# Drop Checker for DIY CO2



## DarkCobra (Jun 22, 2004)

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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## jahmic (Jan 30, 2011)

and sometimes...it's the short, simple how-to threads that deserve a sticky vote


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## Hoppy (Dec 24, 2005)

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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## DarkCobra (Jun 22, 2004)

What color was the drop checker before you put it in the tank?


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## Hoppy (Dec 24, 2005)

DarkCobra said:


> 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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## DarkCobra (Jun 22, 2004)

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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## shawnhu (Jan 23, 2009)

What are you guys using to get the .75 dKH solution? Any tips on what measurements to use?


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## Darkblade48 (Jan 4, 2008)

shawnhu said:


> 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.


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## DarkCobra (Jun 22, 2004)

shawnhu said:


> 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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## Hoppy (Dec 24, 2005)

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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## Brian3 (Jan 14, 2008)

so we have 4 ppm of CO2 in atmosphere? wow


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## DrLav (Dec 20, 2010)

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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## DarkCobra (Jun 22, 2004)

Hoppy said:


> 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.



Hoppy said:


> 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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## Hoppy (Dec 24, 2005)

Brian3 said:


> 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.



DarkCobra said:


> 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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## DarkCobra (Jun 22, 2004)

DrLav said:


> However, with 0.75 dKH that might be pushing things.


That's my concern. Assuming the drops are 0dkH, my final drop checker solution was 0.66dkH, in addition to the weak acid effect of the bromothymol blue. Small errors to be sure, but in a solution as sensitive as this it appears to make a noticeable difference.

At the moment, I've managed to squeeze 3ml of 0.75dkH in my drop checker (instead of 2ml), along with only one drop of pH indicator (instead of three). That resulted in the starting color being blue instead of green. But it's so faint it will require putting a white backdrop in the tank to read. I'm just curious how much this will affect the final color, at the same time of day as the last attempt.


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## Hoppy (Dec 24, 2005)

My yellow DC didn't change color in 5 hours, so I made a fresh mix to replace it with - I mixed 2 mL of .75 dKH water with 1 drop of reagent in the pH test test tube. That was blue. Then after about 15 minutes I took a straw and gently blew into the tube to get some CO2 mixed in. It changed to yellow green with 3 gentle puffs. I refilled the DC with this and put it back in the tank. Now the color is hard to see, but it is visible, since I have a white background built into the DC. Once that gets yellow or whatever color after 2-3 hours I will leave it out of the tank overnight to see if it goes back to blue/green. If this fails I will concede that .75 is too low a KH, and try it with 1.0 dKH.


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## Brian3 (Jan 14, 2008)

Hoppy said:


> 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.
> 
> .


you are saying that the air have 388 ppm of CO2! so a non CO2 tank is beter aereate it? wow


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## DarkCobra (Jun 22, 2004)

Brian3 said:


> you are saying that the air have 388 ppm of CO2! so a non CO2 tank is beter aereate it? wow


Don't get too excited yet.  CO2 PPM in air and water are measured two very different ways which makes direct comparison of the two numbers incompatible. Well-aerated water at best will be no more than 0.5ppm, or 2-4ppm depending on who you ask.


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## Brian3 (Jan 14, 2008)

DarkCobra said:


> Don't get too excited yet.  CO2 PPM in air and water are measured two very different ways which makes direct comparison of the two numbers incompatible. Well-aerated water at best will be no more than 0.5ppm, or 2-4ppm depending on who you ask.


OK that said, so in water we can get higher concentration of CO2 adding fish and no surface agitation? Interesting


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## Hoppy (Dec 24, 2005)

My latest test worked. I had 1 drop of indicator solution in 2 ml of .75 dKH water. It turned yellow in the tank. I pulled it out and stuck it to the outside of the tank. Within 3 hours it was green again. So, it is now back in the tank. I'm convinced this works. But, I think with that small a DC you need to mix the water and indicator in a test tube before squirting it into the DC.


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## DarkCobra (Jun 22, 2004)

It's working great here too.

I might tweak it more eventually, but overall I'm very happy with it.  And it's surprising how much CO2 a single 2L will provide in a low-light 25G, even with the Eclipse filter probably offgassing quite a bit.

Thanks Hoppy!


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## Tamelesstgr (Jan 11, 2008)

This is very interesting to me as I will soon be starting up my 2L on my 20H.


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