This article is from the Water for coffee FAQ, by Jim Schulman with numerous contributions by others.
Water authorities must publish their waters' composition, and
many put up the analyses on municipal websites. Bottled waters also
will provide a complete water analysis on request. Many bottlers state
the composition of their water at this web site:
http://www.bottledwaterweb.com/bott/ Unfortunately, some don't, and
some analyses are incomplete.
Mineral levels are stated in a number of different measurement
units: milligrams per liter (mg/l) is standard. Parts per million
(ppm), and the milligrams per decimeter cubed (mg/dM3) used by MKS
fanatics, are identical to mg/l. French degrees is mg/l divided by 10.
Grains per US gallon is mg/l divided by 17.2. English degrees is mg/l
divided by 14.3 (grains per Imperial gallon). German degrees (DH) or
mmeq/l (the ueberscientific millimoles equivalent per liter) is mg/l
divided by 17.9. Feel free to invent your own unit and add it here.
Finally, bottled waters mostly report their minerals as
straight elemental mg/l or ppm rather than mg/l CaCO3 equivalents. To
get the alkalinity, multiply the bicarbonate by 0.82; to get hardness
multiply the calcium by 2.5, the magnesium by 4.2, and add the two. If
the water is fizzy, and bicarbonate level isn't stated; the alkalinity
will equal the hardness after the water goes flat.
For do it yourself water testing, there are water test kits.
The most accurate are to be found in aquarium supply stores, and
require counting the number of drops of reagent to get a color change
in the water sample. The color change happens when the sample reaches
a preset pH level, the number of drops it takes measures the amount of
buffering minerals present. They are sometimes called "titration to
The best buy I've found of this type are made by Aquarium
Pharmaceuticals. Each one has about a fifty to a hundred hardness and
alkalinity test per $6 kit (exact number depends on how hard the
tested water is), and they have a complete line of other tests.
They're available at
http://www.petsmart.com/fish/shopping/water_test_kits/ The tests use a
simplified (no separate color changing dye needed) titration to
endpoint system, and each drop measures one DH, or 17.9 mg/l, a
resolution eminently satisfactory for coffee water testing. Moreover,
the tests' resolutiom can be adjusted by using different water
volumes. Doing these tests are cumbersome, since the test tube has to
be stoppered and shaken after each drop of reagent is added.
Paper test strips are much easier; all the tests are on one
strip, which is dipped into the water and compared to a color chart.
They work similarly to the pH color strips one finds in highschool
chemistry labs. Unfortunately, the only one I found with the correct
resolution didn't work (3 separate lots, none worked), so avoid test
strips from Continental Hydrodyne Systems. The strips made by
Aquacheck and Jungle Labs work well, but the steps of are too coarse.
Basically, these tests will get you no better than an estimate within
50 mg/l hardness or alkalinity. That's OK for spot checks, but not
accurate enough for a fine tuned water treatment.