Questions about CO2 solubility and rate of dissolution - The Planted Tank Forum
Reply
 
LinkBack Thread Tools Display Modes
post #1 of 35 (permalink) Old 04-16-2015, 05:02 PM Thread Starter
Algae Grower
 
PerfectDepth's Avatar
 
PTrader: (0/0%)
Join Date: Dec 2014
Location: Iowa
Posts: 82
Questions about CO2 solubility and rate of dissolution

I'm not sure if I'm posting this in the right place.. mods please feel free to move it.

I'm using DIY CO2 with an internal reactor connected to a powerhead on a timer. The timer turns the powerhead on 2 hours before the lights turn on. Something I've observed is that the CO2 dissolves more quickly at first, then eventually slows down. The pH drop is one full degree, measured about halfway into the photoperiod. I haven't checked to see exactly when the full 1 degree drop is achieved (hopefully before the lights turn on).

I'm not having any issues, I'm just curious to know exactly why the dissolution rate slows down. Am I correct to assume that it slows down as CO2 concentration increases due to the buffering capacity? Would this be offset by the increased CO2 demand once photosynthesis is in full swing (not to mention degasification)? Other than buffering capacity, are there other factors that will have an effect on the dissolution rate of CO2? I realize the answer is probably complex, but any knowledge shared would be greatly appreciated.
PerfectDepth is offline  
Sponsored Links
Advertisement
 
post #2 of 35 (permalink) Old 04-16-2015, 05:50 PM
Wannabe Guru
 
exv152's Avatar
 
PTrader: (0/0%)
Join Date: Jun 2009
Location: Canada
Posts: 1,845
KH will not inhibit your co2 dissolution. Someone with a pH of 6.6 and a KH of 4 will have the same co2 ppm as someone with a pH of 7 and KH of 10, or a ph of 6 and a KH of 1. The water’s buffering capacity will not inhibit co2 concentration. Otherwise people with a high KH would just blast the co2 without any concern for gassing fish, and we know this is not the case. But there are other factors that will affect co2 concentration, such as o2, water temperature, flow, oily film on surface etc. Also, just because you have really high co2 concentration in a tank, doesn’t mean your plants are effectively using that co2. The plants need a good level of o2, nutrients etc.

Eheim Pimp Club,# 496

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
,
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
,

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
,
7g cube, 12g long
exv152 is offline  
post #3 of 35 (permalink) Old 04-16-2015, 07:09 PM Thread Starter
Algae Grower
 
PerfectDepth's Avatar
 
PTrader: (0/0%)
Join Date: Dec 2014
Location: Iowa
Posts: 82
Thanks for the reply. I understand the relationship between KH, pH and CO2. My question is not about co2 concentration per se, but about why the co2 does not continue to dissolve at the same rate inside the reactor. In other words, what is changing that makes it slow down?

Quote:
Originally Posted by exv152 View Post
But there are other factors that will affect co2 concentration, such as o2, water temperature, flow, oily film on surface etc.
This kind of info is what I'm after.. Could the increasing O2 concentration (from the plants) be slowing down the rate of dissolution?
PerfectDepth is offline  
 
post #4 of 35 (permalink) Old 04-16-2015, 08:22 PM
Wannabe Guru
 
exv152's Avatar
 
PTrader: (0/0%)
Join Date: Jun 2009
Location: Canada
Posts: 1,845
Quote:
Originally Posted by PerfectDepth View Post
My question is not about co2 concentration per se, but about why the co2 does not continue to dissolve at the same rate inside the reactor. In other words, what is changing that makes it slow down?

This kind of info is what I'm after.. Could the increasing O2 concentration (from the plants) be slowing down the rate of dissolution?
I wouldn't worry about plants producing o2 through photosynthesis (pearling). Because the amount of o2 they produce is negligible. But if you don't have enough surface movement in your tank, and you begin to have an oil film build-up, that alone will prevent a huge amount of o2 from getting into the water column (which you want), and you want some co2 to off gas too (not a lot but some). Also higher temps will mean your o2 levels will go down, which is also an issue for your plants. They'll need a lot more co2 to photosynthesize. Also, if the water column doesn't get enough o2 circulation it can lead to anaerobic conditions in the substrate.

Eheim Pimp Club,# 496

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
,
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
,

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
,
7g cube, 12g long
exv152 is offline  
post #5 of 35 (permalink) Old 04-16-2015, 09:01 PM
Planted Member
 
PTrader: (0/0%)
Join Date: Mar 2015
Location: Canadialand
Posts: 161
Quote:
Originally Posted by PerfectDepth View Post
Thanks for the reply. I understand the relationship between KH, pH and CO2. My question is not about co2 concentration per se, but about why the co2 does not continue to dissolve at the same rate inside the reactor. In other words, what is changing that makes it slow down?
It is a pretty standard equilibrium situation. If you are adding CO2 at a constant rate, it will dissolve faster when there is no CO2 in the water, and dissolve slower when CO2 in the water is high. The equilibrium will settle where CO2 input, CO2 usage/loss, pH and kH are all balanced.

So if I had to say what actually limits CO2 dissolving, I would say the principal candidate is CO2.

Quote:
Originally Posted by exv152 View Post
KH will not inhibit your co2 dissolution. Someone with a pH of 6.6 and a KH of 4 will have the same co2 ppm as someone with a pH of 7 and KH of 10, or a ph of 6 and a KH of 1. The water’s buffering capacity will not inhibit co2 concentration
While you are probably right for all practical purposes, it is worth pointing out that CO2 will dissolve incredibly quickly if the pH is somehow high and the kH is low. And if the pH is really low and the kH is really high, you will essentially be bubbling off CO2 from a saturated solution as a result of acid/carbonate reactions. So the rate of dissolution from external sources will be very low.

So while there is a pretty reliable relationship between pH, kH and CO2 levels, this doesn't really fully account for the input and output at those equilibrium levels.
Beefy is offline  
post #6 of 35 (permalink) Old 04-16-2015, 09:15 PM
Planted Member
 
PTrader: (0/0%)
Join Date: Mar 2015
Location: Canadialand
Posts: 161
Quote:
Originally Posted by exv152 View Post
Because the amount of o2 they produce is negligible. But if you don't have enough surface movement in your tank, and you begin to have an oil film build-up, that alone will prevent a huge amount of o2 from getting into the water column (which you want)
I don't entirely agree with this. If your plants are pearling, it means that the water is saturated with oxygen. You will never saturate with just surface movement.

[EDIT] Should specify, pearling means localised areas are saturated with O2, and it doesn't mean the whole tank is super-duper-loaded with O2. I would still argue that it is much more difficult to reach high O2 just with ambient air exposure.
Beefy is offline  
post #7 of 35 (permalink) Old 04-17-2015, 12:15 AM Thread Starter
Algae Grower
 
PerfectDepth's Avatar
 
PTrader: (0/0%)
Join Date: Dec 2014
Location: Iowa
Posts: 82
Thanks Beefy. That makes sense. When I started this DIY adventure, I was worried the internal reactor might not keep up as the plant mass and co2 demand increases, but it's been holding steady so far, which makes sense when you think of it as an equilibrium. I was planning on getting a more powerful pump just to have on hand for the reactor, but I guess there shouldn't be any need as long as the co2 supply is sufficient.
PerfectDepth is offline  
post #8 of 35 (permalink) Old 04-17-2015, 02:25 AM
Wannabe Guru
 
exv152's Avatar
 
PTrader: (0/0%)
Join Date: Jun 2009
Location: Canada
Posts: 1,845
Quote:
Originally Posted by Beefy View Post
I don't entirely agree with this. If your plants are pearling, it means that the water is saturated with oxygen. You will never saturate with just surface movement.

[EDIT] Should specify, pearling means localised areas are saturated with O2, and it doesn't mean the whole tank is super-duper-loaded with O2. I would still argue that it is much more difficult to reach high O2 just with ambient air exposure.
If your plants are pearling it just means they're photosynthesizing, but the o2 they're producing is not going to oxygenate your tank to the o2/co2 "equilibrium" you need. You want to bring the o2 from the air into your system, to create that equilibrium. There are several ways to do this, one is to use a wet/dry filter, or you can get a tank with a large footprint so you increase the surface area and then increase the flow near the surface. You can also reduce your temperature to about 75-76F which will increase your o2 ppm. If you don't allow for this o2/co2 equilibrium, and you have a very low o2 concentration, you'll have to use a lot more co2 to get the plants to photosynthesize. There is no other way to draw in that o2 other than from the air. Pearling plants alone will never give you enough o2.

Tom Barr is the guy who talks about this at great length. He just did a presentation last weekend in Washington DC which I attended. Very informative. You can take if from Tom Barr, or from some guy with 32 posts on TPT. It's up to you.

Eheim Pimp Club,# 496

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
,
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
,

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
,
7g cube, 12g long
exv152 is offline  
post #9 of 35 (permalink) Old 04-17-2015, 03:55 AM
Planted Member
 
PTrader: (0/0%)
Join Date: Mar 2015
Location: Canadialand
Posts: 161
Quote:
Originally Posted by exv152 View Post
Pearling plants alone will never give you enough o2.
Nobody ever said it would, diffusion at the air interface is an important source of O2 in most cases. My issue is with your assertion that plants pearling represents negligible contribution to oxygen - there is just no justification for this. CO2/O2 are consumed/produced at a ratio that is effectively 1:1. If plants use the whole-tank equivalent of 1 ppm CO2, they produce 1 ppm O2. An ideal CO2 level would be 30 ppm during lights-on, and the maximum concentration of O2 in water in 25°C atmospheric air is ~8 ppm.

So lets say your plants actually use just 1 ppm CO2 and produce 1 ppm O2, then plant-generated O2 might contribute as much as 12.5% of the O2 that could exist in the water. But I would guess this is actually pretty conservative...... I wonder if someone has tried shutting off their CO2 and filters during lights-on and monitored actual CO2 levels? Off-gassing should be minimal, so falling CO2 should be related primarily to CO2 consumption. I suspect it would fall very quickly in a well lit planted tank.

Quote:
You can take if from Tom Barr, or from some guy with 32 posts on TPT. It's up to you.
Information is useless when used out of context. For example......

Quote:
and you have a very low o2 concentration, you'll have to use a lot more co2 to get the plants to photosynthesize
...... this is just plain wrong. The plants photosynthesize whether you see the O2 bubbles or not. But you only see the pearling when rate of photosynthesis is high enough that O2 is produced faster than it can dissolve into the water.

The real argument is whether pearling means the water is saturated with O2, or something else. My interpretation is that if you are seeing oxygen, it is being produced faster than it can dissolve, which perfectly fits with the idea of localised saturation. As an extension of this, you will only see pearling in a well-oxygenated tank.
Beefy is offline  
post #10 of 35 (permalink) Old 04-17-2015, 04:05 AM Thread Starter
Algae Grower
 
PerfectDepth's Avatar
 
PTrader: (0/0%)
Join Date: Dec 2014
Location: Iowa
Posts: 82
exv,
That's very interesting and I appreciate the effort (whether you have 32 posts or 1500), but I'd like to understand the science behind it. Can you explain in more detail? Maybe plantbrain can chime in if he's still active here. If I remember correctly, I've seen Tom Barr mention the benefit of sealed wet/dry filters, but maybe that's outdated advice??

Last edited by PerfectDepth; 04-17-2015 at 04:18 AM. Reason: Addressed post to exv152
PerfectDepth is offline  
post #11 of 35 (permalink) Old 04-17-2015, 04:30 AM
Planted Member
 
PTrader: (0/0%)
Join Date: Mar 2015
Location: Canadialand
Posts: 161
Wet/dry filters are great in this context mainly because they expose the biological media directly to atmospheric air with minimal water barrier. This means that oxygen solubility in water is not a factor, availability of oxygen is very high, which is great for the nitrifying bacteria.
Beefy is offline  
post #12 of 35 (permalink) Old 04-17-2015, 08:49 AM
Planted Tank Enthusiast
 
lee739's Avatar
 
PTrader: (0/0%)
Join Date: Oct 2014
Location: Australia
Posts: 532
I presume you are watching the pH drop - the relationship between pH and CO2 concentration is not proportional - it has an inverse log relationship - ie pH drops as CO2 rises, but inverse-log - initially pH drops rapidly, then slows - the CO2 level would be steadily rising though.
lee739 is offline  
post #13 of 35 (permalink) Old 04-17-2015, 09:12 AM
Planted Tanker
 
burr740's Avatar
 
PTrader: (127/100%)
Join Date: Feb 2014
Location: Bama
Posts: 5,776
As mentioned, when there is no or low CO2 in the water, it will absorb faster. As the ppm rises it becomes harder to "inject" more. That is why the dissolution rate is rapid at first then slows down.


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.




burr740 is online now  
post #14 of 35 (permalink) Old 04-17-2015, 03:08 PM
Wannabe Guru
 
exv152's Avatar
 
PTrader: (0/0%)
Join Date: Jun 2009
Location: Canada
Posts: 1,845
Quote:
Originally Posted by PerfectDepth View Post
exv,
That's very interesting and I appreciate the effort (whether you have 32 posts or 1500), but I'd like to understand the science behind it. Can you explain in more detail? Maybe plantbrain can chime in if he's still active here. If I remember correctly, I've seen Tom Barr mention the benefit of sealed wet/dry filters, but maybe that's outdated advice??
CO2 and O2 are not consumed/produced at a ration of 1:1. If that were true then we wouldn’t need to concern ourselves with gassing our fish because, according to that theory, the plants would simply produce enough o2 to keep them alive. We all know this to be untrue.

Tom advocates Fick’s 1st law of diffusion as it relates to Flux. A higher level of flux is ideal for aquarium plant growth. This can be achieved by increasing flow, surface area, surface ripple, no film, and less degassing at any other part of the tank, which will lend to a higher and more stable flux. Aquariums with less of flow in general will have lower and less stable flux.

Temperature also plays a big part of gas concentration in the water. As temperatures rise, the kinetic energy increases at a molecular level. Gases form a weak bond with the water molecules at higher temperatures, which is why you generally see less dissolved gases in higher temps, this is especially true for o2. As water temperatures decrease gases become more soluble and form stronger bonds with the water molecules. This is true for all gases including co2 and o2. The trade off is most plant species do not grow as fast in cold water.

In our aquariums, because it’s similar to nature but not quite, most of the o2 comes from the air, and some comes from the plants via photosynthesis. However, the o2 that plants produce in a tank will not be fully diffused to dissolve in the water column, which is why you often see o2 bubbles rise directly from the plants to the surface. Similar to co2, o2 requires an efficient form of diffusion to be fully dissolved in water. In a high flux tank co2 and o2 are available quite uniformly throughout the tank. The high flow is also good for the distribution of macro/micro ferts to all the plants in addition to the co2 and o2. This accounts for why some folks with high co2, ferts and everything else still experience issues with algae and mediocre plant growth. Nutrient issues aside. Their temperatures may be too high, and/or they may also have poor water flow – low flux, so the plants are not thriving as they’re being told it’s a co2 issue, when in fact it’s a lack of co2 via a lack of proper flow.

BTW none of this is my own personal theory, I’m simply regurgitating someone else’s expertise to the best of my ability, but I can vouch it’s worked for me.

Eheim Pimp Club,# 496

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
,
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
,

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
,
7g cube, 12g long
exv152 is offline  
post #15 of 35 (permalink) Old 04-17-2015, 03:21 PM
Planted Member
 
PTrader: (0/0%)
Join Date: Mar 2015
Location: Canadialand
Posts: 161
Quote:
Originally Posted by exv152 View Post
CO2 and O2 are not consumed/produced at a ration of 1:1. If that were true then we wouldn’t need to concern ourselves with gassing our fish because, according to that theory, the plants would simply produce enough o2 to keep them alive. We all know this to be untrue.
Completely wrong. The most basic representation of the photosynthetic equation is 6CO2 + 6H2O -> C6H12O6 + 6O2. However this doesn't just magically happen in real-time because photosynthesis takes time. It is rate-limited by light availability, biological processes in the plants, etc. etc. So if you suddenly bump things up to 80 ppm CO2, you don't suddenly get 80ppm O2. Flux, as you bring up later, takes time.

Furthermore, high O2 doesn't automagically reduce the effects of CO2 toxicity. If CO2 in the tank is too high, the fish will not be able to lose metabolic CO2 through their gills regardless of O2 levels. Furthermore, part of the problem with CO2 toxicity is the rapidly plunging pH which is completely independent of O2 levels.
Beefy is offline  
Reply

Tags
None

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the The Planted Tank Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome