What is the correlation between alkalinity and pH?

[Roggio]

It could be coincidence but I’ve noticed as the demand goes up in my tank the pH drops below normal levels until I bring the alkalinity back up. I have a lot of stony coral so it seems the demand happens overnight. My pH is usually at a 8.1 low but demand can send it down to 7.9 overnight. That’s usually my indicator I need to increase my two part dose.

 

[Randy Holmes-Farley Verified Community Expert]

There are three effects in play here.

1. If you dose a high pH additive, dosing more raises the pH.
2. Formation of calcium carbonate directly lowers pH.
3. When seawater is in equilibrium with any type of air, the higher the alk the higher the pH in the water. A doubling of the alk (6 dKH to 12 dKH) causes about a 0.3 pH unit increase.

This has a great deal more explanation:

Chemistry And The Aquarium: The Relationship Between Alkalinity And pH ? Advanced Aquarist | Aquarist Magazine and Blog
http://www.advancedaquarist.com/issues/may2002/chem.htm

 

[Roggio]

There are three effects in play here.

1. If you dose a high pH additive, dosing more raises the pH.
2. Formation of calcium carbonate directly lowers pH.
3. When seawater is in equilibrium with any type of air, the higher the alk the higher the pH in the water. A doubling of the alk (6 dKH to 12 dKH) causes about a 0.3 pH unit increase.

This has a great deal more explanation:

Chemistry And The Aquarium: The Relationship Between Alkalinity And pH ? Advanced Aquarist | Aquarist Magazine and Blog
http://www.advancedaquarist.com/issues/may2002/chem.htm

Thanks so much Randy!

 

[kramy2017]

The drop in PH at night can also be caused by CO2 being expelled by corals and algea in your display tank. also your skimmer running at night if you have one. Vary small levels of CO2 can drop PH quickly. to offset this swing of ph you can add kalkwaaser of setup a refugium with algeas growimg on an opposite light cycle from the display tank.

 

[Scorpius]

My alkalinity rises and ph drops anytime the house is closed up.

 

[Randy Holmes-Farley Verified Community Expert]

My alkalinity rises and ph drops anytime the house is closed up.

Just to clarify, the alk rise is likely be due to ongoing dosing being paired up against reduced demand due to the lower pH, not that closing up the home itself will directly cause any rise in alkalinity.

 

[Scorpius]

Just to clarify, the alk rise is likely be due to ongoing dosing being paired up against reduced demand due to the lower pH, not that closing up the home itself will directly cause any rise in alkalinity.

I knew that already. My Ph sits around 7.6-7.9 all winter and my alk/ca demand drops so if I don't adjust my 2-part dosing my Alk/Ca goes up. My house has a Co2 problem lol.

 

[Randy Holmes-Farley Verified Community Expert]

I knew that already. My Ph sits around 7.6-7.9 all winter and my alk/ca demand drops so if I don't adjust my 2-part dosing my Alk/Ca goes up. My house has a Co2 problem lol.

I figured that, but folks reading what you wrote may have incorrectly thought the alkalinity was rising without dosing, like pH changes without dosing.

 

[Scorpius]

I figured that, but folks reading what you wrote may have incorrectly thought the alkalinity was rising without dosing, like pH changes without dosing.

Understood.