Drop Checker for DIY CO2 - The Planted Tank Forum
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post #1 of 23 (permalink) Old 02-21-2011, 08:34 PM Thread Starter
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Drop Checker for DIY CO2

Most of the time DIY CO2 will not produce enough CO2 to gas the fish, and for larger tanks that is probably never. So, there is no need for a drop checker to be used to help you adjust your bubble rate - you can barely do that anyway. It would be nice to use a drop checker to see if your DIY CO2 system was doing any good at all, raising the amount of CO2 in the water enough to make it worth the effort. For that you are more interested in CO2 ppm's from about 5 to 20 ppm, and you can't get that using 4 dKH water in the drop checker. You can get that if you use .75 dKH water.

I am now using DIY CO2, one 3L bottle on a 45 gallon tank, so I made a simple DIY drop checker and made some .75 dKH water to use in it. This is how it will read out:


I will want to see the DC color be yellow, but even a light green would be an indication that my DIY CO2 efforts are worth the effort.
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post #2 of 23 (permalink) Old 02-21-2011, 09:05 PM
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Hoppy, you are the man!

I was just wondering how I was going to get a decent CO2 measurement on my 25G with a single 2L, and here this is.

Not to mention I've been wondering if anyone else is intentionally running below 20ppm.

Going to go do this now.
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post #3 of 23 (permalink) Old 02-21-2011, 10:02 PM
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and sometimes...it's the short, simple how-to threads that deserve a sticky vote
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post #4 of 23 (permalink) Old 02-21-2011, 10:26 PM Thread Starter
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About 2 hours later, after putting the drop checker in the tank! I apparently have around 20+ ppm of CO2, with a single bottle, and a simple good quality airstone under the filter inlet. I'm getting one bubble per second from the bottle.

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post #5 of 23 (permalink) Old 02-21-2011, 11:58 PM
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What color was the drop checker before you put it in the tank?
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post #6 of 23 (permalink) Old 02-22-2011, 12:26 AM Thread Starter
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Originally Posted by DarkCobra View Post
What color was the drop checker before you put it in the tank?
It was a dark green color, from atmospheric CO2, I assume. That was what I expected.

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post #7 of 23 (permalink) Old 02-22-2011, 05:58 AM
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So was mine, indicating about 4ppm CO2; which you know was not what I expected.

I mixed a new 0.75dkH solution, refilled the drop checker, and added three drops of pH indicator. Still dark green.

So I tried something different. I put 10ml of the 0.75dkH solution in a test vial, and added two drops of pH indicator. The color was faint, so I stood the vial on a white surface and looked straight down. This time it was blue, at about 7.2 on the chart.

The pH indicator comes out of the dropper a deep yellow, which means it's at pH 6.0 or lower. I'm thinking that the pH indicator itself is enough to throw the result in a volume as small as a drop checker.

I poured out the drop checker and measured its actual contents as 2.0ml. Three drops of pH indicator is 0.25ml, making the drop checker's contents 12.5% pH indicator by volume.

It appears that to get an accurate result, especially around the low end, the initial reading before it's put in the tank should be subtracted from all subsequent readings. Or use a bare minimum of pH indicator.
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post #8 of 23 (permalink) Old 02-22-2011, 09:52 AM
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What are you guys using to get the .75 dKH solution? Any tips on what measurements to use?
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post #9 of 23 (permalink) Old 02-22-2011, 03:04 PM
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Quote:
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What are you guys using to get the .75 dKH solution? Any tips on what measurements to use?
It can be made from a 4 dkH reference solution relatively easily.

Anthony

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post #10 of 23 (permalink) Old 02-22-2011, 04:09 PM
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Quote:
Originally Posted by shawnhu View Post
What are you guys using to get the .75 dKH solution? Any tips on what measurements to use?
And the measurements for making it from a 4dkH solution are printed on the bottom of the card Hoppy made.
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post #11 of 23 (permalink) Old 02-22-2011, 04:14 PM Thread Starter
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I have always wondered about the effect of the reagent on the readings in a drop checker. To check this I did this simple test: the test tube for a pH test kit holds 2 ml of water. With 4 dKH distilled water in the tube, I added one drop of reagent - blue. 2 drops - blue. 3,4,5 etc. drops - all still blue. This convinced me that the reagent is neutral as far as any effect on pH is concerned. I did some reading, and found a DIY method for making pH reagent from bromothymol blue, a dye. In that method, the last step was to add a significant amount of a alkaline substance - I forget which one, but perhaps it was sodium hydroxide. I presumed that this is what makes the reagent neutral.

Another test I did was with a gas permeable membrane drop checker, where there was only a few drops of 4 dKH water in it, plus a drop of reagent. In my aquarium, that DC agreed with my blown glass model DC. That reinforced my conclusion that the reagent doesn't adversely affect the results. But, it is clear that if you had one drop of 4 dKH water, and 4 drops of reagent, for example, it would likely not be very accurate. That just seems logical.

This morning I removed my DC from the tank and stuck it on the outside of the tank, to see how long it takes for the color to go back to green. So far, after 2 hours, it is still yellow.

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post #12 of 23 (permalink) Old 02-22-2011, 04:26 PM
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so we have 4 ppm of CO2 in atmosphere? wow
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post #13 of 23 (permalink) Old 02-22-2011, 05:07 PM
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Bromothymol blue is a weak acid - which is why it is yellow in the bottle. I think that normally this doesn't influence things much until the indicator concentration is significant relative to whatever else is changing the pH. The indicator dyes are fairly strongly absorbing so the concentrations are low. In most cases I think it shouldn't interfere with the drop checker color. However, with 0.75 dKH that might be pushing things. Be interested to see how Hoppy's experiments turn out.
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post #14 of 23 (permalink) Old 02-22-2011, 05:12 PM
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Quote:
Originally Posted by Hoppy View Post
To check this I did this simple test:
I repeated this, using the API pH test. Their test tubes hold 5ml, so I used a graduated ml vial instead and measured out 2ml of 4dhK. Then started adding drops of pH indicator.

Got up to twelve drops and it still appeared blue. At that point, I wondered if the darkness of the solution was altering color perception. I drew a bit up into a thin pipette, and a slight green tint was noted.

Quote:
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In that method, the last step was to add a significant amount of a alkaline substance - I forget which one, but perhaps it was sodium hydroxide. I presumed that this is what makes the reagent neutral.
I wonder if all bromothymol blue pH indicators are created equal? My API indicator drops are yellow, so they're definitely not neutral; and I think I've shown these at least can alter the result. But the indicator drops included with the drop checker were blue green, close to neutral.

What pH indicator are you using, and what color are the drops?
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post #15 of 23 (permalink) Old 02-22-2011, 05:24 PM Thread Starter
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so we have 4 ppm of CO2 in atmosphere? wow
No. The concentration of CO2 in the water, when the rate of escape of CO2 from the water to the atmosphere is equal to the rate of absorption of CO2 from the atmosphere is X ppm. It takes more experimenting than this, to determine what X is. And, X is not the concentration of CO2 in the atmosphere. That is a well researched number, which is constantly rising, about 388 ppm by volume right now.

Quote:
Originally Posted by DarkCobra View Post
I wonder if all bromothymol blue pH indicators are created equal? My API indicator drops are yellow, so they're definitely not neutral; and I think I've shown these at least can alter the result. But the indicator drops included with the drop checker were blue green, close to neutral.

What pH indicator are you using, and what color are the drops?
I know they are not all created equal. I'm using the API pH kit, and the drops are definitely yellow, a clear, transparent yellow. But, I also have a small container of test reagent that came with an Ebay DC, which is a dirty green color, very opaque.

My DC, now attached to the outside of the tank is still yellow, after about 3 1/2 hours. Odd! One thing I'm aware of, is that KH is not going to buffer the pH if it is much below 1 dKH, so .75 dKH may be too low. If that is the case, the color will be irrelevant.

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