Reef Chemistry Question of the Day #57 Limewater (kalkwasser)

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #57

Assume that you are keeping a reef aquarium by supplying all of the alkalinity needs using limewater (kalkwasser) made from pure calcium hydroxide. That is, the alkalinity is maintained at 9 dKH by slow and steady dosing throughout the day and night.

You are also not doing any water changes or adding any other supplements.

Which of the following does NOT take place in your aquarium compared to before you started limewater dosing?

A. Calcium declines over the long term.
B. Calcium rises over the long term.
C. Magnesium declines over the long term.
D. Bicarbonate declines over the short term.
E. Carbonate increases over the short term.

Assume that long term means months since initiating dosing and short terms means hours since initiating dosing.

Good luck!























.

 

[sawdonkey]

Guessing. A, because kalk is bicarbonate?

 

[ctopal]

B. Magnesium declines

 

[Rjramos]

A. Calcium will not decline over long term. It would maintain with kalk addition.

 

[Riddler05]

C. Magnesium declines over the long term.

 

[Nano sapiens]

A. Calcium declines over the long term.

I'll take a stab at why: If alk levels are maintained at a stable level, then calcium would rise slowly due to some magnesium/strontium (mainly) being incorporated into calcarious organisms' matricies. The incorporation of these other elements then skews the cal/alk ratio slightly and excess calcium in the system would be the long term end result.

 

[leptang]

C. magnesium ends up into the structure of the corals when the coral grow.

 

[DFW]

It cannot be A because without adding magnesium, calcium will precipitate at some point. This assumes that there are corals making use of these elements. It cannot be C because it says you are not adding magnesium. I say B because without adding magnesium, calcium will not rise over the long term. It will drop!

 

[Randy Holmes-Farley]

I can't tell for sure from some of the answers, but remember the question is "Which of the following does NOT take place"

 

[sawdonkey]

Guessing. A, because kalk is bicarbonate?

I meant D, because alk is bicarbonate.

 

[Heatherrom]

Ok I'm going to say that D. Bicarbonates decline in the short term DOES NOT HAPPEN and here is why:

A. Calcium CAN decline over the long term if you only dose kalkwsser because you can only dose So much at a time without raising the pH too much. So a tank heavily stocked with fast growing stony corals and lots of coraline algae may demand more calcium then can safely be dosed with kalk. Also compounding this problem is the fact that you are not supplementing magnesium. The Mg++ ion prevents free calcium ions from bonding to the free carbonate ions and then becoming insoluable and useless to the corals. No Mg leads to lower Ca!

B. Calcium CAN rise over the long term in a tank that is not well stocked with stony corals and lacks ample coraline algae. Since you are dripping in just Ca++ and OH-, you could get an excess of calcium.

C. Magnesium WILL decline over the long term. It can precipitate out of the solution and it is also used during photosynthesis and DNA replication.

D. Bicarbonates WILL NOT decrease over the short term. The hydroxide ion reacts with CO2 that is dissolved or at the surface to form the bicarbonate ion. Addition of kalkwasser can increase the concentration of bicarbonates.

E. Carbonats CAN increase over the short term because more bicarbonates have been introduced. Carbonate and bicarbonate constantly fluctuate in an equilibrium, so more bicarbonate leads to more carbonates in the solution!

Phew! And that's my best theory lol

 

[Cory]

E because I thought calcium and alk are supplied perfectly and magnesium is in trace too. But we wills see!

 

[ctopal]

Oops, I take back my answer. Should have read more carefully!

 

[beaslbob]

I say A

Calcium will not decline

 

[beaslbob]

How about it depends on what is in your top off water?


:tongue:

 

[salmon alley]

Well, I don't know the answer, but I'm guessing it's either A or B, since they appear to be opposite of each other....If I guessed, I would say 'B' that Calcium will NOT rise over the long term

 

[Randy Holmes-Farley]

I'll give the answer later tonight or tomorrow morning, but I'll note that while I know the theoretical answer and the reason, my Triton testing on my system showed it experimentally as well.

 

[Triggreef]

I'm going with D.

 

[Randy Holmes-Farley]

And the answer is...A. Calcium declines over the long term.

It turns out that calcium actually rises over the long term. The reason is fairly simple, but many folks don't think about it.

Limewater adds calcium and alkalinity in exactly the same proportions as in pure calcium carbonate. So if that is what was used by corals to make skeletons, and what precipitated on pumps and such, then it would stay constant.

But both magnesium and strontium get into growing calcium carbonate crystals in place of some of the calcium. A few percent of the calcium is replaced this way. Consequently, since the same amount of alkalinity (the carbonate) is used over time, and somewhat less than the balancing amount of calcium, calcium rises.

I use ordinary Instant Ocean, which is a lowish calcium salt mix, and I use limewater for all calcium and alkalinity. I previously boosted the calcium in the salt mix, but a number of years ago IO changed the recipe slightly and put more calcium into it. At that time, I stopped adding calcium to it, and haven't done so in at least 5 years, recognizing the the limewater was probably boosting it substantially over time. I also haven't measured calcium in many years.

So when I recently got my water tested by Triton, I saw that the calcium was 472 ppm, which is well above what IO would normally have in it at the salinity I use (35 ppt or so), confirming in my case that this theoretical occurrence was reproduced in my aquarium.

As to the other answers:

Magnesium declines over time for the reason given above: it is incorporated and not supplemented.
Bicarbonate does decline in the short term because alkalinity remains the same and pH rose, shifting some of the bicarbonate into carbonate.
Carbonate does rise in the short term for that same reason.

Happy Reefing!

 

[Triggreef]

So how does the Ca not precipitate eventually if their is no mg being supplemented, and the mg is being used up. I thought that if the mg got too low the water could not hold Ca and it would precipitate.

I guess just by doing water changes is adding enough mg to keep it in check?