Refinement for CO2 Measurement Technique - The Planted Tank Forum
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post #1 of 18 (permalink) Old 08-03-2006, 03:24 AM Thread Starter
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Refinement for CO2 Measurement Technique

On Tom Barr's website, http://www.barrreport.com/forums/, we have been discussing how to get a more accurate measure of ppm of CO2 in our tanks. I think Tom came up with a winning idea:

The problem with the 1.0 pH drop technique is that it depends for accuracy on the "degassed" water having 3 to 4 ppm of CO2 in it, but testing has shown that not to be a good assumption. In fact, it looks like it takes 48 hours or more to get the pH to stop rising in the "degassing" sample. So, here is a way to work around this.

1. Measure the pH of the water in the tank. Record it.
2. Take a sample of water from the tank and let it sit in the open for 48 hours. The sample needs to be big compared to how much water will evaporate in that time.
3. Make up a standard solution of RO water with bicarbonate of soda in it to get a small KH. Take a same size sample of this water and let it sit alongside the tank sample to outgas at the same time for the same time.
4. Add a third sample, this time of some RO water with nothing added to it and let it also sit for the same time in the same area.
5. Add enough of the third sample to the tank water and standard solution water to replace what has evaporated. (This is to restore the original KH)
6. Measure the pH and KH of the standard solution sample. Find how much CO2 is in that water, based on the standard equations/ table on Chuck Gadd's website, for example, and record it.
7. Measure the pH of the tank water degassed sample and record it.
8. Now you can calculate what your tank water pH, measured in step 1 indicates by using the following equation:

Tank Water CO2 ppm = CO2 ppm from step 6 times 10exp[pH(step 6)-pH(step 1)]

Most computers have a good calculator built in that will do this calculation easily. But, if it is a problem for anyone, just provide the numbers here and some of our good number crunchers will do the calculation for you.

Write down the ppm of CO2 you found in step 6 where you won't lose it, and you won't have to repeat all of the above steps for further tests. Just let the tank water sample sit out in the open along with some RO water for 48 hours. Add enough of the degassed RO water to the tank water sample to replace the evaporated water, measure pH and repeat the calculation per the equation above.

I have to admit I haven't tried this yet (sorry about my red face). But if a lot of us do try it we should get a good idea how effective this is. Note, that it still depends on making a very accurate measurement of three pH's and one KH, so a pH meter and good KH kit are almost essential.

Hoppy
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post #2 of 18 (permalink) Old 08-03-2006, 01:07 PM
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Geez O Pete!!!

That is a serious method!!

And I thought people in Tom's camp didnt like testing and fiddling with numbers?

HAHA

jB
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post #3 of 18 (permalink) Old 08-03-2006, 04:02 PM Thread Starter
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I don't like testing. But, people tend to put way too much trust in the pH/KH method for measuring CO2, and that method nearly always gives a number that is higher than the actual ppm of CO2, so they are reluctant to raise their CO2 bubble rate. This is just a way to try to get closer to the actual ppm of CO2 in the tank. I think Tom has been consistent in saying if you are going to do testing, do it right, with calibrations where necessary.

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post #4 of 18 (permalink) Old 08-03-2006, 04:02 PM
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At issue is the measurement of CO2 pm without relying on KH on the tap or the tank(How many here have reported 200ppm CO2 and healthy fish plants? Or whining about ADA As and not being able to measure CO2-generally from those who are not using ADA AS :-) ?).

I'm no fan of the equation for the hobbyists, but it does work.
But I'm not suggesting folks do that unless they are all into that.

I just make a simple ref solution. DI water and a KH of 1.
Then take the tank sample.

Wait 48 hours, measure the difference in pH.
I make an assumption: the partial pressures of CO2 will be the same for both samples.

That is a reasonable assumption.

Any reduction of the pH in the tank sample is then assumed not to be due to CO2 gas. also, a reasonable assumption.

So say the difference between the tank and ref sample is .5 pH units.
You can look on the pH/KH chart and see what the reduction in pH will be required.

So say the pH/KH of the ref solution is:
7.8 pH/KH= 1 and you need to add enough CO2 to get 30ppm which will be about 6.0 pH.

7.8 is the ref sample and the tank sample measures at 7.3 pH.

7.3-1.8 = 5.5 pH is your targeted pH for 30ppm.


That's it which is a simple method to get around things like whacky KH issues and 100-200ppm supposed CO2 readings, ADA or peat based reductions that skew the pH/KH table.

If the pH KH works well for you, stick with it, this is for folks havign troubles with that method and still want to measure the CO2.
1. Make sure to top off with DI water for the ref smaple and tank sample after 48 hours , evaporational losses will influence things, also use a good size sample, say 500mls, not 5mls which will be entirely evaporated after 2 days in many places.

There is also another method I use to measure CO2 in tanks with no KH at all.
It relies on another assumption, the CO2 ppm addition rate is the same between making a tank have a KH= 1 and then do a water change/s soon thereafter and knock the KH down to zero or very close it. The CO2 ppm injection rate and rate of plant uptake will be the same in both cases, you just will not be able to measure the pH/KH once the KH goes to zero, but this is a indirect method also and gets around the measuring need for KH for the actual Planted tank itself and relies on ref sampling, either for test or use the entire tank for the reference.

It's just an alternative method to avoid some pitfalls folks are now recently realizing, but uses the pH/KH method as a back up reference still.

I'm not a fan of tweaking the fish to the surface, then backing off a tad for a method. Many do it, but there's something none too ethical about it either.

Anyway, I wanted to know what the CO2 was or at least pretty close and the many folks on these forums that report 100-200ppm of cO2 ands healthy fish, used pH calibrated probes, good KH test kits etc and the folks still have issues unrelated to tannins, namely from non carbonate alkalinity I'd suspect, can figure a way around this as well as folks running no KH tanks.

It's certainly not nearly as hard nor as confusing as it sounds, much like making a CO2 reactor.

Regards,
Tom Barr




Regards,
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post #5 of 18 (permalink) Old 08-03-2006, 05:14 PM
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I am sitting here trying to figure this out and why you cant just calculate the co2 by the pH change of standing tank water alone?

Want to see more...check out my website
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post #6 of 18 (permalink) Old 08-03-2006, 05:34 PM
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Quote:
Originally Posted by plantbrain
It's certainly not nearly as hard nor as confusing as it sounds, much like making a CO2 reactor.

Regards,
Tom Barr
If you can do that, you could certainly DIY a CO2 probe using your existing pH probe. I know this was published as a chemistry education article at some point - I have a copy somewhere but can't lay my hands on it right now. Basically, a membrane covers the end of the pH probe. There is a solution between the membrane and the probe. CO2 diffuses across the membrane, changing the pH of the solution. The probe is calibrated in air and something else (don't remember what). It has a linear response (mV) to CO2 concentration. Since the internal solution is fixed, no worries about different sample KH's, TDS, etc. Haven't done it myself yet, but seems like a reasonable thing (also don't have a CO2 reactor either . . .) Then your only math would be a calibration curve (slope, intercept).

K

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post #7 of 18 (permalink) Old 08-03-2006, 06:22 PM
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Just to broaden the discussion, the majority of pH meters for CO2 seem to read in 0.1 increments (which is much better than "test kits"). In the world of Log functions this equals 1 significant digit (the first digit is the power). A single measurement you may measure CO2 to be 30+-10 ppm. With two measurements the error 0.2 units or 20ppm above etc.

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post #8 of 18 (permalink) Old 08-03-2006, 07:24 PM
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Quote:
Originally Posted by KevinC
If you can do that, you could certainly DIY a CO2 probe using your existing pH probe. I know this was published as a chemistry education article at some point - I have a copy somewhere but can't lay my hands on it right now. Basically, a membrane covers the end of the pH probe. There is a solution between the membrane and the probe. CO2 diffuses across the membrane, changing the pH of the solution. The probe is calibrated in air and something else (don't remember what). It has a linear response (mV) to CO2 concentration. Since the internal solution is fixed, no worries about different sample KH's, TDS, etc. Haven't done it myself yet, but seems like a reasonable thing (also don't have a CO2 reactor either . . .) Then your only math would be a calibration curve (slope, intercept).

K

pH can also be measured by measuring the cell potential of the sample:
(ε) is the electromotive force (EMF) or cell potential of a galvanic cell.
The formula for pH was derived from the application of the Nernst Equation to concentration cells, or galvanic cells where the half cells are at different concentrations.

So using a mV meter works fine once calibrated for CO2.

That should work well also, better than pH really.
That's how we measure other ions like NH4, NO3 etc with ISE meters.

Make a set of known standards and correlate these with the mV readings.
Same with the pH meter and the calibration solutions.


Regards,
Tom Barr




Regards,
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post #9 of 18 (permalink) Old 08-03-2006, 10:04 PM
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I will likely try this, as I have been failed numerous times with the pH controller method. I have it as ny back up right now, but I only keep my pH controller and CO2 on s timer to come on right before the lights, then turn off right after. I have no CO2 running when the lights are off, period. I woke up to too many gasping fish. I am not sure if this is adding to recent bout with hair algae, though it is small, but I see no reason to run the CO2 when there is no photosynthesis happening. I use the chart too, but I think it is an approximation at best. This is an interesting alternative.
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post #10 of 18 (permalink) Old 08-03-2006, 11:35 PM
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Well, if you use pH controllers, this can help you target a good ph range independently of tannins influence or non carbonate alkalinity(provided it stays stable in the tank, but at least you may check that with this).

This is a good reason not to use a pH controller along with folks not keeping better tabs on the their KH even if it is fine and they don't have any wood/peat, ADA AS, weird tap water KH issues etc

Once you set the flow rate to achieve a certain CO2 ppm, then the valve is set, no pH/KH changes will change that and simply turning the CO2 off at night is a simple process.

As long as the CO2 gas being added is stable, the pH/Kh/tannins can bob all over the place.

A pH controller can only account for pH, nothing more.
And the pH controller needs to be set at a higher CO2 bubble rate than you'd use if did 24/7 CO2 injection without one, this way the system can throttle on/off to target the CO2 ppm that you want , if you set the CO2 bubble rate too low using a pH controller, it does not matter what you do, that pH controller will never add enough nor be responsive enough.

It takes time for the pH controller to respond to the rising pH.
You only have one location for the pH probe and a larger tank will have some spots that are not a rich in CO2, you can see this is many larger tanks, the outflow closest to a CO2 reactor, the plants all pearl very strong and farther away they don't.




Regards,
Tom Barr




Regards,
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post #11 of 18 (permalink) Old 08-03-2006, 11:54 PM Thread Starter
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Quote:
Originally Posted by fresh_newby
I will likely try this, as I have been failed numerous times with the pH controller method. I have it as ny back up right now, but I only keep my pH controller and CO2 on s timer to come on right before the lights, then turn off right after. I have no CO2 running when the lights are off, period. I woke up to too many gasping fish. I am not sure if this is adding to recent bout with hair algae, though it is small, but I see no reason to run the CO2 when there is no photosynthesis happening. I use the chart too, but I think it is an approximation at best. This is an interesting alternative.
Do you have a powerhead in the tank to provide more water circulation? Or do you adjust the filter return so as to make sure there is good water circulation? If you don't do either you can have high CO2 concentration in part of the tank and low in other parts, and that may contribute to the fish gasping you see. With good circulation you get good oxygen content throughout the water too.

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post #12 of 18 (permalink) Old 08-04-2006, 03:45 PM
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Found the article: DIY CO2 sensor. It actually measures the total of all CO2 species: CO2(aq) + HCO3-(aq) + CO3-2(aq), but at pH=5 (buffer added to samples), 97.5% of the total is CO2(aq).

S. Kocmur, E. Corton, et al, "CO2 Potentiometric Determination and Electrode Construction, a Hands-on Approach", Journal of Chemical Education, Vol 76 No. 9, Sept 1999, pp. 1253-1255.

Summarizing: A gas-permeable membrane is wrapped around the end of a pH electrode (0.1mm thick teflon or silicone). A rice paper (cigarette paper) soaked in an electrolyte (sodium chloride, sodium bicarbonate) solution is placed between the membrane and the electrode.

Ionic strength and pH are adjusted in the samples/standards by a concentrated sodium citrate buffer (pH=5). Standards are sodium bicarbonate solutions. The calibration curve is delta-E (change in mV reading compared to a zero-bicarbonate solution) versus pCO2 (-log of CO2 concentration). delta-E is measured for an unknown and pCO2 is determined from the calibration curve. They show a linear calibration between 1.4ppm and 140ppm - but they also measure freshly opened soda (approx 4200ppm CO2!)

Unfortunately, this is still a non-deployable probe: Samples must be removed from the aquarium and added to the buffer solution. Doesn't seem any harder than the proposed method though.

Perhaps a DIY journal is in my future . . .

Kevin

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post #13 of 18 (permalink) Old 01-22-2007, 04:30 AM
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I hate to resurrect an old thread, but I'm interested in what was said here. I plan on perfroming both tests; the one hoppy suggests and the one Tom Barr added later in the thread. The only problem I have is figuring out what RO water and DI water is. Help?

I have other questions that I was going to start a new thread with, but I'll ask it here in hopes that this thread will get read again.

I started my co2 on 01/18/07 at one bubble every 6 seconds and dosing on 01/19/07. I measured my ph/kh on Friday to get: 6/5. On Saturday I kicked the co2 up to one bubble every 2 seconds. Tonight, 01/21/07, I measured ph/kh to get: 5/1, then dosed and slowed the co2 down to a bubble every 4 seconds. I know I'm overdoing it on the co2, but I have no fish and wanted to get things kick started and stave off any algae. Plus, I have some Tonina fluviatilis and
Eriocaulon Cinereum that I'm under the impression as needing low ph (more so the Eriocaulon, for as far as I can tell is picky and needs a ph of between 5 - 5.6).

I guess my misunderstanding is that co2 shouldn't change kh, but yet my kh dropped quite a bit? Also, once I figure out how to measure and get dialed in to 30ppm of co2, what will a 50% water change do to co2 levels and how long does it take for the levels to be restored after a water change, assuming the co2 input is the same?

I hope my questions get attention here, since it's related to this thread and I hate starting new threads when there are already perfectly good ones to work from.

By the way, I think my bacopa and ancharis grew over an inch in one day! No love from the riccia though (as far as I can tell).

**Edit**

I guess I should include my dosing if I'm asking questions about kh:

+/-1/8Tsp-KN03 3x a week
+/-1/32+Tsp-KH2P04 3x aweek
About 15 grains of CSM+B (I'm not slick enough to get this into a ml does, so I do a very light "smidgeon").

And while I'm at it, should there be a "target kh" for the long run? And once my co2 is at 30ppm, I'd guess that my ph will rise above 6, so how would I keep ph down?
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post #14 of 18 (permalink) Old 01-22-2007, 05:47 PM
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Damn... I'm looking at Tom's example of of a ref ph of 7.8 and tank ph of 7.3. I only have whole numbers to use on my test kit, and I wouldn't trust myself eyeballing decimal increments. There has got be a less frustrating way...
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post #15 of 18 (permalink) Old 01-22-2007, 11:40 PM
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Quote:
Originally Posted by edacsac View Post
Damn... I'm looking at Tom's example of of a ref ph of 7.8 and tank ph of 7.3. I only have whole numbers to use on my test kit, and I wouldn't trust myself eyeballing decimal increments. There has got be a less frustrating way...
For this kind of work, get a pH meter. The cheap ones on Ebay are fine - you get a resolution of 0.1 pH units. Is your test kit an aquarium kit? - every 1 pH unit is way too coarse.

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