When coffee brewers talk about water, pH often gets most of the attention.
But for coffee, another measurement can be more useful:
alkalinity.
pH and alkalinity are related, but they are not the same thing.
Understanding the difference helps explain why two waters with similar mineral content can produce very different expressions of acidity in the cup.
What is alkalinity?
Alkalinity describes a water's capacity to neutralise acids.
In natural waters, bicarbonate is often one of the main contributors.
That means alkalinity gives us an idea of how strongly the water can resist changes in acidity when acids enter the brewing system.
This is why alkalinity matters when brewing coffee.
Coffee contains many organic acids.
The water is part of the chemical environment in which those acids are expressed.
Alkalinity is not pH
This distinction is important.
pH tells us about the current hydrogen-ion activity of a solution.
Alkalinity tells us about its acid-neutralising capacity.
A water can therefore have a relatively similar pH to another water while having a very different alkalinity.
For coffee brewing, that difference can matter.
Bicarbonate and alkalinity
Bicarbonate — HCO₃⁻ — is one of the most important contributors to alkalinity in typical drinking water.
When bicarbonate encounters acids, it participates in reactions that reduce their acidity.
This is useful.
Without enough buffering, some coffees can taste extremely sharp.
But too much buffering can also become a problem.
Why too much alkalinity can flatten coffee
Imagine a coffee with a beautiful citrus acidity.
If the brewing water contains a high amount of buffering capacity, some of that acidity can become less pronounced.
The coffee may feel:
softer
rounder
less vibrant
or even flat.
This doesn't mean high alkalinity is always bad.
A coffee with aggressive acidity may actually benefit from more buffering.
The point is that alkalinity changes the balance.
Why low alkalinity can also be useful
A delicate washed coffee may benefit from water with relatively low buffering capacity.
This can allow its acidity to remain expressive.
But extremely low alkalinity is not automatically better.
If the water provides almost no buffering, some coffees can become excessively sharp or unbalanced.
Again, the goal is not maximum or minimum.
The goal is appropriate buffering.
Hardness and alkalinity are different
This is one of the most common sources of confusion.
Calcium and magnesium are associated with hardness.
Bicarbonate contributes strongly to alkalinity.
So you can have:
high hardness + low alkalinity
or:
low hardness + high alkalinity.
Those waters can behave very differently.
This is one reason we separate hardness from buffer when designing brewing water.
TDS doesn't tell you alkalinity
Two waters can have similar TDS but different alkalinity.
For example, two waters might both measure around 60 ppm.
One could contain relatively little bicarbonate.
Another could contain substantially more.
The TDS number alone cannot tell you that.
This is another reason why TDS should be treated as a checkpoint rather than a complete description of water.
What does this mean for coffee?
Think about alkalinity as a kind of volume control for acidity.
It doesn't create acidity.
It influences how strongly acidity is expressed.
A coffee with bright citrus and floral acidity may respond differently from a naturally processed coffee with heavier fruit and fermentation character.
The same water doesn't have to be ideal for both.
Water and brewing method
Alkalinity also interacts with brewing conditions.
Temperature, contact time, agitation, grind size and brew ratio all influence extraction.
Water chemistry exists inside that system.
Changing alkalinity while changing several other variables at the same time makes it difficult to understand what actually caused the difference.
This is why controlled experiments are useful.
A better question
Instead of asking:
"What is the perfect bicarbonate level?"
ask:
"How much buffering does this coffee need?"
That question leaves room for different coffees, roast levels and brewing methods.
The Water Artist approach
We treat hardness and buffer as separate dimensions.
Hardness describes one part of the mineral environment.
Alkalinity describes another.
Together they help us understand the water more completely.
But neither should be treated as a magic number.
The right water is the one that works with the coffee and the brewing method.
The Water Artist
Hardness gives the water structure.
Alkalinity determines how strongly it resists acidity.
The art is finding the balance.
Water is the canvas.
You're the artist.