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Old 06/21/2007, 10:23 AM   #1
rachelcb80
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Converting mg/L into mEq/L

I've been searching online for how to do this but my head is starting to hurt. I've never been good at math and forget chemistry! I found a conversion formula but it was for Ca and you have to know the valence or something like that so I thought I'd try looking up "valence of CaCO3" but that didn't work. I probably wasn't even on the right track. This is what I was looking at;

milliequivalents per liter (meq/L) - meq/L is another method of expressing concentration, when the analytes are dissolved and disassociated in solution. meq/L is also equal to millimoles of charge per liter (mM+/L or mM-/L depending on valence). To calculate meq Ca/L from the reported value in mg/L, we must know something about calcium.


Calcium has a molecular weight of 40.08 grams/mole
Calcium has a valence of +2
The equivalent weight = (40.08grams/mole)/(2 equivalents/mole) = 20.04 grams/eq
To convert to mg/meq you simply multiply g/eq by 1000 mg/g and divide by 1000 meq/eq, thus g/eq = mg/meq
If your sample contains 30 mg Ca/L, what is the concentration in meq/L?

Meq Ca/L = (30 mg Ca/L)/(20.04 mg/meq) = 1.50 meq Ca/L

Anyways, I just want to know if you have alkalinity test results of 110 mg/L, what is that in meq/L? (I'd like to know how to solve the problem too for future reference)


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Old 06/21/2007, 10:37 AM   #2
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mg/L is about the same as ppm, so divide by 50 for ppm CaCO3 equivalents. That's about 2.05 meq/L. The valence isn't critical; the atomic weights are what's used. This article covers a lot of units of measurement:

http://reefkeeping.com/issues/2005-08/rhf/index.php


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Old 06/21/2007, 10:55 AM   #3
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Jon

The valence is critical when dealing with ions with a charge of more than 1.

I mol with a valence of 1 = 1 meq / l

1 mol with a valence of 2 = 2 meq / l

1 mol with a valence of 4 = 4 Meq /l

10 mmol of Ca++ = 20 meq /l Ca++

1 mmol of HCO3- = 1 meq / l

1 mmol of CO3-- = 2 meq / l

Mol / L x MW = ppm or mg / l

It all depends on what your are calculating

If your sample contains 30 mg Ca/L, what is the concentration in meq/L?

Mol / L x MW = ppm or mg / l

So, 30 ppm / 40 MW = .75 mmol

.75 mmol x its charge of 2 = 1.5 meq/ l

In this hobby the only thing we use in meq/ l is Alk. So forget about everything else unless you have a reason for it.


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Old 06/21/2007, 10:57 AM   #4
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I was assuming she wanted ppm CaCO3 equivalents.


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Old 06/21/2007, 11:16 AM   #5
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I know but I just wanted CaCO3 equivalents clarified. And now I will have to clarify this CaCO3 equivalents

This is really not a REAL value. It is an expression conversion and the value does not really give you a true value of how much Ca++ there is.

It goes like this. IF water could hold 150 ppm CaCO3, which it absolutely can not, it would yield a concentration of 150 ppm CaCO3 if all the CO3-- was attached to a Ca++ ion. It is a poor expression from long about that gave the "potential" of you much CaCO3 could leave solution. Be it FW or SW only a very small amount of CaCO3 can held in solution. It is only a few ppm. Remember here we are talking about CaCO3 in solution and NOT Ca++ and CO3-- in solution. The Ksp in FW is only 8.3 and vs the Ksp for Kalk which is 5 and something like KCl is 0.98. The lower the # the more it can hold. Dolomite is like 15


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Old 06/21/2007, 11:19 AM   #6
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Thank you Jon, you make it so simple. I was kind of hoping you'd post some big, long formula and I wouldn't feel so bad for not getting it on my own.

Boomer, for now I'm just worried about what is used in this hobby but hey, who knows when you might run in to the need for the other info!

Thanks!


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Old 06/21/2007, 11:25 AM   #7
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I wanted to be simple Rachel like Jon but you made if difficult . See the other post.


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Old 06/21/2007, 12:15 PM   #8
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I make simple things difficult on a pretty regular basis! Thank you both for your time!


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Old 06/21/2007, 10:23 PM   #9
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isn't 1 meq/l = 50ppm = 2.8 dKh and not 2.05?


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Old 06/21/2007, 10:41 PM   #10
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That equation is correct. The 2.05 is the conversion of 110 ppm by incorrect division. I must have been tired. 110 ppm is about 2.2 meq/L.


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