Which water tests are effected by tank pH?

[rishma]

@Randy Holmes-Farley - I sometimes test morning and evening and I am always curious about the variation in parameters. You mentioned in another thread that an alkalinity test could be impacted by tank pH. I am wondering what tests are likely to be impacted by pH and therefore time of day. And which direction the pH is likely to influence the results.

I am assuming my tests methods for a particular parameter use similar chemistry, but that could be wrong, making this a difficult question to answer.

Tests in particular that I do with some regularity are Alkalinity (Hanna checker), -and phosphorus (ULR checker). I see swings day and night and I wonder if some of it is due to pH changes.

 

[Randy Holmes-Farley]

True alk titrations should not be impacted by pH changes caused by changes in CO2. But real alk hobby tests may be impacted in complicated ways.

The true endpoint of an alk titration does depend to some extent on the CO2 level, so a test that goes to a specific pH or color may have a small impact in its accuracy.
A titration that analyzed the shape of the titration curve and calculated both the endpoint and the amount of titrant added does not suffer that effect.

Think of this extreme: you can drive pH down to the color endpoint by adding CO2, and thus would think there’s no alk present by simplistically assuming the endpoint did not change.

We have a fairly sophisticated discussion involving a number of experts here long ago:

Effect of CO2 on alkalinity? - Reef Central Online Community

Effect of CO2 on alkalinity? The Reef Chemistry Forum

www.reefcentral.com

 

[Randy Holmes-Farley]

ORP certainly is impacted by pH, but I’m not sure any others we regularly use will be.

Of course, tests that try to distinguish different forms of an acid base pair will be. Free vs total ammonia is a common one.

 

[Dan_P]

ORP certainly is impacted by pH, but I’m not sure any others we regularly use will be.

Of course, tests that try to distinguish different forms of an acid base pair will be. Free vs total ammonia is a common one.

One perspective about pH might be that in testing water in pursuit of maintaining healthy conditions in a reef tank, the pH effect is smaller than the margin of error of hobby test kits, and definitely, smaller than the tolerance range of the organisms in the aquarium. As for ORP measurements, in general they are useless, and therefore, the substantial pH effect is a “so what”. As for ammonia tests, pH does not affect the results of the salicylate method. The pH does affect free ammonia concentration but not the measurement of free ammonia.

 

[rishma]

Thanks @Dan_P and @Randy Holmes-Farley for the responses.

I agree its academic but interesting to think about. I wonder if my Hanna checker alkalinity readings are affected by pH. I actually don’t know what kind of test it is, it’s not like the manual titration test kits I used for years. I could aerate the samples outdoors and see if that changes anything, but as @Dan_P pointed out the changes I observe morning/evening are likely within the margin of error so I might not be able to tell anyway.

I’ll wade through that reef central thread and learn some things.