Can low PH artificially influence Alkalinity test kit results?

[senorpyro]

Question:
After seeing some suspiciously low alkalinity readings on a new Red Sea pro test kit, I started thinking about some of the RHF articles I'd previously read on the topic. Specifically:

In effect, pH test kits do a pH endpoint titration. They all include pH indicating dyes (providing a color change) and an acid (frequently dilute sulfuric acid) to lower the pH. You typically add acid until the dyes turn color. Since these dyes are selected to have a color change in the pH = 4-to-5 range, what you get is a measurement of how much acid it takes to lower the pH to that range. This color change is used to approximate the endpoint of the titration.

That explanation of how standard alkalinity test kits work seems to be mirrored many places across the internet, but left me wondering...

If the alkalinity test is simply measuring the amount of acid required to drop the PH down to 4.X, how can it be expected to produce reliable results, given that PH can vary significantly without simultaneously impacting alkalinity, such as with depressed PH due to high c02?

I have to assume there's something more complicated happening than determining the difference between the current PH and 4.X. My understanding of chemistry is fairly basic, but I'm having a tough time reconciling how that would work; my first thought is that it probably has something to do with the alkalinity buffering the change somehow, but I have no idea if that's even close to right, and I'd like to have some grasp on the interactions at work here.

Background:
We built a new house this summer, and I was lucky enough to also pick up a Red Sea Reefer 750 (essentially 180 gallons) in August. I've been slowly getting things rolling, filling it with water and some appropriately cleaned dry BRS rock in September. I moved in a couple pieces of rock from an established tank, set up the lights on low and let the tank do its cycling thing in peace while I sat back and watched. Pod populations are healthy, the normal new-tank algae blooms have come and gone, the small handful of snails I put in to start munching away at the algae are all doing well, and things are generally looking pretty good with about 2.5-3 months of maturity. I recently moved over a couple of small sacrificial sps frags from my other tank out of curiosity, and they seem to be doing quite well so far. Their addition prompted me to start looking at Ca/Alk/Mag levels, rather than just watching PH/temp/conductivity/ORP on the Apex.

Because this is a brand new house, I expect high c02 levels during the months where our windows are closed. My PH readings as per my Apex with new probes, calibrated in September have tracked my expectations pretty closely: troughs in the vicinity of 7.4 when the lights on the chaeto in the fuge are off, and highs of 7.65 or so with the windows closed and 7.8'ish with them open. I also see marked drops when the oven is used, and noted a very significant drop during the time lots of family was present on Thanksgiving, all aligning with low ph resulting from excess c02, but not dangerously so.

My test results from Sunday evening using fresh red sea pro kits were:
Ca: 450
Alk: 6.4 dkh
Mg: 1520
and a PH of 7.76

This is with zero water changes or additions other than plain RODI topoff in 3 months - the water was mixed in a 50/50 ratio with IO and Red Sea Coral Pro.

After looking at the BRS calculator, it seemed to indicate I need to add 160ml or so of the alkalinity portion of their 2 part to bring it up to 7.8. I decided to go slowly, and add 30ml twice a day... I did so for two days, testing nightly, but kept coming back to the essentially the same low alk value which prompted me to stop and evaluate before continuing. I have a terribly hard time imagining my mostly bare tank consuming that much alkalinity, so I'm now left waiting to try a second test kit to confirm the readings, and in the meantime wondering if my low PH could possibly be impacting my alk readings.

 

[JimWelsh]

Alkalinity is not affected by addition or removal of CO2. See the section titled "Alkalinity Facts" in this article: http://www.advancedaquarist.com/2002/2/chemistry

You could try this: Take two samples of your tank water in separate containers. Let one sit inside, while you place the second one outside, with an air pump and air stone in it, aerating it for a couple of hours. If your problem is excessive CO2 (which it almost certainly is), then you will find the pH of the second container rises significantly. Then, test both for alkalinity. You should find that they both test out at the same alkalinity level.

Another experiment goes something like this: Take two samples in separate containers of some freshly made salt mix that typically mixes with a high-ish alkalinity level (Instant Ocean should work nicely). Add a small amount of seltzer water (just CO2 in water), say 10 mL in a gallon, to one sample, and then add that same volume of RODI water to the other sample. You should find that the pH of the seltzer sample will be significantly lower than the pH of the RODI sample. Then, test both for alkalinity. You should find that they both test out at the same alkalinity level.

 

[senorpyro]

Right; I understand that alkalinity isn't affected by addition or removal of co2, I just got to wondering about the chemical process by which we mere mortals approximate our bicarbonate levels... I kept coming back to the statement that we're measuring the amount of acid needed to drop the PH of the sample to 4.x, and couldn't quite wrap my head around how factors like co2 wouldn't impact the readings of these test kits, since we're measuring by proxy. That the actual alkalinity would remain stable makes good sense, I'm just thinking about the possibility of errant results from the test. I suppose this is more of a 'explain a bit more of the chemistry involved in the test process, because my layman's reading makes it seem like it wouldn't be reliable' question than anything.

 

[Randy Holmes-Farley]

Right; I understand that alkalinity isn't affected by addition or removal of co2, I just got to wondering about the chemical process by which we mere mortals approximate our bicarbonate levels... I kept coming back to the statement that we're measuring the amount of acid needed to drop the PH of the sample to 4.x, and couldn't quite wrap my head around how factors like co2 wouldn't impact the readings of these test kits, since we're measuring by proxy. That the actual alkalinity would remain stable makes good sense, I'm just thinking about the possibility of errant results from the test. I suppose this is more of a 'explain a bit more of the chemistry involved in the test process, because my layman's reading makes it seem like it wouldn't be reliable' question than anything.

No, it would not impact the results at all. It just pushes more of the titration down to lower pH values.

For example, if the alkalinity is 7 dKH and the pH is 8.2, it takes a certain amount of acid to drop the pH to 7.8.

You are suggesting, reasonably, that this amount of the needed acid is somehow missed if the titration actually starts at pH 7.8 and alkalintiy of 7 dKH.

But that ends up not being true. I can show mathematically why that is the case, but here's how to think of it...

That amount of acid added between pH 8.2 and 7.8 is real, and corresponds to, say, "X" dKH.

Then the titration continues down to the endpoint in the 4's.

The titration that started at pH 8.2 will take correspondingly less acid to continue from pH 7.8 to 4.3 than the titration that started at pH 7.8 to get to the endpoint.

There is no lost alkalinity or lost titration of acid. When CO2 messes with the pH before the titration, it just moves around how much acid is needed at each pH, not how much is needed to get to the endpoint.

As an extreme example, suppose we take ALL of the CO2 out of seawater. Ignoring precipitation that might take place, the pH will rise to over pH 11, but the total alkalinity will stay at about 2.5 meq/L. There is a long way to go down in pH during the titration, but it goes faster with each drop of acid.

 

[sghera64]

No, it would not impact the results at all. It just pushes more of the titration down to lower pH values.

For example, if the alkalinity is 7 dKH and the pH is 8.2, it takes a certain amount of acid to drop the pH to 7.8.

You are suggesting, reasonably, that this amount of the needed acid is somehow missed if the titration actually starts at pH 7.8 and alkalintiy of 7 dKH.

But that ends up not being true. I can show mathematically why that is the case, but here's how to think of it...

That amount of acid added between pH 8.2 and 7.8 is real, and corresponds to, say, "X" dKH.

Then the titration continues down to the endpoint in the 4's.

The titration that started at pH 8.2 will take correspondingly less acid to continue from pH 7.8 to 4.3 than the titration that started at pH 7.8 to get to the endpoint.

There is no lost alkalinity or lost titration of acid. When CO2 messes with the pH before the titration, it just moves around how much acid is needed at each pH, not how much is needed to get to the endpoint.

As an extreme example, suppose we take ALL of the CO2 out of seawater. Ignoring precipitation that might take place, the pH will rise to over pH 11, but the total alkalinity will stay at about 2.5 meq/L. There is a long way to go down in pH during the titration, but it goes faster with each drop of acid.

If I understand the relationships right: Carbonate alkalinity (ignoring other contributors like Borates for simplicity) is a related to the total amount of bicarbonate and carbohydrate present. pH is related to the ratio of of the two. The amount of titrant needed depends on the combined concentration of bicarbonate plus carbonate and not on their ratio.

Is that one way to explain it?

 

[Randy Holmes-Farley]

If I understand the relationships right: Carbonate alkalinity (ignoring other contributors like Borates for simplicity) is a related to the total amount of bicarbonate and carbohydrate present. pH is related to the ratio of of the two. The amount of titrant needed depends on the combined concentration of bicarbonate plus carbonate and not on their ratio.

Is that one way to explain it?

Close.

"The amount of titrant needed depends on the concentration of bicarbonate plus twice the concentration of carbonate.

When you have a lower pH to start with (at the same alkalinity), you just have more bicarbonate and less carbonate.

 

[Randy Holmes-Farley]

That all said, it is true that alkalinity is only a surrogate measure for bicarbonate, and if bicarbonate is really what you care about (and not carbonate), then the pH is a concern in how it alters the relationship between pH and bicarbonate concentration.

A lower pH has more bicarbonate at a given alkalinity.

 

[senorpyro]

Thank you much for taking the time to respond - I'm glad to know that my logic was sound, even if my chemistry was not!

 

[Randy Holmes-Farley]

You're welcome.

Happy reefing.

 

[Cory]

One real thing that can confound alk tests is boron. Too much f it and your carbonate alk will run short. Too little and the ph wil be lower.

 

[Randy Holmes-Farley]

You certainly can be confounded by too much borate and trying to know how much carbonate alkalinity you have based on total alkalinity, but low or no borate won't cause noticeably low pH if total alk is fine.

 

[Gator1970]

You certainly can be confounded by too much borate and trying to know how much carbonate alkalinity you have based on total alkalinity, but low or no borate won't cause noticeably low pH if total alk is fine.

I’ve said it before and I’ll say it again - this forum is so fortunate to have an expert of your caliber on board, Randy!

 

[Randy Holmes-Farley]

Ha, thanks!

Happy reefing.