Bicarbonate, Carbonate, and PH

[ReefHunter006]

I’ve always heard that our testing kits measure the level carbonate and not bi carbonate.

PH directly impacts the levels of bi carbonate versus carbonate as those two transfer the hydrogen ion between them.

So my question is for those of us with a PH swing of .2, .3, .4 a day, doesn’t this impact the reliability of our alkalinity results as there is a build up of carbonate versus bi carbonate in the system as PH climbs?

Or to put another a way. In a system with no consumption or precipitation of alkalinity then at different PH levels I’ll see different alkalinity readings. So keeping alkalinity at 9dkh through a 24 hour PH swing isn’t really as stable as I would have thought.

As a heads up I’ve read the articles from Randy and the Alkalinity stability post but I felt that was focused more on whether to keep alk stable or PH stable at a single point. I don’t have any systems that could keep PH from swinging less than .1 a day. That means finding a way to keep the level of total alkalinity stable at different PH levels.

Happy Holidays!

 

[arking_mark]

Not quite right.

There is a mathematical relationship between Alk/pH/CO2.

Another property of this relationship is that the addition or subtraction of dissolved CO2 doesn't impact Alkalinity.

Thus your pH swings are due to just changes in CO2 levels.

 

[ReefHunter006]

Not quite right.

There is a mathematical relationship between Alk/pH/CO2.

Another property of this relationship is that the addition or subtraction of dissolved CO2 doesn't impact Alkalinity.

Thus your pH swings are due to just changes in CO2 levels.

That’s not what I intended to imply.

The issue is that tests only test for carbonate and not the bi-carbonate and PH does affect the ratio of carbonate to bi-carbonate so we would see different test results at different PH levels even if none was utilized or percipitated.

 

[arking_mark]

That’s not what I intended to imply.

The issue is that tests only test for carbonate and not the bi-carbonate and PH does affect the ratio of carbonate to bi-carbonate so we would see different test results at different PH levels even if none was utilized or percipitated.

That's correct. The pH determines the ratio of Carbonate and Bicarbonate...

Also, dialed in pH is possible... +/- 0.025

 

[ReefHunter006]

That's correct. The pH determines the ratio of Carbonate and Bicarbonate...

Also, dialed in pH is possible... +/- 0.025

That’s impressive job pegging PH. I’ve been unable to achieve that in my larger systems (any system greater than 50 gallons) i would really like to emulate.
However, given I’m seeing much larger swings I am having trouble knowing if alkalinity is truest stable given the limitation of the testing only looking at one side of the bicarb / carb mixture.

 

[arking_mark]

That’s impressive job pegging PH. I’ve been unable to achieve that in my larger systems (any system greater than 50 gallons) i would really like to emulate.
However, given I’m seeing much larger swings I am having trouble knowing if alkalinity is truest stable given the limitation of the testing only looking at one side of the bicarb / carb mixture.

Lessons learned in trying to maintain pH stability

For those interested in maintaining pH stability around +/- 0.05 I have a couple of lessons learned. These are things that I found out on my journey to pH stability. Prior to seeking pH stability my tank pH was consistently around ~7.9. However, much of this journey went on before the tank was...

www.reef2reef.com

 

[Randy Holmes-Farley]

I’ve always heard that our testing kits measure the level carbonate and not bi carbonate.

really? Well, you have never heard the truth then.

Ignore anyone making that claim.

It is utterly and completely wrong, and any concerns with testing that stems from it are also wrong.

 

[Randy Holmes-Farley]

I discuss exactly what an alkalinity test measures here, likely in far more detail than you want:

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

and here's a more simplified description of why we measure alkalinity. I highlighted a pertinent section:

Alkalinity

Like calcium, many corals also use "alkalinity" to form their skeletons, which are composed primarily of calcium carbonate. It is generally believed that corals take up bicarbonate, convert it into carbonate, and then use that carbonate to form calcium carbonate skeletons. That conversion process is shown as:

HCO3- → CO3-- + H+

Bicarbonate → Carbonate + proton (which is released from the coral)

To ensure that corals have an adequate supply of bicarbonate for calcification, aquarists could just measure bicarbonate directly. Designing a test kit for bicarbonate, however, is somewhat more complicated than for alkalinity. Consequently, the use of alkalinity as a surrogate measure for bicarbonate is deeply entrenched in the reef aquarium hobby.

So, what is alkalinity? Alkalinity in a marine aquarium is simply a measure of the amount of acid (H+) required to reduce the pH to about 4.5, where all bicarbonate is converted into carbonic acid as follows:

HCO3- + H+ → H2CO3

The amount of acid needed is equal to the amount of bicarbonate present, so when performing an alkalinity titration with a test kit, you are "counting" the number of bicarbonate ions present. It is not, however, quite that simple since some other ions also take up acid during the titration. Both borate and carbonate also contribute to the measurement of alkalinity, but the bicarbonate dominates these other ions since they are generally lower in concentration than bicarbonate. So knowing the total alkalinity is akin to, but not exactly the same as, knowing how much bicarbonate is available to corals. In any case, total alkalinity is the standard that aquarists use for this purpose.

Unlike the calcium concentration, it is widely believed that certain organisms calcify more quickly at alkalinity levels higher than those in normal seawater. This result has also been demonstrated in the scientific literature, which has shown that adding bicarbonate to seawater increases the rate of calcification in some corals. Uptake of bicarbonate can consequently become rate limiting in many corals. This may be partly due to the fact that the external bicarbonate concentration is not large to begin with (relative to, for example, the calcium concentration, which is effectively about 5 times higher).

For these reasons, alkalinity maintenance is a critical aspect of coral reef aquarium husbandry. In the absence of supplementation, alkalinity will rapidly drop as corals use up much of what is present in seawater. Water changes are not usually sufficient to maintain alkalinity unless there is very little calcification taking place. Most reef aquarists try to maintain alkalinity at levels at or slightly above those of normal seawater, although exactly what levels different aquarists target depends a bit on the goals of their aquaria.

Interestingly, because some corals may calcify faster at higher alkalinity levels, and because the abiotic (nonbiological) precipitation of calcium carbonate on heaters and pumps also rises as alkalinity rises, the demand for alkalinity (and calcium) rises as the alkalinity rises. So an aquarist generally must dose more calcium and alkalinity EVERY DAY to maintain a higher alkalinity (say, 11 dKH) than to maintain 7 dKH. It is not just a one-time boost that is needed to make up that difference. In fact, calcification gets so slow as the alkalinity drops below 6 dKH that reef aquaria rarely get much below that point, even with no dosing: natural calcification has nearly stopped at that level.

In general, I suggest that aquarists maintain alkalinity between about 7-11 dKH (2.5 and 4 meq/L; 125-200 ppm CaCO3 equivalents). Many aquarists growing SPS corals and using Ultra Low Nutrient Systems (ULNS) have found that the corals suffer from "burnt tips" if the alkalinity is too high or changes too much. It is not at all clear why this is the case, but such aquaria are better served by alkalinity in the 7-8 dKH range.
As mentioned above, alkalinity levels above those in natural seawater increase the abiotic precipitation of calcium carbonate on warm objects such as heaters and pump impellers, or sometimes even in sand beds. This precipitation not only wastes calcium and alkalinity that aquarists are carefully adding, but it also increases equipment maintenance requirements and can "damage" a sand bed, hardening it into a chunk of limestone. When elevated alkalinity is driving this precipitation, it can also depress the calcium level. An excessively high alkalinity level can therefore create undesirable consequences.

I suggest that aquarists use a balanced calcium and alkalinity additive system of some sort for routine maintenance. The most popular of these balanced methods include limewater (kalkwasser), calcium carbonate/carbon dioxide reactors, and the two-part/three part additive systems.

For rapid alkalinity corrections, aquarists can simply use baking soda (sodium bicarbonate) or washing soda (sodium carbonate; baked baking soda) to good effect. The latter raises pH as well as alkalinity while the former has a very small pH lowering effect. Mixtures can also be used, and are what many hobby chemical supply companies sell as "buffers". Most often, sodium carbonate is preferred, however, since most tanks can be helped by a pH boost.