Does Tank pH affect KH director results?

[jschultzbass]

So I've had my Director for about year. Shortly after I got it my tank crashed and I lost all SPS. I never found the reason for the crash but haven't put any corals in since then. Only some zoas survived and they are still in the tank. Right now the tank looks great with EVERYTHING covered in coraline. I'm thinking of adding SPS back into the tank but first wanted to raise my tank pH. I set my KH director back up and was getting readings around 9.5 for about a week. My tank pH was ranging from 7.8 - 8.0 during this time. I added CO2 scrubber and pH came up to 8.1 - 8.25. When pH came up, KH director value fell to 8.5. So my question is since the KH director uses a pH endpoint to test alkalinity does the pH of the tank itself affect these results? @Vinny@GHLUSA

 

[Lasse]

IMO not. Only if the higher pH depend on higher alkanity not if the pH depends on CO2 content in the water (at least not in a value you cam measure)

Sincerely Lasse

 

[FactoryKTMmotocross46]

Do you get the ph reading from the kh director? I'm looking into getting one and interested to know if it will track your ph as well. I know it uses a ph probe.

 

[Lasse]

No - it will not track your external pH (in the tank) - only the pH inside the unit.

Sincerely Lasse

 

[lmm1967]

IME - Nope - I have a KH director and I check it's accuracy using the same method - calibrated PH meter, 500 ml of tank water and a known acid.

Starting PH does not seem to affect the reading - although it could just be that it's so minuscule an impact that it's within the normal margin of error.

@Randy Holmes-Farley can probably provide a detailed scientific description of why it works this way.

 

[jschultzbass]

Thanks for the reply guys. Honestly it goes the opposite way I would expect anyway. If my Tank pH goes up, it should take more reagent to bring it down which should give a higher KH reading in the director, not lower. It just got me thinking is all. I'm not sure why my KH dropped though with higher pH and no stony corals. Maybe coralline is taking more up with higher pH?

 

[Randy Holmes-Farley]

This is a far more complicated question than it might seem, and it uncovers one of the dirty little secrets of reef chemistry, starting with the suitability of alkalinity alone being an imperfect measure of the bioavailability of bicarbonate.

The starting pH does not impact the ability of a pH titration to give a perfect measure of the total alkalinity.

BUT, if a coral cares about bicarbonate availability (and not carbonate, or even the other way around, or even if it cares about both...), total alkalinity DOES NOT give an accurate measure of bicarbonate.

At a fixed pH, there is a fixed ratio of bicarbonate and carbonate. At higher pH, more carbonate and less bicarbonate.

At pH 8.0, 89% of total alk of seawater is bicarbonate. So if the total alkalinity is 7 dKH, bicarbonate is about "6.2 dKH".

Same 7 dKH tank at pH 8.5, what is the level of bicarbonate? It is WAY LOWER.

At pH 8.5 in seawater, the relative amounts of bicarbonate and carbonate are about 7:3. Since carbonate counts twice to alkalinity, it is contributing nearly as much alkalinity as the bicarbonate, and instead of "6.2 dKH" of bicarbonate, the value is much lower, closer to "3.75 dKH".

Thus we see that if a coral cares about bicarbonate, a change in pH can have a big effect on the level, even when total alkalinity is unchanged.

 

[Randy Holmes-Farley]

Thanks for the reply guys. Honestly it goes the opposite way I would expect anyway. If my Tank pH goes up, it should take more reagent to bring it down which should give a higher KH reading in the director, not lower. It just got me thinking is all. I'm not sure why my KH dropped though with higher pH and no stony corals. Maybe coralline is taking more up with higher pH?

That isn't true. At fixed total alkalinity, it takes a fixed amount of mineral acid to reach the CO2 endpoint.

i can raise or lower the starting pH of that solution by adding or removing CO2, and it has zero impact on the measured value of total alkalinity.

I discuss it here:

Chemistry and the Aquarium: What is Alkalinity?

Randy provides an overview of alkalinity as to why it's important, how it's measured, and how can it be tested.

reefs.com

from it:

Alkalinity Facts
There are several facts about total alkalinity that follow
directly from the definition. Unfortunately, some of these have
been misunderstood by some hobby authors.

One of these facts is termed The Principle of Conservation
of Alkalinity by Pankow (“Aquatic Chemistry
Concepts”, 1991). He shows mathematically that the total
alkalinity of a sample CANNOT be changed by adding or subtracting
CO2. Unfortunately, there is an article available on
line, which claims otherwise, and encourages people to
“lower alkalinity” by adding CO2 in the form
of seltzer water. This is simply incorrect.

Forgetting the math for the moment, it is easy to see how this
must be the case. If carbonic acid is added to any aqueous sample
with a measurable alkalinity, what can happen?

Well, the carbonic acid can release protons by reversing
equations 1 and 2:

(5) H2CO3 ==> H+ + HCO3–

(6) HCO3– ==> H+ + CO3—

These protons can go on to reduce alkalinity by combining with
something that is in the sample that provides alkalinity
(carbonate, bicarbonate, borate, phosphate, etc). However, for
every proton that leaves the carbonic acid and reduces
alkalinity, a new bicarbonate or carbonate ion is formed that
adds to alkalinity, and the net change in total alkalinity is
exactly zero. The pH will change, and the speciation of the
things contributing to alkalinity will change, but not the total
alkalinity.

This is not true for strong acids, however. If you add
hydrochloric, sulfuric or phosphoric acids (or any acid with a
pKa lower than the carbonic acid endpoint), there will be a
reduction in the alkalinity.

 

[tamanning]

Thanks for the reply guys. Honestly it goes the opposite way I would expect anyway. If my Tank pH goes up, it should take more reagent to bring it down which should give a higher KH reading in the director, not lower. It just got me thinking is all. I'm not sure why my KH dropped though with higher pH and no stony corals. Maybe coralline is taking more up with higher pH?

What is your magnesium level. the level if off can cause problems with calcium.

 

[jschultzbass]

This is a far more complicated question than it might seem, and it uncovers one of the dirty little secrets of reef chemistry, starting with the suitability of alkalinity alone being an imperfect measure of the bioavailability of bicarbonate.

The starting pH does not impact the ability of a pH titration to give a perfect measure of the total alkalinity.

BUT, if a coral cares about bicarbonate availability (and not carbonate, or even the other way around, or even if it cares about both...), total alkalinity DOES NOT give an accurate measure of bicarbonate.

At a fixed pH, there is a fixed ratio of bicarbonate and carbonate. At higher pH, more carbonate and less bicarbonate.

At pH 8.0, 89% of total alk of seawater is bicarbonate. So if the total alkalinity is 7 dKH, bicarbonate is about "6.2 dKH".

Same 7 dKH tank at pH 8.5, what is the level of bicarbonate? It is WAY LOWER.

At pH 8.5 in seawater, the relative amounts of bicarbonate and carbonate are about 7:3. Since carbonate counts twice to alkalinity, it is contributing nearly as much alkalinity as the bicarbonate, and instead of "6.2 dKH" of bicarbonate, the value is much lower, closer to "3.75 dKH".

Thus we see that if a coral cares about bicarbonate, a change in pH can have a big effect on the level, even when total alkalinity is unchanged.

So at a higher pH you have more carbonate, less bicarbonate and it isn't a one for one change. So is the director not measuring total alkalinity or does the bicarbonate resist pH change more than carbonate?

 

[Randy Holmes-Farley]

So at a higher pH you have more carbonate, less bicarbonate and it isn't a one for one change. So is the director not measuring total alkalinity or does the bicarbonate resist pH change more than carbonate?

A pH titration for total alkalinity drives the pH down to the low 4's, which is the point called the carbonic acid equivalence point or the carbonic acid endpoint. At that pH, all bicarbonate and carbonate are converted into H2CO3 which is essentially CO2.

Bicarbonate (HCO3-) takes up one H+ (one equivalent of acid) to get to that point: HCO3- + H+ --> H2CO3

Carbonate (CO3--) takes up one H+ (one equivalent of acid) to get to bicarbonate: CO3- - + H+ --> HCO3- and another H+ (one more equivalent of acid) to get to carbonic acid: HCO3- + H+ --> H2CO3

Thus, carbonate counts twice to alkalinity while bicarbonate only counts once.

As you raise pH by removing CO2, you convert two bicarbonates into one carbonate and one CO2 (that you remove). No change in tiotal alk, but a big drop in bicarbonate.

As you lower pH by adding CO2, you convert one CO2 and one carbonate into two bicarbonates (no change in alk but a big boost in bicarbonate).

 

[lmm1967]

holy cow - I knew he'd scramble my brain.

@Randy Holmes-Farley - silly question I'm guessing you have a link to an answer for.....

how do we now if a coral cares about carbonite or bi-carbonite? Or do we just ignore that and maintain steady alk as tested as long as you are testing the same way all the time?

And Thank You - I imagine you as having a vast amount of knowledge just waiting to be unleashed - and I, for one, appreciate you sharing so much for so little!

 

[jschultzbass]

holy cow - I knew he'd scramble my brain.

@Randy Holmes-Farley - silly question I'm guessing you have a link to an answer for.....

how do we now if a coral cares about carbonite or bi-carbonite? Or do we just ignore that and maintain steady alk as tested as long as you are testing the same way all the time?

And Thank You - I imagine you as having a vast amount of knowledge just waiting to be unleashed - and I, for one, appreciate you sharing so much for so little!

From what I understand from reading some articles is Corals actually prefer carbonate, when they use bicarbonate they have to expel the extra hydrogen, with the carbonate they can readily use it with the calcium for their calcium carbonate skeleton. This is why higher pH usually means faster coral growth...more carbonate available. Is this correct Randy?

 

[Randy Holmes-Farley]

holy cow - I knew he'd scramble my brain.

@Randy Holmes-Farley - silly question I'm guessing you have a link to an answer for.....

how do we now if a coral cares about carbonite or bi-carbonite? Or do we just ignore that and maintain steady alk as tested as long as you are testing the same way all the time?

And Thank You - I imagine you as having a vast amount of knowledge just waiting to be unleashed - and I, for one, appreciate you sharing so much for so little!

I think that the consensus is that they take up bicarbonate. There is much more of it present.

Bicarbonate transporters in corals point towards a key step in the evolution of cnidarian calcification

The bicarbonate ion (HCO[3] [−] ) is involved in two major physiological processes in corals, biomineralization and photosynthesis, yet no molecular data on bicarbonate transporters are available. Here, we characterized plasma membrane-type HCO[3] ...

www.ncbi.nlm.nih.gov

The abstract has details for free, but the upshot is that bicarbonate is taken up, converted into CO2, the CO2 diffuses through the animal to a place where calcification takes place, and the CO2 is converted back into bicarbonate and carbonate.

 

[Randy Holmes-Farley]

From what I understand from reading some articles is Corals actually prefer carbonate, when they use bicarbonate they have to expel the extra hydrogen, with the carbonate they can readily use it with the calcium for their calcium carbonate skeleton. This is why higher pH usually means faster coral growth...more carbonate available. Is this correct Randy?

High pH does accelerate calcification, but it could do it by making more CO3-- available, or by making it easier to pump out the H+.