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Alkalinity

Alkalinity is your water's capacity to neutralise acid, driven mostly by dissolved bicarbonate. It sounds like a niche measurement, but of everything on a water report it correlates most directly with what happens in your cup: brewed tea is itself acidic, so what actually decides the final colour and catechin content is how hard the water pushes back against that acidity, not the pH the water started with. Two waters can share an identical pH and behave completely differently once tea leaf hits them.

What it is

Alkalinity is measured by titrating a water sample with acid until it reaches pH 4.5, the point at which its buffering capacity runs out (US Geological Survey). In most tap water that buffer is almost entirely bicarbonate, so alkalinity and 'bicarbonate content' are close to interchangeable in practice. It is usually reported in the same unit as hardness, milligrams per litre as calcium carbonate, which invites confusing the two: hardness counts dissolved calcium and magnesium, alkalinity counts the acid-neutralising anions behind them. A water can run high in one and low in the other.

What it does to your tea

The clearest evidence: Cabrera et al. (2021) tested a sodium bicarbonate water carrying no calcium or magnesium at all, at pH 7.36, and it still browned the tea and cut catechin content the same way their hard water samples did. That isolates bicarbonate buffering, not calcium, as the active ingredient. Separately, Zhang et al. (2025) found bicarbonate concentration inversely tracked how fresh and clean a tea's aroma scored (r² of -0.92): more buffering capacity, duller aroma. In Cabrera's data, visible browning needed pH above 6.3 and hardness above 42 ppm as CaCO₃, together, neither alone.

How to find out what yours is

Aquarium and pool test kits measure alkalinity directly, by the same acid-titration method water labs use, and are cheap and accurate enough for a kitchen. Your water utility's public quality report often lists 'total alkalinity' too, in mg/L as CaCO₃. A pH meter or strip is not a substitute: it reads the water's current pH, not its resistance to being pushed off that pH, and two waters that read identically on a pH strip can hold very different buffering capacity underneath.

What to aim for

The SCA's coffee-brewing standard sets alkalinity at 40 mg/L as CaCO₃ and pH at 7.0, with 6.5 to 7.5 given as acceptable, borrowed from coffee since no tea standard sets numbers. On pH specifically, the US EPA lists 6.5 to 8.5 as a secondary, aesthetic guideline rather than a health-based one, while the EU drinking water directive and the German Trinkwasserverordnung both allow 6.5 to 9.5, a full pH unit higher at the top. None of these ranges was written with tea in mind.

What you can do about it

Reverse osmosis strips out nearly all dissolved bicarbonate along with everything else, which is why RO or distilled water needs remineralising before it tastes like much. Cutting high-alkalinity tap water with distilled or RO water dilutes the buffering capacity in proportion, the same trick used for hardness. Boiling water that also carries hardness minerals lowers alkalinity somewhat, since bicarbonate precipitates out alongside the calcium carbonate (see hardness); the faster effect is the pH shift itself, 0.5 to 1.0 units upward from CO₂ loss, measured by Cabrera et al. (2021).

What people get wrong

A tap water pH sitting inside the EPA's 6.5 to 8.5 range gets treated as proof the water is fine for tea. It is not: that range was set for taste and pipe corrosion, not for catechins. Cabrera et al.'s (2021) five brewed samples spanned pH 6.0 to 7.1, entirely inside or just under that 'fine' band, yet catechin content varied roughly threefold across them and visible browning showed up once pH passed 6.3. The effects this page describes are not waiting outside the normal drinking water range: they start inside it.

teabert, the tealytics teapot, keeper of the kettle
pH tells you how the water tastes right now. Alkalinity tells you how hard it will fight your tea's acidity to keep tasting that way, and that fight is the one that actually shows up in the cup.

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