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Nearly every acid you’ll ever eat, drink or find in a living cell is a weak acid. Vinegar, lemon juice, cola, yoghurt, aspirin, vitamin C, the carbon dioxide dissolved in your blood: all weak. Strong acids are the exception, not the rule. (For those few, see the strong acids.)
This guide collects the weak acids that come up most often in chemistry courses and everyday life, with their pKa values and a note on why each one matters. Remember the rule: the lower the pKa, the stronger the acid, and each unit is a factor of ten. If pKa is new to you, start with Ka and pKa.
All values are approximate, for water at 25 °C. Where an acid has more than one acidic proton, the first pKa is listed first.
Acids in food and drink
1. Ethanoic (acetic) acid, CH₃COOH: pKa 4.76 The acid in vinegar, typically about 5% by mass. It’s the standard example of a weak acid in almost every textbook, and it gives vinegar its sharp smell as well as its sourness. Its conjugate base, ethanoate (acetate), is used with it to make buffers around pH 4.8.
2. Citric acid, C₆H₈O₇: pKa 3.13, 4.76, 6.40 A triprotic acid found in all citrus fruit; lemon juice contains roughly 5% citric acid. It’s added to sweets, soft drinks and jams as a sour flavour and preservative, and to descalers because it dissolves limescale while being safe to handle.
3. Malic acid, C₄H₆O₅: pKa 3.40, 5.20 The sour taste of green apples and rhubarb. Sour sweets often use it because it gives a longer-lasting sourness than citric acid.
4. Tartaric acid, C₄H₆O₆: pKa 2.98, 4.34 Found in grapes. Its potassium salt, cream of tartar, crystallises inside wine barrels and is used in baking powder. Tartaric acid has an important place in history: Louis Pasteur discovered molecular chirality by separating its crystals.
5. Lactic acid, CH₃CH(OH)COOH: pKa 3.86 Made by bacteria when they ferment milk into yoghurt and cheese, and by your muscles during intense exercise. It’s used in food as a preservative and acidity regulator.
6. Carbonic acid, H₂CO₃: pKa 6.35, 10.33 Formed when carbon dioxide dissolves in water. It gives fizzy drinks their slight tang and makes normal rainwater mildly acidic, around pH 5.6. The carbonic acid/hydrogencarbonate pair is the main buffer in your blood.
7. Phosphoric acid, H₃PO₄: pKa 2.15, 7.20, 12.35 Added to cola drinks for a sharp taste, which is why many colas have a pH around 2.5. It’s also used in rust converters and fertiliser manufacture. Its middle species, H₂PO₄⁻ and HPO₄²⁻, buffer the inside of your cells.
8. Ascorbic acid (vitamin C), C₆H₈O₆: pKa 4.10, 11.6 An unusual acid: its acidic protons are on an “enol” group in a ring, not a carboxylic acid. It’s an essential vitamin and a useful antioxidant in food.
9. Benzoic acid, C₆H₅COOH: pKa 4.20 Occurs naturally in cranberries and some spices. Its sodium salt, sodium benzoate (E211), is one of the most common food preservatives, working best in acidic drinks where the undissociated acid form dominates.
Acids in medicine and the body
10. Acetylsalicylic acid (aspirin): pKa about 3.5 In the acidic stomach it’s mostly un-ionised, which helps it cross cell membranes. That’s also partly why it can irritate the stomach lining.
11. Salicylic acid: pKa 2.97 The parent compound of aspirin, found in willow bark. It’s used in skin treatments for acne and warts because it gently dissolves dead skin.
12. Uric acid: pKa about 5.4 A waste product of breaking down purines. When blood levels are too high, sodium urate crystals form in joints, causing gout.
Laboratory and industrial weak acids
13. Methanoic (formic) acid, HCOOH: pKa 3.75 The simplest carboxylic acid, and about ten times stronger than ethanoic acid. It’s in ant and nettle stings and is used in leather tanning and as a preservative for animal feed.
14. Hydrofluoric acid, HF: pKa 3.17 Weak, but extremely dangerous because fluoride passes through skin and attacks tissue. It dissolves glass and is used to etch it and to clean silicon wafers. Never used in schools.
15. Nitrous acid, HNO₂: pKa about 3.3 Unstable; made fresh in solution by adding acid to sodium nitrite. It’s used in organic chemistry to make diazonium salts, the starting point for many dyes.
16. Oxalic acid, H₂C₂O₄: pKa 1.25, 4.27 The strongest acid on this list for its first proton. It occurs in rhubarb leaves (which is one reason they’re toxic) and is used in rust and stain removers and as a standard in redox titrations.
17. Hypochlorous acid, HOCl: pKa 7.53 Forms when chlorine dissolves in water and is the real disinfectant in swimming pools and treated drinking water. Because its pKa is close to pool pH, small pH changes make a big difference to disinfection.
18. Boric acid, B(OH)₃: pKa 9.24 An unusual acid: it doesn’t donate a proton directly. Instead, it accepts a hydroxide ion from water (acting as a Lewis acid), releasing H⁺ indirectly. It’s used as a mild antiseptic, insecticide and in nuclear reactors to absorb neutrons.
19. Hydrocyanic acid, HCN: pKa 9.21 Very weak but highly toxic. Its salts, cyanides, are used industrially in gold extraction and electroplating. Mentioned here for completeness, not for handling.
20. Phenol, C₆H₅OH: pKa 9.99 A weak acid because the negative charge on its conjugate base (phenoxide) spreads into the benzene ring. It was one of the first antiseptics, used by Joseph Lister in surgery in the 1860s.
Ions that act as weak acids
Some ions are weak acids too:
| Ion | pKa | Where you meet it |
|---|---|---|
| Ammonium, NH₄⁺ | 9.25 | Ammonium salts in fertilisers, which is why they acidify soil |
| Hydrogensulfate, HSO₄⁻ | 1.99 | Sodium hydrogensulfate, used to lower pool pH |
| Hexaaquairon(III), [Fe(H₂O)₆]³⁺ | about 2.2 | Why iron(III) chloride solutions are acidic |
| Hexaaquaaluminium, [Al(H₂O)₆]³⁺ | about 5.0 | Why alum and aluminium sulfate lower soil pH |
The metal ions become acidic because their high charge pulls on the bound water molecules and makes an O–H bond easier to break. See Lewis acids and bases.
Ranking the list
Sorted from strongest to weakest by first pKa:
oxalic (1.25) → phosphoric (2.15) → salicylic (2.97) → tartaric (2.98) → citric (3.13) → HF (3.17) → nitrous (3.3) → malic (3.40) → aspirin (3.5) → methanoic (3.75) → lactic (3.86) → ascorbic (4.10) → benzoic (4.20) → ethanoic (4.76) → uric (5.4) → carbonic (6.35) → hypochlorous (7.53) → HCN (9.21) → boric (9.24) → phenol (9.99)
What the numbers mean in practice
Comparing sourness. At the same concentration, an acid with a lower pKa releases more H⁺ and tastes sourer. That’s why food manufacturers choose different acids for different tastes.
Choosing a preservative. Many preservatives (benzoate, sorbate, propanoate) only work well in their undissociated acid form. They’re used in foods whose pH is below their pKa.
Predicting buffer range. Each weak acid makes a good buffer at pH ≈ pKa ± 1, when mixed with its conjugate base. Citrate buffers work in the pH 3–6 range; phosphate buffers near 7; ammonia/ammonium near 9.
Calculating pH. For a solution of any of these acids, [H⁺] ≈ √(Ka × c) as long as less than 5% ionises. Full examples are in pH of weak acids.
Key takeaways
- Most acids in food, medicine and biology are weak, with pKa values typically between about 1 and 10.
- Lower pKa means stronger: oxalic and phosphoric acids are relatively strong weak acids; phenol and boric acid are very weak.
- Polyprotic acids like citric, phosphoric and carbonic acid have one pKa per acidic proton.
- Some ions, such as NH₄⁺ and hydrated Fe³⁺, are weak acids too.
- Try calculating their pH with the pH calculator.
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