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I am using a drop checker from Hong Kong. I made 4 dKH water from RO/DI, put it in the drop checker along with six drops of AP pH test.

It was dark blue, and within a few hours was light green bordering on yellow. Great!

Well, the weird thing is that the drop checker water stays that color, even after a night of no CO2 and surface aeration with an air stone.

Seems pretty weird to me.. I would expect it to return to a blue color..
 

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That does seem weird. I am finding that the CO2 level in the tank drops very slowly, but by morning my drop checkers all show blue or near blue. And, I don't use an air stone at night, or increase the surface agitation. How are you turning off the CO2 at night? Obviously, I am thinking that you aren't really shutting it off.
A timer turns off a solenoid.. and I use a Red Sea 500 reactor so it's pretty obvious when CO2 is being injected because a bubble will form. There's that and the fact that the bubble counter attached to the canister doesn't show any bubbles :)

I guess it doesn't matter.. it just makes me a little suspicious of what the green color is really telling me..

I think I am going to try using the drop checker in a glass of tank water at equilibrium with the room.

I expect it should stay blue..
 

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You know, I am wondering about something here.. the idea as I understand it is that the liquid inside the drop checker contains no acids and then after it is put in the tank, exchanges CO2 in the air pocket with the water from the tank until they are at equilibrium.

Fine, that makes sense.. but why won't other acids in addition to carbonic acid also enter the water in the drop checker?

I think the point of the drop checker was to isolate carbonic acid from other possible 'contaminating' acids in the tank water that render the usual KH/PH/Co2 calculation erroneous.

It seems like if there was say, tannic acid from driftwood that it too would enter the water in the drop checker..

I'm sure this has been covered.. so help me understand why we are so positive that CO2 is the only thing that can enter the water in the drop checker.
 

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I wasn't asking about the membrane. I was asking about the basic drop checker.

What assurance is there that only CO2 will enter the colored water in the drop checker?

If CO2 can enter, then other acids might be able to as well.

The membrane is supposed to be gas permeable, but not liquid permeable. So, liquids can't pass thru it.
 

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In other news, I removed the drop checker from my tank and stuck it on my mirror in my bathroom. Within a day the color went back to blue.

I had also taken a glass full of water from the aquarium and left it out over night to degass.

I submersed the drop checker (now blue) into the glass of water, and it is staying blue, confirming that indeed the CO2 level in the glass of water went way down.

So I guess I still have no idea why on Earth the drop checker's color stays green even after a night of surface aeration and no CO2.

Weird. But, I am comfortable in believing that green means good CO2 IF it is true that ONLY CO2 can enter the drop checker's water.

I still wonder why other acids in the tank water can't also enter a gas phase and enter the drop checker's water.

Hoppy, can you confirm for me the intent of the drop checker?

In a tank with a giant piece of drift wood, e.g. tons of tannic acid, your drop checker (not the membrane idea) should still give an accurate reading of CO2. Is that correct?

So there is the unstated assumption here that acids other than CO2 cannot pass through the gas pocket inside the drop checker.

I still want to know how we know that...
 

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I'm sure I'm missing something obvious, but as I understand it, atmospheric CO2 concentration is somewhere north of 300 ppm. Why then are drop checkers supposed to be blue when they are sitting out, and not yellow?

I say "supposed" because I think I "broke" mine when I tried adding pH drops to intensify the color, and after a few days it assumed a yellow color that has stuck, whether in or out of the tank.

I wish I paid more attention in chemistry.
No, there's only about 3 ppm of CO2 in the atmosphere.

It should definitely be blue if it's exposed to the air for an hour or so.
 

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My reading indicates that there's ~300ppm atmospheric CO2, ~3ppm of which is willing to dissolve into aquarium water. In streams, lakes etc, you get more than 3ppm CO2 in solution (underground, minerals, other organisms contribute to CO2 more in nature than in aquariums).

Note to MTechnik - I bet that checker didn't change color ove the course of Borat - the test tube appears to be too skinny. I'd suggest a fatter one and a lot less liquid in it (enough to get liquid column height=width should be good).
Where do you get that information?

I thought a liquid would always reach equilibrium with its surrounding atmosphere.. so there should be the same amount in a standing body of water as there is in the surrounding air, up to the point of saturation of the liquid of course.

I would love to know why CO2 magically stays out of equilibrium.
 

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Now I remember why I got a degree in computer science instead of chemistry!

But seriously, instead of all the unnecessary rhetoric and quasi-scientific jargon, let's get to the point:

your drop checker should turn blue if exposed to air for an hour or so.

If it doesn't, you're doing something wrong. End of story.
 
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