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Old 06-13-2006, 05:00 PM   #1 (permalink)
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Default If not Ca/Mg, then what?

I was just taking a look at my local water quality report, and was a bit confused with their totals:

Ca 5800 UG/L
Mg 1100 UG/L
Hardness 19 MG/L (this is confirmed by my GH test)

Can anyone explain where the rest of the hardness is coming from?
I've been out of school for a while now, but I'm pretty sure 5+1 doesn't equal 19.
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Old 06-13-2006, 07:42 PM   #2 (permalink)
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As i understand it the water quality reports that they send out are averages of the water throughout the entire range of their system. Since it is higher and lower in some places their average really doesn't help you out too much with what is in your water. This is why water quality reports are pretty much useless when it comes to our needs.

Some good tests kits should be all you need.
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Old 06-13-2006, 08:36 PM   #3 (permalink)
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I think hardness numbers are always expressed in Calcium equivalents. So, you really can't just add the ppm of Mg and Ca and get ppm of hardness. But, I haven't made any effort to see if converting the ppm of Mg you cited would be raised or lowered by expressing it as Ca equivalents.
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Old 06-14-2006, 12:19 AM   #4 (permalink)
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If you divide the UG/l numbers by 1000 to get mg/l, then use this handy formula:

degrees GH = (ppm Ca/7.2) + (ppm Mg/4.4)

1 degree GH is 17.8 ppm or mg/l.

Plugging in the numbers you gave gives you a GH of 1.05, or 18.78mg, close to the 19mg/l in your report.

There are other elements that make up GH besides Ca and Mg but for our purposes they are in amounts small enough that we can ignore them.
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Old 06-14-2006, 11:56 AM   #5 (permalink)
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Ah, so 7.2 ppm Ca or 4.4 ppm Mg are equal to 17.8 ppm hardness
The world (and fertilator) make so much more sense now, thanks!
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Old 06-14-2006, 08:24 PM   #6 (permalink)
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EDTA hardness chelation tests read in an equivalent of CaCO3. That means when you do a GH test and get an "as CaCO3" ppm reading, it means, "if the only thing dissolved in the water is CaCO3, this is how much CaCO3 there would be in ppm."

"Degrees of German Hardness" is a standard that is equal to 10 ppm of calcium oxide.

Calcium weighs 40.078 grams per mole. Oxygen weighs 15.9994 grams per mole. Therefore, a degree of German Hardness is 71.46908 % calcium and 28.53092 % oxygen, or 7.146908 ppm of calcium.

Since GH test readings are in CaCO3 equivalence, you have to account for the amount of carbonate that will be present along with the 7.146908 ppm of calcium. Carbonate weighs 60.0089 grams per mole. Therefore, calcium carbonate is 40.04320 % calcium, and you would need 2.497303 milligrams of calcium carbonate to raise the calcium content of the water by 1 ppm per liter. (1 / .4004230). Since we know a degree contains 7.146908 ppm of calcium, then 2.497303 x 7.146908 = 17.847993 ppm of calcium carbonate in 1 degree.

There are different ways to apply this to a water report. If the readings are not already in a CaCO3 equivalence, then we don't know exactly what all the calcium compounds are in the water. If they are in fact NOT in a CaCO3 equivalence, it is possible to translate everything over to calcium carbonate. (That's one of the things I like about using RO/DI water... you actually know and control what compounds the GH comes from.) If for example your calcium levels are 77 ppm, then you would need 192.29233 ppm of CaCO3 to achieve that level of calcium.

Another thing about GH tests is that magnesium cations react the same on these tests as calcium cations do. The atomic weight of magnesium is 24.305 grams per mole. That's 60.64424 % of what calcium weighs. Since 1 degree of hardness for calcium is 7.146908 ppm, then 60.64424 % of that is 4.33419 ppm of magnesium in 1 degree of German hardness.

For example, if you have a magnesium reading of 29 ppm, expressing it as a calcium carbonate equivalence is a bit of a conundrum, since we are assuming that the magnesium containing compound that is present is calcium carbonate, which of course contains no magnesium.

Since it takes 1.64896 times as much magnesium to make the GH test read 1 ppm as CaCO3 (40.078 / 24.305), we need to multiply the 2.497303 milligrams of CaCO3 it takes to raise calcium by 1 ppm by 1.64896, which equals 4.11796. So 29 ppm of magnesium as read as calcium carbonate on a GH test would equal to 119.42084 ppm.

From the above examples, water with 77 ppm of calcium and 29 ppm of magnesium expressed as CaCO3 would be 119.42084 ppm + 192.29233 ppm = 311.71317 ppm of CaCO3, or 17.46489 degrees.

To get a much better idea of the actual "total hardness," you would really want to take a TDS (total dissolved solids) reading using a conductivity meter.

Many water reports already have their readings converted to CaCO3. Unfortunately, many of them also do not specify what equivalence the readings are in, making them pretty much useless, in my eyes anyway. If it isn't specified, by all means call your water company and find out (and also complain). For example, if (from the above examples) we are talking about readings which are already in a CaCO3 equivalence, then the GH would be 108 ppm as CaCO3 (instead of 312), which is 6.05110 degrees of German hardness, with a 2.65517:1 calcium:magnesium ratio.

"GH" is an abbreviation for gesamthärte, which is German for "total hardness." It was decided that it is best to keep the German abbreviation of "GH" instead of creating a new English abbreviation ("TH" for total hardness), and to find an appropriate English word that begins with the letter G. The best fitting word was "general," so that's what was used. However, this really isn't accurate, it's total.

GH is intended to be a measure of exactly what it's supposed to be - TOTAL hardness. This includes ALL dissolved solids in the water.

When you test GH using EDTA (which for quite a long time was the main way of testing GH), all of the dissolved solids will not react. The top two ions which react are calcium and magnesium. The rest don't react much at all unless they are in great concentrations. Since the main dissolved solids in tap water tend to be calcium, magnesium, and carbonates, "GH" as it has come to be known in the USA is a measure of calcium and magnesium ions, since for all practical purposes, those are the only ions that show up on the EDTA chelation tests.

Since the advent of accurate testers which test the conductivity of most all ions in a given water column, TDS or "Total Dissolved Solids" is now a true representation of the water's actual "gesamthärte."
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