Coffee water is often reduced to a few numbers: TDS, hardness and alkalinity.
But behind those numbers are individual ions.
Understanding those ions gives us a more useful way to think about water.
Not every ion has the same level of scientific evidence, and not every sensory claim should be treated as established fact.
Some relationships are supported by chemistry research.
Others come from sensory work.
Some are still uncertain.
This atlas keeps those ideas separate.
Magnesium — Mg²⁺
Magnesium is one of the most discussed ions in coffee water.
Hendon and colleagues proposed strong molecular interactions between magnesium and coffee compounds.
Sensory work from APAX has associated magnesium-rich profiles with brightness, fruitiness and sweetness.
But newer experimental work has challenged the assumption that realistic magnesium concentrations necessarily produce large increases in extraction.
The useful takeaway is not:
"More magnesium equals more extraction."
Instead:
Changing magnesium changes the mineral environment, and that may change how the coffee is expressed.
Calcium — Ca²⁺
Calcium is another major contributor to water hardness.
It is often associated with a fuller or rounder sensory direction in practical coffee-water discussions.
But calcium should not simply be labelled as a "body ion."
Its behaviour depends on concentration, accompanying ions, alkalinity and the coffee itself.
Calcium also behaves differently from magnesium chemically, so replacing one with the other is not a neutral substitution.
Sodium — Na⁺
Sodium is naturally present in many waters and can also be introduced deliberately through mineral salts.
Sensory work has suggested that sodium can influence perceived sweetness and bitterness.
But sodium should not be treated as a universal flavour enhancer.
Concentration matters, and excessive sodium can quickly become undesirable.
Potassium — K⁺
Potassium is another naturally occurring cation found in water.
APAX sensory work has associated potassium with sweetness and a lingering finish.
However, the evidence base for direct potassium-driven sensory effects in coffee is less established than the broader literature surrounding calcium, magnesium and alkalinity.
This is a good example of why sensory observations and mechanistic evidence should be kept separate.
Bicarbonate — HCO₃⁻
Bicarbonate is closely associated with alkalinity.
Its most important role in coffee water is not simply "flavour."
It contributes to the water's buffering capacity and therefore influences how acids in coffee are perceived.
Too much alkalinity can suppress brightness and make a coffee feel flatter.
Too little buffering can allow acidity to become overly sharp.
Chloride — Cl⁻
Chloride is often overlooked because it is commonly introduced as the counter-ion in salts such as calcium chloride and magnesium chloride.
That is important.
When we add MgCl₂, we don't add magnesium alone.
We add:
Mg²⁺ + 2Cl⁻
Sensory work has associated chloride with sweetness, roundness and intensity.
But again, these observations should not be treated as universal scientific laws.
Sulfate — SO₄²⁻
Sulfate is another counter-ion that can become important when using salts such as magnesium sulfate.
MgSO₄ gives us:
Mg²⁺ + SO₄²⁻
APAX sensory work has associated sulfate with certain floral, citrus and bitterness directions.
The practical lesson is that the salt used to introduce an ion matters.
Same magnesium.
Different accompanying ion.
Different water chemistry.
Phosphate — PO₄³⁻
Phosphate appears in some specialised water formulations and has been associated with acidity and clarity in sensory work.
Its role in everyday coffee-water preparation is less established than the more common ions.
It is therefore better treated as a specialised formulation variable rather than a universal coffee-water ingredient.
Citrate
Citrate is another specialised anion that can appear in mineral formulations.
It may influence acidity and citrus-like sensory directions, but evidence for its practical role in everyday coffee brewing remains limited.
The ion is only half the story
This is one of the most important ideas in coffee-water formulation.
MgCl₂ is not simply magnesium.
It is magnesium plus chloride.
MgSO₄ is magnesium plus sulfate.
CaCl₂ is calcium plus chloride.
So when we change a mineral salt, we may be changing multiple ions at once.
That means water formulation is not simply about chasing individual ion targets.
It is about understanding the complete ionic composition.
Research, sensory evidence and interpretation
At The Water Artist, we separate three things:
Research tells us what mechanisms are supported.
Sensory work tells us what experienced tasters observe.
Brewing experiments tell us what happens under specific conditions.
Those are related, but they are not identical.
An observed sensory association is not automatically proof of a chemical mechanism.
The Water Artist approach
Don't ask:
"Which ion is best?"
Ask:
"What does this combination of ions do to this coffee?"
That shift moves coffee water away from rigid recipes and toward experimentation.
The goal isn't to make every coffee taste the same.
The goal is to understand how water changes its expression.
The Water Artist
Science tells us what we know.
Sensory work tells us what we observe.
The cup tells us what matters.
Water is the canvas.
You're the artist.