The total hardness of the water is given in "degrees of German hardness", abbreviated to " °dH" (often also " °dGH").
This is the equivalent concentration (molar concentration) of the alkaline earth metal ions dissolved in the water: mainly calcium + magnesium, as well as the strontium and barium ions , which are usually only present in trace amounts . If a lot of calcium and magnesium ions are dissolved in the water, it is hard, otherwise it is soft.
One °dH corresponds to 10 mg CaO (calcium oxide) per liter of water.
| °dH | °e (°Clark) | °fH | °rH | ppm (°aH) | mEq/l | mmol/l | ||
|---|---|---|---|---|---|---|---|---|
| German degrees | 1 °dH = | 1 | 1,253 | 1.78 | 7,118 | 17.8 | 0.357 | 0.1783 |
| English Degrees (Clark Degrees) |
1 °e = | 0.798 | 1 | 1.43 | 5,695 | 14.3 | 0.285 | 0.142 |
| French degrees | 1 °fH = | 0.560 | 0.702 | 1 | 3,986 | 10 | 0.2 | 0.1 |
| Russian degrees | 1 °rH = | 0.140 | 0.176 | 0.251 | 1 | 0.146 | 0.050 | 0.025 |
| ppm CaCO 3 (American degrees) |
1 ppm = | 0.056 | 0.07 | 0.1 | 6,834 | 1 | 0.02 | 0.01 |
| mval/l alkaline earth ions | 1 mEq/l = | 2.8 | 3.51 | 5.00 | 20,040 | 50 | 1 | 0.50 |
| mmol/l alkaline earth ions | 1 mmol/l = | 5.6 | 7.02 | 10.00 | 40,080 | 100.0 | 2.00 | 1 |
The unit 1 ppm is used here contrary to the actual meaning of the word in the sense of 1 mg CaCO 3 per liter of water. CaCO 3 has a molar mass of 100. This means that 1 ppm CaCO 3 = 1/100 mmol/l and is not dimensionless.
If the values for magnesium (Mg) and calcium (Ca) are known, the hardness of the water (e.g. mineral water) can be calculated as follows:
- Hardness of water in
- or in
![{\displaystyle \mathrm {mmol/l} \approx [{\text{Ca value in mg/l}}]/40+[{\text{Mg value in mg/l}}]/24{,}3}](https://wikimedia.org/api/rest_v1/media/math/render/svg/8b3028041af95ee755ded1d4ad066ff0af807571)
![{\displaystyle {\text{°dH}}\approx 0{,}14\cdot [{\text{Ca value in mg/l}}]+0{,}23\cdot [{\text{Mg value in mg/l}}]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/f2e9bfdef3cfcf850632c6750fe585a3176efe6b)
Hardness levels (°dH) in Germany
Below is a list of the current hardness levels (for Germany) for your orientation:
| Hardness level | Millimoles calcium carbonate* | °dH |
|---|---|---|
| I = soft water | < 1.5 | < 8.4 |
| II = medium hard water | ||
| III = hard water | > 2.5 | > 14 |
(*Calcium carbonate per liter)
This table contains the values of the new version of the Detergents and Cleaning Products Act (WRMG) of 5 May 2007.
To measure the total hardness, there are various reagents (drop test with color indicators) or electronic measuring devices available from specialist retailers. These tests only take the calcium + magnesium ions into account.
Sources, among others: https://de.wikipedia.org/wiki/Wasserh%C3%A4rte
