Alkalinity Titration in relation to PH

[friendlyAlien]

First, thank you, Randy, for all the knowledge you share with us! I am learning so much.
As I learn about alkalinity and its testing, I stumble over a simple question.

Looking at this article: https://www.reefedition.com/a-diy-alkalinity-test-by-randy-holmes-farley/

It seems that to measure alkalinity, I just need to reduce the pH to 4.5 and then determine the alkalinity from that.
What is puzzling to me is that it is independent of the starting pH. I can have an alkalinity of 10 with a pH of 7.5ish (thanks to my Thanksgiving dinner party and snow outside) and the same alkalinity but at a pH of 8.3 (thanks to my CO2 scrubber). Why is the pH not a factor in the measurement of alkalinity? What am I missing?

 

[Garf]

First, thank you, Randy, for all the knowledge you share with us! I am learning so much.
As I learn about alkalinity and its testing, I stumble over a simple question.

Looking at this article: https://www.reefedition.com/a-diy-alkalinity-test-by-randy-holmes-farley/

It seems that to measure alkalinity, I just need to reduce the pH to 4.5 and then determine the alkalinity from that.
What is puzzling to me is that it is independent of the starting pH. I can have an alkalinity of 10 with a pH of 7.5ish (thanks to my Thanksgiving dinner party and snow outside) and the same alkalinity but at a pH of 8.3 (thanks to my CO2 scrubber). Why is the pH not a factor in the measurement of alkalinity? What am I missing?

Dunno if this helps;

https://www.reefcentral.com/forums/showpost.php?p=19861400&postcount=2

 

[hoffmeyerz]

Ok, first not a chemist lol!
As I understand from learning in this hobby, Alk is basically a buffer to PH and how we measure it is how hard or easy it is to change PH to a specific point. With that understanding it doesn't matter where PH starts from but how much acid it takes to move it to a specific point.
Hope I'm close in my understanding!

 

[Randy Holmes-Farley Verified Community Expert]

It’s a very complicated question.

1. By the true definition of alkalinity, changes in oh caused by changes in CO2 do not have any impact on alkalinity.

2. By the sophisticated ways that a scientist would measure alkalinity, involving either analysis of the pH vs acid added curve shape, or by titration to a specific pH while allowing all CO2 to leave the water during the titration, there will no effect from variable CO2 levels.

3. By the two main ways reefers use to measure alk (titration to a specific pH or by specific acid addition and subsequent pH measurement (Hanna) ) there will be some CO2 removal but not all (unless you proceed unusually slowly). In these cases, there may well be some pH effect. I think Dan_P was thinking of running some tests.

 

[friendlyAlien]

Ok, this makes more sense now. My naive mind assumed that the titration acid has two jobs. It has to neutralize the alkalinity and the high pH.
A little naive testing (turn the CO2 scrubber off for two hours) seemed to agree with that theory,vif I just were to trust my testing to be accurate enough (alk reading increased as measured by the Hydros iV and Hannah) ever so slightly.