How to measure CO2 amount in my tank?

[liuhao]

i see this question have been floating around for some time but still no answer so far. so here i raise it again to see if it indeed has no answer.

i know we can use KH and PH to calculate CO2 amount in a tank where CO2 injection is only acidic source.

however, in a tank with peat based soil such as Amazonia, the KH and PH chart is not valid anymore, due to presence of tannin and etc. same for a tank with add in acid buffer.

for my tank, when there is no CO2 injected, the PH is 6.67 (measured using a carefully calibrated PH Pen) and KH is 4 (AP testkit). how could i measure the CO2 amount when CO2 is injected?

i was searching for a CO2 testkit but just now i am shocked to see that both Hach and LaMotte testkit wont be able to measure CO2 amount accurately when tannin is present (see http://www.thekrib.com/Plants/CO2/test-kits.html). they are basically screwed up by tannin and any other acid source too.

for the valid KH and PH chart,

co2 (in PPM) = 3xKHx10(7-PH)

i am thinking of a modified version like:

co2 (in PPM) = 3xKHx10(PH0-PH) where PH0 is the PH value when there is no CO2 is injected. for my case, it would be
co2 (in PPM) = 3xKHx10(6.67-PH). but i have no way to validate it because no testkit can measure.

but i am almost sure that it doesnt work in that way because co2 change and PH change is not linear like that.

any advise?

[liuhao]

there is a very inaccurate way of determain ph value when co2 is 10-20ppm in your water (any kinda water). i got it from http://www.thekrib.com/Plants/CO2/co2-meter.html

here it is:

Krause describes a method in his book on aquarium water that is supposed to work with any kind of water. Not absolute pH is the key, but the change of pH by two units is used to determine correct CO2 concentration.

Take a sample of your water and aerate it for some time until all CO2 is removed. In that case the concentration of CO2 is in equilibrium with the surrounding air (0.6ppm). Measure pH of the water (=X).

Next exhale through a pipe into the water sample. After a while the concentration of CO2 in the water will assume 60ppm. Measure pH of the water (=Y).

The optimum CO2 concentration of 10-20ppm is at the pH value about 2/3 of the difference between X and Y: pH,opt = X +.67*(Y-X).

This will work even with buffered water, although the change in pH might be small and only detectable with an electronic pH meter.

but...i need a more accurate way, please.

[liuhao]

another method from: http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html

log(CO2) = pH(a) - pH(t) - 0.3

PH(a) is when no CO2 injected, PH(t) is when CO2 is injected.

but personally i dont think it makes sense at all. because you cant calcuate amount of co2 simply based on delta of PH change. there is another factor like KH that also affects PH dissolving. i.e., same delta of PH but different KH will lead to difference CO2 amount.

[bclee]

[quote:94e561b628="liuhao"]another method from: http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html

log(CO2) = pH(a) - pH(t) - 0.3

PH(a) is when no CO2 injected, PH(t) is when CO2 is injected.

but personally i dont think it makes sense at all. because you cant calcuate amount of co2 simply based on delta of PH change. there is another factor like KH that also affects PH dissolving. i.e., same delta of PH but different KH will lead to difference CO2 amount.[/quote:94e561b628]

Same delta pH irregardless of KH (except for KH=0) will always give you fix amount of CO2 concentration. Trust me, I did the calculations.

BC

[bclee]

BTW, I feel that accuracy is limited to your pH and KH test kits. If you want to do the calculations, I can provide you with all the Henry's law constants and the equilibrium constants. With the accuracy of the KH kit at ±0.5°KH, any more accurate equations is meaningless.

BC

[liuhao]

[quote:43736491d6="bclee"][quote:43736491d6="liuhao"]another method from: http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html

log(CO2) = pH(a) - pH(t) - 0.3

PH(a) is when no CO2 injected, PH(t) is when CO2 is injected.

but personally i dont think it makes sense at all. because you cant calcuate amount of co2 simply based on delta of PH change. there is another factor like KH that also affects PH dissolving. i.e., same delta of PH but different KH will lead to difference CO2 amount.[/quote:43736491d6]

Same delta pH irregardless of KH (except for KH=0) will always give you fix amount of CO2 concentration. Trust me, I did the calculations.

BC[/quote:43736491d6]

there must be somewhere wrong, because the PH(t) calcuated from above equation is too far away from PH/KH chart.

let's say my KH=6, PH=7.4 without CO2 injected and any acid buffer. i wanna achieve 30ppm of CO2. therefore, the PH(t) i calculated from above equation is:

PH(t) = PH(a) - 0.3 - log(co2) = 7.4-0.3-1.477 = 5.6

however using PH/KH chart, i got:

PH(t) = 7 - log(co2/(3*kh)) = 6.77

how could be such big difference?

[bclee]

[quote:5af6fc5592="liuhao"]let's say my KH=6, PH=7.4 without CO2 injected and any acid buffer. [/quote:5af6fc5592]

Your statement here is already wrong. At KH 6, your pH will not be 7.4 without CO2 injection.

Without CO2 injection and buffers, at KH6, your pH should be around 8.6~8.7.

You need to take the delta pH between water in equilibrium with atmosphere and water with CO2 injected.

BC

[liuhao]

[quote:cefaceb114="bclee"][quote:cefaceb114="liuhao"]let's say my KH=6, PH=7.4 without CO2 injected and any acid buffer. [/quote:cefaceb114]

Your statement here is already wrong. At KH 6, your pH will not be 7.4 without CO2 injection.

Without CO2 injection and buffers, at KH6, your pH should be around 8.6~8.7.

You need to take the delta pH between water in equilibrium with atmosphere and water with CO2 injected.

BC[/quote:cefaceb114]

yeah sorry my statement was wrong indeed. but my numbers are correct. my KH is indeed 6, and PH is 7.4 without CO2 injected but WITH acid buffer (possibly tannin and etc from Amazonia Soil).

are you sure that i need to inject CO2 all the way until PH down to 5.6 from 7.4? i need some sorta support from people before i do that. LOL. no obligation really.

thanks.

[bclee]

I have stop using the test kits myself. I adjust my CO2 slowly over a few days lowering the pH little by little watching closely the fishes reaction. If you already have buffers other than carbonates in the water, it makes calculation difficult. All the equations you mentioned will not apply.

I think you can still safely lower your pH down to about 6.2 or 6.0. However, do that slowly, observing the fishes closely. If at that pH the plants are doing ok and your tank is algae free, keep it there. If not, you just have to push the envelop a little further at a time.

BC

[liuhao]

[quote:3981428756="bclee"]I have stop using the test kits myself. I adjust my CO2 slowly over a few days lowering the pH little by little watching closely the fishes reaction. If you already have buffers other than carbonates in the water, it makes calculation difficult. All the equations you mentioned will not apply.

I think you can still safely lower your pH down to about 6.2 or 6.0. However, do that slowly, observing the fishes closely. If at that pH the plants are doing ok and your tank is algae free, keep it there. If not, you just have to push the envelop a little further at a time.

BC[/quote:3981428756]

thanks for your advise.

i cant validate the equation due to my limited knowlege. my wife is a chemistry phd. i believe she can do it if she dedicates her time but so far she is not very supportive for my hobby. hehz.

anyway, i will just do according to the equation. PH is easy to measure bcos i got a PH meter. CO2 i got plenty. plants will be happy for sure but may not be fish. anyway no pain no gain. LOL.

[liuhao]

set my bubble rate to 5bps (for 3 feet tank), it's fairly easy to push PH down from 7.45. when it reaches 6.7, can see plants start bubbling. but when it's down to 6.35, the PH dropping gets slower. i wonder if it can ever go down to 5.7 under 5bps. hehe...

my co2 dissolving is quite efficient. i am using an external reactor. will report the result.

[goh]

Liuhao,
Below is how you can measure the CO2 level if PH has been affected by peat or other stuff:

Aerate some water from tank for about 1 hr. Measure PH/KH:
Eg. PH = 7.0 / KH = 4 / CO2 = 12ppm (from chart)
But CO2 should be ~ 3ppm at equilibrium. Hence the CO2 level is inflated by 12 - 3 = 9ppm.

Now measure PH/KH of tank with CO2 injection:
Eg. PH = 6.6 / KH = 4 / CO2 = 30ppm (from chart)
But the true CO2 should be = 30 - 9 = 21 ppm

[PeterGwee]

[quote:22fa2092="liuhao"]set my bubble rate to 5bps (for 3 feet tank), it's fairly easy to push PH down from 7.45. when it reaches 6.7, can see plants start bubbling. but when it's down to 6.35, the PH dropping gets slower. i wonder if it can ever go down to 5.7 under 5bps. hehe...

my co2 dissolving is quite efficient. i am using an external reactor. will report the result.[/quote:22fa2092]

You seem to have a wide variation in your CO2 readings. A high flow external reactor with good in tank mixing will reduce the variation that you will see and also faster response time. Most folks did not really looked into this critical issue and start barking up the wrong tree.

Regards
Peter Gwee

[bclee]

[quote:3d601c36a4="goh"]Liuhao,
Below is how you can measure the CO2 level if PH has been affected by peat or other stuff:

Aerate some water from tank for about 1 hr. Measure PH/KH:
Eg. PH = 7.0 / KH = 4 / CO2 = 12ppm (from chart)
But CO2 should be ~ 3ppm at equilibrium. Hence the CO2 level is inflated by 12 - 3 = 9ppm.

Now measure PH/KH of tank with CO2 injection:
Eg. PH = 6.6 / KH = 4 / CO2 = 30ppm (from chart)
But the true CO2 should be = 30 - 9 = 21 ppm[/quote:3d601c36a4]

Errr.... no... this is not an accurate way of calculations. If you understand the science and math behind that, you will know why.

There are many factors affecting the accuracy. I just point out one.

When there is buffer in the water, the KH reading you measured will be totally out from the actual KH (concentration of carbonates). This is because the test kits we have uses titration method to measure KH. Weak acids in the water will totally throw off the reading and the KH reading do not reflect the true concentration of carbonates.

The CO2 charts we have uses the pH and KH values for calculations based on the following reactions.

CO2(g) + H2O <---> CO2(aq) + H2O <---> H+ + HCO3- <---> 2H+ + CO3--

The KH value need to be actual carbonate concentration for the CO2 ppm to make any sense at all. In this case, you will not get the carbonate concentration.


BC

[liuhao]

[quote:e6f90b4f67="liuhao"]set my bubble rate to 5bps (for 3 feet tank), it's fairly easy to push PH down from 7.45. when it reaches 6.7, can see plants start bubbling. but when it's down to 6.35, the PH dropping gets slower. i wonder if it can ever go down to 5.7 under 5bps. hehe...

my co2 dissolving is quite efficient. i am using an external reactor. will report the result.[/quote:e6f90b4f67]

after two days of experiment, i dun think i can push ph down to 5.7. it's just too difficult. even my bubble rate was set to 8bps...the lowest i can go is 6.20. and i still believe my reactor is very efficient.

just dont know how much co2 i have at PH 6.20.

this evening, i add some NaHSO4 in to my tank (to lower my KH a little bit. my KH is almost 7). the PH goes down to 6.00. suddenly i see my tonina's bubbling accelerated like hell. well i would call it shooting rather than bubbling. i have never seen it's bubbling like that. a bit scary though. i guess the NaHSO4 converts all the dissolved CaCO3 into CaSO4, Na2SO4 and CO2. probably the released CO2 suddenly push the water CO2 level to very high level. hehz.

[bclee]

[quote:9365b3773d="liuhao"]
after two days of experiment, i dun think i can push ph down to 5.7. it's just too difficult. even my bubble rate was set to 8bps...the lowest i can go is 6.20. and i still believe my reactor is very efficient.

just dont know how much co2 i have at PH 6.20.

this evening, i add some NaHSO4 in to my tank (to lower my KH a little bit. my KH is almost 7). the PH goes down to 6.00. suddenly i see my tonina's bubbling accelerated like hell. well i would call it shooting rather than bubbling. i have never seen it's bubbling like that. a bit scary though. i guess the NaHSO4 converts all the dissolved CaCO3 into CaSO4, Na2SO4 and CO2. probably the released CO2 suddenly push the water CO2 level to very high level. hehz.[/quote:9365b3773d]
How are your fishes coping?

What type of filtration do you have? Do you have agitation at the water surface? Surface agitation will cause you to lose CO2 even if you are able to get CO2 dissolve efficiently.

You should not add any more NaHSO4. Na if accumulated in the tank will cause problems to your plants. Yes, NaHSO4 being acidic will cause more CO2 to be release from the carbonates. That is only temporary. The CO2 will fall to the equilibrium level after a while.

Why do you have NaHSO4 around? It is quite acidic and corrosive. Be careful when handling it.

BC

[liuhao]

fishes seem fine. only yamato shrimps start jumping up and down.

yea i have moderate surface agitation which is caused by wind passing through. my tank is located between my main door and main window. but as i said, the PH goes down from 7.4 to 6.4 very quickly. plants start bubbling at 6.6. just after 6.4, it gets slower. when it reaches 6.2, there are lots of very fine bubbles in my tank water. maybe co2 got saturated? hehe.

NaHSO4 i got from... oh i better keep the secret. LOL. only have a little and use it for experiment. yea will handle it with care and change my tank water.

[quote:97016e66bd="bclee"]
How are your fishes coping?

What type of filtration do you have? Do you have agitation at the water surface? Surface agitation will cause you to lose CO2 even if you are able to get CO2 dissolve efficiently.

You should not add any more NaHSO4. Na if accumulated in the tank will cause problems to your plants. Yes, NaHSO4 being acidic will cause more CO2 to be release from the carbonates. That is only temporary. The CO2 will fall to the equilibrium level after a while.

Why do you have NaHSO4 around? It is quite acidic and corrosive. Be careful when handling it.

BC[/quote:97016e66bd]

fish

[liuhao]

still no concrete answer.

my tank now KH=3 PH=6.45 when no CO2 injected.

yesterday tried to push PH down to 5.7. got some causalties. 3 rasbora gone. anyway when PH reaches 5.9, tonina bubbles a lot so now keeping PH at 5.9.