Water Chemistry for Coffee

A filter coffee at 1.3 % TDS is, by definition, about 98.7 % water¹, and the minerals in that water are not passive. They change how compounds extract, how acids taste, and whether your kettle and espresso machine survive the year².

The two numbers that matter

General hardness (GH) is the concentration of calcium and magnesium ions, expressed in ppm as CaCO₃². These ions bind to flavour compounds and help carry them into the cup: a 2014 study by Hendon and the Colonna-Dashwoods modelled how strongly Mg²⁺, Ca²⁺ and Na⁺ bind to representative coffee compounds and found magnesium and calcium both bind strongly, magnesium slightly more so, while sodium barely does³. The common description of magnesium as "brighter, fruitier" and calcium as "body and sweetness" comes from tasting rather than from that chemistry, and is a practitioner claim⁴.

Alkalinity (KH) is the water's buffering capacity, mostly bicarbonate, also expressed as ppm CaCO₃. It neutralises acids². Too much and a bright Kenyan tastes flat and chalky; too little and the same coffee is sharp and unbalanced⁴.

Neither is captured by a cheap "TDS pen", which measures total conductivity and can't tell you which minerals are present². Use test strips, a titration kit, or your utility's water report.

Reference targets

The SCA column is from the SCA Water Quality Handbook². The practical column is the narrower band most specialty brewers use today⁴⁵: moderate hardness, lowish alkalinity, magnesium-forward.

What goes wrong

• Distilled / RO water alone — nothing to carry flavour; brews taste hollow and thin³, and pure water is aggressive toward metal boilers².
• Very hard tap water — heavy scale, muted acidity, often a metallic edge. Kills espresso machines².
• High alkalinity — flattens acidity and makes light roasts taste dull and "baked"⁴.
• Softened water (ion exchange) — sodium replaces calcium; sodium extracts poorly³ and the coffee tastes salty and flat⁴.

Building your own water

Start from distilled or RO water and add minerals. Two well-known approaches:

Rao / Perger style (per litre of distilled water)⁵

• 0.1 g magnesium sulphate heptahydrate (Epsom salt) contributes ~41 ppm hardness
• 0.05 g sodium bicarbonate contributes ~30 ppm alkalinity

Concentrate approach

Dissolve 24 g Epsom salt in 500 g water for a magnesium concentrate and 17 g baking soda in 500 g water for a buffer concentrate. Each gram of either concentrate added to a litre of distilled water adds about 20 ppm (GH from the first, KH from the second), so a few grams of each lets you tune GH and KH independently. See the arithmetic below.

Commercial mineral packets exist for people who'd rather not weigh milligrams.

Temperature and gas

Freshly boiled water has driven off dissolved oxygen and CO₂; that's fine. What you want to avoid is water that has been kept simmering for hours — it concentrates minerals through evaporation and tastes flat. This is practitioner advice⁵. Fresh water, brought to temperature once.

For espresso machines

Scale is calcium carbonate precipitating out of hard water at high temperature². Aim for the lower end of hardness (60–80 ppm) and alkalinity (~40 ppm), or use a magnesium-forward recipe — magnesium carbonate is far more soluble than calcium carbonate and scales far less⁴. Check the manufacturer's water spec: some warn explicitly against very low alkalinity because it becomes corrosive to brass and copper; that is manufacturer guidance and the handbook makes the same point about corrosivity².

How the numbers are computed

Everything here is stoichiometry, so you can check it with molar masses.

Converting ion concentrations to ppm as CaCO₃ (molar mass of CaCO₃ = 100.09 g/mol; each mg/L of an ion is scaled by 100.09 divided by the ion's equivalent mass):

GH (ppm CaCO₃) ≈ 2.497 × Ca (mg/L) + 4.118 × Mg (mg/L)
KH (ppm CaCO₃) ≈ 0.820 × HCO₃⁻ (mg/L)

What the two salts contribute per gram dissolved in a litre:

Epsom salt, MgSO₄·7H₂O   molar mass 246.47 g/mol; 1 g contains 98.6 mg Mg
                          → 98.6 × 4.118 ≈ 406 ppm GH as CaCO₃ per g/L
Baking soda, NaHCO₃      molar mass 84.01 g/mol;  1 g contains 726 mg HCO₃⁻
                          → 726 × 0.820 ≈ 596 ppm KH as CaCO₃ per g/L

So 0.1 g of Epsom salt in a litre gives 41 ppm GH and 0.05 g of baking soda gives 30 ppm KH, which is where the recipe above comes from. For the concentrates: 24 g Epsom in 500 g water is 4.8 % by mass, so 1 g of concentrate carries 0.048 g of salt, or 0.048 × 406 ≈ 19.5 ppm GH per gram per litre; 17 g baking soda in 500 g is 3.4 %, so 1 g carries 0.034 g, or 0.034 × 596 ≈ 20 ppm KH. To hit a target from distilled water: mg of Epsom salt per litre = GH ÷ 0.406; mg of baking soda per litre = KH ÷ 0.596. The interactive map above uses exactly these factors. Note that baking soda also adds sodium (about 274 mg per gram), so a 30 ppm KH recipe adds roughly 14 mg/L of sodium, comfortably near the SCA guidance².