kalkwasser and magnesium together

[Randy Holmes-Farley]

I'll run through some creep calculations later today to show how the levels translate to creep.

 

[Miami Reef]

I'll run through some creep calculations later today to show how the levels translate to creep.

Thank you so much, Randy.

 

[Randy Holmes-Farley]

OK, lets look at the extremes of the data I show in the article, from coralline ( Corallina pilulifera) at 4.4% to Dendrophyllia at 0.05% magnesium by weight.

pure calcium carbonate is 40% calcium by weight, and contains 2 moles of alkalinity per mole of calcium, exactly like pure calcium hydroxide (kalkwasser). If we make pure calcium carbonate using pure calcium hydroxide, everything matches exactly and there's no creep in calcium if kalkwasser is dosed to match the alk demand.

If we swap out calcium that is 40% by weight with magnesium (which is lighter than calcium) to 4.4% by weight magnesium, we have removed 40 (mw calcium)/24.3 9mw magnesium) x 4.4% = 1.64 x 4.4% = 7.2% of that calcium, so now the final material is 4.4% magnesium and 32.8% calcium by weight (remaining 60% is carbonate, unchanged).

Thus, instead of making a material with 1 mole of calcium per 2 moles of alkalinity, we are making a material that has less calcium, 32.8/40 = 0.82 moles of calcium for each 2 moles of alkalinity (and, incidentally, 0.18 moles of magnesium).

So, what does this do over time in a reef aquarium?

Suppose we dose pure calcium hydroxide every day, and it is all going into calcification at a rate of 2 dKH (0.71 meq/L) per day. Then 14.3 ppm of calcium per day is dosed, which comes from 0.71 meq/L per day divided by 2 to give meq/l of calcium dosed per day, and since molecular weight of calcium is 40 mg/meq, 0.71/2 x 40 = 14.3 mg/L ~ ppm.

How much calcium is consumed each day if all of that alk goes into making calcium/magnesium carbonate in coralline? Instead of incorporating 14.3 ppm per day that is dosed, we take up 0.82 times as much, or 11.7 ppm per day.

That process leaves a residue of 14.3 dosed - 11.7 consumed = 2.6 ppm per day, or 81 ppm per 31 day month.

That's quite a big rise. Water changes typically won't stop that.

OK, let's look at the other extreme, the Dendrophyllia at 0.05% magnesium by weight. All calculations are going to be done the same way, but since it is 88x lower in magnesium than the coralline example, the final effect will be 88x lower, or ~ 1 ppm per month.

That rise will easily be swamped out with water changes, and wouldn't be easy to see even after a year without water changes.

Thus, the degree of creep you experience will depend heavily on the corals present, and most clearly, on the relative amount of coralline-driven demand.

IMO, many tanks have a lot of their demand for alk and calcium from coralline, but, obviously, for some it is a low fraction. In my tank, coralline was a big user since lots of real estate was taken up by noncalcifying organisms such as anemones and soft corals.

 

[Miami Reef]

OK, lets look at the extremes of the data I show in the article, from coralline ( Corallina pilulifera) at 4.4% to Dendrophyllia at 0.05% magnesium by weight.

pure calcium carbonate is 40% calcium by weight, and contains 2 moles of alkalinity per mole of calcium, exactly like pure calcium hydroxide (kalkwasser). If we make pure calcium carbonate using pure calcium hydroxide, everything matches exactly and there's no creep in calcium if kalkwasser is dosed to match the alk demand.

If we swap out calcium that is 40% by weight with magnesium (which is lighter than calcium) to 4.4% by weight magnesium, we have removed 40 (mw calcium)/24.3 9mw magnesium) x 4.4% = 1.64 x 4.4% = 7.2% of that calcium, so now the final material is 4.4% magnesium and 32.8% calcium by weight (remaining 60% is carbonate, unchanged).

Thus, instead of making a material with 1 mole of calcium per 2 moles of alkalinity, we are making a material that has less calcium, 32.8/40 = 0.82 moles of calcium for each 2 moles of alkalinity (and, incidentally, 0.18 moles of magnesium).

So, what does this do over time in a reef aquarium?

Suppose we dose pure calcium hydroxide every day, and it is all going into calcification at a rate of 2 dKH (0.71 meq/L) per day. Then 14.3 ppm of calcium per day is dosed, which comes from 0.71 meq/L per day divided by 2 to give meq/l of calcium dosed per day, and since molecular weight of calcium is 40 mg/meq, 0.71/2 x 40 = 14.3 mg/L ~ ppm.

How much calcium is consumed each day if all of that alk goes into making calcium/magnesium carbonate in coralline? Instead of incorporating 14.3 ppm per day that is dosed, we take up 0.82 times as much, or 11.7 ppm per day.

That process leaves a residue of 14.3 dosed - 11.7 consumed = 2.6 ppm per day, or 81 ppm per 31 day month.

That's quite a big rise. Water changes typically won't stop that.

OK, let's look at the other extreme, the Dendrophyllia at 0.05% magnesium by weight. All calculations are going to be done the same way, but since it is 88x lower in magnesium than the coralline example, the final effect will be 88x lower, or ~ 1 ppm per month.

That rise will easily be swamped out with water changes, and wouldn't be easy to see even after a year without water changes.

Thus, the degree of creep you experience will depend heavily on the corals present, and most clearly, on the relative amount of coralline-driven demand.

IMO, many tanks have a lot of their demand for alk and calcium from coralline, but, obviously, for some it is a low fraction. In my tank, coralline was a big user since lots of real estate was taken up by noncalcifying organisms such as anemones and soft corals.

Thank you so much, Randy. It makes so much sense.

Question, if calcium hydroxide is 1 mole of calcium to 2 moles of alkalinity, what is the ratio of Calcium to alkalinity in the sodium hydroxide/calcium carbonate recipe, assuming one doses a 1 to 1 ratio of it, and doses it per alkalinity consumption?

 

[Troylee]

I was in another group, and I mentioned how kalkwasser has the same proportions of calcium and alkalinity in the uptake of calcifying organisms. I said that using kalkwasser over a long period of time will have calcium creep up because some magnesium will get in place of calcium during calcification.

Someone responded by saying this: ”
Might or could. Depends on the types of calcifying organisms present. Not all calcification at the same ratio.”

Is that true, do different organisms have different calcification rates? Does that mean kalkwasser won’t always have a slight creep of calcium? I was under the impression that calcification/precipitation of calcium carbonate always uses the same ratio.

My calcium def creeps up with kalk.. the only way I’ve found to change this is by doing a water change.

 

[Randy Holmes-Farley]

Thank you so much, Randy. It makes so much sense.

Question, if calcium hydroxide is 1 mole of calcium to 2 moles of alkalinity, what is the ratio of Calcium to alkalinity in the sodium hydroxide/calcium carbonate recipe, assuming one doses a 1 to 1 ratio of it, and doses it per alkalinity consumption?

I didn't design them perfectly, and know more now than I did then, but the basic recipes are close to calcium hydroxide (2:1, see bolded sentence below)

An Improved Do-it-Yourself Two-Part Calcium and Alkalinity Supplement System by Randy Holmes-Farley - Reefkeeping.com

The Design of the Calcium and Alkalinity Parts

The Dowflake material is supposed to contain 77-80% calcium chloride. From the Dow Flake website, it has a bulk density of 0.82 - 0.96 g/dry mL or 194 - 227 grams/level measuring cup. We will assume that it is 78.5% calcium chloride by weight and weighs 200 grams per level measuring cup. Because calcium comprises 36% of calcium chloride, by weight, each cup contains 200 x 0.785 x 0.36 = 56.5 grams of calcium.

Consequently, dissolving 2 ½ cups (500 g) of Dowflake per gallon = 141 grams of calcium per gallon, or 37,300 mg/L. The final concentration will vary with how much moisture was actually in the calcium chloride, and how well it packed in your measuring cup. A concentration of 37,300 ppm calcium is equivalent to 0.93 molar.

When calcification takes place, two moles of alkalinity are lost for every one mole of calcium. So, we need to match the calcium above with 1.86 molar baking soda (sodium bicarbonate) equivalents (before or after baking, the baking doesn't change the alkalinity). As I measure it, Arm & Hammer baking soda weighs about 264 grams per level measuring cup. Because sodium bicarbonate has a molecular weight of 84 g/mole, we need to dissolve 1.86 x 84 = 156 grams/L, or about 594 grams (2 ¼ level measuring cups) of baking soda per gallon. Note that it doesn't matter how many grams the 594 grams of baking soda becomes after baking. All baking does is change the amount of carbon dioxide and water in the baking soda:

2 NaHCO3 --> Na2CO3 + H2O + CO2

More, or less, baking will only alter the pH increase upon addition to the aquarium. However, substantial under-baking may make it impossible to fully dissolve the solid material in the recipe, as sodium bicarbonate is less soluble than sodium carbonate (which is why Recipe #2 is more dilute). Overbaking with respect to time or temperature has no negative effect.

Residual Ions from the Calcium and Alkalinity Parts

Adding 1 gallon of each of these additives will result in a residue of ions remaining after calcification. These are mostly sodium and chloride, and the amounts of those two added are equal in numbers (i.e., moles), but slightly different in weight-based concentrations such as ppm because they do not weigh the same.

After adding 594 grams of baking soda (1 gallon of Recipe #1), we will have added 163 grams of sodium. In natural seawater, magnesium is present at about 12.0% of the sodium concentration (by weight). In order to match the magnesium additions to the sodium additions to leave them in a natural ratio, we need to add 12% of 163 grams, or 19.5 grams, of magnesium for every gallon of the two-part additive that we add.

Additionally, we may want to account for magnesium that is actually incorporated into the coral skeletons. For this calculation, I have assumed that the amount of magnesium incorporated is about 6.5% of the calcium level (by weight), or about 2.5% of the skeleton by weight. In the course of adding this gallon of both parts of the two part supplement, we added 141 grams of calcium, so we need to add 0.065 x 141 = 9 grams of magnesium to account for this deposition.

The magnesium parts of the recipe are designed to add enough magnesium so that it is not depleted by either of the two means described above. Because the magnesium supplement (either version) is 47,000 mg/L in magnesium, we need to add (9 +19.5) grams/47 g/L = 610 ml of the magnesium solution for each gallon of the other parts of Recipe #1.

Interestingly, the potassium present as an impurity in the Dowflake works to our advantage in this use. Recipe #1 has 1,342 ppm potassium in its calcium part. That amount puts it in the right ratio relative to other ions in the recipe (chloride, sodium, etc.) so that it is neither boosted nor depleted significantly over time based on salinity changes (see modeling below).

Residue Remaining from Recipe #1 when using Recipe #1, Part 3A

After one year of adding 8 ppm of calcium and the accompanying 0.4 meq/L (1.1 dKH) of alkalinity per day (41 mL of both parts per day or 4 gallons of both parts per year in a 50-gallon aquarium, including the effect of the magnesium part #3A, 2440 mL/year), the following residue (Table 2) would remain after calcification and adjustment for salinity (there is roughly a 32% rise in salinity over a year using this addition rate without water changes).

Note that in this recipe, all of the ions match NSW fairly closely (green), but without using Part 3A, the magnesium and sulfate are severely depleted (red).