On this page
Stomach acid has a fearsome reputation. It’s often said to be strong enough to dissolve razor blades or metal. That’s partly true, partly exaggerated, and wholly interesting. Your stomach produces a genuine mineral acid, hydrochloric acid, the same substance found on laboratory shelves, and it does so every day, from nothing more than salt and water in your blood.
What stomach acid is
Gastric juice is a mixture of:
- hydrochloric acid (HCl)
- water
- pepsinogen, an inactive enzyme that acid converts into pepsin
- mucus
- intrinsic factor, a protein needed to absorb vitamin B12
- salts such as potassium chloride and sodium chloride
The acidic component is hydrochloric acid, a strong acid that’s fully ionised into H⁺ and Cl⁻ ions.
How strong is it?
The acid secreted by the stomach’s parietal cells is about 0.15–0.16 mol/dm³ HCl. That gives a pH of about 0.8 at the point of secretion. By the time it mixes with the stomach’s contents, the pH of the stomach is typically:
- 1.5–2.0 on an empty stomach
- up to 3–5 shortly after a meal, because food buffers the acid
For comparison:
| Solution | Approximate pH |
|---|---|
| Laboratory 1 mol/dm³ HCl | 0 |
| Freshly secreted gastric acid | about 0.8 |
| Empty stomach | 1.5–2.0 |
| Lemon juice | 2.0–2.6 |
| Cola | 2.5–3.0 |
| Stomach after a meal | 3–5 |
So stomach acid is roughly as concentrated as the dilute hydrochloric acid used in school labs (often 0.1–1 mol/dm³). It’s a real strong acid, but it isn’t “stronger than battery acid”, which is around 4–5 mol/dm³ sulfuric acid.
How the stomach makes it
The stomach lining contains millions of parietal cells. Each one pumps hydrogen ions out into the stomach against a colossal concentration difference: the H⁺ concentration inside the stomach can be about a million times higher than in the blood.
The key player is an enzyme called the proton pump (H⁺/K⁺-ATPase). It uses energy from ATP to swap H⁺ ions out of the cell for K⁺ ions coming in.
Where do the H⁺ ions come from? From carbon dioxide and water, catalysed by carbonic anhydrase:
CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
The H⁺ is pumped into the stomach. The hydrogencarbonate ion goes the other way, into the blood, in exchange for a chloride ion. The chloride then follows the H⁺ into the stomach. The net result: HCl in the stomach, and a little extra hydrogencarbonate in the blood. After a big meal, the blood leaving the stomach is slightly more alkaline, an effect called the alkaline tide.
What the acid does
- Kills microbes. Most bacteria and viruses in food don’t survive the stomach’s acidity.
- Denatures proteins. Acid disrupts the folded shape of proteins, exposing their chains to digestive enzymes.
- Activates pepsin. Pepsinogen is converted into pepsin by acid, and pepsin works best around pH 1.5–2.
- Helps absorb some minerals. Acid helps convert dietary iron into a more easily absorbed form and helps release vitamin B12 from food proteins.
Why doesn’t it digest the stomach?
The stomach wall is made of protein, and it’s bathed in acid and a protein-digesting enzyme. The reason it survives:
- A thick mucus layer coats the stomach lining. It’s a gel that slows the acid reaching the cells.
- Hydrogencarbonate secreted into the mucus neutralises acid near the cell surface, keeping it close to neutral right next to the cells, even when the stomach contents are at pH 2.
- Tight junctions between cells stop acid leaking between them.
- Rapid renewal: the cells of the stomach lining are replaced every few days.
- Prostaglandins, signalling molecules, keep mucus and hydrogencarbonate production up.
When this protection breaks down, ulcers can form. The two biggest causes are infection with the bacterium Helicobacter pylori and regular use of non-steroidal anti-inflammatory drugs (NSAIDs) like aspirin and ibuprofen, which reduce prostaglandin production. The discovery of H. pylori’s role earned Barry Marshall and Robin Warren the 2005 Nobel Prize in Physiology or Medicine; Marshall famously drank a culture of the bacteria to prove it could cause stomach inflammation.
Can it dissolve metal?
Hydrochloric acid at around 0.1 mol/dm³ does react with reactive metals such as zinc, iron and magnesium, releasing hydrogen gas:
Zn + 2HCl → ZnCl₂ + H₂
This is why swallowing coins (especially modern zinc-core coins) or button batteries can be dangerous: over time, acid attacks the metal, and a button battery can also cause serious tissue damage by electrolysis. But stomach acid can’t quickly dissolve steel razor blades or stainless steel; those claims are exaggerations. It dissolves reactive metals slowly, not everything instantly.
Heartburn and acid reflux
The oesophagus, the tube from mouth to stomach, doesn’t have the stomach’s protective lining. A ring of muscle at its lower end normally stops stomach contents flowing back up. If this muscle relaxes at the wrong time, acid splashes up and irritates the oesophagus. That burning feeling is heartburn. Frequent reflux is called gastro-oesophageal reflux disease (GORD, or GERD in American English).
Triggers include large meals, lying down after eating, fatty foods, alcohol, caffeine and being overweight.
How antacids work
Antacids are bases that neutralise excess acid:
| Antacid | Reaction |
|---|---|
| Calcium carbonate | CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ |
| Magnesium hydroxide | Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O |
| Aluminium hydroxide | Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O |
| Sodium hydrogencarbonate | NaHCO₃ + HCl → NaCl + H₂O + CO₂ |
Most use weak, mostly insoluble bases. They neutralise acid steadily without making the stomach strongly alkaline, which would disrupt digestion and trigger the stomach to make even more acid. The carbonate-based ones produce carbon dioxide, which is why they can cause burping.
Magnesium compounds tend to have a laxative effect and aluminium compounds a constipating one, which is why some products combine the two.
How much acid can one tablet neutralise? A typical 500 mg calcium carbonate tablet contains 0.500 ÷ 100.1 = 5.0 × 10⁻³ mol CaCO₃, which can neutralise 1.0 × 10⁻² mol of HCl. That’s the acid in about 100 cm³ of 0.1 mol/dm³ gastric juice.
Acid-reducing medicines
Instead of neutralising acid after it’s made, some medicines reduce how much acid the stomach produces:
- H₂-receptor antagonists (such as famotidine) block a histamine signal that tells parietal cells to secrete acid.
- Proton pump inhibitors (PPIs, such as omeprazole) switch off the H⁺/K⁺-ATPase pump directly. They’re activated by the acid itself, in the parietal cells, which makes them highly targeted.
Key takeaways
- Stomach acid is hydrochloric acid, secreted at about 0.15 mol/dm³ (pH about 0.8); the stomach itself is usually at pH 1.5–3.5.
- Proton pumps in parietal cells move H⁺ against a million-fold concentration gradient.
- The acid kills microbes, denatures proteins and activates pepsin.
- Mucus, hydrogencarbonate and rapid cell renewal protect the stomach from its own acid.
- Antacids neutralise acid with weak bases; proton pump inhibitors stop it being made.
- See the whole-body picture in pH inside the body.
Advertisement