should I be worried... rapid alk uptake

[pdxmonkeyboy]

ok, lame title but here is the situation.

I had to fallow my tank because of ich. Had to take all the rock work out and put in my frag tank to catch fish, then put back in the display. I lost a couple small colonies.

flash forward a month, I purchased some reef brite xho blue strips and another gyre and about 5 frags and 4 mini colonies.

Numbers are all where they should be.. mag is high. 400 gallons total system volume.

alk 8.0
cal 460
mag 1600+
no3 8
phos .06

I check my alk every day and run a doser. For the last 5 days, every time I check alk it is at 7.8 or 7.7 and i have dialed in 10 to 15 ml more of cal and alk every other day. I am trying to maintain it at 8. I am running a little less cal to bring it down a bit.

corals look great, good extention. it just seems odd that consumption would rise that much..

I tested mag like 5 times as I thought maybe it was low and limiting alk numbers.

oh yeah, I am also dosing vinegar. I went from 80ml to 100ml last week.

thoughts? worries?

 

[pdxmonkeyboy]

anyone??

 

[Gregg @ ADP]

There’s another side of alkalinity ‘consumption’ that, for whatever reason, doesn’t seem to get much play in these discussions. It has to do with decaying organics within our closed systems and the impact of that on our buffering system.

Consider the following equilibrium rxns:

CO2 + H2O <- -> H2CO3 <- -> HCO3- + H+

In the case of your tank, it’s possible that by moving rocks and exhuming areas where detritus has settled and organic decay is occurring, you have released a lot more H+ into your water column than you would typically have.

Le Chatelier’s Principle tells us that as we ^ the [H+] (which could easily happen with a disruption in the tank and rock/sand bed), there is more of a likelihood of those ions coming into contact with our bicarbonates, which will then serve to kind of soak up the H+ and the reaction then shifts to the left.

Long story short, you might have to keep dosing higher levels of HCO3 until the [H+] more or less normalizes.

 

[Gregg @ ADP]

Oops, missed the vinegar dosing part. Same chem applies.

 

[Randy Holmes-Farley Verified Community Expert]

The numbers look fine to me, except the magnesium which is likely test error. I'd stop measuring it.

 

[Randy Holmes-Farley Verified Community Expert]

There’s another side of alkalinity ‘consumption’ that, for whatever reason, doesn’t seem to get much play in these discussions. It has to do with decaying organics within our closed systems and the impact of that on our buffering system.

Consider the following equilibrium rxns:

CO2 + H2O <- -> H2CO3 <- -> HCO3- + H+

In the case of your tank, it’s possible that by moving rocks and exhuming areas where detritus has settled and organic decay is occurring, you have released a lot more H+ into your water column than you would typically have.

Le Chatelier’s Principle tells us that as we ^ the [H+] (which could easily happen with a disruption in the tank and rock/sand bed), there is more of a likelihood of those ions coming into contact with our bicarbonates, which will then serve to kind of soak up the H+ and the reaction then shifts to the left.

Long story short, you might have to keep dosing higher levels of HCO3 until the [H+] more or less normalizes.

I'm not sure what you are actually suggesting.

I have discussed many times the effects of organics being metabolized. It can lower pH (increase H+) by adding CO2 as the endproduct of metabolism, and the production of nitrate will deplete alkalinity (just as the consumption of nitrate adds it back).

But the lowering of pH by adding CO2 CANNOT lower bicarbonate. In reality, it will raise it in reef tank water (by converted carbonate into bicarbonate) and have no net effect on alkalinity. In fact, there is a principle in natural waters chemistry called The Principle of Conservation of Alkalinity which shows this (discussed below).

Chemistry and the Aquarium: What is Alkalinity? ? Advanced Aquarist | Aquarist Magazine and Blog
https://www.advancedaquarist.com/2002/2/chemistry

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.

Another interesting result of the Principle of Conservation of Alkalinity is the equation for determining the total alkalinity when two different aqueous solutions are mixed together. If you mix (a) parts of a solution with total alkalinity A with (b) parts of a solution of total alkalinity B, the resulting alkalinity is just the weighted average of the two samples:

TAmix = [a(A) + b(B)]/[a + b]

Equation 7 can be used to calculate changes in TA for water changes in a tank, for additions of limewater, for dilution of tank water with pure water, and a host of other situations where you might want to know what the final alkalinity will be. It can also be used for calculating reductions in alkalinity caused by strong acids, where the alkalinity of the acid is just the normal strength of the acid as a negative number.

 

[Gregg @ ADP]

I'm not sure what you are actually suggesting.

I have discussed many times the effects of organics being metabolized. It can lower pH (increase H+) by adding CO2 as the endproduct of metabolism, and the production of nitrate will deplete alkalinity (just as the consumption of nitrate adds it back).

But the lowering of pH by adding CO2 CANNOT lower bicarbonate. In reality, it will raise it in reef tank water (by converted carbonate into bicarbonate) and have no net effect on alkalinity. In fact, there is a principle in natural waters chemistry called The Principle of Conservation of Alkalinity which shows this (discussed below).

Chemistry and the Aquarium: What is Alkalinity? ? Advanced Aquarist | Aquarist Magazine and Blog
https://www.advancedaquarist.com/2002/2/chemistry

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.

Another interesting result of the Principle of Conservation of Alkalinity is the equation for determining the total alkalinity when two different aqueous solutions are mixed together. If you mix (a) parts of a solution with total alkalinity A with (b) parts of a solution of total alkalinity B, the resulting alkalinity is just the weighted average of the two samples:

TAmix = [a(A) + b(B)]/[a + b]

Equation 7 can be used to calculate changes in TA for water changes in a tank, for additions of limewater, for dilution of tank water with pure water, and a host of other situations where you might want to know what the final alkalinity will be. It can also be used for calculating reductions in alkalinity caused by strong acids, where the alkalinity of the acid is just the normal strength of the acid as a negative number.

I was posting that while eating lunch, and was thinking about this point:

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.

Then I got side-tracked trying to set up the concept with carbonic acid stuff.

Correct me if I’m wrong, but isn’t there plenty of organic decomposition of nitrogenous compounds that produces stronger acids w/lower pKa than carbonic acid? And dosing acetic acid (with a lower pKa) would also serve to lower alkalinity, correct?

 

[pdxmonkeyboy]

update...

the tank has leveled off at 110ml per day cal and alk.

The corals look great. if you pushed me into a corner and demanded why everything is growing and looking better I would have to say the two new reefbrite xho blues.

thanks for the help everyone!!

 

[Randy Holmes-Farley Verified Community Expert]

Correct me if I’m wrong, but isn’t there plenty of organic decomposition of nitrogenous compounds that produces stronger acids w/lower pK than carbonic acid? And dosing acetic acid (with a lower pKa) would also serve to lower alkalinity, correct?

I'm not sure I understand the stronger acids question. I don't think there is any substantial process messing with alkalinity in reef tanks that relates to metagbolism of organics aside from the conversion of ammonia to nitrate.

Acetic acid temporarily lowers alkalinity until it is metabolized. The amount it lowers it is complicated since the acetate is partly counted in an alk test. If it allw as counted, there would be no impact on total alk, although there would be a small conversion of carbonate alkalinity into acetate alkalinity. But the pKa of acetic acid is not that far from a total alk titration endpoint (see link below) so not all gets counted) and there will be a small lowering to total alkalinity temporarily when dosing acetic acid (until it is metabolized and the acetate disappears, leaving behind bicarbonate).

It is definitely not like adding HCl or any mineral acid. Every bit of HCl added instantly and permanently depletes alkalinity.

It also takes a large amount of vinegar addition to make a substantial difference.

I discuss and show data here:

Chemistry and the Aquarium: What is Alkalinity?
https://www.advancedaquarist.com/2002/2/chemistry

 

[Gregg @ ADP]

I'm not sure I understand the stronger acids question. I don't think there is any substantial process messing with alkalinity in reef tanks that relates to metagbolism of organics aside from the conversion of ammonia to nitrate.

Acetic acid temporarily lowers alkalinity until it is metabolized. The amount it lowers it is complicated since the acetate is partly counted in an alk test. If it allw as counted, there would be no impact on total alk, although there would be a small conversion of carbonate alkalinity into acetate alkalinity. But the pKa of acetic acid is not that far from a total alk titration endpoint (see link below) so not all gets counted) and there will be a small lowering to total alkalinity temporarily when dosing acetic acid (until it is metabolized and the acetate disappears, leaving behind bicarbonate).

It is definitely not like adding HCl or any mineral acid. Every bit of HCl added instantly and permanently depletes alkalinity.

It also takes a large amount of vinegar addition to make a substantial difference.

I discuss and show data here:

Chemistry and the Aquarium: What is Alkalinity?
https://www.advancedaquarist.com/2002/2/chemistry

I guess my thinking was that, in the case of an established reef tank, there would be sufficient anaerobic decomposition under rocks to acidify that region and there would be acids with a low enough pKa to then lower the alkalinity if (as OP did) the rock structure was moved and those areas were exposed to the main water column.

Then I dug around and it looks like that even then, stonger acids are converted into acetic acid, NH4, and CO2...which goes back to substantiate your original statement.

One thing I mentioned in my thread in the meet and greet forum, as a science teacher, I will teach chemistry...if there is a gun to my head. And even then, I often consider just taking the bullet

 

[Randy Holmes-Farley Verified Community Expert]

I guess my thinking was that, in the case of an established reef tank, there would be sufficient anaerobic decomposition under rocks to acidify that region and there would be acids with a low enough pKa to then lower the alkalinity if (as OP did) the rock structure was moved and those areas were exposed to the main water column.

Then I dug around and it looks like that even then, stonger acids are converted into acetic acid, NH4, and CO2...which goes back to substantiate your original statement.

One thing I mentioned in my thread in the meet and greet forum, as a science teacher, I will teach chemistry...if there is a gun to my head. And even then, I often consider just taking the bullet

The pH may certainly drop in sand and other low flow regions due to production of CO2 that gets trapped. I measured my sand and it was well below tank pH. I'm not sure if the alk drops in those regions, but just adding CO2 won't do it. The amount of organic matter in seawater is usually well below the amount of bicarbonate, so the effects of organics in either direction are usually small unless one adds large amounts (like vinegar dosing).

 

[Gregg @ ADP]

The pH may certainly drop in sand and other low flow regions due to production of CO2 that gets trapped. I measured my sand and it was well below tank pH. I'm not sure if the alk drops in those regions, but just adding CO2 won't do it. The amount of organic matter in seawater is usually well below the amount of bicarbonate, so the effects of organics in either direction are usually small unless one adds large amounts (like vinegar dosing).

Good stuff, man. Thanks.

I like your point about the amount of acids from organic decomposition in relation to bicarbonates. I have always thought of it along the lines of acidification of soils, but never really thought about the math side of it.

I do wonder, given what you’ve said about your sand pH, if a tank with a lot of rock structure and deep enough sand bed would have sufficient acids to actually lower alkalinity if one were to move it all around.

You’ve given me some great ideas for class activities. I would be willing to bet that the vast majority of predictions would be similar to what my assumption was. It will be interesting to see if they can figure out why their results don’t match their predictions.