁵ mol/L. This constant defines the standard buffering window for sodium acetate/acetic acid systems between pH 3.76 and 5.76 across analytical and biochemical workflows.

Acetic Acid Physical and Thermodynamic Property Lookup Table

Acetic acid (ethanoic acid, CAS number 64-19-7) functions as a weak acid and polar protic solvent. Reference values below compile data from the NIST Chemistry WebBook (SRD 69), PubChem (CID 176), and the CRC Handbook of Chemistry and Physics.

Parameter	Standard Value	Conditions	Source
Acid Dissociation Constant (pKa)	4.756 (4.76)	Water, 25 °C	CRC Handbook / PubChem
Acid Ionization Constant (Ka)	1.75 × 10⁻⁵ mol/L	Water, 25 °C	CRC Handbook / PubChem
Buffer Range	pH 3.76 – 5.76	pKa ± 1.0 unit	Standard Practice
Molecular Weight	60.052 g/mol	C₂H₄O₂ molar mass	NIST WebBook SRD 69
Boiling Point	117.9 °C	101.325 kPa (760 mmHg)	NIST WebBook / PubChem
Freezing Point	16.6 °C	Glacial liquid	NIST WebBook / PubChem
Density	1.049 g/cm³	25 °C (1.051 g/cm³ at 20 °C)	PubChem HSDB
Vapor Pressure	15.7 mmHg (2.09 kPa)	25 °C (11.4 mmHg at 20 °C)	PubChem HSDB / NIST
Dynamic Viscosity	1.056 mPa·s	25 °C (1.22 mPa·s at 20 °C)	PubChem HSDB
Refractive Index (nD²⁰)	1.3716 – 1.3720	Sodium D-line, 20 °C	PubChem HSDB
Dielectric Constant (εr)	6.15	20 °C (6.20 at 25 °C)	NBS Circular 514 / PubChem
Polarity Index (P’)	6.2	Snyder classification	CRC Handbook
Flash Point (Closed Cup)	39 °C (102 °F)	Glacial acetic acid	PubChem CAMEO Chemicals

Temperature effect: Aqueous pKa varies slightly with temperature: 4.781 at 0 °C, 4.756 at 25 °C, and 4.788 at 50 °C.

Dissociation Formulas and Buffer Mechanics

Acetic acid ionizes reversibly in water according to Bronsted-Lowry equilibrium:

CH₃COOH (aq) + H₂O (l) ⇌ CH₃COO⁻ (aq) + H₃O⁺ (aq)

Ka = ([CH₃COO⁻][H₃O⁺]) / [CH₃COOH] = 1.754 × 10⁻⁵ mol/L ➔ pKa = -log₁₀(Ka) = 4.756

Henderson-Hasselbalch Equation for Acetate Buffers

Acetate buffer pH is governed by the Henderson-Hasselbalch relation:

pH = pKa + log₁₀([CH₃COO⁻] / [CH₃COOH]) = 4.756 + log₁₀([Conjugate Base] / [Weak Acid])

Maximum buffer capacity occurs at pH = pKa = 4.76 when [CH₃COO⁻] = [CH₃COOH]. Useful buffering spans pH 3.76 to 5.76. For buffer selection principles, see our guide on what is a biological buffer.

Weak Acid Equilibrium and Degree of Ionization

For unbuffered aqueous acetic acid at total concentration C (mol/L), hydronium concentration is calculated via quadratic mass balance:

[H⁺] = (-Ka + √(Ka² + 4KaC)) / 2 ➔ α = [H⁺] / C ➔ Percent Dissociation = α × 100%

Carboxylic Acid pKa Comparison and Substituent Trends

Acetic acid is the reference standard for aliphatic carboxylic acids. Electronegative alpha-substituents withdraw electron density inductively (-I effect), stabilizing the conjugate carboxylate anion and lowering pKa.

Acid	Formula	pKa (25 °C)	Ka (mol/L)	Substituent Effect
Formic Acid	HCOOH	3.75	1.77 × 10⁻⁴	No alkyl group
Acetic Acid	CH₃COOH	4.76	1.75 × 10⁻⁵	Aliphatic baseline
Propionic Acid	CH₃CH₂COOH	4.88	1.32 × 10⁻⁵	Weak ethyl donation
Chloroacetic Acid	ClCH₂COOH	2.86	1.38 × 10⁻³	Monochloro -I stabilization
Dichloroacetic Acid	Cl₂CHCOOH	1.29	5.13 × 10⁻²	Dichloro -I stabilization
Trichloroacetic Acid	Cl₃CCOOH	0.65	2.24 × 10⁻¹	Trichloro strong -I
Trifluoroacetic Acid (TFA)	CF₃COOH	0.23	5.89 × 10⁻¹	Trifluoro strong -I
Oxalic Acid (pKa1 / pKa2)	HOOC-COOH	1.25 / 4.27	5.62 × 10⁻² / 5.37 × 10⁻⁵	Diprotic dicarboxylic

Laboratory Worked Calculations

Worked Calculation 1: Preparing a 0.100 M Acetate Buffer at pH 5.00

Goal: Formulate 1.000 L of 0.100 M total acetate buffer at pH 5.00 at 25 °C using glacial acetic acid and anhydrous sodium acetate.

Calculations:

• Molar Ratio: 5.00 = 4.756 + log₁₀(Ratio) ➔ Ratio = 10^(0.244) = 1.754.
• Speciation: 2.754[CH₃COOH] = 0.100 M ➔ [CH₃COOH] = 0.0363 M; [CH₃COO⁻] = 0.0637 M.
• Reagents: Glacial acetic acid = 0.0363 mol × 60.052 g/mol = 2.180 g (2.08 mL at 1.049 g/mL). Sodium acetate = 0.0637 mol × 82.034 g/mol = 5.23 g (or 8.67 g trihydrate).

Dissolve 5.23 g sodium acetate in 800 mL water, add 2.08 mL glacial acetic acid, and dilute to 1.000 L. For titration curves, see how to find the equivalence point on a titration curve.

Worked Calculation 2: Determining pH and Percent Dissociation of 0.100 M Acetic Acid

Goal: Calculate hydronium concentration, pH, and percent ionization of unbuffered 0.100 M aqueous acetic acid at 25 °C.

Calculations:

• Equilibrium: x² + (1.754 × 10⁻⁵)x - (1.754 × 10⁻⁶) = 0 ➔ x = [H⁺] = 1.316 × 10⁻³ mol/L.
• pH and Dissociation: pH = -log₁₀(1.316 × 10⁻³) = 2.88; Percent Dissociation = (1.316 × 10⁻³ M / 0.100 M) × 100% = 1.32%.

At 0.100 M, 1.32% of molecules ionize. For commercial vinegar comparisons, see vinegar molarity.

Computational Code Snippets: Excel, R, and Python

Excel Formulas

Buffer pH from sodium acetate in A2 and acetic acid in B2:

=4.756 + LOG10(A2 / B2)

Equilibrium pH for pure acetic acid of analytical concentration C in C2:

=-LOG10((-0.00001754 + SQRT(0.00001754^2 + 4 * 0.00001754 * C2)) / 2)

R Script

pKa <- 4.756
Ka  <- 10^(-pKa)
acetate_buffer_pH <- function(salt_M, acid_M) pKa + log10(salt_M / acid_M)
acetic_acid_pH <- function(c_tot, Ka_v = Ka) {
  h <- (-Ka_v + sqrt(Ka_v^2 + 4 * Ka_v * c_tot)) / 2
  c(pH = -log10(h), alpha_pct = (h / c_tot) * 100)
}

Python Script

import math
PKA = 4.756
KA = 10 ** (-PKA)
def acetate_buffer_ph(salt_m: float, acid_m: float) -> float:
    return PKA + math.log10(salt_m / acid_m)
def acetic_acid_speciation(c_tot: float, ka: float = KA) -> dict:
    h = (-ka + math.sqrt(ka**2 + 4 * ka * c_tot)) / 2
    return {"pH": round(-math.log10(h), 2), "percent_dissociation": round((h / c_tot) * 100, 2)}

Bench Solvent Behavior: Evaporation, Extraction, and Chromatography

• Rotary Evaporation: Boiling point is 117.9 °C; vapor pressure is 15.7 mmHg at 25 °C. Evaporation requires bath temperatures of 50–60 °C and vacuum (< 20 mbar). Co-evaporation with toluene facilitates azeotropic removal.
• Liquid-Liquid Extraction: Miscible with water, ethanol, and ethyl acetate, but insoluble in hydrocarbons. Density is 1.049 g/cm³. Un-ionized acid partitions into organic layers; raising aqueous pH above 6.0 forms acetate, trapping it in water.
• HPLC Mobile Phase Additive: Added at 0.1% v/v (pH ~3.3) in reversed-phase HPLC and LC-MS. Its high polarity index (P’ = 6.2) and dielectric constant (εr = 6.15) suppress silanol interactions while yielding volatile ions for mass spectrometry.
• Purity Checks: Reagent quality is verified by refractive index (nD²⁰ = 1.3716–1.3720) and viscosity (1.056 mPa·s at 25 °C). Water absorption depresses the 16.6 °C freezing point.

Chemical Safety, SDS, and Handling Precautions

Glacial acetic acid is an occupational corrosive and moderate fire hazard, governed by prudent practices in the laboratory:

• Freezing Point and Bottle Breakage: Glacial acetic acid freezes at 16.6 °C (61.9 °F). Uneven expansion during thawing can shatter glass bottles. Store stock bottles above 18 °C. Thaw frozen bottles slowly in warm water (20–25 °C); never apply direct heat.
• GHS Hazard Classifications: Concentrated solutions (≥ 90%) are classified as Flammable Liquid Category 3 (H226), Skin Corrosion Category 1A (H314), and Serious Eye Damage Category 1 (H318). Containers bear the flame pictogram and the corrosive symbol.
• Exposure Limits: OSHA enforces a PEL of 10 ppm (25 mg/m³) TWA. NIOSH recommends an REL of 10 ppm TWA and an STEL of 15 ppm (37 mg/m³). Use inside a certified fume hood.
• Exothermic Dilution: Dilution releases heat. Always add acid slowly to water with stirring, never adding water to concentrated acid.

Frequently Asked Questions

Who is the king of all acids?

Industrially, sulfuric acid (H₂SO₄) is termed the “king of chemicals” (or king of acids) due to its massive production volume in fertilizers and refining. In physical chemistry evaluating proton-donating strength, superacids hold the distinction: fluoroantimonic acid (HSbF₆) is the strongest known superacid, with acidity quadrillions of times greater than pure sulfuric acid.

What is the difference between acetic acid and vinegar?

Acetic acid (CH₃COOH) is a distinct chemical substance, with glacial acetic acid being the pure anhydrous liquid (≥ 99.7%, freezing at 16.6 °C). Culinary vinegar is a dilute aqueous food solution (4% to 8% acetic acid, ~0.7 to 1.3 M) with fermentation byproducts and a pH of 2.4 to 3.4.

What is the Ka value of oxalic acid?

Oxalic acid (HOOC-COOH) is a diprotic dicarboxylic acid with two dissociation constants at 25 °C: Ka1 = 5.62 × 10⁻² mol/L (pKa1 = 1.25) and Ka2 = 5.37 × 10⁻⁵ mol/L (pKa2 = 4.27). The strong initial dissociation reflects stabilization from the adjacent carboxyl group.

Is acetic acid an organic compound?

Yes, acetic acid (CH₃COOH) is an organic carboxylic acid containing a methyl carbon covalently bonded to a carboxyl carbon, with carbon-carbon and carbon-hydrogen bonds characteristic of organic compounds.

Sources

• National Institute of Standards and Technology (NIST). (2026). Acetic acid. NIST Chemistry WebBook, SRD 69. https://webbook.nist.gov/cgi/cbook.cgi?ID=C64197&Units=SI (Verified 2026-11-07).
• National Center for Biotechnology Information (NCBI). (2026). Compound Summary: Acetic Acid (CID 176). PubChem. https://pubchem.ncbi.nlm.nih.gov/compound/Acetic-acid (Verified 2026-11-07).
• CRC Handbook of Chemistry and Physics. (2026). Dissociation Constants of Organic Acids and Bases. ChemnetBASE / CRC Press. https://hbcp.chemnetbase.com (Verified 2026-11-07).
• Centers for Disease Control and Prevention (CDC) / NIOSH. (2026). Acetic Acid: Pocket Guide to Chemical Hazards. https://www.cdc.gov/niosh/npg/npgd0003.html (Verified 2026-11-07).