Reef Chemistry Question of the Day: Precipitation

[Miami Reef]

Which of the following is or are most likely to cause precipitation of calcium carbonate? Assume magnesium is held constant for all.


1)
pH = 8.2
Calcium = 410 ppm
Alkalinity = 5.0 meq/L or 14dKH


2)
pH = 8.2
Calcium = 820 ppm
Alkalinity = 2.5 meq/L or 7dKH

3)
pH = 8.7
Calcium = 410 ppm
Alkalinity = 2.5 meq/L or 7dKH

4)
pH = 8.0
Calcium = 410 ppm
Alkalinity = 8 meq/L or 22.4dKH


I’ll be surprised if someone gets the answer. I don’t even think Randy can get it. (Just kidding on that last part.)

 

[Fish Styx]

#3

 

[Pod_01]

#3 elevated pH is my guess…

Out of curiosity, is #4 even possible!!!

 

[BeanAnimal]

I feel like I should not have to attempt an answer!

 

[Cichlid Dad]

#1 that high of ALK with higher calcium. Calcium brought down to mid 300 and you will be ok

 

[FrugalReeferJon]

All of the above

 

[taricha]

I think I talked myself into #3 because 8.7 vs 8.2 on a log scale is bigger than a factor of 2x. (But there's a bunch of reasons I could be wrong.)

 

[Miami Reef]

The answer is “all of the above.”


Here’s the rational by @Randy Holmes-Farley . I found his post in an old Reef Central post, and I couldn’t resist making a Chemistry Question of the Day with it.

For example, these 5 combinations are all equally ikely to cause precipitation of calcium carbonate. The last one matches many tanks, but the first one, where calcium only is elevated, is very unusual for reefers.

​

pH = 8.2​	pH = 8.2​
Calcium = 410 ppm​	Calcium = 820 ppm​
Alkalinity = 5.0 meq/L​	Alkalinity = 2.5 meq/L​
​	​
pH = 8.0​	pH = 8.7​
Calcium = 410 ppm​	Calcium = 410 ppm​
Alkalinity = 8.0 meq/L​	Alkalinity = 2.5 meq/L​
​	​
pH = 8.45​
Calcium = 410 ppm​
Alkalinity = 4.2 meq/L​

http://reefkeeping.com/issues/2005-07/rhf/index.php#13

It is based solely on the calculated supersaturation of calcium carbonate as an example. In reality, it takes higher than those by quite a bit to cause a precipitation event.

Can magnesium inhibit coral calcification?? - Reef Central Online Community

Can magnesium inhibit coral calcification?? The Reef Chemistry Forum

www.reefcentral.com

Happy reefing!

 

[FrugalReeferJon]

 

[taricha]

4)
pH = 8.0
Calcium = 410 ppm
Alkalinity = 8 meq/L or 22.4dKH

By the way, this implausible scenario is possible - you just might need to breathe into the the tank through an airstone for a while to drive the CO2 way way up so 22 dKH is only at 8.0 pH.

 

[EricR]

By the way, this implausible scenario is possible - you just might need to breathe into the the tank through an airstone for a while to drive the CO2 way way up so 22 dKH is only at 8.0 pH.

Interesting, and thanks for the perspective.
My first glance at this (#4) had me thinking impossible,,, as a non-chemist.

 

[Miami Reef]

edit. I misunderstood.

 

[Randy Holmes-Farley]

By the way, this implausible scenario is possible - you just might need to breathe into the the tank through an airstone for a while to drive the CO2 way way up so 22 dKH is only at 8.0 pH.

Or add dry ice to it. My brother and I use to play with dry ice my mom brought home from the lab until the bathroom was cloudy with an acidic fog. lol

 

[Miami Reef]

Or add dry ice to it. My brother and I use to play with dry ice my mom brought home from the lab until the bathroom was cloudy with an acidic fog. lol

Your mom was also a chemist?