CO2/pH Data

neil1973

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Dec 17, 2005
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Re: CO2/pH Data

The idea of getting an exact level of co2 in water by blowing through a straw seems highly unlikely to me, there are just way too many variables. People will have different co2 contents in exhaled air, especially when not breathing normally. Also as has been demonstrated in this thread the time needed for co2 in water to reach an equilibrium is considerable.

I think this thread is extremely useful and the ph change method of co2 determination is worth pursuing. Firmly establishing the average amount of co2 in water at equilibrium with air and at average tank temperature will be extremely helpful. The 0.5ppm figure seems to be coming up a lot (a couple more references are given below although I’m sure most people have seen these) but it would be really good if someone with a good grasp of physics could step in here and verify this. Another consideration is the co2 content of air in the room that the sample is been left to stand in. It seems likely that there will be considerably more co2 in a room with closed windows that is being used by people, and therefore there the level in water at equilibrium would be greater than 0.5ppm.

http://www.hallman.org/plant/huebert.html
http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html
 

yme

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Nov 30, 2005
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Re: CO2/pH Data

well I am.... and did some experiments.

first the complete graph of my degassed tank:

degassed.JPG


now for the CO2 levels/blowing etc..
I took 0.5 liter RO water added a bit of carbonates to get a KH of around 3. I had to shake the 0.5 liter flask to dissolve the carbonate. Immediately after this, I put the pH electrode in. the stable level after 30 minutes was pH:7.80. with a KH of 3 this gives a CO2 level of 1.4 mg/l.
Then I took an airline started to blow. Due the first time that I did the experiment, I wasn´t ready to note the first time points, that´s why they are lacking. Subsequently I started to blow untill a couldn´t get a lower pH with a stable blow level/speed (at least I tried).
I couldn´t get below 5.73, which gives a CO2 level of 168 mg/l.

co2-KH3.JPG


now I connected the airline to an airpomp and measured the pH untill the pH was stable: pH 7.36. because this was much lower than the pH I started with, I disconnected the airpump and measured the pH again. as you see I measure a rapid increase in pH and I end up with a pH of 7.82

O2.kh3.JPG


because it is now around midnight, I did one last experiment. I repeated the lowering of the pH by blowing in the water, but now the KH was around 8-8.5. for some reason the stable pH was lower than 7.8 after adding the carbonates. The pH was 7.18. It was the same sample, I only added carbonates and i have shaken the flak to dissolve the carbonates. a pH of 7.18 and a KH of 8 gives CO2 level of 16 mg/l (shouldn´t be stable right?). I started blowing and got a stable pH level at pH 6.48. this means a CO2 level of 80-85 mg/l.

co2.kh8-8.5.JPG


conclusions???? :( :( :(

greets,

yme
 

VaughnH

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Re: CO2/pH Data

Two things your data suggests is that the minimum pH you can drop the water to by blowing in it depends on KH, and the minimum time to reach that pH is about 5 minutes. I tried doing this too, for two KH's, with distilled water and a tiny bit of bicarbonate of soda in it. It took me about 7 minutes to reach the minimum and I reached 6.64 with KH=3.0 and 6.78 with KH= 6.0. I plotted minimum pH vs KH for both of our data sets and got:
pHvsKHblowhard-1.jpg

I'm still wondering how best to convert all of this to a method for measuring the tank CO2.
 

VaughnH

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Re: CO2/pH Data

How about this as a method:
1. Pour about 500 ml of distilled or RO water into a beaker or measuring cup.
2. Add a tiny bit of bicarbonate of soda to the water.
3. Measure the KH of the water and adjust by diluting or adding more soda to get KH between 3 and 8.
4. Place the end of a piece of air line or a straw into the water and blow from the lungs for 5 to 7 minutes, or until the pH of the water no longer drops.
5. Measure the pH of that water
6. Calculate the ppm of CO2 using Chuck Gadd's website calculator or equivalent.
7. Dip out about a 500 ml sample of water from the tank.
8. Measure the pH of that sample.
9. Place the end of a piece of air line or a straw into the water and blow from the lungs for 5 to 7 minutes, or until the pH of the water no longer drops.
10. Measure the pH of the sample.
11. Calculate ppm of CO2 in the tank using this equation:
CO2 ppm=ppm from step 6 * 10exp(pH from step 10 - ph from step 8)

This lets each user find his own lung blowing ppm of CO2 and use it to calculate the ppm CO2 in his tank water. All it assumes is that the lung power CO2 ppm will be reasonably the same for both samples.
 

Tom Barr

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Jan 23, 2005
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Re: CO2/pH Data

Yme, the real issue is not so much the dgassing of the glass of water, a small smaple, but rather, the time it takes for the tank to drop to get a certain relative PPM of CO2 and the amount after.

40-60 hours seems like a long time to get the flat range of the curve. I would have expected it to happen in less time. 12-24 hours.

Using the small samples to see what the true CO2/pH drop etc is anmd blowing into the water itself, as you can see, this does not = 30ppm, I've heard claim this ppm range, some now claim 60ppm, well, it depends alot and if you think about that, it makes sense it's going to have some variation, it's nothing I'd use as a calibration certainly.

The response times for the CO2 system's pull down and rise: how long it takes under light and without light may be more useful graphs.

Think about the CO2 ppm in context of plant growth and optimalization when looking at a graph like that.

It's tough to tell between 1 ppm nor less of CO2.
Most things for that matter.

Regards,
Tom Barr
 

vidiots

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Apr 29, 2006
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Re: CO2/pH Data

I think you are on the right track with your attempt to calibrate your CO2 measurements. I do think that using your breath as a consistant source of CO2 may have too many variables involved to get consistant results. You may want to try a bubble counter and a pressurized CO2 system. With the bubble counter you should be able to ensure that you are delivering the same rate of CO2 to the two samples.