BrewWater
How do you remineralise distilled water for coffee?
Short answer
Weigh 0.101 grams of Epsom salt and 0.067 grams of baking soda into one litre of distilled water, then stir until fully dissolved. That reaches ten milligrams per litre of magnesium and forty of alkalinity. Scale both figures linearly for larger volumes.
Start from distilled or reverse osmosis water
Begin with water that has effectively no mineral content, so the dose you add is the whole mineral content. Dosing on top of tap water stacks a known quantity onto an unknown one and the arithmetic stops meaning anything.
Weigh the Epsom salt
Weigh 0.101 grams of magnesium sulfate heptahydrate per litre. Magnesium is 9.861 percent of the crystal by mass, so this delivers ten milligrams per litre of magnesium. A scale reading to three decimal places is required — spoon measures cannot resolve a tenth of a gram.
Weigh the baking soda
Weigh 0.067 grams of sodium bicarbonate per litre. Bicarbonate is 72.63 percent of it by mass, and one mole of bicarbonate buffers as half a mole of calcium carbonate, which lands alkalinity at forty milligrams per litre as calcium carbonate.
Dissolve each addition fully before adding the next
Add one salt, agitate until no crystals remain, then add the second. Undissolved crystals settling at the bottom of a bottle mean the water you pour off is weaker than the figure you calculated, and the residue makes the next batch stronger.
Consider a concentrate if you brew often
At single-litre scale these doses are awkward to weigh repeatedly. Mixing a deliberately concentrated batch and measuring that by volume is more repeatable — but record the dilution ratio, because misquoting a concentrate as a direct dose is exactly how the wrong figures end up circulating.
| Measure | Value | Unit | Source |
|---|---|---|---|
| Magnesium content of Epsom salt by mass | 9.861 | % by mass | IUPAC Commission on Isotopic Abundances and Atomic Weights · verified |
| Bicarbonate content of sodium bicarbonate by mass | 72.63 | % by mass | IUPAC Commission on Isotopic Abundances and Atomic Weights · verified |
| Target total alkalinity for brewing water | 40 | mg/L as CaCO3 | Specialty Coffee Association of America · verified |
| Target calcium hardness for brewing water | 68 | mg/L as CaCO3 | Specialty Coffee Association of America · verified |
Why weigh rather than measure by spoon?
The doses are around a tenth of a gram per litre, one to two orders of magnitude below a level teaspoon. The error from eyeballing it is larger than the quantity itself. This is also why several published recipes are wrong: figures intended as a concentrate get repeated as direct per-litre doses.
Where do these numbers come from?
From standard atomic weights, not from a recipe. Magnesium sulfate heptahydrate has a molar mass of 246.466 and contains one magnesium atom at 24.305, giving the 9.861 percent figure. Sodium bicarbonate is 84.006 with a bicarbonate ion at 61.016, giving 72.63 percent. The arithmetic is shown so you can check it.
Should you add calcium as well?
The brewing standard specifies calcium hardness, not magnesium, so a fuller recipe would include a calcium salt. Magnesium is commonly used instead because it is easier to source and dissolves readily. Be aware that this is a practical substitution rather than what the standard actually asks for.