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Drop Checkers/CO2 Indicators-Why and How

Discussion in 'Articles' started by VaughnH, Feb 9, 2007.

  1. VaughnH

    VaughnH Lifetime Charter Member
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    Using a Drop Checker/CO2 Indicator​

    Why do so?

    Look back thru the posts here and on other forums and you see many cases where people report having 100-200 ppm of CO2 in their tanks, with no fish problems. Others report they have “good CO2”, 20 or even 40 ppm, but they can’t seem to get rid of BBA. Still others have endless algae problems, but using the KH/pH measurements of their tank water they are afraid to raise the CO2 level any more because the fish will all die.

    These are all signs that the KH/pH measurement method just doesn’t work for typical aquarium water. That shouldn’t be a surprise, because all of the write ups about that method, including Chuck Gadd’s, mention that it is only good if there is no phosphate or other ions in the water that affect its KH or pH, and we dose phosphates, add wood and peat which decrease pH, and who knows what else in the water is affecting the KH or pH readings.

    Carbon is the primary element plants are made of. When a plant grows it has to have a supply of carbon, and that is almost always from CO2. While we can measure with considerable accuracy how much nitrogen, phosphorous, and potassium we are making available to our plants, the amount of carbon we make available is largely a guess. It makes good sense to try to know that nutrient’s concentration in the water at least as well as we know the others.

    Someone determined that when they dropped the pH of the tank water by 1.0 (from 7.2 to 6.2, for example), they knew they had enough CO2 in the water, or about 30 ppm. This test was done by taking a sample of tank water, letting it sit until the CO2 in the water “degassed”, reading its pH and then subtracting 1 from that reading as a target for the tank pH. This method depends on the fact that a change of 1 in pH equals a 10 to 1 change in ppm of CO2 in the water. So, if the degassed water ended at 3 ppm of CO2 this would give you 30 ppm of CO2 in the tank. But, those of us who measured the amount of CO2 in water that sits out in the open to degas, found that the amount of CO2 in that water continued to drop for 2 or 3 days, down to as low as .5 ppm. And, a degassed ppm of .5 means the tank water would only be at 5 ppm with a pH drop of 1. So, this method doesn’t work well either.

    The in-tank CO2 indicator has been available for several years, most recently as an elegant glass device sold by ADA and as a look alike from a Hong Kong dealer on ebay. But, the makers of those devices have always told us to use tank water in them, which makes them no better than just testing the tank water directly.

    We need to know how much CO2 we are supplying to our tanks. If we don’t know it with some accuracy we will refuse to increase that amount for fear of fish deaths. And/or, we will spread misinformation about how much CO2 can be or should be used in aquariums.

    Why does a “drop checker” work?

    Imagine a jar of water, with a tight lid on it and an air space above the water. Assume the water has some CO2 in it. CO2 goes into and out of solution with water very easily, so molecules of CO2 will constantly be moving from the water to the air and back. This system must reach an equilibrium where the rate of CO2 leaving the air space is exactly the same as the rate entering the air space. If that were not true, all of the CO2 or none of the CO2 would end up in the air or the water. But, we already know the CO2 molecules easily move from air to and from water, so it can’t all end up in one or the other location.

    So, with X ppm of CO2 in the water and Y ppm in the air space, the system is in equilibrium. Now, add a second blob of water connected to that same air space. The same equilibrium must be reached between the air space and the new blob of water - Y ppm in the air and X ppm in the water. So, the new blob of water must reach an equilibrium where it contains the same X ppm of CO2 as the water in the jar.

    That is what a “drop checker” is - a little blob of water connected thru an air space to another bigger container of water.

    How do we make a “drop checker” do what we want it to do?

    First, what do we want it to do? We want it to show us that we have a certain ppm of CO2 in the water. This gets us back to the KH/pH/CO2 tables and equation. We need to eliminate the reasons why that equation and table don’t work for aquarium water. The way to do that is to use water that meets the requirements of the equation - no ions that affect the KH or the pH except CO2 and carbonates.

    Distilled water has nothing dissolved in it, so no ions are there to affect either KH or pH. Deionized water also meets that criteria. So, if we use either distilled or deionized water in the drop checker the KH/pH/CO2 table works just fine. All we need to do is add some carbonates or bicarbonates to get some KH in the water, and some pH reagent so the water color will tell us its pH.

    Since we want to get a clear indication that we have the right amount of CO2 in the water, we need to select a KH that will make the pH indicator give us the most accurate reading possible. If you look at the color chart for the commonly used pH reagent, Bromothymol blue, you notice that the middle of the chart is green, and a small pH change changes the color to a bluish or yellowish green. That is the easiest point on the color chart to identify. And, that occurs at a 6.6 pH. So, we want the pH to be 6.6 when the water has the desired amount of CO2 in it.

    The hard part is deciding what the desired amount of CO2 is. Up until now everyone has measured their ppm of CO2 using the tank water’s KH and pH, but we know that isn’t accurate. The desired ppm has grown from 10 to 15 to 20 to 30 ppm over the past few years, based on those measurements. For now it seems prudent to assume 30 ppm is the desired amount, but in the future that number might end up being just about any number from 20 to 50 ppm.

    At 30 ppm of CO2, with a pH of 6.6, the KH will be 4 dKH. So, our distilled water should be dosed with sodium bicarbonate to a KH of 4 dKH. That isn’t much bicarbonate of soda!
    You can mix up a batch of 4 dKH water using distilled or DI water as follows:
    • [*]Add 6 grams of bicarbonate of soda to one liter of distilled water. This gives you one liter of water with a KH of 200 dKH. Now take 10 ml of that water and mix with 490 ml of distilled water (a dilution of 1 in 50) and you get 500 ml of 4 dKH water.
      [*]We don’t all have a gram scale capable of measuring 6 grams accurately. Most of us do have a KH test kit. So, it is also possible to arrive at 4 dKH water by mixing a very small amount of sodium bicarbonate into distilled water and repeatedly diluting it with more distilled water until the test kit says we have 4 dKH. Relying upon test kits isn’t a good idea, but in this case it is a good enough idea if we just want to monitor the ppm of CO2 in the water. Buying certified 4 dKH water is a better idea, and that product will very likely be available soon.​
    Setting up the “drop checker” / CO2 indicator
    • [*]Add enough 4 dKH water to the bulb of the drop checker to fill it to where the manufacturer recommends, or half full for the ADA style glass device. A syringe works very well for this. Flushing out the bulb with some of the 4 dKH water is very desirable. Add enough pH reagent to get a strong blue color, but not enough to make the water opaque. The exact amount of reagent isn’t critical.
      [*]Place the drop checker in the tank, under water, so that an air bubble is trapped in it to separate the water inside from the outside, and wait about 2 hours for it to reach equilibrium. It will be green if you have 30 (actually 25 to 40) ppm of CO2 in the water. If it is yellow-green, you have too much CO2, about 50 ppm. If it is blue green, you have too little CO2, about 20 ppm.​

    Another type of drop checker

    This is a slow working device. It takes a couple of hours to reach equilibrium with the tank water. If that is too slow, it is possible to speed it up considerably by replacing the air space separating the tank water from the drop checker water with a gas permeable membrane. Many thin plastic or rubber materials are gas permeable. One such material, that works surprisingly well, is Tyvek, which is the material Priority Mail envelopes are made of. A small container of water capped with that material will act as a drop checker.

    To achieve a rapid response requires that the volume of water in the drop checker be at a minimum and the area of the membrane that allows CO2 to pass thru be at a maximum. For a disc shaped blob of water, that means a thin disc, shaped like a coin. To further minimize the volume of water a piece of sponge can be fitted in the water chamber to displace some of the water. If the disc of water is loosely filled with sponge and is only about a sixteenth of an inch thick, the device will reach equilibrium in about a half hour. It will be much harder to read the color of the water in it, but it will work just as the air gap type drop checker works. No commercial version of this design is now available.
     
  2. dcheese44

    dcheese44 Junior Poster

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    In an earlier post you had said that you had found a drop checker on ebay, do you what type or brand it was?
    And I don't wish to be rude in any manner, how long have you had this experiment going on?
    After reading and following your posts on the drop checker did you test all of the levels simultaneously in seperate tanks or were adjustments made to the same tank over time? How did you compare colors or was it a judgement call, or did you take pictures?
    I am very interested in tracking/mointoring CO2 content but I have doubts about most of the methods out there. Can one just decide if there is enough CO2 in the tank by the appearance of it's inhabitants(fish /plants/algae)?
     
  3. neil1973

    neil1973 Prolific Poster

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  4. VaughnH

    VaughnH Lifetime Charter Member
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    The drop checker I purchased is similar to this one on ebay: eBay: Co2 Drop Checker-monitoring proper dosage of CO2 (item 250082840577 end time Feb-16-07 08:24:41 PST) There is another style also sold on ebay: eBay: NEW!! Co2 Drop Checker-Highest quality handmade glass (item 250082505885 end time Feb-15-07 08:21:05 PST) And the Red Sea unit is some times sold on ebay, but doesn't seem to be offered now.

    I started looking at using the drop checker about 6 months ago, as I recall without checking back. During that time I have done many different tests ranging from in-tank comparisons to counter top testing in a measuring cup while blowing into the water to get CO2 in it. I've tested many DIY versions of the device. And, it is obvious that this is such a simple device that any version will work, and most work very well, whether DIY or purchased. I took a lot of photos during that period. One place I posted a lot is on APC, at:DIY Drop Checker - Aquatic Plant Central

    You can do all fertilizing or CO2 injecting by just watching the plants and fish, but that takes experience in order to understand what you are seeing. Most of us don't have that experience.
     
  5. Frolicsome_Flora

    Frolicsome_Flora Guru Class Expert

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    Hoppy: I had an idea while standing in the shower this morning, like you do.

    What do you think to the idea of making a drop checker, with a ph controller probe glued into it? That way a PH controller might actually become useful instead of so hit and miss like they are at the moment, because of all the other tank born PH altering elements.

    Dont know how feasable it would be to do, because frankly I dont know how big a PH probe is, but it seems to me that if it could be done, you would have a pretty amazingly accurate way of controlling things.
     
  6. VaughnH

    VaughnH Lifetime Charter Member
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    Tom is working on getting someone to make just that type of pH probe, converting it to a CO2 probe. This only works if the response time of the device is a lot faster than our drop checkers. So, it requires that the pH probe be in a very tiny amount of reference KH water, and have a very large ratio of membrane area to water volume. (Obviously, it has to be done with a gas permeable membrane to have any hope of achieving the rapid response time.) If the response time is too long the pH controller would be constantly overshooting and undershooting the desired pH, so as to be totally useless.

    Also, any membrane device that is left in the water for very long will see the membrane pores becoming increasingly plugged, due to biofilm. So, this would not be a "stick it in and forget it" device.
     
  7. Frolicsome_Flora

    Frolicsome_Flora Guru Class Expert

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    Yeah, Id not thought of that.. the response time would get you into trouble over and over. :)

    I wonder though if the volumn of the air between the 2 fluids could be kept to a minimum, whether that would increase response time more so.. less volumn to equalise and wotnot. Even then though, like you say, it'd still be too slow.
     
  8. Bartman

    Bartman Prolific Poster

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    Well, diffusion in air is orders of magnitude faster than diffusion in water so I'm not sure that would give you much advantage over the current designs.

    The mass transfer across a gas-liquid boundary and within the liquid phase itself are probably the rate limiting steps.

    Reducing the volume of liquid and increasing the mass transfer across the gas-liquid boundary would probably have much more impact. The mass transfer across the boundary can be enhanced by using the proper membrane or by maximizing the gas-liquid interfacial area. In other words, rather than shrinking the volume of gas, you’d be better off with a very wide, thin disk of reference solution in contact with the gas phase or a semi-permeable membrane.

    I think I saw some comments along this line of thought by Hoppy or someone in another thread where they were looking at using a membrane rather than an air gap between the tank water and reference solution.
     
  9. Tom Barr

    Tom Barr Founder
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    There is no gas-liquid transfer in the pH probe method, it's liquid-liquid across the DO membrane.

    The membrane just maintains the separation of the KH solution,(non gas), the gases can move freely.

    The accuracy of the color for the pH inn the drop checker is limited by your color perception, which is rather poor.

    The pH meter is not.

    So the pH meter method will give you much more accurate measure either way.
    since the phase change is not required, and the volume of KH ref solution is small relative to surface area, the effect is rapid.

    Note, the tip can have very little KH ref between it and the outside water, a thin layer is all that is required.

    Regards,
    Tom Barr
     
  10. rcalzadilla

    rcalzadilla Prolific Poster

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    BTB (Bromothymol Blue) source

    Where can one buy, reasonable priced:
    BTB (Bromothymol Blue) solution
    4dKH certified water solution.
    For Drop Checker use, of course.
    thanks

    raul
     
  11. VaughnH

    VaughnH Lifetime Charter Member
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  12. rcalzadilla

    rcalzadilla Prolific Poster

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    Drop checker doesn't turn blue

    Just received a drop checker I purchased on eBay. The glass type with the bell, and bulb at the other end.

    The instructions read:

    How to use it :
    1. Fill the checker bulb with 2/3 of distill water then add 3 to 5 drops of indicator solution to blue color
    2. Careful put the drop checker about 3 inches under the water level.
    3. After a couple of hours, you can match the color of the indicator solution with the test card



    The distilled water only turns amber/yellow as is the color of the regent.
    Contacted seller buy insists it should turn blue.

    Does anyone have any idea why it doesn't turn blue?

    I tried using 4gKH water but only turns the water yellow.



    PS: Seller now says this is a new regent, that it is supposed to be amber and not blue. I don't feel comfortable with this answer and his not knowing what color his product should be
     
  13. mrkookm

    mrkookm Guest

    That new reagent sounds suspect to me, ditch it! Pick up a new PH test kit and use 2 drops of it's reagent in your DC in which you should have 4KH solution, you cannot use just DI water in your DC.

    If you do not have a ref 4KH solution reach out to the guy or gal here for a 500ml bottle which will easily last for almost 2 years and thats factoring changing the DC weekly.
     
  14. rcalzadilla

    rcalzadilla Prolific Poster

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    Thanks for reply.
    I was wrecking my mind with this, but doubting my understanding on the subject which is as you concur.

    I ordered 4 bottles of regent and each with the same result: amber/yellow and does not change color in the tank. The tank has, at least, 19ppm CO2.

    Is it allowed in this forum to warn other by giving out the name?

    raul
     
  15. rcalzadilla

    rcalzadilla Prolific Poster

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    Thanks for reply.
    I was wrecking my mind with this, but doubting my understanding on the subject which is as you concur.

    I ordered 4 bottles of regent and each with the same result: amber/yellow and does not change color in the tank. The tank has, at least, 19ppm CO2.

    Is it allowed in this forum to warn other by giving out the name?

    raul

    PS Here is a picture of the DC 6 hrs in the tank.
     
  16. rcalzadilla

    rcalzadilla Prolific Poster

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    Thanks for reply.
    I was wrecking my mind with this, but doubting my understanding on the subject which is as you concur.

    I ordered 4 bottles of regent and each with the same result: amber/yellow and does not change color in the tank. The tank has, at least, 19ppm CO2.

    Is it allowed in this forum to warn other by giving out the name?

    raul
     
  17. rcalzadilla

    rcalzadilla Prolific Poster

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    Is my DIY 4dKH accurate ?

    While I wait for my certified 4dKH I have made my own batch using kitchen measuring utensils and TB's instructions.
    It turned out over 4dKH so I added distilled water and tested, distilled water and tested again until it tested out 4dKH.
    How accurate is this?

    I am using it in my DC and it's looking good by using regent from an API PH test kit. Three drops regent + my 4dKH.
    First it turned blue and after about 4 hrs turned green.
     
  18. VaughnH

    VaughnH Lifetime Charter Member
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    There is no reason not to make your own 4 dKH water, provided you use distilled or deionized water, and have a good KH test kit. Since the measured ppm of CO2 is directly proportional to the KH of the drop checker water, even a 20% error in KH still gives a more accurate CO2 measurement than trying to do it with tank water KH and pH. I find it extremely difficult to arrive at 4 dKH water when trying to make it myself - I usually go through a half gallon of water before finally getting close. That's why I gave up the DIY method.
     
  19. mrkookm

    mrkookm Guest

    .......X 2
     
  20. rcalzadilla

    rcalzadilla Prolific Poster

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    Tks for reply.

    Actually I know that the 4dKH I was able to make was pure luck.
    I just poured some distilled, without measuring, and it tested out 4dKH !
    Don't know if I'll be able to do it again but, I'll see when I run out of my present supply.

    As to the accuracy of my PH test kit, it's Aq. Pharmaceuticals. Is this considered a good test?
     
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