# CO2 levels in my NPTs



## dwalstad (Apr 14, 2006)

I am currently measuring CO2 levels in all my 7 aquariums. My particular LaMotte kit measures CO2 directly using the titration method and seems to be giving me reliable and interesting information.

Starting CO2 levels (before lights come on) have been between 4 and 13 ppm, depending on tank. Steep CO2 decrease at 2 and 4 hr, but then uptake rate levels off. At the end of 12 hrs, CO2 levels vary from 0 to 5 ppm depending on the tank. In comparison, most High-tech tanks are maintained with _consistent_ levels of 13 ppm to about 30 ppm CO2.

My 50 gal (see recent photo) is doing very well with 5 ppm before light and about 2-3 ppm after 12 hr light. Almost no algae. However, I see that _Rotala macrandra_ is not quite as vigorous as when tank was set up. [Demanding plants like the beautiful red-leaved _R. macrandra _ thrive in my new NPTs where there's plenty of CO2 released by decomposition of fresh soil organic matter.]

My Kitchen 55 gal is doing okay but had algae (diatoms?) on glass. Measurements were 4 ppm (before lights on) and 1 ppm after 12 hr of light. A few days later and after scraping algae off glass, CO2 was 6 ppm (before lights on) and 2.2 ppm after 12 hr of light.

In general, algae can get CO2 easier than plants. Simply by cleaning glass, there was more CO2 for plants. Also, the algae I scraped off probably released a little CO2 during its decomposition in the tank.

This suggests to me that it is worth it to do a little algae removal.


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## Emily6 (Feb 14, 2006)

That's a really cool mini-study! Are there organisms alive in bio-film which would compete for nutrients against plants?


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## JeffyFunk (Apr 6, 2006)

dwalstad said:


> I am currently measuring CO2 levels in all my 7 aquariums. My particular LaMotte kit measures CO2 directly using the titration method and seems to be giving me reliable and interesting information.


I saw a similar titration kit from Hach which also "measures" CO2 levels by titration w/ an alkali reference solution. How accurate are they? I know that, generally speaking, titrations are usually pretty accurate for simple systems (i.e. lab experiments), but I wonder how they fare in systems that are not made from DI water? Do you know how much organic acid(s) you have for example? I would expect organic acids (and Total Organic Carbon in general) to also have an effect on both the analysis results and the available carbon that the plants have access to. Is it even possible to distinguish the results from the titration on the CO2 and the other acids, perhaps via. looking at a titration curve of pH vs. titrant amount? Just curious as to the background on analysis method here and thought that you, Ms. Walstad, would have some information on the methodology.


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## dwalstad (Apr 14, 2006)

JeffyFunk said:


> Just curious as to the background on analysis method here and thought that you, Ms. Walstad, would have some information on the methodology.


Actually, I do have information on the CO2 titration method. It is fully described by the AWWA (American Water Works Assoc) textbook.* The bicarbonate buffering system typically predominates in normal waters over minor buffers like phosphates, silicates, humic acids, etc. I don't see anything in my tapwater that would invalidate the results of my test. For example, textbook warns against ferrous iron levels greater than 1 ppm, high salt content of seawater, and buffer ions that exceed 5% of the CO2 concentration.

Before purchasing the LaMotte kit I made my own "kitchen recipe" with a Phenolphthalein indicator and the titrant (0.02N NaOH) described in the textbook. All you do is add colorless indicator to sample and then start adding the titrant drop-by-drop until solution turns pink. Pink color for phenolphthalein means you've reached pH 8.3 and all the CO2 in the water has been converted to sodium bicarbonate. Phenolphthalein added to a solution of baking soda (sodium bicarbonate) will turn pink immediately, because pH is automatically 8.3.

The kitchen measurements basically told me the same thing about my tanks and weren't too far off from the LaMotte kit.

However, the LaMotte kit allowed me to put an actual number (mg/l or ppm) on CO2 and confirm what I had been seeing earlier. While the information is relevant to me and my tanks, I'm not sure that someone else testing their own tanks would get the exact same values. For example, when the solution turns pink is going to vary depending on who is doing the testing. There's no way that I could tell anyone else what level of pink is "pink". But I've done it enough times that I have a very good idea of when pink is "pink"-- for me.

Distilled water equilibrated with air in my home invariably gives me 2 to 2.5 ppm CO2. I use that to gauge my testing consistency week to week.

I decided against the KH/pH method. I don't like the fact that you've got to measure two things and consult a chart to get your answer. Every time you measure something visually like this, you introduce another potential for error. With the titration method, standardizing the pink color is all I have to worry about. 

___________________
*Greenberg AE, Clesceri LS, and Eaton AD (Eds). 1992. Standard Methods for the Examination of Water and Wastewater. (18th Edition). American Public Health Association (Washington, D.C.)


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## JeffyFunk (Apr 6, 2006)

dwalstad said:


> Actually, I do have information on the CO2 titration method. It is fully described by the AWWA (American Water Works Assoc) textbook.* The bicarbonate buffering system typically predominates in normal waters over minor buffers like phosphates, silicates, humic acids, etc. I don't see anything in my tapwater that would invalidate the results of my test. For example, textbook warns against ferrous iron levels greater than 1 ppm, high salt content of seawater, and buffer ions that exceed 5% of the CO2 concentration.


Hmm... I have a copy of the Standard Methods textbook @ work... I'll check it out this week. That said, I wonder how different your aquarium water is in comparison to tap water. I know that the standards for "stuff" in drinking water is typically very low. Also, I would think that for a low tech tank that there would be a significant quantity of total organic carbon (TOC) for the plants to uptake. For a high tech tank, I would think that there would be a significant quantity of phosphates in the water column. E.I dosing suggests target levels of up to 2 ppm. Although this is probably higher than what's in a low tech tank, the CO2 concentration would also be much higher.

[If any high tech people have measured CO2 levels via alkali titration, please chime in here and let us know about the results and if the presence of phosphates (or any other potential interferences) affect the results. Not to divert this thread into a random tangent, but I do think it's important to possibly discuss when talking about the analysis of CO2.]


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## dwalstad (Apr 14, 2006)

I'll be interested to hear what you have to say. Everyone I've talked to seems as confused as I am. I suspect that many things will interfere with free CO2 measurement.

Perhaps the important thing may be the _difference_ between readings before and after 12 hr light in each tank. That difference _has_ to be free CO2, because it represents CO2 taken up by plants/algage for light-fueled photosynthesis. Number may not represent all free CO2 because plant growth may not be good enough to take up all CO2. Nevertheless, it gives one some idea of how much free CO2 is actually in the tank.

Tricky stuff!


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## JeffyFunk (Apr 6, 2006)

Okay, i had a chance to read both the Standard Methods Handbook on CO2 analysis via titration and the Hach Water Analysis handbook on their CO2 analysis kit. Both of them say essentially the same thing (although the Standard Method's handbook is a little more detailed on the background and potential interferences). Again, my concern would be the fact that this is simply a titration and limited in scope by the aquarium water matrix.

Indeed, it has been noted that these kits for CO2 analysis are pretty much only suited for drinking water matrices. See the following online observation:

http://www.thekrib.com/Plants/CO2/test-kits.html

Another thing to keep in mind is that alkali solutions may go bad over time due to the absorbence of CO2 from the atmosphere, lowering the actual concentration of hydroxide anions in solution!

The only other information i could really find about measuring CO2 in the aquarium was over at The Barr Report. Tom Barr reports measurements of CO2 is high tech tanks as a function of location relative to where the CO2 distribution point is and water circulation:

http://www.barrreport.com/co2-aquat...-ppm-readings.html?highlight=CO2+measurements

In this posting, he reports that CO2 concentration was measured in situ using the following device:

http://www.w-m-t.com/Products/Oxyguard/CO2_Portable.php

[I was trying to figure out how exactly this device works and what type of biases it may have, but that's beyond the scope of my ability as a function of work and time at this moment... If anyone would care to explain how this works and what type of potential interferences it may have, please do.]

The observations that he makes that really stick out to me are (1) the importance of good water circulation for getting CO2 to the plants (for High Tech and Low Tech tanks) and (2) that the concentration of CO2 is dependent upon location in the tank relative to the CO2 input location (for High Tech tanks mainly). As far as I can tell, he does not measure CO2 concentrations as a function of time during the day (as you have).

I guess my take away message from all of this would be to (1) accept that there are going to be interferences in the CO2 titration due to the aquarium matrix and the nature of acid/base titrations, (2) just try to use the data to point to any trends or qualitative observations about CO2 concentrations in the aquarium as a function of time, paying attention to where you sample the water from and (3) note when you do water changes as that would be expected to dramatically change the aquarium matrix, not only from the addition of fresh CO2 but also from the removal of titration interferences.


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## JeffyFunk (Apr 6, 2006)

Credit to Tom Barr for finding the analytical paper that I wish I had found myself in their own discussion of this topic (http://www.barrreport.com/general-plant-topics/6399-co2-meter-low-tech.html). Here's the link to a wet chemistry method cited in the above forum post for the determination of CO2 levels in water that is able to effectively compensate for any interferences that may be present.

http://pubs.acs.org/doi/abs/10.1021/i560129a012

Essentially, the method involves the titration of the water sample measured between two pH values (in this case, between pH 4.4 to 8.3 using Methyl Red & phenolphthalein indicators, respectively). The water sample is then boiled to remove any dissolved CO2 present, cooled to ambient temperature, and the titration repeated. The difference in the amount of titrant used is proportional to the the quantity of dissolved CO2 present in the water sample.


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## dwalstad (Apr 14, 2006)

JeffyFunk said:


> I guess my take away message from all of this would be to (1) accept that there are going to be interferences in the CO2 titration due to the aquarium matrix and the nature of acid/base titrations, (2) just try to use the data to point to any trends or qualitative observations about CO2 concentrations in the aquarium as a function of time, paying attention to where you sample the water from and (3) note when you do water changes as that would be expected to dramatically change the aquarium matrix, not only from the addition of fresh CO2 but also from the removal of titration interferences.


Hello JeffyFunk,

It took some time to digest all that you've written.  I agree with you that the titration method would be influenced by many things. Now, I accept relevance of the book's nomograph where final CO2 calculation is influenced by total dissolved salt (TDS), pH, temperature, and alkalinity.

For my CO2 measurements, I took all samples from the mid-tank-- same place every time. I did not take measurements until two days had passed after _any_ tapwater additions (my tapwater contains 13 ppm CO2; after its degassed 2 ppm). You're right that water additions would have skewed the results.

My 50 gal has been set up the longest (16 mon) and is very stable. CO2 measurements taken on 3 separate days over a period of 10 days are almost identical. Mean values show 5.5 ppm (before lights) and 2.5 ppm 12 hr later with lights. Plants take most of it during the first 4 hr (CO2 is 3 ppm at 4 hr timepoint).

I'm sure that fish and bacteria are generating some CO2 during that 4 hr, but not that much. (If they were generating a lot, my starting measurement (only 5.5 ppm) would be _much_ higher.) Thus, I conclude that my 50 gal NPT doesn't have that much CO2 and my plants aren't really taking up that much after all. That's _sort of _interesting. 

Most importantly, I want to thank you so very much for the time you've put into answering my questions. You have given me the information (LaMotte correspondence, Barr Report, etc) that I needed to make sense of the literature. I will keep this in mind when I bring my data all together.

Diana


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