Lowering KH for CO2?

[Biollante]

Hi,

This is part of the problem with these kinds of issues. It is all chemistry, terms like pH, alkalinity, the use of equivalents, the reactions that take place…

I do not know if I am smart enough to explain this.

---

What always intrigues me is why the thing that can be easily observed,

• in this case that CO₂ in solution cannot be increased or decreased by manipulating pH without the use of CO₂,
  
  
  • has to be explained in depth.
    
  
  
• However, the position that contradicts simple observation does not require explanation.
  

Let me give this a little thought, painful though that may be.

It might help me if someone could explain why anyone thinks, based on your observations or explanations why the CO₂ should be manipulated by adjusting pH.

Biollante

[Tug]

I'm sure I know the same character that likes to say lowering the KH will increase the amount of CO2. This was about two years ago when I had my experience. I'm surprised that he is still at it. I think (ouch, yes it hurts) a small part of what Bio is saying is that in theory it can be done but in practice it will kill your fish. I'm sure if you ask this individual if they have tried lowering KH to raise CO2 levels they will excuse themselves from the table. Most likely they seized upon this chemical relationship as a way to force a debate that is of little use to hobbyists like us.

I have been looking for a laymen's explanation of KH and pH for a long time. I'm not sure it exists. The closest thing I can offer you was provided to me recently and I am still struggling with it. If we keep at it, after a few more years, we might have enough sense to recognize a nefarious argument and walk away. If all you realize is that the pH/KH tables were not intended to be used to lower KH but to use the pH/KH reading as a guide for adding CO2 you will know all you need to. The tables, as Bio stated, are reasonably accurate but tend to error by indicating higher levels of CO2 then is actually present. Depending on what else is in the water the KH/pH tables can be off by as much as 20% but that gets us close to where we can eyeball any further additions of CO2.

KH-CO2-pH equilibrium
http://www.seachem.com/Library/Artic...d_Aquarium.pdf

[Biollante]

Hi,

I think the explanations in Understanding the General Chemistry of the Planted Aquarium by Gregory Morin, PhD that Tug provided are about as layman as they get.

So let me step back a moment, I am going to try this with a little math and a little chemistry.

I suspect the thing I missed is the appeal of the pH, KH, CO₂ chart. The pH, KH, CO₂ chart appeal lies in its apparent simplicity, a simple answer to a complex and mysterious question.

Try not to get hung up in the numbers, let me give the formula that most of the charts are based on.

• CO₂ = (3 X KH) X 10^(7-pH)
  
  • KH is in German degrees,
    
    
    • most charts and calculators seem to use 17.85-ppm=1-dKH.
      
  
  
• CO₂ = (3 X (KH X 0.056)) X 10^(7-pH)
  
  • (KH in part per million with dKH=KH_(ppm) X 0.056)
    
    
    • (We get the 0.056 conversion factor since we already decided 17.85-ppm=1-dKH, so 1 ÷ 17.85 = 0.056)
      
      
      • (I know your fancy calculators say 0.056022408 and I can tell many nitwit chart makers use this number, but this is a whole other level of stupidity we can discuss another day.)
        

Over my many revolutions around the sun one of the lessons I have learned is that nothing that is too good to be true is.

The pH, KH, CO₂ chart is too good to be true. As I said in an earlier post that there is nothing wrong with the chart itself, I believe in most cases it is good enough to get most people in the ballpark (in the area, reasonably close).

• In a sense, the beauty of it is that most of the time it will over report CO₂.
  

It does work quite well as long as the principal pH buffer is bicarbonate and carbonate.

Now here is the bit the morons omit,

• notice the equation only accounts for three actors, pH, KH, CO₂,
• since we need to measure pH and KH to discover the amount CO₂ in solution,
  
  • to make the equation valid we must measure the system at equilibrium.

It is foolish to measure pH and KH, then change the KH and only measure the pH and expect a valid answer.

• Therefore, if you decide to add Hydrochloric acid to your aquarium to reduce the fish population and reduce the alkalinity
  
  • The equation will still report the same amount of CO₂ as long as you measure the pH and KH after the adjustment by any means other than CO₂ has stabilized.
    
  
  
• The same is true if you add baking soda, after the system stabilizes,
  
  • Measure the pH and KH again and you will find that the CO₂ levels remain the same.
    

Go ahead run the numbers yourself this is a simple demonstration I'll wait.

As I mentioned earlier it (the equation/chart) works quite well as long as the principal pH buffer is bicarbonate and carbonate.

Tom Barr reports other interferences based on actual tap water measurements across the country, I will stick to my experience, the most common problem in newer aquariums I have seen, are the use pH buffering agents, pH Up, pH Down, pH sideways, Discus Buffer, pH something or other, these are phosphates. Phosphate buffers are, like me, cheap and easy, more information.

Immediately the astute reader may see where this leads us.

• If you take a quick look, you will notice that everything is multiplied by 10 raised to the power of 7 minus pH (10^(7-pH)), this means very small changes in pH have large effect on the outcome.

Since the equation accounts for resistance to change in pH by alkalinity, if someone is using a phosphate buffer whose job it is to resist changes in pH by means other than alkalinity then the equation is not sufficient to cover the case.

It is also possible that the use of phosphates as fertilizers can affect the accuracy, though generally if you maintain alkalinity over 2-KH this should not be noticeable. Generally higher KH levels the less effect the interferences have.

As tanks mature the interferences increase, life in general lowers pH, I believe this is why people who depend on pH controllers (or charts) suddenly find they are having trouble maintaining proper CO₂ levels, the failure to account for something other than CO₂ acting on pH.

As Nipat has pointed out even with interferences, as long as they are consistent, we really calibrate ourselves to the system. (Sorry for the bad paraphrase, were I not a lazy, miserable wretch of a potted-plant I would have looked up and linked his quote.)

I hope this helps, I think from here on I need specific questions to deal with this topic further.

Biollante

[BigFlusher]

O verbose one,

What you are saying is that you cannot use the equation testing both KH AND pH, get an answer, make a bunch of changes then test only pH and get the answer you want.

Okay

[Tug]

Pretending to be obtuse?
This is one of those topics that is easily misunderstood. A misunderstanding, I didn't realize that it would be mine to understand or not. the syntax and the meaning of a word, i.e., Verbose one, in title honorific is funny; being verbose is not. My apologies for any dust up.

You have three measurements, pH, KH and CO2.

[BigFlusher]

Yes, the Verbose One is saying that if you need to know 2 things to know the 3rd ignoring a change to one of the things is only fooling yourself.

Also, if the water is buffered with bicarbonate and you add a phosphate buffer, the equation does not account for this.

[ismenio]

Hi,

Okay my earlier post is harsh.

I understand why folks might think that lowering alkalinity might increase CO₂ in solution and assuming sufficient buffering I suppose it a one degree drop in pH will add CO₂ to the water, kill critters, but add CO₂.

I also recall I have written about CO₂ generators using vinegar and baking soda. Baking soda produces 1-mole of CO₂ for each mole of baking soda, which, if we recall our 8^th grade chemistry is at standard pressure 22.4-liters of gas.

Therefore, I still advise you not hang out with morons; I will simply be nicer about it.

Biollante

Hello Biollante

Can you tell how to do do a CO2 generator using vinegar and baking soda ?

Regards

[Biollante]

Hi,

The easiest version of a baking soda-vinegar, CO₂ generator is an IV bag over a bottle; two holes in the cap, one hole for the “drip-line” from the IV bag the other for the CO₂ gas to the tank.

IV bags work well because it is easy to control the flow of vinegar into the bottle with the baking soda.

Some folks use peristaltic pumps, it really doesn’t matter as long as you can control the flow of vinegar.

As always it does not matter the source of the CO₂, it is getting it into solution.

The main drawback is that even though it is mole for mole acetic acid for baking soda, vinegar is only 4-9% acetic acid, here in the USA, most distilled white vinegar is 5% acetic acid.

So for a 3-mole CO₂ (~132-grams) requires 252-grams of baking soda and 189-ml of acetic acid (for those of you who came up with 180-ml, remember density). Assuming 5% acetic acid, this means 3.7-liters of vinegar.

At a drip rate of 18-ml an hour (0.3-ml/minute) 10-hours a day, depending on efficiency getting CO₂ into solution this should keep a 180 to 210-liter water column, aquarium happy for 21 days.

If you are in a wet climate, you may need as much as 10% more baking soda.

I think it is prudent to have some sort of pressure relief, even if it just a tube that blows off.

Take care this is closer to pressurized CO₂; it is easy to gas your fish.

Biollante

[aquabillpers]

The relationship among pH, KH, and CO2 is something that I don't really understand but I accept the fact that pH is a dependant variable, determined (mainly) by KH and/or CO2, which are themselves independant of each other.

That is generally correct, right? <g>

Another approach to this problem is to avoid it completely by cutting back on the light, using a soil substrate, and stocking lots of fishes. Then one doesn't have to think about it.

Bill

[jcgd]

There is a lot going on in this thread. Biollante, it seems like you are explaining a whole whack load of things that while are very informative, it is confusing.

Why doesn't the co2 level in water increase as you decrease the kh?

Well, in normal conditions the level of co2 in water is usually around 3ppm. Maybe a bit more, maybe a bit less. This is the case regardless of the kh. Even if you drop the kh, that co2 concentration won't change because it has to do with the concentration of co2 in the air and the properties of water. The ph will change, however, to reflect the change in kh. If you compare the new ph and new kh to the ph chart, it should but you around 2.8-3 ppm of co2. Approximately. I've tried to find a reference but haven't found anything and I have to run.