What Is Alkaline Water? A pH Figure, and What It Describes

Water reaches a pH above 7 either naturally, by dissolving minerals underground, or artificially, usually by electrolysis in an ioniser.

pH is a real and precisely defined measurement, so the category has an unambiguous definition even where the marketing around it does not. What that measurement means, and what it does not, are two separate questions, and only the first belongs on a drinks site.

Alkaline water at a glance

What it is

Water measuring above pH 7, from minerals or from electrolysis

Typical range sold

pH 8 to 10, against pH 7 for neutral water

Two main routes

Carbonate rock underground, or an electrolysis cell on a worktop

The stable one

Naturally alkaline water, which is buffered by dissolved carbonate

The unstable one

Ionised water, whose pH drifts back towards neutral

Taste

Soft and faintly sweet when mineral-rich; close to the input water when ionised

In brewing

Alkalinity suppresses acidity, which flattens coffee and tea

The pH scale, and measuring it on your own worktop

Logarithms are the part of this scale that gets glossed over, and skipping them makes every figure on a bottle look smaller than it is.

pH measures the concentration of hydrogen ions, and each whole number is a tenfold change. Water at pH 9 holds a tenth the hydrogen ion concentration of water at pH 8 rather than slightly less, so a jump that looks like one point on a label is a large chemical move.

Where things sit on the scale

Most acidicMost alkaline

1. ColaAround pH 2.5
2. Orange juiceAround pH 3.5
3. CoffeeAround pH 5
4. Pure waterpH 7, neutral by definition
5. Alkaline waterTypically pH 8 to 10

Bottled still water is commonly slightly alkaline already. A great many ordinary mineral waters sit between pH 7.5 and 8 because of dissolved bicarbonate, without any of them being sold on that basis, and the same is true of most tap supplies drawn from chalk.

Against that, a glass of orange juice at pH 3.5 has about three thousand times the hydrogen ion concentration of neutral water.

Home testing gives you two different numbers, and confusing them is a common error. The first is pH, where cheap paper strips read to roughly half a unit and will separate tap water from a pH 9 product without ever distinguishing 8.2 from 8.6, while a digital meter reads finer but is meaningless until it has been calibrated against buffer sachets.

The second number is alkalinity, which no strip reports and which is measured by titration, sold cheaply as an aquarium KH test kit. It counts how much acid the water can absorb before its pH starts to fall.

That second number is the one that predicts behaviour. A water at pH 9 with low alkalinity shifts the moment it meets a coffee, whereas a water at pH 8 with high alkalinity flattens the same coffee completely.

Carbonate rock, and why alkaline usually means hard

Two quite different processes arrive at the same reading on a meter, and they produce water that behaves nothing alike.

The natural route is geology and nothing more mysterious. Rain arrives slightly acidic, having taken up carbon dioxide on the way down, and where it then works through limestone or dolomite it dissolves calcium and magnesium carbonate that consumes the acidity and lifts the pH.

Alkaline water and hard water arrive together for that reason. The dissolved carbonates that raise the pH are the same ones that fur a kettle, so a naturally alkaline bottle is usually a hard one.

The same carbonate keeps the pH steady, since a carbonate-buffered water holds a reservoir of it ready to neutralise whatever acid reaches it. A bottled spring water therefore reads the same in a year, whereas an ionised glass does not.

Inside an ioniser, and the half that goes to the drain

The machine turns out to be simpler than the language around it, and knowing what it does explains why its output behaves as it does.

Water passes over charged plates in an electrolysis cell, which splits the incoming stream in two. Positive ions such as calcium and magnesium collect at the negative plate and leave as the alkaline fraction, while the negative ions go the other way and leave as an acidic fraction.

What that means in practice

Input matters

An ioniser can only separate what is already there. Distilled water barely ionises at all

Waste stream

Roughly a third to a half of the water goes to the drain as the acidic fraction

Flow rate

Slower flow gives higher pH. The number on the display depends on the tap

Scaling

The plates scale up like a kettle and need periodic cleaning

Drift

The pH falls back over hours as the separation re-equilibrates

Drift is the property that separates the machine's output most sharply from a spring's. A glass you pour now and measure tomorrow reads closer to neutral, because nothing was added to hold the pH where the display put it, and a glass that reads low straight from the tap usually means the input water had little to separate.

The acidic half is the part rarely discussed on the box. It leaves at somewhere around pH 4 to 6, carrying the ions the alkaline side rejected, and most machines simply plumb it to the drain: a household drinking two litres a day is discarding another litre or more, which matters where water is metered or short.

Some owners collect it for cleaning surfaces, rinsing produce or watering acid-loving plants, all of which are sensible uses for mildly acidic water and none of which need a machine, since white vinegar in tap water reaches the same pH for a fraction of the cost.

What it tastes like, and what the label is telling you

There is a genuine, describable taste difference here, and it comes from the dissolved minerals rather than from the pH figure itself.

Naturally alkaline mineral waters often read as soft, round and faintly sweet, because that is what bicarbonate does to water, and it is the same effect that makes a high-bicarbonate bottle taste gentler than its carbonation would suggest. Ionised water, having had nothing added to it, tastes much like whatever went into the machine, though some drinkers report a slightly slippery or soapy mouthfeel at the upper end of the pH range.

If you compare the two side by side, you are really comparing mineral content, and the taste of alkaline water follows the analysis rather than the number on the front.

What each term on a label tells you

pH 8 to 10

The claimed range for typical products. Plain water sits near 7

Ionised or electrolysed

Made by running water through an electrolysis cell

Naturally alkaline

Alkalinity from dissolved minerals at source, not from a machine

Added minerals

Bicarbonate, calcium or magnesium salts added to raise pH

TDS

Total dissolved solids. The number that actually predicts taste

Alkalinity

Buffering capacity, which is a different measurement from pH

pH and alkalinity are the two lines most often read as one, and only the second is stable. A water at pH 9 with almost nothing dissolved in it heads back towards neutral as soon as it meets anything at all, while a mineral-rich water holds its reading against whatever it is mixed with.

Total dissolved solids is the line that predicts the taste, and it is the one alkaline branding tends to leave smallest.

In a coffee pot and in a pan of noodles

Alkalinity does something measurable to two things a kitchen makes, and the two outcomes point in opposite directions.

In brewing it is unhelpful, and the mechanism is straightforward: alkalinity buffers acidity, so water with high alkalinity neutralises the acids that give a bright coffee its definition and leaves the cup flat. The same buffering dulls a delicate green tea, so hard water and alkaline water cause the same complaint by the same route, and no adjustment to steeping temperature recovers what the water has already taken out of your cup.

Brewing water sets out the targets for coffee and tea: a moderate mineral content and a near-neutral pH.

Colour moves with pH as well, visibly. Anthocyanin-rich infusions such as hibiscus shift from red towards purple in alkaline water, which is the cheapest demonstration of the chemistry there is and a useful check that a bottle is doing what it claims.

An espresso pulled with strongly alkaline water tastes hollow where the acidity should have been, the same chemistry showing up in flavour rather than colour.

Cooking is where alkalinity earns its place. It greens vegetables, which is why a pinch of bicarbonate in blanching water keeps beans vivid, and it softens dried pulses by breaking down pectin faster.

The clearest case of all is noodles, where kansui, an alkaline solution, is what gives ramen its yellow colour, springy bite and particular smell, and substituting plain water produces a completely different noodle. None of that helps in a glass, since the alkalinity that builds a good noodle is the same chemistry that flattens a coffee.

The claims this page does not evaluate

Alkaline water is marketed with a large volume of claims attached to it, so the category cannot be described without naming them.

We make no health claims for any drink, so nothing here says alkaline water does anything beneficial, and equally nothing says it is harmful or that any particular assertion about it is false.

Weighing those claims is work for regulators, researchers and qualified professionals, and a drinks reference that took a position either way would be pretending to an authority it has not got.

The questions an editorial site can answer honestly are the ones with checkable answers: what pH measures, how the two production routes differ, how the water tastes, what the label means and what happens when it meets a coffee or a pan of noodles.

If you arrived with a question about your own circumstances, this is the wrong sort of page, and no rearrangement of what is on it would change that.

A spring, a machine, or a spoon of bicarbonate

Three routes reach the same reading on a meter, and they differ in price by more than an order of magnitude.

Bicarbonate stirred into tap water raises the pH for almost nothing and produces a drink that tastes faintly of soda. It is the cheapest way to reach the measurement and the least pleasant to drink, and it is the simplest way of making alkaline water at home.

Between the other two, a naturally alkaline mineral water hands you a stated analysis, a stable pH and a taste of its own, while an ioniser hands you an adjustable number from any input at a low running cost, with alkalinity that fades and no mineral content to speak of.

For the taste, the bottled spring water is the more informative purchase. For brewing anything at all, neither is what you want, and a moderate low-bicarbonate water beats both.

The fair comparison is against an ordinary bottle with a good analysis printed on it, since a great many sit at pH 7.5 to 8 already, cost less than either, and say exactly what is dissolved in them.

Alkaline and sparkling pull opposite ways

Two properties that appear together on some labels are, chemically, working against each other.

Dissolving carbon dioxide in water produces carbonic acid, which is what drops sparkling water to somewhere near pH 5. Carbonation is an acidifying process by definition, so a still water at pH 9 does not stay at pH 9 once gas goes into it.

A sparkling water sold as alkaline is therefore one of two things. Either it was alkaline before carbonation and is mildly acidic in the bottle you are holding, or it is so heavily mineralised that the bicarbonate absorbs most of the drop.

The second case is real and it is exactly what high-bicarbonate springs do, so a few genuinely mineral sparkling waters still measure above 7 with gas in them.

That same buffering explains why soda water tastes softer than plain seltzer at identical carbonation. The added bicarbonate is neutralising part of the carbonic acid in the glass, performing on a bottling line the reaction a limestone aquifer performs underground.

The three things sold as alkaline water

Set against the rest of the bottled-water shelf, the category turns out to be narrower than its marketing presence suggests.

Only the first three are alkaline water in any useful sense, and of those the first is usually the most informative, since a spring water with a printed analysis tells you both its pH and everything else dissolved in it. The added-mineral route sits in the middle, with a composition that is a formulation rather than a place, which makes it stable and predictable while removing the one thing a source guarantee is for.

A sports drink and an infused water both turn up in the same aisle and belong to neither conversation, since one is built around dissolved salts at a stated concentration and the other around flavour, and neither is designed around a pH figure at all.