Alkaline Water: What the Chemistry Says

Walk down the drinks aisle of many supermarkets and you’ll find bottled “alkaline water” with a pH of 8, 9 or even 9.5, sometimes at several times the price of ordinary water. Home ionisers that make alkaline water from tap water sell for hundreds or thousands. The marketing often promises that it will “neutralise acid in your body”, “balance your pH”, boost energy, slow ageing or prevent disease.

Chemistry gives us a clear way to test these claims. Here’s what alkaline water is, what actually happens when you drink it, and where the evidence stands.

What is alkaline water?

Alkaline water is water with a pH above 7. There are two main types:

1. Naturally alkaline mineral water. Water that has passed over rocks such as limestone picks up dissolved minerals, especially calcium, magnesium and hydrogencarbonate ions. Hydrogencarbonate is a weak base, so the water ends up slightly alkaline, often pH 7.5–8.5.
2. Artificially alkaline water. Made by adding alkaline mineral salts (such as potassium hydrogencarbonate or calcium and magnesium compounds), or by electrolysis in a home “ioniser”. In an ioniser, water flows past electrodes: near the cathode, water is reduced and hydroxide ions form, raising the pH of that stream. A separate acidic stream is produced at the anode.

For comparison, most tap water is between about pH 6.5 and 8.5, a range many drinking water regulations consider acceptable. So some ordinary tap water is already “alkaline water”.

What happens when you drink it?

This is the key question, and the chemistry is straightforward.

Step 1: the stomach

Your stomach contains hydrochloric acid at a pH of about 1.5–3.5 (see stomach acid chemistry). Alkaline water at pH 9 has a hydroxide concentration of about 1 × 10⁻⁵ mol/dm³. A glass (250 cm³) therefore contains roughly 2.5 × 10⁻⁶ mol of OH⁻.

The stomach typically contains tens of cm³ of gastric juice even when “empty”, and around 0.01–0.1 mol/dm³ of H⁺. Even 50 cm³ at 0.01 mol/dm³ contains 5 × 10⁻⁴ mol of H⁺, about 200 times more than the hydroxide in a whole glass of pH 9 water. The alkalinity is neutralised almost instantly:

H⁺ + OH⁻ → H₂O

Any hydrogencarbonate in mineral water is neutralised too, releasing a little CO₂. The stomach simply produces more acid if needed.

Step 2: the intestine and blood

What leaves the stomach is acidic; the pancreas then neutralises it with its own hydrogencarbonate. By the time water is absorbed into the blood, its original pH is long gone. What remains are the dissolved minerals (calcium, magnesium, potassium, sodium), which are absorbed like minerals from any other food or drink.

Step 3: the blood’s pH doesn’t change

Blood pH is held between 7.35 and 7.45 by powerful buffers, the lungs and the kidneys. Healthy people can’t meaningfully change blood pH by what they drink. If they could, it would be dangerous: blood pH above about 7.45 is alkalosis, a medical problem. See the blood buffer system.

Checking the common claims

Claim: “Alkaline water neutralises acid in your body.” The body isn’t “acidic” in a way that needs neutralising. Each body fluid has its own controlled pH (see pH inside the body). Drinking alkaline water doesn’t change blood or cell pH.

Claim: “It changes your body’s pH, as shown by urine tests.” Urine pH does vary with diet, because the kidneys excrete excess acid or base to keep blood pH constant. Changes in urine pH show the kidneys doing their job, not a change in the body’s overall pH.

Claim: “An alkaline diet or water prevents cancer.” Tumours can create acidic microenvironments around themselves, but drinking alkaline water doesn’t change that environment. Major cancer research organisations have found no good evidence that alkaline water or alkaline diets prevent or treat cancer.

Claim: “It hydrates better.” Hydration comes from water itself. A few small studies have reported minor differences, often funded by manufacturers or with limitations, and there’s no convincing evidence that alkaline water hydrates better than ordinary water for the general population.

Claim: “It contains antioxidants” or “has smaller water clusters”. Some ioniser marketing claims electrolysed water has special “micro-clustered” molecules. Water molecules in liquid form constantly make and break hydrogen bonds in trillionths of a second; there are no stable clusters to make smaller. Any dissolved hydrogen gas from electrolysis escapes quickly once the water is poured.

Where there might be something real

To be fair to alkaline water, a few points have some basis:

• Minerals. Some alkaline mineral waters are good sources of calcium and magnesium, which are useful nutrients. That’s a property of the minerals, not the pH.
• Reflux. A small laboratory study found that water at pH 8.8 could inactivate pepsin, an enzyme involved in reflux damage to the throat. That’s an interesting early finding, not proof of a clinical benefit.
• Taste. Some people prefer the taste of mineral-rich water. That’s a perfectly good reason to drink it.

Are there risks?

For most healthy people, drinking moderately alkaline water is harmless; it’s just water with minerals. However:

• People with kidney disease can have trouble excreting excess minerals such as potassium, so high-mineral water may need caution. Anyone with a medical condition should ask a doctor.
• Very high pH water (above about 10) can irritate the mouth and stomach.
• Relying on expensive water instead of proven health measures has an opportunity cost.

A simple experiment you can try

1. Add a few drops of universal indicator or red cabbage indicator to a glass of alkaline bottled water. Note the colour (probably blue-green).
2. Add a teaspoon of vinegar or lemon juice (representing a very mild version of stomach acid).
3. Watch the colour switch to acidic immediately.

A single teaspoon of a weak food acid overwhelms the alkalinity of a whole glass. Stomach acid is stronger and more plentiful. (See make a red cabbage pH indicator.)

How to evaluate health claims about chemistry

Alkaline water is a useful case study for thinking critically about any product that uses chemistry in its marketing. Ask a few questions. Does the claim describe a mechanism that fits what we know about the body? Is the effect big enough to matter once real quantities are compared, as we did with stomach acid? Is the evidence from independent, well-designed studies in people, or from small, company-funded trials and testimonials? And would the claimed effect be dangerous if it were true? Applying those questions here quickly shows why scientists remain sceptical.

Key takeaways

• Alkaline water is water with a pH above 7, from dissolved minerals or electrolysis.
• Its alkalinity is tiny compared with stomach acid and is neutralised almost immediately.
• Blood pH is tightly controlled and isn’t changed by what you drink; urine pH changes because the kidneys are keeping blood pH constant.
• There’s no good evidence for claims about disease prevention, detox or special hydration.
• Mineral content and taste are real, but they don’t depend on “pH magic”.