Alkalinity and pH swing

[IntrinsicReef]

I don’t think consumption of alkalinity has much if anything to do with why we get pH swings. It’s variable amounts of CO2.

After our conversation last week I did some reading and am hoping for a little clarification on why you made the above statement. I read your excellent article on alkalinity. https://reefs.com/magazine/chemistry-and-the-aquarium-what-is-alkalinity/
In the article you provide a graph that shows you adding acid to a saltwater sample and an un-buffered pure freshwater sample. The pH declines gradually in the saltwater sample but drops precipitously in the freshwater sample.

My line of reasoning is that corals remove alkalinity from the water column and incorporate it as calcium carbonate into their skeletons. As the alkalinity ( mostly carbonates and bicarbonates as I understand) is removed from the water, the pH buffering capacity of the water is reduced. Therefore the water is subject to more violent pH swings with the introduction of carbon dioxide from the respiration of organisms and other biological processes.
I guess a more specific question is that if you add a set amount of CO2 to 7dkh water and the same amount to 10dkh water, will there be a more significant pH change in the less buffered water?
Thank you in advance for you time.

 

[Randy Holmes-Farley]

The size of the daily pH swing is directly tied to the alkalinity, but not to the consumption. THere would be a similarly sized swing even if there was no consumption.

If you allow alk to decline a lot in a single say, the ph will also decline,a nd depending on how you add back alkalintiy, it might make the swing smaller or larger.

But the fact that there is a daily pH swing is not primarily due to consumption of alk, but rather to production and consumption of CO2. That is what I was saying.


Here's more discussion:


Higher alkalinity gives a clearly smaller pH swing for the same change in CO2 (or other acid added) because there is more pH buffering. THe size of the daily swing is also tied to the absolute pH, and will be smaller at higher pH: I discuss that in great detail here:

Chemistry And The Aquarium: Boron In A Reef Tank

This article also discusses the sources and sinks for boron in reef tanks, and shows that while typical tanks may be slightly depleted in boron with respect to natural levels, most are not so significantly depleted that any correction is required. Nevertheless, how to test for and supplement...

reefs.com

"Buffering of Normal Seawater

...

In the case of normal seawater at pH 8.2, b = 0.19 meq/L/pH unit for the boric acid/borate system, and 0.63 meq/L/pH unit for the bicarbonate/carbonate system. These values are additive, and result in a total buffering of b = 0.82 meq/L/pH unit. Under these conditions, the boric acid/borate system provides about 23% of the total buffering, while the bicarbonate/carbonate system provides about 77%.

If the pH of normal seawater is raised to 8.5, the total buffering is b = 1.2 meq/L/pH unit, or about 40% greater than at pH 8.2 (because both systems are closer to the pKa). At this pH, the relative contribution of the two systems to the total capacity is only slightly different than at pH 8.2, with 20% from borate and 80% from carbonate.

If the pH of normal seawater is lowered to 7.8, the total buffering is b = 0.42 meq/L/pH unit, or about half that at pH 8.2 (because both systems are farther from the pKa). At this pH, the relative contribution of the two systems to the total capacity is also only slightly different than at pH 8.2, with 29% from borate and 71% from carbonate."

 

[IntrinsicReef]

Thank you for the reply and reference material. I will read it today.
I know that you are chemically correct, but I am still going to disagree with you from a practical husbandry standpoint. I was originally speaking about a tank with almost 100% stony coral cover. If I maintain a stable alkalinity ( whether 7 or 10dkh) the pH swing will be around .2 to.4 based on the ambient carbon dioxide's effect on the water. I have run the tank at both of these dkh readings and monitored the pH. If I shut off all forms of alkalinity additions to the system the consumption of carbonates is so high that the pH will plummet. It will exceed the normal daily pH swing in a few hours. I have had to make many emergency trips to this tank because of a blocked line or a failed doser.
I understand that CO2 is the driving force of the pH change, but I think stony coral colonies consuming the water's buffer can have a more dramatic effect on daily pH in this instance.

 

[IntrinsicReef]

I realize I just restated much of what you already stated. My original point was about tightening the pH swing with an elevated alkalinity.
"Personally, I think many hobbyists keep alk too low. The ocean has vast amounts of water to constantly replace minerals in water around the corals. The corals use these carbonates quickly in our small water volumes, and we get wide pH swings as a result."
I didn't distinguish between the daily pH swing due to CO2, and the swing that happens with the consumption of alkalinity and then adding it back.

 

[Randy Holmes-Farley]

Thank you for the reply and reference material. I will read it today.
I know that you are chemically correct, but I am still going to disagree with you from a practical husbandry standpoint. I was originally speaking about a tank with almost 100% stony coral cover. If I maintain a stable alkalinity ( whether 7 or 10dkh) the pH swing will be around .2 to.4 based on the ambient carbon dioxide's effect on the water. I have run the tank at both of these dkh readings and monitored the pH. If I shut off all forms of alkalinity additions to the system the consumption of carbonates is so high that the pH will plummet. It will exceed the normal daily pH swing in a few hours. I have had to make many emergency trips to this tank because of a blocked line or a failed doser.
I understand that CO2 is the driving force of the pH change, but I think stony coral colonies consuming the water's buffer can have a more dramatic effect on daily pH in this instance.

Adding carbonate and then stopping can certainly cause pH to drop because the carbonate was previously propping up pH by binding CO2 to form bicarbonate. If you had been supplementing with bicarbonate, this effect disappears.

Dropping alkalinity also reduces the equilibrium pH for a given CO2 level.

But a tank without any alk consumption snd a similar alk level still has a similar pH swing based on similar photosynthesis.

 

[IntrinsicReef]

I think I am on the same page now. It seems like I was conflating two ( or more) different processes in my article. I do run higher alkalinity in my tanks for higher pH relative to CO2 concentration, tighter daily pH swing, and a buffer pool so the pH takes longer to drop below 8 if the addition of buffer ceases. But, I worded that poorly in my write up. Thank you again for taking the time to discuss it with me and clarifying.

 

[Randy Holmes-Farley]

I think I am on the same page now. It seems like I was conflating two ( or more) different processes in my article. I do run higher alkalinity in my tanks for higher pH relative to CO2 concentration, tighter daily pH swing, and a buffer pool so the pH takes longer to drop below 8 if the addition of buffer ceases. But, I worded that poorly in my write up. Thank you again for taking the time to discuss it with me and clarifying.

You’re welcome.

Happy Reefing