Why is Kalk so hard to dissolve?

[Dennis Cartier]

I have been using kalk in a reactor off and on for years. I started with an Avast K1 and then moved to a K2. I now run a Geo 8" kalk reactor. I switched from the Avast's after I started monitoring the saturation using an EC probe and found that the reactors would start to lose saturation even though plenty of excess kalk was still present and the mixing was occurring 24x7. I thought that being non-pressurized, and dosing fairly slowly, the solution must be getting weakened by being exposed too long to CO2 at the surface. The surface always had a layer of calcium carbonate crystals, but the rotating shaft and overflow elbow always still had a small air gap. So I switched to the Geo as it is sealed and in theory, should not easily allow contact with air containing CO2.

What I have found is that the solution in the Geo also gets progressively weaker even when large amounts of excess kalk remain in the reactor. I had a 1 minute mixing cycle run every 12 hours, which I extended to 10 minutes to see if a longer mix would help. Nope, no difference in saturation, though it did result in a lot more undissolved kalk getting trapped by my settlement stage that allows me to dose continuous even after and during mixing cycles.

I am wondering if kalk may contain particles that are easier to dissolve and ones that dissolve less easily. Whenever I add fresh kalk to the reactor I always hit 10.3 mS/cm, but that is as good as it is going to get. Each day after that, the high points (after a mix cycle) and low points (before a mix cycle) both get progressively lower. Going way overboard on the quantity of kalk seems to slow the slide somewhat, but not to the point where it seems worth it. After a week, I am still going to be looking at highs being in the 70's and lows being in the 50's, even with an inch of undissolved kalk in the reactor.

This can't all be calcium carbonate precipitated from the solution after getting enough CO2 to precipitate out can it? I realize the incoming water will have CO2, but it can't be that much I would think. My guess on this is that the easy to dissolve particles dissolve first giving the initial saturation and then the next 'level' of particle dissolves, but less easily, and there are less of those, so saturation is not guaranteed unless I go way overboard on the kalk. Once those are used up, then the next particles that are even harder to dissolve are used, resulting it less than saturation.

What would the factor be that makes these particles less easy to dissolve? Could it be that they are less able to dissolve in kalk solution that is already partially saturated? Perhaps they are not able to dissolve as easily because of the high PH of the solution? The next time I am ready to dump the excess kalk and re-fill, I hope to try a test. The reason I would do this, dump and re-fill, is because the saturation is not able to get close to full saturation after a mix cycle. It might only hit 60 - 70%. Instead of dumping the reactor contents, if I were to siphon off the liquid and retain the layer of kalk sludge on the bottom and then re-fill it with RODI and see if it can get back to saturation, then that would suggest they are not able to dissolve due to the saturation of the solution they are being mixed in. Although it still could be PH related, even with that test (if successful). They only thing that would be known for sure, is that the kalk sludge could still be dissolved, it just need to be mixed with fresh RODI exclusively.

Has anyone else noticed this?

Dennis

 

[KrisReef]

I’m not sure if this is correct but according to the internet:


“
Create a saturated solution of calcium hydroxide by adding 0.10-0.30g of Ca(OH)2 to 30 mL of distilled water in a 50 mL beaker. You should see some undissolved calcium hydroxide at the bottom of the beaker. If not, add a little more calcium hydroxide. “

——-

In other words, once a solution reaches the saturation level any remaining salt quits dissolving unless you remove some of the salt from the solution, or you add more water to the system, or heat the water up to raise the saturation point. There are limits on how much can dissolve at a given temperature and pressure that would seem to have been reached if you have undissolved Kalk at the bottom of your container.

Stirring and mixing a solution after it is saturated isn’t going to increase the saturation level.

That’s how I understand the saturation point works.

 

[Dennis Cartier]

Yes, but removing some of the saturated solution by pumping RODI into the reactor should result in more kalk being dissolved (when adequately mixed) to again reach saturation ... and it does, to a point. It never quite reaches saturation again. It always stops before saturation is reached when RODI is added to a previously saturated solution.

The amount of extra undissolved kalk within the reactor does not seem to help with this. It always still stops before saturation on the addition of RODI, just the amount that it misses by, and how fast the magnitude of miss grows is affected by the surplus kalk. Have several orders of magnitude of surplus kalk in the reactor and the miss is small and grows slowly, have exactly the correct amount of kalk for n numbers of drains and refills of the reactor, and the miss is obvious and the magnitude of the miss grows rapidly. Making the kalk 'run out' long before it should even with extra kalk added to account for precipitation of carbonate.

It is this inefficiency at dissolving more kalk to again attain saturation that has me scratching my head. That is where my hypothesis that a spoonful of kalk contains parts that are very easy to dissolve and other parts that are progressively harder to dissolve, to the point that they are not able to overcome the saturation level of a partially saturated solution of kalk.

Dennis

 

[Randy Holmes-Farley]

You might find these articles interesting to read on the kinetics of Ca(OH)2 dissolution:

https://onlinelibrary.wiley.com/doi/abs/10.1002/(SICI)1521-401X(199902)27:2%3C72::AID-AHEH72%3E3.0.CO;2-H

(PDF) The Dissolution Rate of Ca(OH)2 in Aqueous Solutions

PDF | The dissolution rate of reagent grade Ca(OH)2 in aqueous solutions has been determined by means of a spinning disc method. The dissolution rate... | Find, read and cite all the research you need on ResearchGate

www.researchgate.net

 

[Dennis Cartier]

What I took from the ReseachGate paper is that the pH affects the dissolution rate, the particle size and the type of limestone the kalk is made from affects the dissolution rate by way of other elements being present. It was pointed out that the dissolution rate at pH above 8.0 has not been widely studied. Which is exactly the situation that occurs in a kalk reactor. The pH is still going to be above 12.0 when attempting to re-establish full saturation by mixing the slurry within the reactor.

The mention on the particle size is interesting as I noticed that while I can have a substantial slurry of surplus kalk on the bottom of the reactor, just adding a few tablespoons of fresh powder will again easily reach saturation. I am speaking of powdery kalk, the kind that emits a little plume of particles that you best avoid breathing when it plops down onto a solid surface. I believe that the fine particles are able to more easily dissolve, and that is why adding fresh kalk always reaches saturation even when the saturation is already high along with the pH.

This leaves me doubting the practicality of reactors ever being able to sustain saturation over even short periods.

I was not able to access the Willey paper behind it's paywall.

Dennis

 

[Randy Holmes-Farley]

Particle size also impacts the settling rate after stirring, which may impact dissolution efficiency in a stop and start reactor.

 

[Dkeller_nc]

If I understand your observations correctly, you've noticed that the saturation point for your kalk reactor/solution, as measured by conductivity, is gradually going down over several days after you've opened/cleaned out/added more fresh kalk powder. There are multiple possible explanations, but yes, when I had a kalk reactor on my system, I noticed something similar - it was necessary to remove the reactor, dump all of its contents out, and refill it with fresh kalkwasser powder and RODI to get a "reset", despite the presence of a large amount of settled precipitate/undissolved particles in the bottom of the reactor.

I need to emphasize that the following is speculation that has a mechanistic analysis behind it, but no direct evidence from experimentation to back it up. Kalkwasser is, of course, powdered calcium hydroxide/oxide, and calcium hydroxide is reactive to CO2 in the air, forming calcium carbonate. Calcium carbonate is only slightly soluble in water, especially water with dissolved substances that result in a high pH.

RODI water enters the reactor with some degree of dissolved CO2 - if you want to look at it this way, it's very weak carbonic acid. This CO2 will of course react with solubilized calcium in the reactor's liquid to form insoluble calcium carbonate that exists as a crust on the liquid surface, and also drops to the bottom of the reactor. That part isn't speculation. But logically, one would expect that some of the CO2 would react with the surface of the solid calcium hydroxide particles and form an insoluble crust on the outside of these particles. That may prevent those particles from dissolving further to keep the reactor liquid at calcium hydroxide saturation. The aquarist observes that there's plenty of undissolved white particulates in the reactor, and interprets that as kalkwasser powder. In a sense, he/she is correct - it's just that it's solid calcium hydroxide with an insoluble coating of calcium carbonate. Stirring acts to break up at least some of this crust, so one would observe the conductivity going up after a stir cycle. However, over time the crust becomes thicker an thicker, and less and less of it is broken up by the stirring, so the high/low points will both go down over time.

Again, this is just armchair thinking here, but since you've a conductivity meter to monitor the reactor output, you could potentially test this theory by removing at least some of the undissolved kalk and crushing it in a mortar/pestle, returning it to the reactor, and observing the conductivity trends. One could, of course, also do this in a couple of beakers on the kitchen counter provided one has a stand-alone conductivity meter.

 

[Randy Holmes-Farley]

Yes, I agree that particulates may get coated with calcium carbonate over time and become less efficient at dissolving.

 

[Dkeller_nc]

One interesting experiment along these lines would be to bubble air that's been scrubbed of CO2 through the RODI water that's being used to feed the kalkwasser reactor. At least in theory, that would prevent calcium carbonate from coating the calcium hydroxide particles in the reactor, and the conductivity of the solution would stay relatively constant until most of the calcium hydroxide was dissolved.

The difficulty with this idea, though, is how effective bubbling no-CO2 air through the RODI reservoir would actually be at removing the CO2 dissolved in the RODI.

 

[Dennis Cartier]

Again, this is just armchair thinking here, but since you've a conductivity meter to monitor the reactor output, you could potentially test this theory by removing at least some of the undissolved kalk and crushing it in a mortar/pestle, returning it to the reactor, and observing the conductivity trends. One could, of course, also do this in a couple of beakers on the kitchen counter provided one has a stand-alone conductivity meter.

I had been thinking along these lines as well. Removing the partially used kalk that was only reaching partial saturation and see if fresh RODI only would allow it to again reach saturation. Crushing it is new though. I never considered that only the surface could have calcium carbonate and the rest of the particle could be usable kalk still.

One interesting experiment along these lines would be to bubble air that's been scrubbed of CO2 through the RODI water that's being used to feed the kalkwasser reactor. At least in theory, that would prevent calcium carbonate from coating the calcium hydroxide particles in the reactor, and the conductivity of the solution would stay relatively constant until most of the calcium hydroxide was dissolved.

The difficulty with this idea, though, is how effective bubbling no-CO2 air through the RODI reservoir would actually be at removing the CO2 dissolved in the RODI.

Again I was wondering the same thing, could the RODI water that I am feeding the reactor with be high in dissolved CO2? I thought about putting an air stone in the RODI barrel, my fish room is normally between low 400's and low 500's for CO2 (ppm), but that will only help if the RODI is already much higher in CO2. When I was setting up some gardening watering, I tested my RO and RODI and found my RO was a pH of 7.00 and the RODI was 6.90. The RO was coming out of the fridge, so cold, and the RODI was from the fish room reservoir (barrel).

Whenever I stumble onto these situations where a basic chemistry problem seems to contradict the effectiveness of a common product (kalk reactors), it makes me curious why no one else has noticed this.

Dennis

 

[Dkeller_nc]

Well, at least some of us have noticed (me, in particular). I never tried to solve the question with experimentation, since kalk is pretty cheap (at least relative to other reefing expenses). And at the time I was using it, continuous-duty peristaltic pumps were all industrial/scientific items, and really expensive. So I just used the ATO through the kalk reactor, and that led to varying amounts of calcium hydroxide added to the system depending on the weather and the daily tank evaporation rate. Since the total dosed kalk varied based on the volume of make-up water, I didn't really worry about the variation from the concentration of the kalk solution itself since I had to do daily testing of alkalinity and adjust the amount of 2-part I was adding anyway.

 

[Randy Holmes-Farley]

Well, at least some of us have noticed (me, in particular). I never tried to solve the question with experimentation, since kalk is pretty cheap (at least relative to other reefing expenses).

Yes, in my reservoir, I always just added an excess and stirred until it got close to saturation.

 

[Dennis Cartier]

Randy just pointed out in another thread that leaving the EC probe in 24x7 may result in getting coated in calcium carbonate and having artificially low readings as a result. Time to take my probe out, clean it and see if the EC readings change.

Dennis

 

[Dennis Cartier]

Lol, OK, the results are in. Not that I ever doubt Randy, but I did not expect to see much change from the before to the after, but .... the reading before the cleaning was 9.3 mS/cm and the reading after the cleaning was 10.7 mS/cm. Which told me both that Randy was correct, deposited calcium carbonate could pull the reading lower than it really is, and that I need to calibrate my meter.

Dennis

 

[Randy Holmes-Farley]

Lol, OK, the results are in. Not that I ever doubt Randy, but I did not expect to see much change from the before to the after, but .... the reading before the cleaning was 9.3 mS/cm and the reading after the cleaning was 10.7 mS/cm. Which told me both that Randy was correct, deposited calcium carbonate could pull the reading lower than it really is, and that I need to calibrate my meter.

Dennis

Thanks for the update!

 

[ScottB]

I have been using kalk in a reactor off and on for years. I started with an Avast K1 and then moved to a K2. I now run a Geo 8" kalk reactor. I switched from the Avast's after I started monitoring the saturation using an EC probe and found that the reactors would start to lose saturation even though plenty of excess kalk was still present and the mixing was occurring 24x7. I thought that being non-pressurized, and dosing fairly slowly, the solution must be getting weakened by being exposed too long to CO2 at the surface. The surface always had a layer of calcium carbonate crystals, but the rotating shaft and overflow elbow always still had a small air gap. So I switched to the Geo as it is sealed and in theory, should not easily allow contact with air containing CO2.

What I have found is that the solution in the Geo also gets progressively weaker even when large amounts of excess kalk remain in the reactor. I had a 1 minute mixing cycle run every 12 hours, which I extended to 10 minutes to see if a longer mix would help. Nope, no difference in saturation, though it did result in a lot more undissolved kalk getting trapped by my settlement stage that allows me to dose continuous even after and during mixing cycles.

I am wondering if kalk may contain particles that are easier to dissolve and ones that dissolve less easily. Whenever I add fresh kalk to the reactor I always hit 10.3 mS/cm, but that is as good as it is going to get. Each day after that, the high points (after a mix cycle) and low points (before a mix cycle) both get progressively lower. Going way overboard on the quantity of kalk seems to slow the slide somewhat, but not to the point where it seems worth it. After a week, I am still going to be looking at highs being in the 70's and lows being in the 50's, even with an inch of undissolved kalk in the reactor.

This can't all be calcium carbonate precipitated from the solution after getting enough CO2 to precipitate out can it? I realize the incoming water will have CO2, but it can't be that much I would think. My guess on this is that the easy to dissolve particles dissolve first giving the initial saturation and then the next 'level' of particle dissolves, but less easily, and there are less of those, so saturation is not guaranteed unless I go way overboard on the kalk. Once those are used up, then the next particles that are even harder to dissolve are used, resulting it less than saturation.

What would the factor be that makes these particles less easy to dissolve? Could it be that they are less able to dissolve in kalk solution that is already partially saturated? Perhaps they are not able to dissolve as easily because of the high PH of the solution? The next time I am ready to dump the excess kalk and re-fill, I hope to try a test. The reason I would do this, dump and re-fill, is because the saturation is not able to get close to full saturation after a mix cycle. It might only hit 60 - 70%. Instead of dumping the reactor contents, if I were to siphon off the liquid and retain the layer of kalk sludge on the bottom and then re-fill it with RODI and see if it can get back to saturation, then that would suggest they are not able to dissolve due to the saturation of the solution they are being mixed in. Although it still could be PH related, even with that test (if successful). They only thing that would be known for sure, is that the kalk sludge could still be dissolved, it just need to be mixed with fresh RODI exclusively.

Has anyone else noticed this?

Dennis

I share in your struggle with this. I had a thread going for a bit and link it below.

On the bright side -- even though I don't see full saturation for more than a day -- dosing 3 liters a day is helping me hold a much better pH range (8.15-8.35, avg 8.27). I still don't have a hard / fast rule about when to change out the kalk slurry. Right now the stuff is just over 2 weeks old and I have a full inch on slurry on the bottom. The EC reads 550 to 640 depending on the stir I guess. Mine stirs for 10 minutes every 2 hours. Most of the dosing is over lights out roughly 2 liters. I also dose ALK at night at 110ml versus 200 before kalk.

Perhaps the moral of the story is that full saturation doesn't really matter that much?

*Edit just saw your post about cleaning/calibrating.
- How long was yours running before needing a clean?
- How are you doing with pH management?

When do you add more calcium hydroxide to Kalk stirrer?

My pH has always been 7.9 to 8.1 or so. I bought a Kalk stirrer to offset the added CO2 coming from my calcium reactor to see if I could bump it up a tad. I've been going slow, increasing the amount dosed (via APEX DOS) every few days. I began with the recommended 150 grams of calcium hydroxide...

www.reef2reef.com

 

[Dennis Cartier]

I share in your struggle with this. I had a thread going for a bit and link it below.

On the bright side -- even though I don't see full saturation for more than a day -- dosing 3 liters a day is helping me hold a much better pH range (8.15-8.35, avg 8.27). I still don't have a hard / fast rule about when to change out the kalk slurry. Right now the stuff is just over 2 weeks old and I have a full inch on slurry on the bottom. The EC reads 550 to 640 depending on the stir I guess. Mine stirs for 10 minutes every 2 hours. Most of the dosing is over lights out roughly 2 liters. I also dose ALK at night at 110ml versus 200 before kalk.

Perhaps the moral of the story is that full saturation doesn't really matter that much?

*Edit just saw your post about cleaning/calibrating.
- How long was yours running before needing a clean?
- How are you doing with pH management?

When do you add more calcium hydroxide to Kalk stirrer?

My pH has always been 7.9 to 8.1 or so. I bought a Kalk stirrer to offset the added CO2 coming from my calcium reactor to see if I could bump it up a tad. I've been going slow, increasing the amount dosed (via APEX DOS) every few days. I began with the recommended 150 grams of calcium hydroxide...

www.reef2reef.com

Ah, right I remember the thread. You got the HM Digital inline EC meter.

Lol, I had never cleaned it, other than giving it a scrubbing with a toothbrush. It had been running in the kalk for a couple of years. I have calibrated it in the past though, I guess when it had carbonate build up, because once I calibrated after the cleaning, the reading on the reactor is now 8.0 mS/cm, compared to 9.3 mS/cm before the cleaning and calibration. So it was off a fair bit due to being calibrated while having a build up and then having the buildup removed.

My pH tends to go from 8.0 to 8.18 daily. That is with reasonably low CO2 in the house (420 - 520 ppm) and an outside airline to a needle wheel pump in a fuge barrel. The outside air to the fuge, made the biggest difference. I think this is because my tank tends to have excess CO2 that needs to be blown off. That is the whole reason for running the kalk reactor, to help use up the excess CO2. So having the saturation move around is annoying as I mainly want the kalk for pH help, not dosing, so when the saturation varies, it messes up my nice flat line on my Alkatronic!

The excess CO2 is coming from my calcium reactor, an Aquarium Engineering ACR, which runs at saturation (pH somewhere in the mid to low 5's). I believe you are/were running the same reactor? This causes lots of excess CO2 in the tank. I have a Geo 618 reactor plumbed in after the ACR with no CO2 hooked to it, just to act as a power scrubber and a convenient place to put specialty media (ReMag, etc.). The Geo is running at 6.17 pH as of now. I am only dosing 1,152 ml a day from the CalRx, but it seems to be a drag on the pH.

I am waiting on some parts, but I plan to make an aeration tube to blow off the excess CO2 coming from the ACR and see if that helps the pH. Once that is up in running, the effluent from the CalRx will be locked to 7.3 pH, so there should be less excess CO2 entering the tank. I will have to up my flow through the CalRx though as I expect to lose some dkh in the strength of the effluent after the pH is raised.

Dennis

 

[arking_mark]

Following along.

 

[Randy Holmes-Farley]

Perhaps the moral of the story is that full saturation doesn't really matter that much?

Yes, if you demand is not that high.

Another moral of the story is that dosing from reservoirs is often able to deliver more.

 

[ScottB]

Ah, right I remember the thread. You got the HM Digital inline EC meter.

Lol, I had never cleaned it, other than giving it a scrubbing with a toothbrush. It had been running in the kalk for a couple of years. I have calibrated it in the past though, I guess when it had carbonate build up, because once I calibrated after the cleaning, the reading on the reactor is now 8.0 mS/cm, compared to 9.3 mS/cm before the cleaning and calibration. So it was off a fair bit due to being calibrated while having a build up and then having the buildup removed.

My pH tends to go from 8.0 to 8.18 daily. That is with reasonably low CO2 in the house (420 - 520 ppm) and an outside airline to a needle wheel pump in a fuge barrel. The outside air to the fuge, made the biggest difference. I think this is because my tank tends to have excess CO2 that needs to be blown off. That is the whole reason for running the kalk reactor, to help use up the excess CO2. So having the saturation move around is annoying as I mainly want the kalk for pH help, not dosing, so when the saturation varies, it messes up my nice flat line on my Alkatronic!

The excess CO2 is coming from my calcium reactor, an Aquarium Engineering ACR, which runs at saturation (pH somewhere in the mid to low 5's). I believe you are/were running the same reactor? This causes lots of excess CO2 in the tank. I have a Geo 618 reactor plumbed in after the ACR with no CO2 hooked to it, just to act as a power scrubber and a convenient place to put specialty media (ReMag, etc.). The Geo is running at 6.17 pH as of now. I am only dosing 1,152 ml a day from the CalRx, but it seems to be a drag on the pH.

I am waiting on some parts, but I plan to make an aeration tube to blow off the excess CO2 coming from the ACR and see if that helps the pH. Once that is up in running, the effluent from the CalRx will be locked to 7.3 pH, so there should be less excess CO2 entering the tank. I will have to up my flow through the CalRx though as I expect to lose some dkh in the strength of the effluent after the pH is raised.

Dennis

Yes I was running the ACR until I stupidly over pressurized it when I messed up the regulator. And yes, it was dragging down pH and thus the stirrer. Now back to 2-Part plus kalkwasser. It has taken a little while to dial in but I am really kinda happy with my dkh and pH now.