# Calculating magnesium?



## Glouglou (Feb 21, 2006)

I want to calculate magnesium level...

*The equation:*
So far I know that the way to calcul magnesium is:
GH (ca+mg) in ppm or mg/l - 2.5 x Ca in ppm or mg/l -> the answer divide by 4.1 give you the magnesium in ppm or mg/l

*The problem: *

GH= 70mg/l 
Ca= 40 mg/l

Gh (70mg/l) - 100 (2.5 x 40mg/l of Ca)= -30 divide by 4.1= -7.31 mg of magnesium

How can I have a negative result with this?

Same with the water from my house...

Gh= 180mg/l
Ca= 80mg/l

Gh (180mg/l) - 200 (2.5 x 80 mg/l of Ca) = -20 mg of magnesium?????

Maybe I should listen in my math class...


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## Laith (Sep 4, 2004)

I don't know that formula (what is the 2.5x for?). The one I use is as follows:

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

70ppm GH is the equivalent of 3.92 degrees GH (17.86 ppm per degree of GH).
so 3.92 = (40/7.2) + (ppm Mg/4.4)
3.92 = 5.56 + (ppm Mg/4.4)
17.25 = 5.56 + ppm Mg
ppm Mg = 11.69

180ppm GH is 10 degrees GH
so 10= (80/7.2) + (ppm Mg/4.4)
10 = 11.11 + (ppm Mg/4.4)
44 = 11.11 + ppm Mg
ppm Mg = 32.89

I *think* I've done the math right!


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## Edward (May 25, 2004)

Hi

7.144 ppm Ca = 1 GH degree
17.86 ppm GH = 1 degree GH

The statement GH = 70 mg/l where Ca = 40 mg/l is wrong because 40 ppm Ca = 5.6 degree GH = 100 ppm GH, not 70. Because of this error Mg result in negative number.

The other example is GH = 180 mg/l where Ca = 80 mg/l. Well, 80 ppm Ca = 11.2 degree GH = 200 ppm GH, not 180. 

So obviously something is wrong. In practical applications the error is caused by a mistake in interpreting test kit levels or simply by the accuracy the test kit operates with. If you are sure you read the test kits correctly then your Mg level is zero. 

You can find GH, Ca to Mg calculator in APC FAQ thread and also in PPS Excel.

Edward


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## Glouglou (Feb 21, 2006)

*Thanks!*

By what I can see in the Links
->pps excel, Water hardness->Magnesium and Calcium

The way to calcul Magnesium level is:

(GH in ppm/mg/l) - (2.5 x Ca in ppm/mgl) divide by 4.1

or (17.86 GH in ºdGH) - (2.5 x Ca in ppm/mgl) divide by 4.1

One of my example is:

GH= 70mg/l 
Ca= 40 mg/l

Gh (70mg/l) - 100 (2.5 x 40mg/l of Ca)= -30 divide by 4.1= -7.31 mg of magnesium

If my test is accurate my magnesium level is O.

This formula is given on the Ca freshwater Hagen test kit as the way to calcul magnesium -> (GH in ppm/mg/l) - (2.5 x Ca in ppm/mgl) divide by 4.1


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## dennis (Mar 1, 2004)

Something is still wrong there. Either one or both of your test kits are off. Think of it this way, Gh kits measure the amount of hardness cations (Ca, Mg, K, etc) in the water. The normal kit cannot distinguish between them so the results are "as if" all the hardness comes from CaCO3. In CaCO3, 40% ny molecular weight is Ca. So, you 80mg/l Gh hardness has at most 80*.4= 28mg/l of the hardness that is positive cations. There for if you had nothing but Ca you should get a Ca reading ~28mg/l. By my understanding, 40mg/l reading in your situation is impossible.

Have you calibrated either of your test kits? It is always a good idea. If you have a descent gram scale it should be pretty easy to make a calibration solution. 0.1g CaCO3 in 1 liter of DI H2O will give you 100mg/l (5.60 degrees). If liters are hard to come by, a gallon jug of DI water from the supermarket should be around 3.785liters so, 0.3785 grams of CaCO3 in that gallon will be 100mg/l. 

Now, you can use this same solution to check calcium. As before, Ca makes up 40% of CaCO3 so 100*.4= 40mg/l Ca.


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## BryceM (Nov 6, 2005)

Most test kits read GH, Ca, and Mg as an equivalent value of CaCO3.

Correct me if I'm wrong, but I think the problem is one of keeping the units straight. Leave everything in it's equivalent values as ppm CaCO3 until you're done. Then convert it back to Ca and Mg. I believe the math should go like this:

GH (ppm CaCO3) = Ca (ppm CaCO3) + Mg (ppm CaCO3), since GH is the sum of Calcium hardness and Magnesium hardness.

Thus:

Mg (ppm CaCO3) = GH (ppm CaCO3) - Ca (ppm CaCO3)

Mg (PPM CaCO3) = 70 (ppm CaCO3) - 40 (ppm CaCO3)

Mg = 30 ppm CaCO3

To express 30 ppm CaCO3 as actual Mg, multiply by 0.24:

30 * 0.24 = *7.3 ppm Mg*

To express 40 ppm CaCO3 as actual Ca, multiply by 0.4:

40 * 0.4 = *16.0 ppm Ca*


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## dennis (Mar 1, 2004)

I think you are correct with that Bryce. I was not considering that his Ca of 40mg/l was still as CaCO3 equiv.


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## Salt (Apr 5, 2005)

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.


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## Satirica (Feb 13, 2005)

Hagen Ca test kits report Ca as Ca ppm not as CaCO3. A test result of 40ppm (or mg/l if you prefer) refers to the concentration of only Ca++ ions in solution according to Hagen. This is also true for Aquarium Pharmaceuticals Ca test kit.

The question is what are the units associated with the GH number you have. If it is expressed in ppm CaCO3 then at least one of the numbers is incorrect. The formula on the Hagen test kit assumes that your GH is reported in ppm CaCO3. All GH test kits that I know of report ppm CaCO3 but that doesn't mean that all of them do -- just the ones *I* have looked at. Salt is correct that if you are getting the GH number from your water company's analysis then you need to ask them about the units. Also, if you are getting the number from the water company you should be aware that some water supplies have ionic concentrations that vary seasonally.

When you are doing the tests be aware that you are looking for any color change on the Ca kit, not a pure color endpoint so you are looking for the pink to become any shade of violet, not pure violet or blue (which is in the directions for API's test kit). This can make a big difference in what you measure. Also, it usually works best if you use double the amount of water in the test kit (a tip Edward shares in his posts about PPS) and divide the final results by 2. No one to my knowledge makes a good Ca kit for fresh water -- the scale is more appropriate for measuring Ca in a salt water reef system where Ca levels run much higher and a 20 ppm resolution is more than sufficient.

My *guess* is that your GH is in ppm CaCO3 and is a good number, and that your Ca number is a bit high due to the error associated with using a small volume of water and the error associated with making a judgement about "did the color change enough." It is *very* easy to make a +20 ppm error with Ca test kits. You can buy small translucent measuring devices that are similar to graduated cylinders at drug stores that will allow you to make the test with double the volume of water, which will minimize the error.


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## Glouglou (Feb 21, 2006)

*About the GH...*

The Gh test kit is an Hagen one, and work by titration (one drop until color change) x 20 give you the GH in ppm/mgl.

The formula to find the Magnesium from the Ca Hagen kit refer at the GH find with is own kit (Hopefully)

When you say :


> All GH test kits that I know of report ppm CaCO3


I'm a litle bit lost because in my mind GH represent Ca, Mg, Na and other salt??? I go back to read all the post and try to go slowly on this one...

Well, I will double the resolution on the Ca test kit and come back with the result.


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## BryceM (Nov 6, 2005)

No. GH does not include dissolved salts such as Na and others. It is _only_ a measure of the divalent cations in solution (Ca, Mg, and perhaps strontium & barium).

TDS is a measure of the total dissolved solids, and is generally a measure of all of the various salts.


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## Glouglou (Feb 21, 2006)

*Ok*

Ok, the light is shining at the end of the tunnel...

I keep editing my post because I'm tryng to Chew what you give me!

No more question , for now!


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