Why does pH increase with Kalk take time to stabilize?

[rishma]

When restarting nightly Kalkwasser dosing, why does it take a week or more for average pH to stabilize at the higher value? What is happening chemically that takes a week+? See chart below.

I have seen this trend before, and I don’t understand why this happens. I’m curious and would like an explanation if someone can educate me.

Here is the story behind the chart…
I turned off my nightly kalk dosing and my average pH went down over several days/nights. I expected this. There were other things happening like water changes so I didn’t monitor the trend.

At the beginning of the chart above, I restarted Kalkwasser at 1/2 my maintenance dose (alkalinity demand). The reason for the lower dose is I found my alkalinity much higher than my target so I was allowing it to drift down to my target.

My alkalinity is falling a little every day, but with Kalk added each night the average pH trends up over time. I know from past experience it will level out and the day/night swing will become fairly consistent. The average pH will level out little under 8.2. My tank is aerated with outside air so windows open/closed have less effect on pH.

What is happening in the water that takes time to reach the new average?

 

[Randy Holmes-Farley Verified Community Expert]

Do you know if the amount dosed is unchanged in this period? Or is it matched to alk demand and may increase as demand increases due to the pH increase?

 

[rishma]

Do you know if the amount dosed is unchanged in this period? Or is it matched to alk demand and may increase as demand increases due to the pH increase?

The amount of kalk dosed is constant each night during this period.

 

[Randy Holmes-Farley Verified Community Expert]

I expect that it is slowly depleting the more acidic compounds, such as bicarbonate, and converting them to basic versions such as carbonate. It just takes time to reach a new equilibrium between CO2 consumption and CO2 addition. :)

 

[rishma]

ok! Thank you. It surprises me that the timeframe is so long in a small circulating closed system.

But honestly it still surprises me how much swing happens between day and night even with aeration. PH is fascinating.

 

[rishma]

I expect that it is slowly depleting the more acidic compounds, such as bicarbonate, and converting them to basic versions such as carbonate. It just takes time to reach a new equilibrium between CO2 consumption and CO2 addition. :)

@Randy Holmes-Farley, I have a new theory. I think the slowly increasing pH might be due to increasing algae and photosynthesis in addition to kalkwasser. The CO2 in the tank reaches lower levels each day with more algae cells photosynthesizing.

I scrubbed the tank and changed water this week. The alkalinity of the new water was the same as the old water, but I removed a bunch of algae from glass walls. The average pH is lower and the swing is much smaller….both very slowly increasing. My nutrients are low but algae grows.

I saw something similar previously when I reduced lighting intensity to acclimate new corals. Lower pH and smaller swings.

More algae = more photosynthesis = higher average pH.

What say you?

 

[Randy Holmes-Farley Verified Community Expert]

@Randy Holmes-Farley, I have a new theory. I think the slowly increasing pH might be due to increasing algae and photosynthesis in addition to kalkwasser. The CO2 in the tank reaches lower levels each day with more algae cells photosynthesizing.

I scrubbed the tank and changed water this week. The alkalinity of the new water was the same as the old water, but I removed a bunch of algae from glass walls. The average pH is lower and the swing is much smaller….both very slowly increasing. My nutrients are low but algae grows.

I saw something similar previously when I reduced lighting intensity to acclimate new corals. Lower pH and smaller swings.

More algae = more photosynthesis = higher average pH.

What say you?

That is certainly plausible. :)