When Do Calcium and Alkalinity Demand Not Exactly Balance?

[Randy Holmes-Farley]

Therefore, seawater alkalinity is viewed as the conversions between HCO3− and CO32−. But is that the case in the confined space of a seawater aquarium with typically fluctuating CO2 production? In practice, the salinity of ocean water is determined by measuring the total chlorinity, something that would lead to large fluctuations in the aquarium.

Your oversimplification to carbonate and bicarbonate only is not ever correct, but it is absolutely true that CO2 changes CANNOT change the total alkalinity. I gave the detailed reason earlier in this thread. It is called The Principle of Conservation of Alkalinity by Pankow (“Aquatic Chemistry Concepts”, 1991). He shows mathematically that the total alkalinity of a sample CANNOT be changed by adding or subtracting CO2. Unfortunately, there is an article available on line, which claims otherwise, and encourages people to “lower alkalinity” by adding CO2 in the form of seltzer water. This is simply incorrect.

Forgetting the math for the moment, it is easy to see how this must be the case. If carbonic acid is added to any aqueous sample with a measurable alkalinity, what can happen?

Well, the carbonic acid can release protons by reversing equations 1 and 2:

(5) H2CO3 ==> H+ + HCO3–

(6) HCO3– ==> H+ + CO3—

These protons can go on to reduce alkalinity by combining with something that is in the sample that provides alkalinity (carbonate, bicarbonate, borate, phosphate, etc). However, for every proton that leaves the carbonic acid and reduces alkalinity, a new bicarbonate or carbonate ion is formed that adds to alkalinity, and the net change in total alkalinity is exactly zero. The pH will change, and the speciation of the things contributing to alkalinity will change, but not the total alkalinity.

This is not true for strong acids, however. If you add hydrochloric, sulfuric or phosphoric acids (or any acid with a pKa lower than the carbonic acid endpoint), there will be a reduction in the alkalinity.

 

[Randy Holmes-Farley]

Many local processes have an immediate effect on the local pH and real alkalinity and are not without consequence and are therefore not conservative.

There certainly are many processes that impact alkalinity, and I listed the most prevalent ones. There are many, many others that are minor processes that I do not believe ever rise to the point where aquarists can detect them, but in special scenarios they might become detectable.

 

[Randy Holmes-Farley]

Research has shown that corals are able to extract calcium ions from the water and use CO2 produced by the symbiodinium and can thus produce the necessary CO3. And then the connection between calcium consumption and measured alkalinity based on calcium carbonate is not really clear.

It is clear to chemists, and it should be clear and understood by all reefers that there is zero ambiguity here.

Corals deposit calcium carbonate. They absolutely MUST use roughly 2 units of alkalinity for every calcium ion to make calcium carbonate, regardless of where that carbon and oxygen come from.

A calcifying organism can potentially do any of these three thigns, but they cannot do what you claim.

1. Take up carbonate.
2. Take up bicarbonate and release one H+ to the bulk water (HCO3- --> CO3-- + H+)
3. Take up CO2 and release two H+ to the water. CO2 + H2O --> CO3-- + 2H+

No organism can take up CO2 and deposit calcium carbonate without releasing that H+. It cannot keep accumulating inside. It would quickly kill itself from low internal pH.

 

[Randy Holmes-Farley]

It is also wrong to assume that denitrification mainly takes place in a deep sand bed or that autotrophic denitrification using sulfur and sulfur compounds requires special equipment such as denitrators, as is sometimes suggested. These essential processes take place everywhere and have important consequences in the small space of an aquarium. For example, almost half of the bacteria present in a normal biofilm follow an anaerobic lifestyle and or have the ability to do so.

I certainly do not mean to claim that denitrification cannot happen in other environments. It certainly can occur in other scenarios, such as biofilms, live rock pores, etc.

If you do not add elemental sulfur, there will be little to no processes involving elemental sulfur. If you do add it, then yes, the processes I described for a sulfur denitrator might happen anywhere you put it, at least to some extent. I've not heard of any reefers adding sulfur outside of a denitrator, but if they do, then that may be an detectable process.

Sulfate reduction to hydrogen sulfide (shown below for a "typical organic" happens and adds alkalinity. But most reef tanks have little hydrogen sulfide production, and in those that do, a calcium to alkalinity demand imbalance is the least of their concerns since hydrogen sulfide is quite toxic.

(CH2O)106(NH3)16(H3PO4) + 53 SO4-- --> 56 CO2 + 50 HCO3- + 53 HS- + 16 NH3 + 53 H2O + PO4---

Hydrogen Sulfide and the Reef Aquarium by Randy Holmes-Farley - Reefkeeping.com

 

[Randy Holmes-Farley]

Much research has been done regarding the effect of BADES, autotrophic denitrification using elemental sulphur, on the environment, but mainly for the purification of fresh water. With regard to seawater and the seawater aquarium, Hignette's findings and publications by Marc Longouet in particular have been groundbreaking. According to these findings, the sulfur/lime ratio in the reactor shown in the article is far from optimal and is therefore a bad example.
In a BADESS with a combination of a sulfur reactor and a lime reactor.
As applied in the SLAD system.
55S + 44CaCO3 + 50NO3 - + 18H2 O + 4NH4 + → 4 C5 H7 O2 N + 25N2 + 55SO4 2- + 44Ca2 + + 24HCO3 - (T.C.Zhang2004)
Calcium and carbonate are added and the negative effect of H+ on the pH is neutralized. Buffering is supported. This when using fresh water pH +- 6.5-7.

I know you love to push the BADES system, and I properly addressed the use of calcium carbonate after a sulfur denitrator. It tends to shift the problem with a sulfur denitrator from alkalinity depletion to calcium addition.

The imbalance in BADES is identical to a bare sulfur denitrator, just shifting the problem from alk depletion to calcium accumulation, as I said in the article. Even your own equation posted in the quote above is greatly overloaded with calcium produced relative to alkalinity, proving the point.

 

[Randy Holmes-Farley]

Personally, I find that in a small and confined environment of an aquarium, Total alkalinity cannot be reduced to a measurement of mg/liter CaCO3. And how does one make the 1 to 1 connection between calcium consumption or production and alkalinity?

I'm not sure what that first sentence means, but the whole point of the article is to discuss ways that alkalinity might be impacted separately from calcium. I believe I accounted for all important possibilities, but I'm certainly open to hearing of more.

 

[Hans-Werner]

I have an anecdote more of the strange kind: In the early 2000s the observation of unequal development of calcium and alkalinity in tanks with calcium carbonate CO2 reactors was made and the understanding that it might be introduced with salt etc. was underdeveloped. One explanation was, that more calcium than carbonate ions could dissolve from the calcium carbonate.

Upon hearing this, at once the image of an ionic lattice made up of carbonate ions only with calcium ions lacking appeared in my head. I found this idea very funny. The idea of an ionic lattice made up of ions of the same charge only is too funny.

 

[Randy Holmes-Farley]

I have an anecdote more of the strange kind: In the early 2000s the observation of unequal development of calcium and alkalinity in tanks with calcium carbonate CO2 reactors was made and the understanding that it might be introduced with salt etc. was underdeveloped. One explanation was, that more calcium than carbonate ions could dissolve from the calcium carbonate.

Upon hearing this, at once the image of an ionic lattice made up of carbonate ions only with calcium ions lacking appeared in my head. I found this idea very funny. The idea of an ionic lattice made up of ions of the same charge only is too funny.

Yes, charge balance is a wonderful truism that most reefers never think about.

I once saw someone post an ICP result here that listed all the major ions, and showed there was at least one serious error somewhere because the positive charges and the negative charges were not close to balanced. Normally one cannot look at an ICP result and know with certainty that there is an error, as opposed to a real chemistry problem in the water, but that was one case where I could.

 

[GARRIGA]

For those of us seeking zero WC then is it safe to assume that when all other concerns being equal and the only desire is to raise or lower alkalinity then the following two steps can be performed without impacting the other elements such as calcium and trace?

Lowering alkalinity: Apply an acid such as Seachem Acid Buffer or Sodium Bisulphate which converts carbonate to co2. I'm assuming best balance that sodium with calcium chloride and Balling C although no clue how material this would be but makes sense to me. No clue what's in Seachem's product therefore don't know if that needs to be balanced were it used enough. Obviously overdosing kalkwasser would possibly cause enough used to create an imbalance.

Increase alkalinity: Add nitrates and convert that to nitrogen which adds base. My approach would be macroalgae assisted with carbon dosing. Key is keeping it simple.

Am I correct in my thinking?

BTW, awesome article. Easy enough for one like myself who doesn't want to delve into the details to understand.

 

[spsick]

@Jonty a relevant article

 

[Randy Holmes-Farley]

For those of us seeking zero WC then is it safe to assume that when all other concerns being equal and the only desire is to raise or lower alkalinity then the following two steps can be performed without impacting the other elements such as calcium and trace?

Lowering alkalinity: Apply an acid such as Seachem Acid Buffer or Sodium Bisulphate which converts carbonate to co2. I'm assuming best balance that sodium with calcium chloride and Balling C although no clue how material this would be but makes sense to me. No clue what's in Seachem's product therefore don't know if that needs to be balanced were it used enough. Obviously overdosing kalkwasser would possibly cause enough used to create an imbalance.

Increase alkalinity: Add nitrates and convert that to nitrogen which adds base. My approach would be macroalgae assisted with carbon dosing. Key is keeping it simple.

Am I correct in my thinking?

BTW, awesome article. Easy enough for one like myself who doesn't want to delve into the details to understand.

Thanks!

I believe that any imbalances from using sodium bisulfate (DIY or Seachem's product) or HCl (muriatic acid) to lower alkalinity are always insignificant. I would not do anything to offset it.

If the nitrate you add is consumed, that adds alkalinity. If it is done with calcium nitrate, it causes no imbalances since the alk and calcium will be used eventually. If you use sodium nitrate, then presumably you are using calcium chloride to add the calcium that the alk eventually combines with, and thus you are adding a small amount of sodium chloride. The effect is typically small due to low doses of nitrate usually used, but the accumulating sodium chloride can be offset with Balling Part C.

 

[GARRIGA]

Thanks!

I believe that any imbalances from using sodium bisulfate (DIY or Seachem's product) or HCl (muriatic acid) to lower alkalinity are always insignificant. I would not do anything to offset it.

If the nitrate you add is consumed, that adds alkalinity. If it is done with calcium nitrate, it causes no imbalances since the alk and calcium will be used eventually. If you use sodium nitrate, then presumably you are using calcium chloride to add the calcium that the alk eventually combines with, and thus you are adding a small amount of sodium chloride. The effect is typically small due to low doses of nitrate usually used, but the accumulating sodium chloride can be offset with Balling Part C.

Reason I asked is because I'm leaning AFR to avoid the 2 part issue with rising salinity which in my situation would be enhanced due to the lack of WC. Seems calcium nitrate the best solution based on my goal although now I'm adding more calcium therefore wouldn't that also need Balling C? Obviously dependent on how much calcium nitrate needed to raise alkalinity.

One benefit of zero WC and consumption via macroalgae or carbon dosing being that alkalinity isn't lost to nitrification. I assumed then my loss would be equal to calcium loss as only change would be corals building skeleton or other such organisms. Isn't that partially correct and I should then experience less of an imbalance between alkalinity and calcium?

 

[Randy Holmes-Farley]

Reason I asked is because I'm leaning AFR to avoid the 2 part issue with rising salinity which in my situation would be enhanced due to the lack of WC. Seems calcium nitrate the best solution based on my goal although now I'm adding more calcium therefore wouldn't that also need Balling C? Obviously dependent on how much calcium nitrate needed to raise alkalinity.

One benefit of zero WC and consumption via macroalgae or carbon dosing being that alkalinity isn't lost to nitrification. I assumed then my loss would be equal to calcium loss as only change would be corals building skeleton or other such organisms. Isn't that partially correct and I should then experience less of an imbalance between alkalinity and calcium?

AFR is a fine way to go, but removing tank water when needed to maintain salinity is also pretty simple.

If you keep nitrate low, alk loss from nitrification and water change is low, but it will become significant, as noted in the article, if nitrate is high. The article example was 10 water changes of 20% each at 50 ppm nitrate giving alk loss of 4.5 dKH.

if instead, it is 5 ppm nitrate, the alk loss is only 0.45 dKH from 10 water changes, and that effect will likely be lost in the noise of trying to match the tank water to the old water, and issues from magnesium incorporation in calcium carbonate.

 

[DGGuaglianone]

Excellent article thank you for sharing. I’ve been having some issues with my tank consuming alkalinity faster than calcium and this article gives a lot of insight as to why this could be happening. I do have a question though, I listened to the live interview “Rappin With ReefBum: Guest Dana Riddle” on YouTube and about the 47:30 mark Dana Riddle begins discussing how in short supply of CO2, corals can potentially use carbonate and bicarbonate for photosynthesis. My understand is that aqueous carbon dioxide (carbonic acid), bicarbonate-, and carbonate- are all an equilibrium reaction in solution and the equilibrium point is based on pH. Just curious as to your perspective on this alternative source use of alkalinity in reef aquarium.

 

[Randy Holmes-Farley]

Excellent article thank you for sharing. I’ve been having some issues with my tank consuming alkalinity faster than calcium and this article gives a lot of insight as to why this could be happening. I do have a question though, I listened to the live interview “Rappin With ReefBum: Guest Dana Riddle” on YouTube and about the 47:30 mark Dana Riddle begins discussing how in short supply of CO2, corals can potentially use carbonate and bicarbonate for photosynthesis. My understand is that aqueous carbon dioxide (carbonic acid), bicarbonate-, and carbonate- are all an equilibrium reaction in solution and the equilibrium point is based on pH. Just curious as to your perspective on this alternative source use of alkalinity in reef aquarium.

Adding or removing CO2 has no impact on alkalinity. It is a chemical impossibility (see below).

It is likely many organisms do use bicarbonate to produce CO2 internally, including many species of macroalgae. But that DOES NOT deplete alkalinity. These organisms strip out the CO2 and leave the alkalinity:

HCO3- ---> CO2 + OH-

The OH- retains the alkalinity in the water, just like dosing kalkwasser adds hydroxide to boost alk.

Chemistry and the Aquarium: What is Alkalinity?

Randy provides an overview of alkalinity as to why it's important, how it's measured, and how can it be tested.

reefs.com

Alkalinity Facts
There are several facts about total alkalinity that follow directly from the definition. Unfortunately, some of these have been misunderstood by some hobby authors.

One of these facts is termed The Principle of Conservation of Alkalinity by Pankow (“Aquatic Chemistry Concepts”, 1991). He shows mathematically that the total alkalinity of a sample CANNOT be changed by adding or subtracting CO2. Unfortunately, there is an article available on line, which claims otherwise, and encourages people to “lower alkalinity” by adding CO2 in the form of seltzer water. This is simply incorrect.

Forgetting the math for the moment, it is easy to see how this must be the case. If carbonic acid is added to any aqueous sample with a measurable alkalinity, what can happen?

Well, the carbonic acid can release protons by reversing equations 1 and 2:

(5) H2CO3 ==> H+ + HCO3–

(6) HCO3– ==> H+ + CO3—

These protons can go on to reduce alkalinity by combining with something that is in the sample that provides alkalinity (carbonate, bicarbonate, borate, phosphate, etc). However, for every proton that leaves the carbonic acid and reduces alkalinity, a new bicarbonate or carbonate ion is formed that adds to alkalinity, and the net change in total alkalinity is exactly zero. The pH will change, and the speciation of the things contributing to alkalinity will change, but not the total alkalinity.

This is not true for strong acids, however. If you add hydrochloric, sulfuric or phosphoric acids (or any acid with a pKa lower than the carbonic acid endpoint), there will be a reduction in the alkalinity.