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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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Well, I recieved my Red Sea checker today. Unfortunately didn't have any distilled water left after making the last batch of CSM+B - and forgot to replace it. But there is a bypass on my well house's water conditioner and my KH is near zero there.
So I thought I'd try a little experiment untill I get the distilled water. I used the well water and after adding a very tiny amount of baking soda, finally got a KH of 5 (after a few more trips to the well house). Put one ml. of the KH 5 water into the little Red Sea container. Next I added about 6 drops of AP pH test solution and it came out quite green. Oh well... I suppose there is a bit of CO2 acid in my well water which generally rises to a standing pH of 6.6 from pH 5.2 out of the tap. Tommorrow, with distilled water, I'm hoping I'll get the blue color as this method requires.

Is it possibly CO2 acids which caused the KH5 well water to be green from the get go??
 

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Discussion Starter · #184 ·
I don't think it is likely that well water will contain 40 ppm of CO2, and I know it won't if that water is allowed to sit in the open for very long. You just have to use either DI or distilled water for this method to be any better than just measuring the tank pH and KH.
 

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The well water could contain a lot of dissolved gases from ground sources, pressurizing and pumping. There may be other reasons for your results but it is certainly possible to have lots of dissolved gases present in a freshly drawn sample.
 

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Yeah, it was just an experiment. It actually turned a bit blue by morning. I run a airstone at night. And it was a lighter green (approaching greenish-yellow) tonight when I got home from work. I have distilled water now so I'll fix that tonight.These "drop checkers" are a cool concept and I appreciate everyone's efforts on this thread.
 

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How often do you have to change the indicator solution? Is there any advantage using a different buffering solution over baking soda?
 

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Discussion Starter · #188 ·
I am in a phase where I am neglecting my aquarium now - too busy with other things for now. So, I have noticed that the indicator solution gets dimmer in color over time, but still reaches the same color as it originally reached for the same bubble rate. My best guess now is that two weeks is about as long as I would let it go if I wanted accuracy. As far as the KH solutions goes, sodium bicarbonate works very well, is very cheap, and is widely available, so I don't see any advantage to trying anything else. The only things that would work would be carbonates or bicarbonates.
 

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I still have my original batch in the drop checker. It has been in there for about 2 months now. I can glance in the aquarium and see the solution. If the solution looks blue, I know I'm out of CO2 (which has happened a couple of times). If the solution looks kinda clear (with the lights and background, etc., it looks clear), I know I have CO2 in the water.

When I do my weekly water changes, the water level falls below the drop checker. By the time I am filling my tank up again, the solution is bluish looking. It turns clear by the end of the day.
 

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Do you mix up a "fresh" batch of buffer solution every change? Or can I mix up 100ml at a time and store it for a while?
 

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Discussion Starter · #193 ·
A DIY HOB Drop Checker and pH Controller from Sept. '92:

The Krib

HTH
Don't be misled by that article. You can't hang this on the back and have part of it outside of the tank and part inside. If you try that you will get a difference in temperature between the tank water and the drop checker water, which will cause slow distillation of the warmer water to transfer water to the cooler water, changing the KH of the reference water. Also, it doesn't work with tank water in place of KH reference water, and it reacts so slowly to changing tank water conditions that it is useless as a pH controller or a CO2 controller.

If you make up a bottle of KH reference water, and seal it tightly, it should be good for several months at least. Since the indicator solution is a dye it can't be completely stable, so if you add the indicator solution to a bottle of reference KH solution, at least do it in a small bottle, and store it out of direct light. Don't forget that some plastic bottles allow water vapor to leave the bottle very slowly, which would change the KH of the water. (Store a plastic bottle of water for a couple of years and it starts to collapse.)
 

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This has been a very good thread, long however.

Anywya, I consider myslef pretty new to the CO2 injection and am at a point where I have to decide where to go with CO2.

Some questions I have about this CO2 dropper is:
1. The red sea model looks nice and appears to be pretty cheap, is it recommended for those that aren't creative enought or have the time to DIY?

2. The KH and the Distilled water, how do you test it? I have a test strip kit but not a liquid test kit for KH. If I mix a mason jar worth, would the water last?

3. how often do you need to refill the dropper?

4. If I go with the red sea, once the reagent runs out, can I use normal PH reaent from my AP test kit? do you use the high range or low range ph indicator?
 

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Discussion Starter · #196 ·
This has been a very good thread, long however.

Anywya, I consider myslef pretty new to the CO2 injection and am at a point where I have to decide where to go with CO2.

Some questions I have about this CO2 dropper is:
1. The red sea model looks nice and appears to be pretty cheap, is it recommended for those that aren't creative enought or have the time to DIY?

2. The KH and the Distilled water, how do you test it? I have a test strip kit but not a liquid test kit for KH. If I mix a mason jar worth, would the water last?

3. how often do you need to refill the dropper?

4. If I go with the red sea, once the reagent runs out, can I use normal PH reaent from my AP test kit? do you use the high range or low range ph indicator?
The Red Sea model will work as well as any of them. The principle this works on is so simple even crude DIY units work fine. In my opinion the only advantage of the glass ones is that they look nice in the tank.

You can mix the KH solution without a test kit, but then you need a digital balance that will measure accurate to +/- .01 grams, and a volumetric flask that will measure at least a liter of water accurately. If you mix 4.99 grams of bicarbonate of soda that you have baked at low heat for several minutes to dry it out, with 5 liters of distilled or DI water, you will have 5 liters of 40 dKH water. Then mix 10 ml of that with 90 ml of distilled or DI water, and you will have 100 ml of 4 DKH water. This should be more accurate than depending on a test kit, and you can store both the 40 dKH and the 4 dKH solutions in airtight bottles for an indefinite time.

You can use the reagent from any pH test kit, if the kit gives yellow at about pH=6 and blue at about pH=7.2.

The solution in the drop checker will last at least two weeks, with no problems. But, eventually you will need to clean off the "biofilm" on the drop checker, and add fresh solution to it.
 

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Hello,
I'm very new to all this. I'm not sure what you mean by distilled water for your drop checker and why that's important. Could you explain please? I'm assuming from your discussion that you can change the KH of this by adding bicarb of soda. KH is the water hardness right?! So it will be very different depending where you live in the counrty (I'm in the UK).

Sorry new to website and didn't realise there was more after 1st page!!!! Maybe there are answers in the other 19 pages!!!! Sorry if I have interupted your discussion on something else.....can I delete an entry and how?

Thanks
Claire :confused:
 

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Hello,
I'm very new to all this. I'm not sure what you mean by distilled water for your drop checker and why that's important. Could you explain please? I'm assuming from your discussion that you can change the KH of this by adding bicarb of soda. KH is the water hardness right?! So it will be very different depending where you live in the counrty (I'm in the UK).

Sorry new to website and didn't realise there was more after 1st page!!!! Maybe there are answers in the other 19 pages!!!! Sorry if I have interupted your discussion on something else.....can I delete an entry and how?

Thanks
Claire :confused:
You use distilled water to make your 4dKH solution. If you use plain water, pH could be off as a result of something other than the bicarbonate.

PS - for you "real chemists" 4dKH corresponds to 0.06648mM Na Bicarbonate (if my math is right).
 

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Discussion Starter · #199 ·
NEW: Membrane style DIY Drop Checker

I have a workable, DIY, easy to make design now for a membrane version of the drop checker. Tomorrow I will make a couple to do some more testing. Here is the sketch:


This design will allow you to reduce the thickness of the KH water solution to as thin as you can use and still see color differences, thus making it react rapidly. It also will seal well, so leakage of the solution will not be a problem. And, you can use either Tyvek or Cole Parmer membrane, or just about any other membrane on it. Installing the membrane will be as easy as I can make it be. All of the materials are readily available from a Tap Plastic store (clear acrylic tubing) and a hardware store (for O-rings), plus a 1/2" diameter probe holder/vacuum cup from the LFS. I feel good about this one!!

This is a result of a lot of experimenting I did and reported on the Barr Report, see: Gas Permeable Membrane Drop Checker - Barr Report

To explain this some more, the top O-Ring is to seal the drop checker fluid in the chamber (the blue green color on the sketch), and the bottom O-ring is just like a rubber band to hold the membrane on the device. I know from testing that if the thickness of the fluid layer is only around 1/16 inch this will react in less than an hour, probably less than 30 minutes. I haven't tried to make the checker solution strongly colored enough to go below 1/16 inch thickness, but it would be worth trying it thinner, to see if a 5 minute response time is achievable. I expect to be able to make one of these in less than 30 minutes with hand tools.
 

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Nice - I've been playing w/ some different membranes that are more commonly available (no luck yet).

Please post some part numbers or specs for the membranes you've got working!
 
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