pH/Alk/CO2 question

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My tanks pH is usually locked in at 8.3 +/- 0.02. I've been trying to stabilize Alk which has been between 7 and 7.7dKH.

I was making the assumption that with a stable pH, my target Alk should be around 7.7dKHish to have natural CO2 levels. If my dKH is below 7.7dKH, my tank would be below natural levels.

Yesterday I had all the windows open in the house. I assumed that since my dKH was below 7.7dKH, at around 7.3dKHish, I wouldn’t see a pH swing. I'd just see my skimmer working less to drive out CO2.

Instead, my pH went as high as 8.37. This leads me to believe that my CO2 levels may instead be higher than normal.

I have two explanations:
1. Kalk drip plus outside air is able to raise pH and further drop me below normal CO2 levels
2. GHL pH probe is off and my real pH is lower than its showing

If #2 is correct, and say pH is closer to 8.2, then I'm targeting the wrong AlK levels and should be targeting something closer to 7dKH for natural CO2 levels.

Interestingly enough, I can't seem to keep my Alk at 7.7dKHish as it keeps trending down. I'm using AFR which doesn't show up immediately in Alk readings and have gone from 7ml to 22ml over the last week (0.1dKH to 0.3dKH addition).

So any thoughts on what I'm seeing?
 

Randy Holmes-Farley

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Even in a tank sitting in outside air, there is often a pH swing as organisms use CO2 during the day and produce it at night.

CO2 equilibration is fairly hard to achieve since it doesn't really like to enter or leave the water (meaning it is technically a slow step).

Depending on several factors (feeding, dosing nutrients, alk additives), your tank may produce more CO2 than it consumes, or it may consume more than it produces, so even with the swing, the average pH may still be above or below the equilibrated expectation for the air and the alkalinity.
 
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Even in a tank sitting in outside air, there is often a pH swing as organisms use CO2 during the day and produce it at night.

CO2 equilibration is fairly hard to achieve since it doesn't really like to enter or leave the water (meaning it is technically a slow step).

Depending on several factors (feeding, dosing nutrients, alk additives), your tank may produce more CO2 than it consumes, or it may consume more than it produces, so even with the swing, the average pH may still be above or below the equilibrated expectation for the air and the alkalinity.

Thanks for that explanation Randy. When you say slow step for CO2 equilibrium, is that minutes or hours?

Here is a graph. My pH is normaly between 8.29 and 8.31 with a blip or two above and below those numbers...then the outside air and a larger pH swing:

Screenshot_20210511-081932_GHL Connect.jpg


Interestingly enough, it corresponds roughly with my light cycle, but I think that's just coincidence here. I think I opened the windows sometime in the morning and closed them around 6ish. I am not sure about those times though...
 

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Thanks for that explanation Randy. When you say slow step for CO2 equilibrium, is that minutes or hours?

Here is a graph. My pH is normaly between 8.29 and 8.31 with a blip or two above and below those numbers...then the outside air and a larger pH swing:

Screenshot_20210511-081932_GHL Connect.jpg


Interestingly enough, it corresponds roughly with my light cycle, but I think that's just coincidence here. I think I opened the windows sometime in the morning and closed them around 6ish. I am not sure about those times though...

Time is a tricky concept at the molecular level. The half life of H2CO3 dehydrating to CO2 (a necessary step for it to leave the water) is about 0.03 seconds. CO2 hydrating to HCO3 has a half life of about 30 seconds.

Those sound fast, but is slow relative to typical molecular interactions. It slows the equilibration of CO2 in, say, a skimmer, substantially.


"The conversion of CO2 into H + and HCO3- ions in an aqueous solution is an important chemical reaction in many geological processes, such as CO2-transfer across air-water interfaces"
...
"The hydration of CO2 in reaction I is slow, whereas the dissociation of carbonic acid is so fast that in practically all cases H2CO3 ,H + and HCO3- are in equilibrium. "
 
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Time is a tricky concept at the molecular level. The half life of H2CO3 dehydrating to CO2 (a necessary step for it to leave the water) is about 0.03 seconds. CO2 hydrating to HCO3 has a half life of about 30 seconds.

Those sound fast, but is slow relative to typical molecular interactions. It slows the equilibration of CO2 in, say, a skimmer, substantially.


"The conversion of CO2 into H + and HCO3- ions in an aqueous solution is an important chemical reaction in many geological processes, such as CO2-transfer across air-water interfaces"
...
"The hydration of CO2 in reaction I is slow, whereas the dissociation of carbonic acid is so fast that in practically all cases H2CO3 ,H + and HCO3- are in equilibrium. "

Very interesting. Next time I'm home for the day, I'll time my window opening and pH flucuations.
 
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@Randy Holmes-Farley
Since this conversation, my tank has substantially matured and now a has more normal pH swings due to photosynthetic organisms. Larger now that we have been on vacation and there is less CO2 in the house.

SmartSelect_20210622-074751_GHL Connect.jpg


This got me thinking more about CO2 levels in the tank. During this time, my Alk was a very steady 7.7.

With a low of pH of 8.28 and a high of 8.39, I calculate that my effective tank CO2 goes from
about 350 to 260 (effective partial pressure).

Given the accuracy of the measurement devices...the CO2 swings could be 350 to 470.

My questions:
1. Does natural sea water day vs night have these pH/CO2 swings? (I think no)
2. Is there any benefit to corals in keeping the pH more stable? (I think yes)
 

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Yes, the ocean does show a pH swing, especially in shallow lagoons.

Diurnal fluctuations in seawater pH influence the response of a calcifying macroalga to ocean acidification​


"Coastal ecosystems that are characterized by kelp forests encounter daily pH fluctuations, driven by photosynthesis and respiration,"
 

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As to pH stability, we have less data on the benefit:

 

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@Randy Holmes-Farley
Since this conversation, my tank has substantially matured and now a has more normal pH swings due to photosynthetic organisms. Larger now that we have been on vacation and there is less CO2 in the house.

SmartSelect_20210622-074751_GHL Connect.jpg


This got me thinking more about CO2 levels in the tank. During this time, my Alk was a very steady 7.7.

With a low of pH of 8.28 and a high of 8.39, I calculate that my effective tank CO2 goes from
about 350 to 260 (effective partial pressure).

Given the accuracy of the measurement devices...the CO2 swings could be 350 to 470.

My questions:
1. Does natural sea water day vs night have these pH/CO2 swings? (I think no)
2. Is there any benefit to corals in keeping the pH more stable? (I think yes)
some of us would commit felonies to get that kind of pH stability and levels haha. Good for you.
 

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