CO2 measurement - when all things are taken into account

[Bradders]

Hi All,

I have been learning and experimenting more with soil. As a result, I have a CO2 measurement question for the experts!

My tap water has a pH of 7.5+. When I added Tropica Aquarium Soil, it seemed to buffer the water to around 7.0 when degassed.

1. When aiming for the 1pH drop for Co2, do you take the new buffered soil level (pH7.0) as a starting point or the actual tap level (pH7.5)? And this question is based on the next question ......
2. I am slightly confused by how the Drop Checker will perform. During the tuning of the CO2, the pH went down to 6.3, and the drop checker showed yellow, which implies 'too much'. But its only a pH drop of 0.7. So is it too much or is there some calibration needed due to the 0.5 pH drop due to the buffered soil?

The ultimate aim is to get to 30PPM without making the environment unsafe for fish. So thought I had better check in with you good people so I can understand this!

Thanks,
Brad

 

[MrClockOff]

Hey @Bradders

Forget about tap water. We have to measure pH drop relative to aquarium water.
If your drop checker is yellow when pH drop is 0.7 that could mean

• drop checker is above CO2 defuser and CO2 mist is getting into drop checker
• aquarium water got accidentally into drop checker
• uneven CO2 distribution in your aquarium
• unreliable pH checker

Cheers
Dan

 

[Bradders]

Hey @Bradders

Forget about tap water. We have to measure pH drop relative to aquarium water.
If your drop checker is yellow when pH drop is 0.7 that could mean

• drop checker is above CO2 defuser and CO2 mist is getting into drop checker
• aquarium water got accidentally into drop checker
• uneven CO2 distribution in your aquarium
• unreliable pH checker

Cheers
Dan

I think you may be right about the pH probe. I had to recalibrate it YET AGAIN using a pH7 solution. I need to keep my eye on that. However, a comparison now with the API test kit indicates that the aquarium water pH is ~6.5 on both the probe and API tests.

Rechecking my current aquarium water, it has a pH of 6.56. After shaking the water vigorously for 2 minutes in a bottle to de-gass it, the pH settles at around 7.10. (I checked with a pH 7.0 solution after these tests, and it settled at a pH of 7). So, assuming this thing stays calibrated, I am looking at a drop to a pH of 6.10. And the drop checker (CO2 Art) should remain green at that level?

 

[MrClockOff]

I think you may be right about the pH probe. I had to recalibrate it YET AGAIN using a pH7 solution. I need to keep my eye on that. However, a comparison now with the API test kit indicates that the aquarium water pH is ~6.5 on both the probe and API tests.

Rechecking my current aquarium water, it has a pH of 6.56. After shaking the water vigorously for 2 minutes in a bottle to de-gass it, the pH settles at around 7.10. (I checked with a pH 7.0 solution after these tests, and it settled at a pH of 7). So, assuming this thing stays calibrated, I am looking at a drop to a pH of 6.10. And the drop checker (CO2 Art) should remain green at that level?

Yeah, all sounds correct except shaking bottle for degassing the water? We tend to leave it alone for 24h to degass as shaking water would mean you mixing it with air and air contains atmospheric CO2.

 

[Bradders]

Yeah, all sounds correct except shaking bottle for degassing the water? We tend to leave it alone for 24h to degass as shaking water would mean you mixing it with air and air contains atmospheric CO2.

Right, OK, will collect some water and leave that on the side for 24 hours and test again. Thank you.

 

[dw1305]

Hi all,

1. When aiming for the 1pH drop for Co2, do you take the new buffered soil level (pH7.0) as a starting point or the actual tap level (pH7.5)? And this question is based on the next question ......

You can basically ignore <"all the pH readings for the tap, tank water etc">.

That one. It is only the carbonate hardness of the <"4 dKH solution in the drop checker"> that matters. The base in solution (sodium bicarbonate (NaHCO3) etc.) is the <"proton acceptor">.

Because the drop checker has an air gap, the colour change of the bromothymol blue, narrow range, pH indicator is entirely dependent upon CO2 that diffuses across that air gap, and the small proportion of that CO2 that becomes carbonic acid (H2CO3). That is why you have the 2 hour lag period, it gives time for the drop checker to "catch up". I still like the <"bouncy castle analogy">.

The carbonic acid then disassociates (into HCO3- and H+ ions) and it is the "extra" proton (H+) that causes <"the pH indicator to change"> from blue (deprotonated) to yellow (protonated). When you have a mixture of protonated and deprotonated bromothymol blue? It is green, simply because you've mixed together yellow and blue.

1. I am slightly confused by how the Drop Checker will perform. During the tuning of the CO2, the pH went down to 6.3, and the drop checker shows yellow.

The drop checker has 4 dKH fluid and a narrow range pH indicator, bromothymol blue. Only gases can diffuse across the air gap and when the fluid in drop checker is yellow? You have at least 30 ppm CO2 dissolved in the tank water, nothing else really matters.

Cheers Darrel

 

[Bradders]

Thanks you @MrClockOff and @dw1305 - always really appreicate your guidence.

After the re-caibration (I need to keep an eye on that PH probe) i am seeing some sucess. The CO2 got to my target level, but slowly busted it so need to adjust. I was say its lime green, but a tad to yellow. So onwards and upwards for adjustment tomorrow!

 

[dw1305]

Hi all,

After the re-caibration (I need to keep an eye on that PH probe)

You need to <"recalibrate the pH meter every time you use it">. They are quite complicated bits of kit. Have a look at <"C02 injection with already low ph"> and subsequent posts.

I'm not a CO2 user for a <"number of reasons">, but if I was I would use a drop checker <"and aim for ~15 ppm CO2">.

cheers Darrel

 

[Zeus.]

I trust the DC myself, esp if using a CO2 reactor, as no CO2 bubbles in tank. I actually stop calibrating my pH probe and just used them to check if the pH was stable from lights on till CO2 off. as any I see it as the actual pH is irrelevant- as if the DC turns the same light green every day and the pH reading says the same from lights on till CO2 off is all that matters, any drift in the probe reading the pH during the day will be very minimal. If the next day the probe gives a slightly different reading but is stable and DC is same colour as day before the [CO2] is the same.

Shaking the water to degas works, but I prefer leaving a glass of tank water 24 hrs. If using the shaking method you have to keep shaking it until there is no further change in pH ( then you could always leave the sample over night and check again)

 

[sparkyweasel]

I think you may be right about the pH probe. I had to recalibrate it YET AGAIN using a pH7 solution.

Is it calibrated at just one point, ie pH7? Not with two different solutions?

 

[Bradders]

Is it calibrated at just one point, ie pH7? Not with two different solutions?

There is a 7 and 4 solution for calibration. Is it important to do both?

 

[Bradders]

Recalibrated again this morning with 0.10 drift. But calibrated back to 7 with the solution. These are the results.

• pH7 solution - 7.0 pH reading. [As expected]
• 24-hour degassed water by standing - 7.45 pH reading. [The broadly aligns with water out of my tap]
• Aquarium Water - 6.27 pH reading. [I assume this it the new Tropica Aquarium Soil causing this disparity, plus some non-gassed out CO2 from previous day]
• Reading at time of new soil being added with Tap Water but before any CO2 was injected - ~pH7.0 [For clarity, 24 hours after water change and soil addition]

Notwithstanding the comments about trusting the drop checker colour (I am fully subscribed to that science provides on this thread), you can see that the pH is all over the place really. Furthermore, the added variable of the Tropica Aquarium Soil acidifying the water means the aquarium 1 pH drop would be around pH6.0 to achieve 30PPM. But I am not sure how 'stable' the soil will be at keeping the non-CO2 levels around pH7.0.

 

[Andy Pierce]

Yeah, all sounds correct except shaking bottle for degassing the water? We tend to leave it alone for 24h to degass as shaking water would mean you mixing it with air and air contains atmospheric CO2.

"Degassing the water" actually means "equilibrate the sample with atmosphere". Your best way to reach this equilibrium quickly is to insert a small air bubbler into your sample water and push air through it. You can alternatively let the sample sit for 24 hours but then you're hoping passive diffusion will do the 'degassing' for you and passive diffusion in water isn't very good.

 

[Bradders]

And just for full transparency, here is the DC checker this morning. You can see that it remains an olive green, with a current pH of 6.31

 

[dw1305]

Hi all,

There is a 7 and 4 solution for calibration. Is it important to do both?

Yes it is, the better meters have this <"two point calibration">.

cheers Darrel

 

[dw1305]

Hi all,

you can see that the pH is all over the place really.

The "problem" is really the pH meter, you just don't know if any of those readings are correct or incorrect.

Because of the log10 nature of the pH scale when you are around pH7 small differences in water chemistry make large differences in pH <"CO2 tuning - need help">. You need @Andy Pierce for <"the chemistry">.

Furthermore, the added variable of the Tropica Aquarium Soil acidifying the water means the aquarium 1 pH drop would be around pH6.0 to achieve 30PPM

Notwithstanding the comments about trusting the drop checker colour

Just trust the drop checker, it doesn't have any "moving bits" that may mislead.

pH meters are really good at sorting out acids from bases in liquids with high conductivity, and telling you that orange juice is acidic and sea water alkaline, but as you get towards "shades of grey" neutrality (pH 7) things become a lot less straightforward - <"Reccomended PH Probe">.

cheers Darrel

 

[Zeus.]

And just for full transparency, here is the DC checker this morning. You can see that it remains an olive green, with a current pH of 6.31

Which is normal for must tanks which are injected with CO2 for the time of day. Takes longer than you think for tank to fully degas. So the tank may have 10 -15ppm CO2 when you start the CO2 injection. So trying to get a 1.0pH drop from CO2 on till lights on will lead to too much CO2 in tank - hence the reason we trust the DC colour and just monitor the pH from lights on till CO2 off . If the pH doesnt change from lights on till CO2 off and the DC colour goes 'light green' during that period the stable pH indicates its stable and the DC indicates the CO2 concentration

 

[Andy Pierce]

And just for full transparency, here is the DC checker this morning. You can see that it remains an olive green [...]

I would just declare victory right here.

 

[Andy Pierce]

The "problem" is really the pH meter, you just don't know if any of those readings are correct or incorrect.

Totally agree this.

Because of the log10 nature of the pH scale when you are around pH7 small differences in water chemistry make large differences in pH <"CO2 tuning - need help">. You need @Andy Pierce for <"the chemistry">.

For the chemistry piece, this goes the other way... large differences in acid/base water chemistry make small differences in pH. This is true at all pH values (there's nothing special about pH 7). You don't have to worry about running out of alkalinity with CO2 injection because CO2 injection does not change alkalinity. I think where @dw1305 might be going with this is that at low ionic strength (low TDS) pH probes don't work very efficiently. If you had pure water never exposed to atmosphere it would have a pH of 7, but measuring that pH with instrumentation would be difficult. If the pH is not close to 7, that means the water is not pure and there is probably lots of other "ionic stuff" in there which will help your pH probe work better.

Just trust the drop checker, it doesn't have any "moving bits" that may mislead.

Totally agree this as well. I found even expensive laboratory grade pH meters a faff to use correctly back in the day.

 

[dw1305]

Hi all,

I think where @dw1305 might be going with this is that at low ionic strength (low TDS) pH probes don't work very efficiently. If you had pure water never exposed to atmosphere it would have a pH of 7, but measuring that pH with instrumentation would be difficult. If the pH is not close to 7, that means the water is not pure and there is probably lots of other "ionic stuff" in there which will help your pH probe work better.

That is what I meant.

Totally agree this as well. I found even expensive laboratory grade pH meters a faff to use

I think that is the real issue, I really want to know what the tank water pH is, but I'm very wary of the values given by <"any single pH meter">. A drop checker, or a conductivity meter, doesn't give you fine scale resolution, but it does give you a value that you know will be accurate.

Because I don't use CO2 I'm not too bothered about real time CO2 values, this means that I can use the <"Snail Shell Index"> as a measure of carbonate hardness dKH and we know that dKH, <"atmospheric CO2"> and pH are in equilibrium, so it gives me an "estimation" of mean pH levels as well.

It doesn't sound very "scientific", but it is based on probability and there isn't really anything that go "wrong".

Personally I like visual methods, they cut <"straight to the chase"> - As in <"Climate stripes - University of Reading">.

cheers Darrel