What Is It?
In practice, permanent hardness is the measure of all of the remaining magnesium and calcium ions that aren’t removed by boiling. Calcium and magnesium can form compounds with anions other than carbonate and bicarbonate, and when these are dissolved into water, they won’t fall out of suspension if you just boil the water. Chlorides and sulphates can bond with calcium and magnesium ions to form salts such as magnesium sulphate and calcium chloride. These minerals don’t cause limescale and they don’t act as a buffer to the acids in the coffee, but they do interact with the flavours in coffee.
How to Measure Permanent Hardness
You can quickly estimate permanent hardness by subtracting your KH measurement from your GH. In practice, the KH is a measurement of the temporary hardness, and the GH measures the total concentration of calcium and magnesium ions in the water. By means of some simple math, you can obtain a close estimate of the permanent hardness. Here’s the basic formula:
GH − KH = permanent hardness
For example, if your GH reading is 100 mg/L and your KH is 40 mg/L, just subtract the KH from the GH to estimate your permanent hardness:
100 − 40 = 60 mg/L of permanent hardness
A more advanced application of the titration method allows you to obtain separate magnesium and calcium ion measurements. The advanced titration kits necessary to calculate these measurements can be more difficult to obtain, however, and they are not strictly necessary.
If you don’t have access to an advanced titration kit and have only a means to measure GH and KH measurements, there is still a way to estimate the magnesium and calcium balance in your water: Simply use your GH measurements and then infer, from readings provided by your public water authority, the ratio of calcium and magnesium ions in the source water.