Chemistry behind calcium drop after overdosing kalkwasser

[adamwang]

So I just made the mistake of overdosing too much kalk slurry in one go and caused alk and [Ca] to drop a lot. Theres tonnes of other reefers who've posted about making the same mistake, and thanks to them and their advice I've been able to minimize the damage by dosing vinegar.

However, one thing that I do not seem to understand is why dosing kalk over a short vs long period of time should make a difference, at least assuming basic chemical theory. If one doses x amount of kalk, Ca(OH)2, the total amount of Ca2+ and OH- ions being added are the same if I dose this in one night vs over a week. Since the equilibrium constants theoretically do not change, I should see the same amount of precipitation if I did this over one week. Even with coupled equilibria, the constants are multiplicative so the final equilibrium concentrations shouldn't have a "path-dependence" through time. Further, this theory does not predict why adding a Ca containing compound would cause resulting [Ca] to be lower than started with (unless this is temporary).

Is there an accepted explanation to this or is this something that we do not know about yet? I know there are models like MIAMI which are used in academia. Do these models predict this behaviour? My only hypotheses are that the equilibrium constants change fast enough when extremely high [OH-] in one spot causes calcium to precipitate in a way that they become catalysts for more precipitation in a runaway reaction, or that somehow the precipitated calcium and alkalinity physically become unavailable to undergo the reverse reaction. Do people know if this only happens in reef tanks, or in newly made fresh seawater as well? How does temperature play into this?

 

[Randy Holmes-Farley]

I am not aware of any general conclusion that an overdose causes a calcium drop unless you mean the precipitated calcium carbonate, which rises substantially with a pH and alk boost and frequently turns the whole tank to look like milk.

In such an overdose, the precipitation can take alk and calcium lower than it was before the overdosing because seawater is already supersaturated with calcium and carbonate, and lots of new crystal surfaces can eliminate the supersaturation by dropping alk and calcium.

 

[adamwang]

Yes that is what I meant, sorry should have specified. When I woke up and did my measurements the tank was already clear, so I missed the measured alk boost from the suspended particles. However, there was a layer of white dust over everything and I had a drop of ~3dkh and ~30 ppm of [Ca], which is roughly in-line with calcium carbonate precipitate. I saw a substantial rise in pH and possibly a rise in [Mg] of ~30ppm, though the latter may be from testing error.

I didn't realize that calcium carbonate was so supersaturated in seawater. How come people don't see such a drop when adding dusty aragonite sand or coral rocks? Are the calcium carbonate crystals just so small in this case that the surface area is so much more larger? I'm curious if anybody has done experiments to reliably replicate this phenomena so one can avoid this. I also wonder if there are applications to global warming here. A drop in 3dkh is ~40ppm carbon dioxide equivalent being removed.

 

[Randy Holmes-Farley]

Yes that is what I meant, sorry should have specified. When I woke up and did my measurements the tank was already clear, so I missed the measured alk boost from the suspended particles. However, there was a layer of white dust over everything and I had a drop of ~3dkh and ~30 ppm of [Ca], which is roughly in-line with calcium carbonate precipitate. I saw a substantial rise in pH and possibly a rise in [Mg] of ~30ppm, though the latter may be from testing error.

I didn't realize that calcium carbonate was so supersaturated in seawater. How come people don't see such a drop when adding dusty aragonite sand or coral rocks? Are the calcium carbonate crystals just so small in this case that the surface area is so much more larger? I'm curious if anybody has done experiments to reliably replicate this phenomena so one can avoid this. I also wonder if there are applications to global warming here. A drop in 3dkh is ~40ppm carbon dioxide equivalent being removed.

Precipitation does happen all the time, and especially in new rock and sand, but various things tend to bind to the surfaces and reduce precipitation. These include magnesium, phosphate and organics.

The drop you see is very reasonable. Completely removing the supersaturation would give a somewhat bigger drop than you saw.

One would not want to drop the alk in seawater as it is very important for calcifying organisms. Thst said, some folks are looking into precipitation of calcium carbonate. IMO, that is not likely a good solution as the calcium and alk have to be created, and that itself usually generates issues. Kalk is made by driving CO2 off of limestone, for example.

 

[adamwang]

Thanks for the insight Randy.

So how does one guarantee to avoid this? I am hoping to find an explanation to the phenomena where one does not see massive precipitation from dosing kalk slowly over a week, vs in one night.

I'm not sure what you mean by supersaturation in a precise sense. Perhaps you have something else in mind, but I know there is something called the omega constant which measures supersaturation of calcium carbonate and is widely used by oceanographers. It is given by:

Omega = [Ca][CO3] / K_sp

where K_sp is the empirical equilibrium constant for precipitation of Calcium carbonate. If omega is above 1 the solution is supersaturated and precipitation occurs. If kalk, Ca(OH)2, is added, the numerator should rise considerably from increased [Ca], then again slowly over time as OH causes CO2 to be converted into carbonate. If adding x amount of kalk all at once causes Omega to be large enough to precipitate a lot, shouldn't we expect that if one adds kalk overtime we should expect Omega to be even larger and we should see more precipitation? Thus, the equilibrium constant should be changing somehow.

On the other hand, what I originally had in mind was that in a reef tank we always have the following reversible reactions (ignoring charges):

CO2 + H2O <=> CO3H2
CO3H2 <=> CO3 + 2H
CO3 + Ca <=> CaCO3
H + OH <=> H2O

We can add all three to yield:

CO2 + OH + Ca <=> CaCO3 + H

Which is a well defined reaction with a well defined equilibrium constant, that is the multiplicative product of the ones' before. If one adds kalk, the equilibrium is shifted to the right. Standard theory predicts that if the equilibrium constant here does not change, and if one adds the same amount of kalk all at once or over a week, then we should see the same amount of precipitation.

Namely, these theories predict that once one raises values enough, all reef tanks should expect to see sudden precipitation at any time without warning.

However, this is not the case. I know there are reports of this, but the number of successful reef tanks where this hasn't happened far outnumber the ones that suddenly have this that occur. We don't seem to see this happen in the ocean either. Meanwhile overdosing kalk and causing precipitate seems like quite a common thing to happen. Therefore, there should be something else at play here which is changing the equilibrium constants, which is the question I am trying to ask.

My tank had ~8dkh, ~330ppm Ca, and ~1170ppm Mg before dosing. I added about 1 teaspoon of kalk as a slurry into 15 to 20 gallons of water over the course of over 30 minutes. This is ~3g of kalk, which translates to about 25-35ppm of Ca, which is not a large amount. My pH the next morning after the precipitate was above 9, which was the highest reading I could get using my tester. Besides a lower Ca to start with, and a substantially higher pH after, these are all reasonable values for reef aquarium.

 

[Randy Holmes-Farley]

I dosed kalkwasser for 20 years. I had some hardened sand when the tank was new, and several massive precipitation events when dosing went awry, but no lasting problems from it.

When you overdose, alk and even more so, carbonate, rises drastically, pushing omega from about 4 in natural seawater to way over 10, causing a mass event. The omega rise comes mostly from carbonate rising due to both the alk rise and the pH rise. Calcium is a minor player.

If you dose to maintain alk, omega doesn’t change over time except as the pH moves around.

 

[adamwang]

I dosed kalkwasser for 20 years. I had some hardened sand when the tank was new, and several massive precipitation events when dosing went awry, but no lasting problems from it.

When you overdose, alk and even more so, carbonate, rises drastically, pushing omega from about 4 in natural seawater to way over 10, causing a mass event. The omega rise comes mostly from carbonate rising due to both the alk rise and the pH rise. Calcium is a minor player.

If you dose to maintain alk, omega doesn’t change over time except as the pH moves around.

Gotcha, good to know we're talking about the same thing with supersaturation. I've only been in the hobby less than a decade so I'm sure you have much more experience than I do — this section of the forums is named after you after all haha. Though, I can't say I'm as lucky as you, I've lost some corals in the past from incorrectly dosing kalk.

I guess that makes sense that the rise is from KH since calcium is much more concentrated in saltwater.

So if one wants to raise Ca using kalk, as long as they keep alk and pH constant using some some acid, there should not be precipitation? If so what would you suggest using?

 

[Mhoover1481]

I dosed kalkwasser for 20 years. I had some hardened sand when the tank was new, and several massive precipitation events when dosing went awry, but no lasting problems from it.

When you overdose, alk and even more so, carbonate, rises drastically, pushing omega from about 4 in natural seawater to way over 10, causing a mass event. The omega rise comes mostly from carbonate rising due to both the alk rise and the pH rise. Calcium is a minor player.

If you dose to maintain alk, omega doesn’t change over time except as the pH moves around.

 

[Mhoover1481]

I dosed kalkwasser for 20 years. I had some hardened sand when the tank was new, and several massive precipitation events when dosing went awry, but no lasting problems from it.

When you overdose, alk and even more so, carbonate, rises drastically, pushing omega from about 4 in natural seawater to way over 10, causing a mass event. The omega rise comes mostly from carbonate rising due to both the alk rise and the pH rise. Calcium is a minor player.

If you dose to maintain alk, omega doesn’t change over time except as the pH moves around.

Hey Randy, I’m new to R2R and have been trying to find a way to get in touch with you! I’m having a little trouble with salinity and I have been reading your notes on creating a DIY refractometer solution using NaCL, and I have a few questions.
I know this post doesn’t pertain to that but I saw this was the most recent post you commented on so I figured you’d see this!!

I’d be thrilled if you have the time to respond! Thank you!

 

[Randy Holmes-Farley]

Gotcha, good to know we're talking about the same thing with supersaturation. I've only been in the hobby less than a decade so I'm sure you have much more experience than I do — this section of the forums is named after you after all haha. Though, I can't say I'm as lucky as you, I've lost some corals in the past from incorrectly dosing kalk.

I guess that makes sense that the rise is from KH since calcium is much more concentrated in saltwater.

So if one wants to raise Ca using kalk, as long as they keep alk and pH constant using some some acid, there should not be precipitation? If so what would you suggest using?

There’s no need or desire for acid. CO2 serves that purpose.

These have more on these topics:

What is that Precipitate in My Reef Aquarium? by Randy Holmes-Farley - Reefkeeping.com

What Your Grandmother Never Told You About Lime by Randy Holmes-Farley - Reefkeeping.com

Chemistry And The Aquarium: Metals In Limewater ? Advanced Aquarist | Aquarist Magazine and Blog

The Degradation of Limewater in Air - Reefkeeping.com

Chemistry and the Aquarium: Calcium

Randy discusses calcium and its role in helping maintain a healthy marine aquarium.

reefs.com

 

[Randy Holmes-Farley]

Hey Randy, I’m new to R2R and have been trying to find a way to get in touch with you! I’m having a little trouble with salinity and I have been reading your notes on creating a DIY refractometer solution using NaCL, and I have a few questions.
I know this post doesn’t pertain to that but I saw this was the most recent post you commented on so I figured you’d see this!!

I’d be thrilled if you have the time to respond! Thank you!

I’d suggest starting a new thread on that topic. I’m more than happy to help.

 

[Mhoover1481]

I’d suggest starting a new thread on that topic. I’m more than happy to help.

You are awesome! I did start a discussion, is there any way I can tag you in it? Again, super new to R2R lol!

 

[Randy Holmes-Farley]

You are awesome! I did start a discussion, is there any way I can tag you in it? Again, super new to R2R lol!

If it is in the chemistry forum, I’ll see it.

 

[Mhoover1481]

If it is in the chemistry forum, I’ll see it.

Got it, posted!
Apologize for accidentally hijacking this post, won’t happen again lol!

 

[adamwang]

There’s no need or desire for acid. CO2 serves that purpose.

These have more on these topics:

What is that Precipitate in My Reef Aquarium? by Randy Holmes-Farley - Reefkeeping.com

What Your Grandmother Never Told You About Lime by Randy Holmes-Farley - Reefkeeping.com

Chemistry And The Aquarium: Metals In Limewater ? Advanced Aquarist | Aquarist Magazine and Blog

The Degradation of Limewater in Air - Reefkeeping.com

Chemistry and the Aquarium: Calcium

Randy discusses calcium and its role in helping maintain a healthy marine aquarium.

reefs.com

Thanks for linking those. Some of those articles helped me a lot in the past. I took a quick look through them again, but I don't seem to understand what you mean in reference to CO2. Would it be possible for you to elaborate for me?

 

[Randy Holmes-Farley]

Thanks for linking those. Some of those articles helped me a lot in the past. I took a quick look through them again, but I don't seem to understand what you mean in reference to CO2. Would it be possible for you to elaborate for me?

You seemed to be asking if it was desirable to dose an acid to keep calcium carbonate from precipitating. In general, most folks want all the pH boost they can get, but my CO2 comment was that whether you want it or not, CO2 will fairly rapidly enter the system from the air and drive the pH back down, reducing the potential for undesirable calcium carbonate precipitation.

 

[adamwang]

You seemed to be asking if it was desirable to dose an acid to keep calcium carbonate from precipitating. In general, most folks want all the pH boost they can get, but my CO2 comment was that whether you want it or not, CO2 will fairly rapidly enter the system from the air and drive the pH back down, reducing the potential for undesirable calcium carbonate precipitation.

Ah that makes more sense. Thanks for explaining. Responding to each and every thread on this forum must take a lot of time — and I'm sure the hobby appreciates it!