Fully saturated kalkwasser and magnesium

[Wrasse-cal]

I understand kalkwasser does not contain magnesium...obviously—it is calcium hydroxide.

Assuming fully saturated kalkwasser, what is the balanced dose of magnesium to add per ml of kalkwasser?

 

[Wrasse-cal]

I did a search on this, and I was super surprised I couldn’t find this question asked before. Seems to me like it would be a pretty useful thing to know...

If my AP chemistry from a billion years ago wasn’t sequestered somewhere unreachable in my brain, I know this *should* be easily calculable—just not by me!

 

[dwest]

I did a search on this, and I was super surprised I couldn’t find this question asked before. Seems to me like it would be a pretty useful thing to know...

If my AP chemistry from a billion years ago wasn’t sequestered somewhere unreachable in my brain, I know this *should* be easily calculable—just not by me!

I think there is no good answer. Certainly one variable is the amount of magnesium in your salt mix plus how big and often you do water changes. I find that if I do regular water changes, I don’t need to add much magnesium. So, I just test and add some when needed. I don’t worry at all until I get below 1300 ppm.

 

[Wrasse-cal]

I think there is no good answer. Certainly one variable is the amount of magnesium in your salt mix plus how big and often you do water changes. I find that if I do regular water changes, I don’t need to add much magnesium. So, I just test and add some when needed. I don’t worry at all until I get below 1300 ppm.

Well, Triton, Balling, etc. all have ratios for balanced dosing of alk, ca and mg.

I would assume this could be doable as saturated kalkwasser is basically a combined balanced dose of ca and alk.

 

[Randy Holmes-Farley Verified Community Expert]

I understand kalkwasser does not contain magnesium...obviously—it is calcium hydroxide.

Assuming fully saturated kalkwasser, what is the balanced dose of magnesium to add per ml of kalkwasser?

you can’t usefully add any to the kalkwasser itself. Is that the question? It will precipitate.

 

[Randy Holmes-Farley Verified Community Expert]

If the question is how much magnesium to add to the tank itself, that is not exactly known because some corals use more magnesium per calcium than others. Coralline is at the high end. In general, it is about 1 ppm magnesium per 20 ppm of calcium.

 

[Wrasse-cal]

you can’t usefully add any to the kalkwasser itself. Is that the question? It will precipitate.

I'm aware. Thanks.

If the question is how much magnesium to add to the tank itself, that is not exactly known because some corals use more magnesium per calcium than others. Coralline is at the high end. In general, it is about 1 ppm magnesium per 20 ppm of calcium.

This was my question: is there a rough volumetric amount (I'm using the BRS magnesium mix) that I can assume to add based on kalwasser use? I realize there is going to be some variance, but, for example, Triton dosing assumes some ALK/Ca/Mg ratio. I'd like to have a check/comparison on my Mg test kit results.

 

[Randy Holmes-Farley Verified Community Expert]

I'm aware. Thanks.



This was my question: is there a rough volumetric amount (I'm using the BRS magnesium mix) that I can assume to add based on kalwasser use? I realize there is going to be some variance, but, for example, Triton dosing assumes some ALK/Ca/Mg ratio. I'd like to have a check/comparison on my Mg test kit results.

My diy is 47,000 ppm (47 mg/mL) magnesium. BRS uses this.

saturated kalkwasser is about 800 ppm (800 mg/L) calcium. So 10 liters delivers 8 grams of calcium.

if you want magnesium to be about 1/10th of the calcium, you would want to dose 800 mg of magnesium. That comes from about 17 mL of magnesium additive.

So a rough estimate is 17 mL of my diy magnesium additive per 10 L of limewater/kalkwasser.

 

[Randy Holmes-Farley Verified Community Expert]

Here’s a copy and paste from one of my question of the day threads for the selected mag to calcium ratio:


Here's the rationale:

Totally pure calcium carbonate (CaCO3) has one mole of calcium for each mole of carbonate.

Each mole of carbonate is equivalent to 2 moles of alkalinity (to get to the carbon dioxide endpoint):

CO3-- + 2H+ --> 2H2CO3 ---> H2O + CO2

One mole of calcium weighs 40 grams per mole, so for each 2 mole of alkalinity it has 40 grams of calcium.

Thus the ratio of calcium to calcium to alkalinity in pure calcium carbonate is 40 grams of calcium to 2 moles of alkalinity, which is equivalent to 40 mg of calcium to 2 milliequivalents of alkalinity.

For simplicity, if we think of this as coming from one liter of seawater, then the ratio consumed is 40 mg/L calcium for each 2 meq/L (= 5.6 dKH).

BUT, other ions are incorporated into coral skeletons and coralline algae in a reef tank, especially magnesium. This effect is minor and won't change the answer much, but we should work through it. The amount of magnesium incorporated varies with coral type, ranging from close to 0 to about 4.4% by weight in the calcium carbonate (for coralline algae).

The calcium in calcium carbonate comprises about 40% of the weight of calcium carbonate, so magnesium might range from about 0 to about 11% of the calcium value.

Let's assume the magnesium is about 3% by weight of the calcium carbonate. There must then be less calcium in it, by weight.

Calcium weighs 40 g/mole while magnesium weighs 24.3 g/mole. Since the replacement is 1:1 on an ion (mole) basis, then the calcium declines by more than the 3%. The 3% magnesium in the calcium carbonate is swapping into the crystal in place of 40/24.3 * 3 = 5% (calcium as a percentage of the original total calcium carbonate mass).

So the original 100 grams of this calcium carbonate now must have 35 grams of calcium and 3 grams of magnesium along with the 60 grams of carbonate.
Consequently,our new material has the ratio:

35 mg calcium
3 mg magnesium
60 mg = 2 meq/L = 5.6 dKH of alkalinity

or reduced to 1 meq/L:

17.5 mg calcium
1.5 mg magnesium
30 mg = 1 meq/L = 2.8 dKH of alkalinity

 

[Wrasse-cal]

Follow up question:

What is the dKH of fully saturated kalkwasser? I suppose with dilution I could test this, but does anyone have the value offhand?

 

[Hermie]

Follow up question:

What is the dKH of fully saturated kalkwasser? I suppose with dilution I could test this, but does anyone have the value offhand?

about 120dKH according to www.hamzasreef.com/Contents/Calculators/KalkContribution.php

 

[Randy Holmes-Farley Verified Community Expert]

At 25 deg C, fully saturated limewater has a molar concentration of 0.0204 M. Since each mole of calcium hydroxide has 2 hydroxide moieties, that means the hydroxide concentration is 0.0408 M, 40.8 meq/L, or 114 dKH.

The concentration will be a bit higher at lower temperature, and a bit lower at higher temperature.

Here are some additional details from one of my articles:

http://reefkeeping.com/issues/2005-01/rhf/index.htm

Limewater that is saturated with calcium hydroxide has a pH of 12.54 at 25ºC. It is actually recognized as a secondary pH standard. The pH is substantially higher at lower temperature (12.627 at 20ºC and 13.00 at 10ºC), and lower at higher temperature (12.289 at 30ºC; 11.984 at 40ºC). Saturated limewater has a conductivity of about 10.3 mS/cm at 25ºC, and contains about 808 ppm of calcium and 40.8 meq/l of alkalinity. Slightly more calcium and alkalinity dissolve at lower temperatures, and less at higher temperatures. Of interest to chemists, a large fraction of the calcium in saturated limewater is present as the ion CaOH+, with the remainder being Ca++. The CaOH+ will instantly dissociate into Ca++ and OH- upon its addition to aquarium water.