Solubility of Carbonates and Elevated pH

[Gray]

I have been running into an issue maintaining my Alk levels. Wanting some clarification so that I can better maintain my alkalinity levels.

System:
Volume 91g
Age: 4 months
Skimmer w/ Recirculating CO2 Scrubber

Parameters: (Source - Trident, Hannah, Red Sea Kit)
Temp 78-78.5F
pH - 8.2-8.5
Alk - Currently 6.8dKh (8.5 Target)
Ca - 420 ppm average
Mg - 1390 ppm average

Supplemental:
Red Sea Foundation B
BRS Kalkwasser

What I believe to be a mass precipitation event of carbonates as the pH rose started with the switch to kalkwasser. Since alkalinity consumption was low I started with a 3grams/gallon dose of 0.5 gallons per day over 12 doses. This was roughly a half or a third of my daily evaporation. The initial pH changes seemed to be short lived and would return within an hour of the dosed kalkwasser. Within a day or two the alkalinity began to drop at an increased rate.

I will post the pH and Alk graph from apex below. My primary question at this point is if an 8.5dKh and pH of 8.3-8.4 is viable or would these levels be outside the carbonates equilibrium at this pH? Appreciate any feedback. Please let me know if additional information is needed.

This chart from ReefKeeping.com which I believe Randy possibly is the author of makes me question how to manage pH and alk in my system.

An overlayed chart from my apex log of Alk (orange) and pH (blue) from the past week. Ignore the early spike, as that was an artifact from the return pump being off.

Thank you!

 

[Randy Holmes-Farley Verified Community Expert]

I will post the pH and Alk graph from apex below. My primary question at this point is if an 8.5dKh and pH of 8.3-8.4 is viable or would these levels be outside the carbonates equilibrium at this pH? Appreciate any feedback. Please let me know if additional information is needed.

Yes, many people have that pH and alk without problematic precipitation (I did).

But at high pH and high alk, precipitation can start and once started, it can only be stopped by letting alk and pH de line while phosphate, magnesium, organics, and bacteria coat calcium carbonate surfaces.

Do you detect precipitation such as hardening sand?

 

[Gray]

Yes, many people have that pH and alk without problematic precipitation (I did).

But at high pH and high alk, precipitation can start and once started, it can only be stopped by letting alk and pH de line while phosphate, magnesium, organics, and bacteria coat calcium carbonate surfaces.

Do you detect precipitation such as hardening sand?

Ok, so if I'm understanding correctly the presence of magnesium, phosphate, and other organics inhibit the crystalline structure and therefor further precipitation? I did not notice any significant hardening of the sand but I can't be 100% sure. During this point I had a drop in my phosphate as well. Phosphates were reading 0.08 during the beginning of the week and bottom out to undetectable on the Hanna ULR checker.

Should I be wary of anything while raising the alk back up?

 

[Randy Holmes-Farley Verified Community Expert]

Ok, so if I'm understanding correctly the presence of magnesium, phosphate, and other organics inhibit the crystalline structure and therefor further precipitation? I did not notice any significant hardening of the sand but I can't be 100% sure. During this point I had a drop in my phosphate as well. Phosphates were reading 0.08 during the beginning of the week and bottom out to undetectable on the Hanna ULR checker.

Should I be wary of anything while raising the alk back up?

Yes, the bind onto the exposed surfaces of calcium carbonate, and inhibit them acting as seed crystals for more precipitation.

It may just be that "normal" demand has risen as the pH rose (which is also normal).

If you do not see hardening of sand or excessively rapid deposits on pump impellers, than I'd just proceed as if there is not a problem.

 

[killer2001]

Alk reagent probably old. Typically when my Alk reagent reached 1+ month old I started getting lower than normal readings.

 

[Chuck_e]

Late to the party - how are you testing for kH and Ca? I test manually once/week, and the only chart I can get out of Apex has a 7-day range, so my comparison isn't very helpful:

Thanks...