Soda lime and CO2 removal questions.

Z

Zapins

New member
Hello everyone, I need some help with using soda lime.

I'd like to use it to scrub all the CO2 out of a 2.5 g tank for a plant experiment. I want to see what the symptoms are of carbon dioxide deficiency for a database I am making.

My setup will be a series of connected 2 Liter soda bottles that will use a bubbler to send atmospheric air through the 2.5g tank.

The first 2 bottles will contain soda lime which will be set up so the bubbler puts air in at the bottom and allows it to bubble up through the soda lime, then the 3rd bottle will be a fresh bottle of water to catch any chemicals that might be carried through, the 4th chamber will be the 2.5g tank and the 5th chamber will be another bottle of water to check there are no leaks in the system.

My questions (and thanks ahead of time):
1) I'd like to know, how do I use the soda lime?

2) Do I just bubble the air up through the dry soda lime or do I mix it with water?

3) How much water do I need to mix?

4) How much soda lime do I need to ensure all CO2 is removed (in lbs or volume)?

5) Should I add a 3rd bottle of soda lime to ensure all CO2 is removed before the air enters the 2.5g tank? Basically how efficient is soda lime at scrubbing CO2.

6) How long will the soda lime last before it needs changing? Days? Weeks? Months?
 
Scrub all CO2 out? The pH will get very high. Depending on the alkalinity of the water, and whether you are talking salt or fresh, it might get above 10, maybe even 11. and might also cause precipitation of calcium or magnesium carbonate.
 
pH may get higher than normal, but that can be adjusted with buffers and acid. Freshwater plants are not extremely sensitive to pH.
 
Zapins said:
pH may get higher than normal, but that can be adjusted with buffers and acid. .
Click to expand...

No it can't. Take a system that is buffered with carbonate and add any acid and you make CO2. Remove the CO2 and the pH goes up. You can have one or the other but never both.
 
You could use a mineral acid (like HCl or H2SO4) and remove the CO2 by bubbling, but that will reduce the alkalinity to almost zero.
 
Thanks for the discussion & advice.

Randy Holmes-Farley said:
You could use a mineral acid (like HCl or H2SO4) and remove the CO2 by bubbling, but that will reduce the alkalinity to almost zero.
Click to expand...

This is what I will do. Alkalinity is not necessary for growing freshwater plants and can be 0 with no adverse effects.

Still interested in knowing people's experience with soda lime since I have never used it personally. Specifically the 6 questions in my original post.
 
CO2 can't be zero if you expect plants to grow. They need a source of carbon. :)

Do a search of CO2 scrubbers. There are tons of threads. However, they do not typically remove all CO2 from the air. How much is needed depends on the flow rate, etc. The more you have the more effective it will be. Beyond that, I think you just have to experiment. :)
 
Randy Holmes-Farley said:
CO2 can't be zero if you expect plants to grow. They need a source of carbon. :)
Randy, I'm not sure if you read my original post fully, but that is the reason I want to scrub the CO2 out of a 2.5g tank.

Zapins said:
.... I want to see what the symptoms are of carbon dioxide deficiency for a database I am making.
Click to expand...

Randy Holmes-Farley said:
Do a search of CO2 scrubbers. There are tons of threads. However, they do not typically remove all CO2 from the air. How much is needed depends on the flow rate, etc. The more you have the more effective it will be. Beyond that, I think you just have to experiment. :)
Click to expand...

I was hoping that I could get some advice or links to specific threads that can answer my questions from someone with first hand experience using soda lime. I could have done an exhaustive Google search if I had wanted to piece together the information I need, however, I thought it might be nice to sign up for this forum and talk with people who have used it. However, if nobody knows the answers to my 6 questions, then I suppose I have no choice but to figure it out on my own :(.
Click to expand...
Click to expand...
 
I don't think anyone has measured the percentage of carbon dioxide that aquarium scrubbers typically remove, so I don't think anyone can help much.
 
I'm not looking for specific 100% accurate answers, I'm just looking for peoples experiences. I have never used this stuff before and you guys have (I think?).

All I want to know is how to use the stuff, does it need to be mixed in water or do people just use it dry? Is there a specific amount of water to use if you use it with water and roughly how long does it last in your tank before it goes bad? How much do you guys use in your x gallon aquarium?

Part of why I'm asking these questions is because I'm trying to figure out if I need to order a full 5 gallon bucket or if I can do my experiment with a few pounds of the stuff. It isn't extremely cheap and I'd like to save money where I can since I have no use for soda lime after I'm done with the experiment.
 
Your questions:

I've used it for a number of years . It generates a pH boost of 0.15 in my system with the air in my basement. The effect will vary depending on CO2 in the air and tank in a specific situation .It's used dry in a container where air passes through it on the way to the skimmer air intake . It does not go into the tank.

I use about 1 liter total , half in each of 2 containers ,each of which feeds one of my two skimmers on the 650 gallon system. It last about month. That changes plus or minus a week seasonally as CO2 levels in the room vary. Some report less effect and faster exhaustion.;depends on a number of variables in the room air and the tank.

I get a 5 gallon bucket for about $ 85 once every 13 months or so from Air Gas a national supplier . Soda lime is used medically during anesthesia administration . I would guess someone in that field might be able to give you more precise information on the exact CO2 removal capacities. Those who use it for aquariums look at it's effect on pH which speaks to CO2 levels but not with the precision you seem to require.

I does turn a violet color when exhausted ;so ,it's easy to know when to change it.

I don't see how it will hep your experiment since the CO2 level in the water will be effected by the air around it not just the lower CO2 air you add with it.
 
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tmz thank you for your reply that is exactly what I was after. I am following Dr. Hershey's experiment plans here: http://www.angelfire.com/ab6/hershey/stcarbon.pdf

Unfortunately he doesn't give very detailed information on how much, and how to setup everything.

I am surprised you get your soda lime for only $85, the air-gas guys near me want $100+ for a 5 gal tub. Judging by your reply I don't think I will need a huge amount of the stuff.

Despite the way my original posts are worded I wasn't after highly specific information. This experiment should reduce CO2 levels far below anything we can achieve in normal aquarium use. The bubbler alone will jostle a lot of CO2 out of the water column leaving behind only a few ppm of dissolved gas, and the soda lime will scrub the bubbler's air of CO2 further reducing the amount of gas in the water. I suppose if I want to make tripple sure no CO2 gas gets into the test tank I could hook up my O2 cylinder and use 100% pure O2 gas, scrub it, and bubble it up through the test chamber. But I'll first try it with just a bubbler since Hershey was apparently able to get good CO2 deficiency results that way.

The point of this is to finally put to rest the general misconception that a "CO2 Deficiency" is a common problem that affects basically everyone and results in every plant damage symptom under the sun.

I'll add the results to my database at www.DeficiencyFinder.com
 
How are you going to drive this air through the water to ensure that other sources of CO2 (other water surfaces, organic decay, etc) are not contributing or even dominating? Will the tank top be closed?
 
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The method used in the article you cited bubbles the air through a saturated calcium hydroxide and water solution ,ie limewater; not soda lime. Soda lime is mostly dry calcium hydroxide with some sodium hydroxide and potassium hydroxide and color indicators. A tank sealed to the air is noted in the article . The soda lime is pellet like and doesn't pack down or clog allowing free air flow . It is made for use dry; not in solution.
 
I plan to silicon the lid of the 2.5g tank down and seal it once the plants are inside. I will weigh down the roots with metal dies so they are oriented correctly and there will be no substrate and no filter to reduce bacterial CO2 generation. The 5th chamber will show if air flow is traveling through the entire system and if there are any leaks. Also, even if there are leaks it would be outward leaks forcing scrubbed air out of the system and would not allow CO2 in. Furthermore, if I can't get CO2 deficiency symptoms by going to all these lengths then it doesn't bode well for the idea that CO2 deficiencies are common.

Also, I wanted to mention that the pH of pure RO/DI water is 7.0, so even with all the CO2 scrubbed out of it the pH should never reach 10 or 11. Freshwater systems don't have all that calcium/magnesium/other salt water mineral content in them to increase the pH that high.

Tmz – I did notice that he used calcium hydroxide, however, I looked up the reaction for how calcium hydroxide scrubbs out CO2 and it mentioned how a few other chemicals are needed in order to get the reaction going properly. After doing a bit more research I found that all these chemicals are premixed as soda lime which is used instead.

I suppose that is why I was a bit confused if I should use water with soda lime or use it dry. I'll use it dry in my experiment since it will scrub the air of CO2 and that is what I'm going for.

Is there a source of soda lime that might sell me a few pounds of the stuff instead of a 37 pound bucket? Perhaps 5-10 pounds?
 
Also, I wanted to mention that the pH of pure RO/DI water is 7.0, so even with all the CO2 scrubbed out of it the pH should never reach 10 or 11.

No, that is untrue. All bicarbonate and carbonate in the water will be converted into hydroxide if you remove all CO2. Depending on the alkalinity, the pH will rise above 10 or 11. At the alkalinity of seawater (2.5 meq/L), that will convert into 2.5 meq/l of hydroxide, which will drive the pH to 11.4. Fresh water often, but not always, has lower carbonate alkalinity. In any case, it has plenty to raise the pH to 10 or higher.

For example, lake Malawi water, which many people mimic for cichlids, has alkalinity similar to that for seawater and so the pH will be 11+ if you remove all CO2 from it.
 
The scrubber will work a better as a solution than dry since the reactions to form calcium carbonate are a lot faster in solution than in the solid phase.
 
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