~Explanation of how a drop checker works~
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Old 05-09-2011, 02:19 AM   #1
btimmer92
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~Explanation of how a drop checker works~


It has come to my attention that not everyone knows how a drop checker works, or why it should be used. Since this piece of equipment is normally a vital tool to dosing CO2, I thought I would explain in detail.

What is needed to use a drop checker:
glass drop checker
suction cup
Bromothymol Blue (pH drops)
standard reference soltuion

Directions:
Fill the compartment with reference liquid about halfway, if it is a bulb type. Then put about 3 pH drops in, or however many it takes to get the perfect opacity of blue, not too clear, not too dark. Suction the drop checker to the inside wall of the tank, under water. Keep it right side up, so no tank water gets in the bulb. Wait a few hours and compare the color to your target color.

A reference solution comes in different dKH. The most common is 4dKH. If you are using 4dKH, this is how you determine your CO2 levels:

Blue - not enough CO2
Green - 30ppm CO2 (target level)
Yellow - too much CO2, dangerous for inhabitants

Some science behind it.
A drop checker is basically a pH test, but not for your tank water. It measures the pH of your drop checker solution, which indicates CO2 levels. How it works is there is an enclosed air chamber trapped between the tank water and the drop checker liquid. The CO2 in your tank will flow freely into the trapped air, and then intothe drop checker compartment. It will do this until the water in the tank and the water in the drop checker reach equilibrium. This means that there is equal concentrations of CO2 in both chambers of liquid. CO2 is the only variable effecting the pH in the drop checker, therefore pH in the drop checker is directly related to CO2 levels in both the tank.
Effect of CO2 and KH on pH:
The more CO2 dissolved in water, the lower the pH will be.
The lower your hardness, the lower your pH will be
There are several other miscellaneous parameters that could effect pH

A reference solution is a solution that has an exact hardness degree that is known. In addition, there are no other unknown water parameters that could affect the pH reading.

Every water parameter is constant, an known, except the CO2 level. The amount of CO2 is the only thing that is affecting the pH. Therefore, the pH reading in the drop checker solution correlates to an exact amount of CO2 .

For example, in a 4dKH reference solution of water and baking soda, a pH level of ~6.6 indicates 30ppm of CO2. A 6.6 pH with Bromothymol Blue turns this solution green, and therefore when the drop checker is green, you have 30ppm of CO2. In conclusion, a drop checker test is just a pH test, in water that has no other variables beside CO2.
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Old 05-09-2011, 02:35 AM   #2
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thanks!
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Old 05-09-2011, 02:43 AM   #3
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well done, btimmer92!

don't forget to add the relationship between light intensity, fert, and CO2 level.
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Old 05-09-2011, 03:01 AM   #4
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For some reason, I need to get my drop checker closer to yellow than green to get any response from the plants. Fish are happy. So, in my experience the 'green' range wasn't enough. I don't understand why, though.
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Old 05-09-2011, 03:11 AM   #5
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The green range could indicate as little as 20 ppm, since each of us has a different perception of what "green" is, and the ppm of CO2 is very sensitive to that color being exactly "right". And, the green range could indicate as much as 45 ppm, for the same reason. Not all plants react the same to various CO2 concentrations, so what is needed to get a vigorous response can be different for different plants.
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Old 05-09-2011, 03:37 AM   #6
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Another good article.

http://www.njagc.net/articles/co2dropchecker.htm
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Old 05-09-2011, 04:23 AM   #7
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Went and checked my controller and its at 6.6! Thanks!
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Old 05-09-2011, 04:29 AM   #8
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Hold on scape, 6.6 should be the pH of the drop checker. Any number of things could affect the pH of the water. If you are using a controller, you should be focused on the pH drop between CO2-on, and CO2-off. Reference the red and blue highlighted lines.

Very good points to add, everyone!
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Old 05-09-2011, 05:03 AM   #9
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Quote:
Originally Posted by Dave-H View Post
For some reason, I need to get my drop checker closer to yellow than green to get any response from the plants. Fish are happy. So, in my experience the 'green' range wasn't enough. I don't understand why, though.
I get the same thing.

Quote:
Originally Posted by Hoppy View Post
The green range could indicate as little as 20 ppm, since each of us has a different perception of what "green" is, and the ppm of CO2 is very sensitive to that color being exactly "right". And, the green range could indicate as much as 45 ppm, for the same reason. Not all plants react the same to various CO2 concentrations, so what is needed to get a vigorous response can be different for different plants.
That seems like a very valid explanation.
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Old 05-09-2011, 05:06 AM   #10
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-The more CO2 dissolved in water, the lower the pH will be.
-The lower your hardness, the lower your pH will be
-There are several other miscellaneous parameters that could effect pH

You mention in this second point that the lower your hardness, the lower your pH will be. I saw Hoppy say in one post that carbonate hardness (KH) does not affect how much your pH will drop due to CO2 (i.e. 30ppm CO2 always drops the pH by 1.0 no matter if it's in 1dKH water or 15dkH water). Can you elaborate on what you meant when you stated the 2nd point?
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Old 05-09-2011, 05:39 AM   #11
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He was saying that KH doesn't affect the RATE of change of the pH due to CO2. 0 to 30 ppm will always cause a 1 point pH swing. The rate is (1 point per 30 ppm). He is saying that that rate is not affected by KH. However KH does affect the actual pH itself. It is kind of complicated to explain because it involves calculus and derivatives.
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Old 05-09-2011, 05:46 AM   #12
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Ah I see. I thought you were referring also to the CO2 related drop in pH when you posted that. Anyway, is there a source to read about the relationship if it's too complicated to explain on the forum? As long as it doesn't go beyond partial differential equations or transformations of any sort, I should be able to follow.
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Old 05-09-2011, 01:29 PM   #13
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when you guys say "reaction" from your plants to the CO2, what do you mean? i've been wondering if my CO2 diffusion is working well or not, but haven't determined what the visual signs should/would be.
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Old 05-09-2011, 02:46 PM   #14
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Quote:
Originally Posted by VeeSe View Post
-The more CO2 dissolved in water, the lower the pH will be.
-The lower your hardness, the lower your pH will be
-There are several other miscellaneous parameters that could effect pH

You mention in this second point that the lower your hardness, the lower your pH will be. I saw Hoppy say in one post that carbonate hardness (KH) does not affect how much your pH will drop due to CO2 (i.e. 30ppm CO2 always drops the pH by 1.0 no matter if it's in 1dKH water or 15dkH water). Can you elaborate on what you meant when you stated the 2nd point?
Not quite: I believe what I said was that a drop in pH of 1.0 means the ppm of CO2 went up by a factor of 10. So, if the ppm of CO2 in the water started at 3 ppm, the final amount is 30 ppm, no matter what the KH of the water. If I didn't say that, I should have.
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Old 05-10-2011, 02:28 AM   #15
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Quote:
Originally Posted by VeeSe View Post
Ah I see. I thought you were referring also to the CO2 related drop in pH when you posted that. Anyway, is there a source to read about the relationship if it's too complicated to explain on the forum? As long as it doesn't go beyond partial differential equations or transformations of any sort, I should be able to follow.
I am not sure of actual mathematical equations relating these parameters to pH, all I know is what I stated in the post, that certain parameters either have a positive or negative relationship with each other. Not sure what kind, such as direct, quadratic, etc. that could get very complicated, too complicated to bother with. But what I was saying, about the derivatives, if what hoppy was saying is correct, then that means that change in pH (Y) is a function of change in KH (X) (X change in KH results in Y change in pH). This means that the derivative of this function (dy/dx) is independent of CO2. This does not mean that the variable of pH (Y) itself is independent of CO2. If it was, then (dy/dz) would be zero, meaning that CO2 would not affect pH. So, pH has some sort of relationship with CO2 (Z) a very complex one. pH (Y) is dependent on both pH and CO2, but dy/dx and dz/dy are independent of each other.

Ok, hold on, based on what hoppy said, the equation is Z/Zo=10^(Yo-Y) which means Z=Zo[10^(Yo-Y)]
so to calculate CO2(Z), you plug in starting ph for Yo, and current pH for Y and I guess we are assuming that the starting CO2 is 3ppm? That would have to be true for the equation. Is that true Hoppy? is that for aquarium water?

Wow that was a doozy, im gonna forget about math for a few days now...
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