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I used a propane torch to bend the test tube from a pH kit. It takes a little practice but the tubes are like $.50 so you can get a few extras. Teh trick is to spin it as you heat it and to make the bend, keep spinning it while pulling slightly apart and forming the bend.
Oooh, I like this idea! =)
 

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How long do you leave the drop checker in the tank? I understand there is a minimum time that you have to leave it in the tank but I am curious to know if you leave it in the tank over a long period, days or even weeks and use it like a thermometer.
Yes, it's held in there with a suction cup indefinately so that you can always glance over at it and know approximately what your CO2 levels are.
 

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EDIT: I did some googling and found the obvious source of membrane material - Kordon breather bags! Who has some and is willing to supply one or two to me - of course I will pay shipping and cost of the bag(s)?
Naja002 suggested breather bags already: http://www.aquaticplantcentral.com/...checker-14.html?highlight=breather#post256349

=P

I think, for an effective drop checker using a semi-permeable membrane, you could overfill a test tube or container or whatever, and place the membrane over the top so that there is absolutely no air inside the container. This way, the dissolved CO2 within the tank can directly dissolve through to the other side and give faster results.

Of course, since CO2 in H2O yields carbonic acid, I wonder if these molecules will be able to get through the membrane...no time to ponder this for now. Final exam in 2 hours. ;)
 

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Wouldn't something like the Vacuum Sealer work? Do they seal with heat?

Of course, you'd have to trick the machine into thinking that there was suction before the bag was sealed.
 

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I found CO2 probes of this type already exist for blood gas analysis. They have a gas permeable membrane with a carbonate/distilled water solution between the membrane and the pH sensor, just as Tom is proposing. Looks like once again someone has figured it out before us, and probably patented the design.
Some has figured it out before us? That might be quite an underestimation.

This is a list of references from an article I found dealing with a probe-type detector for real-time PCO2 and PO2 in patients. Notice many of these publication dates are least 20 or 30 years old.

References

Bruggen van and Scott, 1962. J.T. Bruggen van and J.C. Scott , Microdetermination of carbon dioxide. Anal. Biochem. 3 (1962), pp. 464-471.

Eigen et al., 1961. M. Eigen, K. Kustin and G. Maass , Die Geschwindigkeit der Hydratation von SO2 in wässriger Lösung. Z. Physik. Chem. (N.F.) 30 (1961), pp. 130-136.

Gibbons and Edsall, 1963. B.H. Gibbons and J.T. Edsall , Rate of hydration of carbon dioxide and dehydration of carbonic acid at 25.0 °C. J. Biol. Chem. 238 (1963), pp. 3502-3507. Abstract-MEDLINE

Harned and Scholes, 1941. H.S. Harned and S.R. Scholes, Jr. , The ionization constant of HCO2−3 from 0 to 50.0 °C. J. Am. Chem. Soc. 63 (1941), pp. 1706-1709. Full Text via CrossRef

Harned and Davis, 1943. H.S. Harned and R. Davis, Jr. , The ionization constant of carbonic acid in water and the solubility of carbon dioxide in water and aqueous salt solutions from 0 to 50.0 °C. J. Am. Chem. Soc. 65 (1943), pp. 2030-2037. Full Text via CrossRef

Jones and Bradshaw, 1933. G. Jones and B.C. Bradshaw , The measurement of the conductance of electrolytes.V. A redetermination of the conductance of standard potassium chloride solutions in absolute units. J. Am. Chem. Soc. 55 (1933), pp. 1780-1800. Full Text via CrossRef

Kempen van, 1972. L.H.J. Kempen van , Estimation of free and hemoglobin-bound CO2. Thesis (1972) Nijmegen .

Kempen van et al., 1972. L.H.J. Kempen van, H. Deurenberg and F. Kreuzer , The CO2-quinhydrone electrode. A new method to measure partial CO2 pressure in gases and liquids. Respir. Physiol. 14 (1972), pp. 366-381.

Lunn and Mapleson, 1963. J.N. Lunn and W.W. Mapleson , The Severinghaus PCO2, electrode; a theoretical and experimental assessment. Brit.J. Anaesthesiol. 35 (1963), pp. 666-678.

Maffly, 1968. R.H. Maffly , A conductometric method for measuring micromolar quantities of carbon dioxide. Anal. Biochem. 23 (1968), pp. 252-262. Abstract

Murakami et al., 1965. I. Murakami, S. Takashima, K. Fujisaku, H. Sasamoto, Y. Takagi and Y. Oota , A new method for determination of PCO2, both in liquid and gas. In: Digest 6th Internal. Conf. Med. Electron. Biol. Engin. (1965), pp. 610-611.

Robinson and Stokes, 1959. R.A. Robinson and R.H. Stokes , Electrolyte Solutions. , Butterworths, London (1959).

Stow et al., 1957. R.W. Stow, R.F. Baer and B.F. Randall , Rapid measurements of the tension of carbon dioxide in blood. Arch. Phys. Med. Rehabil. 38 (1957), pp. 646-650. Abstract-MEDLINE

Taylor, 1953. G. Taylor , Dispersion of soluble matter in solvent flowing slowly through a tube. In: Proc. Roy. Soc. A 219 (1953), pp. 186-203. Full Text via CrossRef

Tsao and Vadnay, 1964. M.U. Tsao and A. Vadnay , A method for continuous measurement of blood PO2, and PCO2. J. Lab. Clin. Med. 63 (1964), pp. 1041-1053. Abstract-MEDLINE
 

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Measurement of ASL pH. We developed a novel technique to measure pH with pH-sensitive microelectrodes (Microelectrodes, Bedford, NH) in small-volume samples that could be quickly temperature, water vapor, and gas equilibrated. Microaliquots (0.3-1.0 µl) were aspirated from the microcapillary tube into the tip of a section (0.5 cm) of CO2-permeable silicone tubing (Helix Medical, Malvern, PA; 0.025 in inner diameter, 0.047 in outer diameter). The pH microelectrode was inserted into the sample by stretching the end of the tubing containing the sample over the microelectrode tip, the tight fit trapping a thin layer of liquid between the tubing wall and the electrode, so that reference and pH electrodes made contact with the sample. The microelectrode and tubing were placed in a water bath that was continually gassed and equilibrated with 5% CO2. A column of air in the tubing, distal to the electrode, prevented water from reaching the sample. CO2 equilibration was complete within 2 min, as evidenced by a stable pH. Measurements were accurate and reproducible within ±0.01 pH units.

From: Abnormal surface liquid pH regulation by cultured cystic fibrosis bronchial epithelium -- Coakley et al. 100 (26): 16083 -- Proceedings of the National Academy of Sciences

And from Helix Medical: Standard Tubing Wacker
We have the same tubing as described within the article:
60-411-42 .025 .64 .047 1.19 .011 .28

CO2-permeable silicone tubing. Probably cheaper then $500 per sample.
 

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I enjoyed dennis' explanation, but I think it lacked a discussion of partial pressures, although he alluded to it with the phrase "nothing forcing a high equilibrium level".

Here's a list of terms and concepts I'll use to try to explain this...I don't want to clutter the text with this, so everything will be listed first.

http://en.wikipedia.org/wiki/Partial_pressure
Gases will dissolve in liquids to an extent that is determined by the equilibrium between the undissolved gas and the gas that has dissolved in the liquid (called the solvent).
The form of the equilibrium constant shows that the concentration of a solute gas in a solution is directly proportional to the partial pressure of that gas above the solution.
http://en.wikipedia.org/wiki/Co2
It is present in the Earth's atmosphere at a low concentration of approximately 0.04% and is an important greenhouse gas.
http://en.wikipedia.org/wiki/Atmospheric_pressure
In 1982, the International Union of Pure and Applied Chemistry (IUPAC) recommended that for the purposes of specifying the physical properties of substances, "the standard pressure" should be defined as precisely 100 kPa (≈750.062 torr) or 29.9230 inHg
http://www.google.com/search?hl=en&q=29.9230+inches+to+mm
29.9230 inches = 760.0442 millimeters
http://www.madsci.org/posts/archives/2001-09/1001605307.Es.r.html
Oxygen makes up about 21% or 210,000 ppm of the atmosphere.
CO2 constitutes 0.04% of atmospheric gases. At 760 mm Hg (sea level pressures), this amounts to 0.304 mm Hg. We're dealing on mm Hg (milimeters of mercury) since solvation of gases in liquids is based upon the concept of partial pressures.

It's true that there is about 300-400 ppm of CO2 in atmospheric air, but how does that compare with 210,000 ppm of oxygen gas? Or roughly 790,000 ppm of nitrogen gas? The partial pressure of CO2 is comparatively very very low in atmospheric air.

That's why even though there's 300 ppm of CO2 in the air, only a few ppm will actually dissolve in our aquariums.

As the CO2 in the aquarium comes out of solution into the air space within the drop checker, the partial pressure of CO2 in that air space rises dramatically because of the confined space. This increased partial pressure of CO2 in that space allows for a speed (within a few hours) equilibration between the drop checker liquid and the aquarium water.

When the drop checker is green, the water contains 30 ppm of CO2, which equates to a very high partial pressure of CO2 within that liquid. When you take it out of the water, this high partial pressure of CO2 within the liquid rapidly forces it out of solution to equilibrate with atmospheric CO2 which contains 300 ppm of CO2 but a very low CO2 partial pressure.

...hope that helps. :)
 

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Less oxygen and possibly less nitrogen in the drop checker air space would lead to an overall higher partial pressure of CO2 even though the CO2 amount is still 30 ppm.
 
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