Gomer

Just use Bromothymol blue and use the 3 images above. Exact CO2 levels aren't important. Getting addiquate CO2 is.

hoppycalif

The images don't tell you what is an adequate level of CO2. They tell you what the pH is. If you have low KH, as I do, the pH needs to be lower for adequate CO2 than it was when I had much higher KH. So, the color would be different for the two cases. Where can I get Bromothymol blue? I just bought one of the ebay devices - the cost is low enough just to play with.

Why wouldn't this device work equally well if you loaded it with distilled water with a tiny bit of baking soda to get some carbonate in it? Then, wouldn't the CO2 ppm in the device be the same as in the tank water? And, with "ideal" water, of known KH, the pH would truly represent the ppm of CO2 in both the device and the water. I don't see the flaw in doing this.

Bert H

Perhaps I am being a little dense here, but aren't you still measuring the pH? You still have all the factors which affect the accuracy/inaccuracy of this methodology. So you will have a ball park figure with it. Isn't that what you have now just by measuring pH?

hoppycalif

You would be measuring a single pH and not two pH's and subtracting them, which can double the error in their measurement. The downside would be that the color of the reagent would be harder to see, since it would be much farther from the color comparison chart. Also, the water whose pH you would be measuring would have a known KH, with no other sources of alkalinity or acidity except the CO2, which would be the same ppm (?) as in the tank water. So, this should give a much more accurate indication of how much CO2 you have, without running into one of those cases where your KH/pH say you have 100 ppm CO2 but the fish are perfectly happy. My chemistry knowledge is skimpy, so I am just assuming that these indicators work because the water in the indicator reaches an equillibrium with the tank water so both have the same ppm of CO2. In any case, I have ordered the ebay version of the "drop checker" and will do some experimenting when I get it.

hoppycalif

I got my ebay version of the ADA Drop Checker today, eBay: Type1 Co2 Drop Checker-monitoring proper dosage of CO2 (item 250036844403 end time Oct-14-06 06:06:32 PDT), and it's in the tank now. I like it! Here's how it works as a CO2 indicator:

First, take some distilled water, mix in a bit of baking soda, measure the KH, and adjust slowly until you get KH of 4 degrees, as close as you can get it. I used 4 times the normal water sample, so each drop of my AP test kit would equal 1/4 degree of KH. I think I got it to within +/-.1 degree of being exactly 4. Now, this water has no other substance that affects alkalinity and no substance that affects acidity in it, so it is "perfect" water for the KH/pH tables to give the CO2 ppm.

I had previously tested my AP pH kit to see if the number of drops of reagent was critical for measureing pH, and found that it isn't at all critical, just as I would expect. This means you can add more reagent to get a more intense color.

If you check the ph/KH charts, or better yet, the calculator, for KH of 4 degrees, Measuring CO2 levels in a Planted Tank, you see that the pH target to have 30 ppm of CO2 is 6.6. And, an error of .1 in pH gives a range of 25-40 ppm of CO2, which is a reasonable range, since you can actually see the difference between pH of 6.5 and 6.7, so you can easily reduce that error range if you wish, with a good careful look at the color.

Now look at the color chart for the AP or any similar test kit, you see that the color corresponding to a pH of 6.5 to 6.7 is green - slightly yellowish for 6.5 and slightly bluish for 6.7. So, it is easy to judge the color as being green, not blue green and not yellow green.

So, water with KH=4.0 will definitely be green at the pH that gives the correct ppm of CO2 for our tanks. And, if the color is yellowish, the pH will be low enough that the ppm will be much too high - about 70-80 ppm. Likewise, if the color is blueish, the ppm will be much too low - about 10 ppm.

So, if we put this KH = 4 water in the bulb of the "Drop Checker" with enough indicator solution to get a strong color, which took about 7 drops for me, we have not a pH indicator, but a good CO2 indicator.

That's what I did. Getting the water and indicator into the bulb is difficult, because the throat of the "Drop Checker" is small and the air trapped in the bulb will easily stop water from flowing into the bulb. I had to use a syringe with a bent needle to squirt the water into the bulb, the use it again to remove a bit when I had too much in there. I had blue water in the bulb.

I stuck it to the tank wall, about 3 inches below the water line, where I could see the bulb easily, and watched it. After one hour, the color was slightly greenish blue. In another 15 minutes it was blue green. Then I had to go do some political activism and when I got back it was 3.5 hours after I stuck it in the tank. Oops, it was yellow, very yellow! About 3/4 of the fish were at the surface, so I shut off the CO2, and adjusted the needle valve to slightly reduce the bubble rate for tomorrow.

My conclusion is that this works very well to monitor CO2 level in the tank, with a time constant of around 2 to 3 hours - the time to reach approximately the final reading. This is fine for my use. I will keep watching this, and adjusting my CO2 bubble rate accordingly, and will report back in a few days.

Laith

Thanks for the results of this experiment Hoppy.

If I understand how this works; CO2 off-gassing from the aquarium water is diffusing into your RO+KH+reagent water sample, the concept being that at one point it reaches equilibrium between the three?

I'm just wondering how accurate a measure that is of the CO2 dissolved in the tank water? Wouldn't there be some distortion due to the fact that it is measuring an amount of CO2 after it has diffused into an air medium and then back into a liquid? Or does the fact that it reaches equilibrium negate any distortion?

Just curious... though it would probably still be more accurate than the standard KH/pH/CO2 tables.

onemyndseye

Great run down Hoppy.... you seem like a really sharp cat You should add this to the APC review archive

I THINK in my very limited knowledge of chemistry that once that equilibrium is reached that the levels would all equal out (though maybe to some degree of variance) ... even the air portion of the Drop Tester would reach equilibrium with the CO2 levels in the tank. If CO2 and O2 were NOT independant of each other is were I would think the big problems would come into play.... *shrug*

-Justin
One Mynds Eye

Bert H

Nice follow-up Hoppy. Sounds like this is indeed, more accurate than other methods which we have tried. Any chance you can post some pics of the system 'at work', so to speak.

hoppycalif

I will try to post a photo of the "drop checker" at work today. Unfortunately I am not educated in the fields that would let me theoretically prove that this does or does not work. My understanding is that the air/water system reaches an equillibrium - no net CO2 transfer from one to the other, and two such systems, connected by the air gap are all in equillibrium with each other, thus the ppm of CO2 is the same in both batches of water. I am passing this by Tom Barr on his website just to get a very critical look at it - and he doesn't think it will work. I hope someone here has the background to offer criticism or comments on it too. Most things that seem too good to be true are.......not true! But, I'm hoping this one is true.

freshreef

i have found only bromothymol blue powder. how do u make stock solution?