CO2 vs pH - The Planted Tank Forum
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post #1 of 13 (permalink) Old 09-28-2015, 05:47 PM Thread Starter
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CO2 vs pH

Since we really cannot accurately test to find how much CO2 is available to plants in water, (see 'How to test for CO2' thread) how do we know it is not the lowered pH as a result of carbonic acid that is helping plants grow healthy? I have read that CO2 quickly forms carbonic acid once introduced to H2O. So maybe it is not the CO2, but the pH that effects plant life.

Too much CO2, large drop in pH = fish kill
Too little CO2, pH can rise without the buffer = fish kill, plant stagnation, algae growth.

So CO2 is not agent for plant growth. pH is the agent?
If this is so, controlling pH is key to plant growth.

Any reason this thinking is wrong?
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post #2 of 13 (permalink) Old 09-28-2015, 06:03 PM
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Too much co2 kills fish b/c you suffocate them.
If co2 drops ph. too little of it causes less of a drop. Not raise it.
Plants use co2 as part of photosynthesis. The more you have available the more the plants can use
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post #3 of 13 (permalink) Old 09-28-2015, 07:59 PM
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Agreed.. it's not the pH that is helping the plants... It is absolutely the CO2.

Plants engage in photosynthesis to produce food. Photosynthesis takes in CO2 and H20, and uses light energy to manipulate it into hydrocarbon sugars and O2. CO2 is a critical factor in the plants ability to make food.

Many plants need to take "extra measures" to draw in CO2 (you can research carbon concentrating mechanisms in plants on your own). The lower the CO2 concentration, the more energy wasted by the plant drawing in CO2. This results in less net sugar produced per day, but is overall better than not making sugar at all because no CO2 is entering the plant.

Some plants can also reap carbon from carbonate, but this takes substantially more energy, and again results in less sugar energy produced per photon hitting the plant.

Abundant CO2 concentrations allow CO2 to more readily enter the plant, increasing the overall net energy production for the plant.. More usable energy yields more growth, unless fertilizer nutrients become a limitation. (fertilizer nutrients aren't needed for photosynthesis, but are needed for growth an cell maintenance).

This is also why CO2 injected tanks generally need substantially higher fertilizer dosing than non-injected. CO2 yields faster growth potential, and faster nutrient use by the plants.


edit:

If you don't believe this, it is possible to create water that is low in pH and low in CO2 by using non-carbonate buffers. The CO2/pH/KH charts all work on the idea of the water having all of its pH buffering being from carbonates. However, using phosphate or organic acid buffers you can create water that has low pH, reasonable alkalinity (which will cause KH tests to claim high KH even though that isn't really carbonate), and low CO2.

Plants in general will not grow particularly quicker in this low ph/low CO2 water than they will in higher pH/low CO2 water. Of course there is some preference to low or high pH by some plants, so you will find a few that do better in one or the other. However, compare the growth rate of either to CO2 injected water and it is just no comparison. And the basic facts of photosynthesis should make the reasons obvious.

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Last edited by mattinmd; 09-28-2015 at 08:15 PM. Reason: added test proposal.
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post #4 of 13 (permalink) Old 09-28-2015, 10:00 PM
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Yup, plants need carbon to make sugars. CO2 is where most plants get their carbon.
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post #5 of 13 (permalink) Old 09-29-2015, 12:09 AM Thread Starter
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And, from what I have read, CO2 pushes the OH- action which lowers pH. Plants also use hydrogen for photosynthesis. As they do, the pH goes up.

'The alkalinity of water also plays an important role in daily pH levels. The process of photosynthesis by algae and plants uses hydrogen, thus increasing pH levels ⁰. Likewise, respiration and decomposition can lower pH levels. Most bodies of water are able to buffer these changes due to their alkalinity, so small or localized fluctuations are quickly modified and may be difficult to detect ⁰.'

pH of Water - Environmental Measurement Systems

So it could be that the greater availability of hydrogen due to CO2, which happens to lower pH, increases plant growth.

'The correct way to notate the dissolving of carbon dioxide into water is:
CO2(g) + 2H2O(l) <-->> H3O(+) + HCO3(-).
Since carbonic acid is (defined as) a weak acid, the amount of the acid, H3O(+) is low, slightly increasing pH. HCO3(-), the strong conjugate base, drives the water <--> acid/base equation, H2O(l) <<--> H3O(+) + HO(-), to the left. greatly decreasing pH (actually, it greatly increases pOH). The small amount of H3O(+) is greatly countered by the large amount of OH(-), and that decreases pH. '
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post #6 of 13 (permalink) Old 09-29-2015, 12:15 AM
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Plants can extract hydrogen readily from h2o at more or less any pH.

It's the carbon that is the problem in aquatics. No really.

Any acid will cause the h3o+ effect to occur, but no other acid improves plant growth like carbonic acid.

edit:
Hence, one should conclude it is not the extra free hydrogen, but rather the carbon.

Also, keep in mind that photosynthesis requires just as much CO2 as H2O to make sugar.

The end result of photosynthesis is glucose.. to simplify the light+dark reactions together you get:

6 CO2 + 6 H2O + energy (sunlight) -> C6H12O6 + 6O2

Your tank without injection has ~3-10ppm of CO2 in water... it has nearly 1,000,000 ppm of H20. Ok, maybe it is closer to 999,500 ppm, assuming 500ppm of other stuff dissolved in water... But really, there's no real shortage of hydrogen around in an aquarium.

CO2 injection typically raises CO2 concentrations to 30ppm.. still scarce, but 3-10 times more concentrated than would happen without it.

Besides, I'm not even sure a plant can take advantage of already freed-up H+. You could look at the light-dependent reactions and try to work out how a plant could use H+ in converting ADP to ATP...

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Last edited by mattinmd; 09-29-2015 at 01:16 AM. Reason: added more
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post #7 of 13 (permalink) Old 09-29-2015, 04:09 PM Thread Starter
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Does H2O readily give up a hydrogen for plant uptake, or does it require an acid such as carbonic or chlorophyll to help release that hydrogen for plant use? I'm thinking water is way too up tight to give up anything without a lot of energy.
I know that chlorophyll breaks down water into hydrogen and oxygen for plant use. CO2 in water creates Hydrogen ions. Do plants use that hydrogen? If so, wouldn't that take less energy than to use chlorophyll to break down water? So then plants grow faster with less energy output?

I am sure CO2 is helpful in plant growth. What I am wondering is by what means is CO2 helpful. :}

Last edited by AWolf; 09-29-2015 at 06:26 PM. Reason: clarification
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post #8 of 13 (permalink) Old 09-29-2015, 06:35 PM
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Quote:
Originally Posted by AWolf View Post
I am sure CO2 is helpful in plant growth. What I am wondering is by what means is CO2 helpful. :}
It provides the carbon to form carbohydrates.
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post #9 of 13 (permalink) Old 09-29-2015, 06:57 PM
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Plant tissue is made up of water, carbon, nitrogen, etc. in that order by percent of weight. Lots of carbon is needed compared to the other nutrients, NPK and trace elements. That carbon has to come from somewhere, and with terrestrial plants, it comes from CO2 in the atmosphere. For aquatic plants it comes from CO2 dissolved in the water, or, with a lot less efficiency it can come from the Excel or metricide we add to the water. With the very limited supply of CO2 in our tanks from the atmosphere and from biologic activity in the substrate and from the livestock, plants are very limited in their growth rate. Add CO2 to the water, from a tank of CO2 or from a sugar/yeast system, and the carbon available goes up so much the plants can grow much faster. This is about as certain as the rising sun in the east.

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post #10 of 13 (permalink) Old 09-29-2015, 08:20 PM
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Quote:
Originally Posted by AWolf View Post
Does H2O readily give up a hydrogen for plant uptake, or does it require an acid such as carbonic or chlorophyll to help release that hydrogen for plant use?
Neither...

Plants use light energy to split water.... it is part of the photosynthesis process, specifically the light-dependent reaction (photosynthesis is a 2 stage process). That same reaction that splits water also captures energy by converting ADP to ATP. The second reaction then uses the hydrogen ions, CO2, and energy from the ATP being converted back to ADP, to form sugar.

Remember, photosynthesis works by taking light energy, and using it to convert simple oxides (H2O and CO2) back into hydrocarbons (glucose sugar).. Those hydrocarbons can later have their energy released by re-oxidizing them back into H2O and CO2.

Quote:
Originally Posted by AWolf View Post
I am sure CO2 is helpful in plant growth. What I am wondering is by what means is CO2 helpful. :}
CO2 is used to provide the carbon needed to form the backbone of sugars..


Earlier I stated the overall input/output of photosynthesis:

6 CO2 + 6 H2O + energy (sunlight) -> C6H12O6 + 6O2

That simplifies the overall process, as there's actually 2 complex reactions going on and several intermediary compounds used (chlorophyll, ADP/ATP, etc), but the intermediaries aren't consumed and just get used over and over again...

Regardless, you an see the glucose molecule, which is food energy in the form of sugar, is C6H12O6... You can't make this molecule without carbon.. It is absolutely critical to the plant to manufacture its own food and store it..

Some of those sugars never get burned by the plant as energy, and instead get bonded together to form cellulose, which is the "fiber" that makes up the physical structure of the plant.

Plants, like animals, are fundamentally a hydrocarbon based life form. Carbon is *very* important to all living things on earth. All sugars, starches, "dietary fibers" and fats are fundamentally just chains or rings of carbon atoms with hydrogen and oxygen hanging off them in different ways.
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Last edited by mattinmd; 09-29-2015 at 08:32 PM. Reason: more
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post #11 of 13 (permalink) Old 09-30-2015, 03:31 AM Thread Starter
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Very good education here today! Still wishing for an accurate CO2 test. I was hoping that there was some process in the in water that would measure CO2 more accurately than pH. If CO2 didn't have such a simultaneous left and right reaction in water maybe it would be easier to detect and measure. Could the Hydrogen ions that are directly related to the CO2 breakdown in water be measured? A hydrogen ion meter? :} To say that my plants are growing very well, and algae is not a problem, is to say that I have enough CO2 to support the amount of light I use and the fertilizer I feed. I'll have to live with that and hope for an app that positively measures CO2 in aquariums. :}

Last edited by AWolf; 09-30-2015 at 03:44 AM. Reason: clarification
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post #12 of 13 (permalink) Old 09-30-2015, 03:44 AM
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A drop checker can help measure the CO2.

Carbon is a required element for plants, as stated several ways in the posts above.
Like building a house, you need lumber, bricks, other structural materials.... Carbon is one of the building blocks of all living organisms (plant, animal and others) on Earth.
CO2 is the most efficient way plants can get their carbon.
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post #13 of 13 (permalink) Old 09-30-2015, 04:09 AM
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Drop checkers work fairly well, just be aware that there's some considerable lag in them. They don't tell you what the CO2 is now, but rather what the average over the past hour or two was. They are also somewhat accuracy limited by your ability to judge color

Another mechanism that is reportedly pretty reasonable is change in pH. Just looking up your CO2 on a pH/KH/CO2 chart doesn't work well unless your water is strictly carbonate buffered, and your KH measurement is good (some aren't). Non-carbonate buffers can change the pH the water presents with no CO2 in it, causing considerable error. You can at least correct out the buffering and KH measurement errors by using change in pH.

What you would do is measure your pH with the lights going really good for at least a couple hours and no CO2 injection running. At that point, the water should be pretty close to 0 ppm of CO2, give or take 1-2ppm. You now know roughly the zero CO2 point for your water, regardless of what kind of buffering you have. From there, dropping the pH is approximately 30ppm of CO2. This isn't entirely perfect either, but this is effectively what everyone with a pH controller is doing, and it works pretty darn well.

But realistically, I don't think any of us really need an accurate way to measure CO2 in a pure scale.. I don't really need to know exactly how many ppm I have.. I just need a way of measuring in some arbitrary unit so I can compare my current levels to past levels. Adjusting the levels based on fish stress, algae and plant growth is really the best way to get it "right" anyway...

I'm going to make up a unit of measure called an "uggwhump" as an example (insert cave troll visage here if you like)... If I was running 2 uggwhumps of CO2 last week, but my plants barely grew, I might try 3 uggwhumps this week... eventually I might find that 5 uggwhumps makes my fish start to gasp at the surface, so I'd back off to 4.5 uggwhumps... At that point I've found the highest CO2 I can safely use in my tank, which is ultimately more important than comparing my numbers against anyone else's.

pH measurement may be a bit of an uggwhump... but that's probably good enough for the task of monitoring your own tank's health..

Most of our aquarium parameters are along the same lines.. there's general rules of thumb about good "starting points" for CO2, light levels, fertilizer levels, etc... but these are all just starting points that need to be tweaked anyway..

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