Reef Chemistry Question of the Day #97 Potential Depletion Rates

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #97

Many chemicals are depleted from the water in reef aquaria by the organisms living in them, and responding to that depletion is an important part of tank care. Recognizing what measured results are impossible and must be testing error is also an important part of the learning curve for a reefer.

Take our friend Julie. She was monitoring her tank closely for a few days. She took two measurements 36 hours apart. She dosed nothing in between, and did no water changes. She was a little confused by her results.

Which of the following of her measurements is the least likely change to have actually taken place in her reef aquarium during this time?

A. Calcium declined by 5%
B. Magnesium declined by 5%
C. Hydrogen ion (H⁺) declined by 50%
D. Alkalinity declined by 25%

Good luck!



























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[tonizzy22]

Answer is B magnesium would be the least likely to change in 36 hours.

 

[beaslbob]

wild guess is b. alk and ph can swing wildly and 5% calcium depletion is reasonable.
But nothing like magnesium.

 

[s2nhle]

I vote for C.

 

[dallasg]

B Mg by 5% is huge

 

[Skydvr]

The sample being 36 hours apart would more than likely put one in the morning and one at night, most likely with the lights off for one of them. If the one in the morning was before the lights came on and the one at night was right before the lights went out, this would show the largest swing. To see that large of a drop, hopefully the second reading was the one that was in the morning before (or right around the time) the lights came on. If pH was initially 8.2, a 50% drop in H+ ions would result in a pH around 7.9. This would also be affected by the drop in alkalinity.

I can't remember the correlation of dKH to meq/L and how that relates to ppm, but I am thinking that a 25% drop in alkalinity, depending on how it is measured, is not as significant as the 5% drop in magnesium. Magnesium is not used up at a rate anywhere near that of calcium.
If I could remember the correlation between ppm and dKh, I could have a better idea about the decrease in alkalinity. I am pretty sure it is two carbonate atoms per calcium atom that gets put into coral skeletons, so estimating a value in the average range for calcium and alkalinity, one could figure out if the alkalinity reduction was approximately in the correct range for the reduction in calcium. Of course sensitivity of the test used and whether or not she can differentiate between whole values would need to be taken into account (like with the Red Sea alkalinity test, which has an intermediate color for the titration, so if one drop goes from starting color to too much titrant color, it is at a half value of dKH).


But the glaring factor is the magnesium dropping by the same amount as the calcium as others have stated.

 

[s2nhle]

The sample being 36 hours apart would more than likely put one in the morning and one at night, most likely with the lights off for one of them. If the one in the morning was before the lights came on and the one at night was right before the lights went out, this would show the largest swing. To see that large of a drop, hopefully the second reading was the one that was in the morning before (or right around the time) the lights came on. If pH was initially 8.2, a 50% drop in H+ ions would result in a pH around 7.9. This would also be affected by the drop in alkalinity.

I can't remember the correlation of dKH to meq/L and how that relates to ppm, but I am thinking that a 25% drop in alkalinity, depending on how it is measured, is not as significant as the 5% drop in magnesium. Magnesium is not used up at a rate anywhere near that of calcium.
If I could remember the correlation between ppm and dKh, I could have a better idea about the decrease in alkalinity. I am pretty sure it is two carbonate atoms per calcium atom that gets put into coral skeletons, so estimating a value in the average range for calcium and alkalinity, one could figure out if the alkalinity reduction was approximately in the correct range for the reduction in calcium. Of course sensitivity of the test used and whether or not she can differentiate between whole values would need to be taken into account (like with the Red Sea alkalinity test, which has an intermediate color for the titration, so if one drop goes from starting color to too much titrant color, it is at a half value of dKH).


But the glaring factor is the magnesium dropping by the same amount as the calcium as others have stated.

So are you picking B?

 

[DFW]

C, because the pH would be sky high if hydrogen ions decline by 50 %?

 

[Randy Holmes-Farley]

The sample being 36 hours apart would more than likely put one in the morning and one at night, most likely with the lights off for one of them. If the one in the morning was before the lights came on and the one at night was right before the lights went out, this would show the largest swing. To see that large of a drop, hopefully the second reading was the one that was in the morning before (or right around the time) the lights came on. If pH was initially 8.2, a 50% drop in H+ ions would result in a pH around 7.9. This would also be affected by the drop in alkalinity.

I can't remember the correlation of dKH to meq/L and how that relates to ppm, but I am thinking that a 25% drop in alkalinity, depending on how it is measured, is not as significant as the 5% drop in magnesium. Magnesium is not used up at a rate anywhere near that of calcium.
If I could remember the correlation between ppm and dKh, I could have a better idea about the decrease in alkalinity. I am pretty sure it is two carbonate atoms per calcium atom that gets put into coral skeletons, so estimating a value in the average range for calcium and alkalinity, one could figure out if the alkalinity reduction was approximately in the correct range for the reduction in calcium. Of course sensitivity of the test used and whether or not she can differentiate between whole values would need to be taken into account (like with the Red Sea alkalinity test, which has an intermediate color for the titration, so if one drop goes from starting color to too much titrant color, it is at a half value of dKH).
.

FWIW, any of the normal units of alkalinity can be used to calculate the 25% drop: dKH, meq/L, or ppm calcium carbonate equivalents.

1 meq/L = 2.8 dKH = 50 ppm calcium carbonate equivalents.

 

[Steven@]

C, my best guess is always 100% wrong.

 

[beaslbob]

Interesting that it is a day and half.
As stated above one could be lights on the second lights out which means ph could have risen from 7.9 to 8.4 as (api test kit) did happen in my old 55g before I dosed alk. and probably similar for alk.
Right now I'll stick with b.

 

[beaslbob]

double post

 

[Cory]

My logic says A because most calcium tests kits have noise issues. But I also guess c because that's a big drop in ph(if that's ph). Final answer is C.

 

[Cal]

since i don't know the answer. always pick C

 

[s2nhle]

since i don't know the answer. always pick C

hahaha, what kind of logic is this?

 

[beaslbob]

hahaha, what kind of logic is this?

military tech school logic. :wink:

 

[Skydvr]

Yes, I am going with B, magnesium.

Thanks Randy, I had considered that since those values are not logarithmic like pH, but I wasn't sure if it was a direct conversion (every x increase in ppm converted directly to a y increase in dKH). I have never taken a chemistry class before (well I technically had one in high school, but my teacher did not seem to grasp the concept that he was no longer teaching third grade science. Our sole experiment/demonstration was tasting things to determine if they were a basic or acidic). I have been learning quite a bit from your section here and previous articles. I have had the opportunity to go back to school and I am hoping that I can make some room in my schedule to take a chemistry class or work with the biology department to make some test equipment (I am in my junior year of EE, time is at a premium).

Yeah, C as the default is military logic. Worked in the Navy as well. You'd be surprised at how well it actually works.

PH is logarithmic (base 10), so if you raise ten to an average reef pH reading, you get the number of H+ ions. If you then subtract 50% of that then take the logarithm of that answer, you actually get a rather reasonable drop if the first reading was taken at the end of the day before lights go out when oxygen levels are at their highest due to photosynthesis (and the house being empty most of the day while people are at work/school/etc, and the second reading was taken in the morning around the time, or just before, the lights come on due to organisms taking up oxygen, including photosynthetic organisms, while the lights were off. People would typically also be home at this time contributing to atmospheric CO2, which would contribute to additional dissolved CO2 in the aquarium water, which drives pH down. The decrease in carbonate also contributes to a lower pH.
If we were to take an average pH of around 8.2, the equation would be log[10^8.2 - (10^8.2 * .5)] which would give you a pH of about 7.9. Not to big of a drop. If pH itself dropped by 50%, that would be quite drastic and catastrophic. Another way to look at it is each integer change of pH is a tenfold increase in the H+ ion concentration. Log[10^8.2 * 10] = 9.2 pH. 10^8.2 gives H+ concentration at pH of 8.2

 

[s2nhle]

military tech school logic. :wink:

Bingo. That is my thought and i was right.

 

[Randy Holmes-Farley]

And the answer is...B. Magnesium declined by 5%

A 5% decline in magnesium would be about 65 ppm. At the typical rate that magnesium is depleted (which is about 1/10th the rate for calcium), that would take a couple of months or more in most tanks.

A 5% decline in calcium (about 21 ppm) could happen in 36 hours in a medium to high demand tank.

A 25% decline in alkalinity (about 2 dKH) would happen in many reef tanks in 36 hours or less.

A 50% change in H+ equates to about a 0.3 unit change in pH. That happens overnight in many aquaria.

Happy Reefing!