Raising Carbonates without raising PH

[Richard Kowalski]

Hello,

I am just starting my flood and drain system. My system cycled, and I added some gold fish to start with. My PH was 6.8. Ammonia 0, Nitrites 0, Nitrates 40, KH 0.

I am amazed at how fast my plants are growing in the grow beds compared to my soil garden.

I added enough Potasium Bicarbonate to raise KH to 4. But now the PH is 8.

How do the experienced Aquapons keep the PH and Carbonates at an acceptable level without raising your PH every time you adjust KH?

Thanks

Rich K

[Vlad Jovanovic]

What a terribly astute question Richard!...I await the answer to such a quandary with nail biting anticipation....How does one keep a pH of 6.4 to 6.8 while maintaining a KH of DH 3-4?

[Richard Kowalski]

Vlad,

Maybe I asked the question the wrong way.

Do you test and adjust for KH? If so, what is the KH number you keep your system at and how do you adjust for it without effecting PH.

Thanks again

[Richard Kowalski]

I have done more reading on the subject and I have found that I should be knocking down the PH to below 6.5 and slowly bring it back up with Calcium carbonate or Potasium carrbonate till I hit my desired PH. The KH will also be going up since I am using a carbonate to raise PH. 

Basicly something to that effect.

Thanks 

[Vlad Jovanovic]

You framed the question perfectly Richard  I've been wondering the very same thing!

It is very unlikely (read impossible, as far as I can tell) to use your potassium carbonate and calcium carbonate to obtain a KH of dH3-4 while having anything resembling even a remotely sane pH. Your pH will be in the pH10.2 or so range if you do that.

The soluable carbonates are wonderful for raising pH, but offer about as much of a "buffer" as hydroxides do (very little).

You probably want to use potassium bicarbonate (not carbonate, but bicarbonate) to afford yourself any noticeable rise in KH (and not have to fiddle with pH daily). Though be aware that even then, you in all reality (or anything resembling it) likely will not be able to hold a KH of dH3-4, and be in the suggested pH range of 6.4 to 6.8 (as you and others have already noticed). With the bicarbonate you will be near pH8 at dH3-4.

The KH does seem to be very important during the cycling stages (when you are running a high pH and KH to allow the bacteria to become established), but beyond that I'd shoot for dH 0.5 or so.

I'm not sure how Steve R is able to hold a pH in the mid 6's and a dH of 3-4, but will be going to Colorado in a couple of months and will hopefully find out.

[Richard Kowalski]

Vlad,

Thanks for the reply. I guess its a balancing act to get the system where you want it. 

[Charles Sublette]

Mr. Jovanovic is correct.

It is impossible to raise carbonates/bicarbonates without raising the pH.

The only way to stop carbonates/bicarbonates from raising the pH is through forcing a pH swing to remain on the low end of the swing, which easily happens even with 3~4dKH, by injecting carbon dioxide, some other acid (i.e., vinegar or any other weak acid), or by a tremendous amount of nitrification. However, if you do this, then you will notice the bicarbonates will be consumed much faster.

When folk are telling you that they have a pH in the mid-to-upper 6's with a 3dKH, then this is them telling you of poor water management or a poorly built system, which is either due to a lack of agitation, lack of proper bio-filter construction, or a lack of attention to the pH throughout the day.

3dKH, carbonate buffed, water should have a pH hovering around 7.4~7.6. The only interference to this would be the abundance of phosphates, which could lower the pH down to around 7.0, while still maintaining a higher KH. However, as plants consume these phosphates and as plant's cation exchange system changes through out the day, then, if the water's KH is around 3 with a 6.8 pH, then I guarantee you it will be swinging upward above 7.0, possibly to 7.4~7.6, at some point during the day even if it is only for a hour.

If your AP system is significantly agitated, either by degassing unit such as a fluidized bio-filter or vertical wet/dry grow bed, or significantly aerated DWC (i.e., raft) grow bed, then most of the carbon dioxide will be expelled before it has a chance to convert into a weak carbonic acid (H2CO3). So, then, the pH swing is ultimately left to feeding volume/frequency and number of fish/plants and the accumulation of plant decay.

Water with 6dKH, carbonate buffed, with little phosphate and little other weak acid interference, then the pH would be around 8.0~8.2.

Water with 5dKH, carbonate buffed, with little phosphate and little other weak acid interference, then the pH would be around 7.8~8.0.

Water with 4dKH, carbonate buffed, with little phosphate and little other weak acid interference, then the pH would be around 7.6~7.8.

Water with 3dKH, carbonate buffed, with little phosphate and little other weak acid interference, then the pH would be around 7.4~7.6.

And so on. However, predicting the pH level, per KH level, is extremely difficult to predict since the KH alkalinity test kits is a test for all resistances (i.e., buffers) in water, as talked about in the potassium bicarbonate thread. So, predicting the pH, per KH level, is extremely difficult (almost impossible) since everyone's water chemistry have different contaminants and element concentrations and plants further contribute to the contaminants.

[Charles Sublette]

The reason why calcium carbonate, such as oyster shells actually helps to create bicarbonate, rather than potassium carbonate is due to solubility and reactivity.

When very slowly introducing carbonate, such as through a complex formation of calcium carbonate (i.e., crushed oyster shells/grit, aragonite sand, crushed coral, crushed limestone, crushed calcified algae, etc), then these carbonates (CO32-) have a chance to interact with the nitrification acids or carbonic acid, which then converts these carbonates into bicarbonates (HCO3-). However, this is a slow process.

The slow reactivity of the carbonate/bicarbonate buffer system is the reason why potassium carbonate acts the way it does. Potassium carbonate is very soluble thus reacts with water quite fast, except not as fast as potassium hydroxides. Since Potassium carbonate equilibriates so fast, a huge portion of the carbonates are converted into hydroxides, which then creates other precipitates, forcing the pH to shoot up fast. Only the small portion of carbonates (CO32-) that is reacting with various weak acids are then converted into bicarbonates (HCO3-). This is why you see only a small bump in KH with potassium carbonate.

So, if you want potassium carbonate to have a better chance at creating more bicarbonates, then only do extremely slow drip doses, such as described by a medical IV drip stand. Also, try to inject this solution at a location before the water is significantly agitated so it can react with any remaining carbonic acids or directly inside the bio-filter so the solution has a chance to react directly with the nitrification acids. However, the drip dosage must be slow. An alternative solution would be to mix the potassium carbonate in some water along with some vinegar, which vinegar is then consumed by bacteria creating carbonic acid so to react with the potassium carbonate to create potassium bicarbonate.

Or.... You can simply dose the water with Potassium Bicarbonate. 

And... This is why Potassium Carbonate is sold mainly as a pH raising solution. Don't let the small amount of KH that potassium carbonate creates fool you to think otherwise.

The same above applies to Potassium Hydroxides as well, except it requires twice as much carbonic weak acid to create bicarbonates. Thus, this is why it is mainly sold as a pH raising solution and is why it is much cheaper, even can be at the price of FREE, than other carbonate/bicarbonate solutions.

If curious on how folk with zero, or near zero, carbonates maintain a stable system, then it all comes down to frequency of management and very simple DIY drip dosage devices that can keep the system stable.

[Charles Sublette]

To clarify, so to avoid confusion, when I am saying "carbonate" buffed, I am actually referring to bicarbonate.

Charles Sublette said:

3dKH, carbonate buffed, water should have a pH hovering around 7.4~7.6. The only interference to this would be the abundance of phosphates, which could lower the pH down to around 7.0, while still maintaining a higher KH. However, as plants consume these phosphates and as plant's cation exchange system changes through out the day, then, if the water's KH is around 3 with a 6.8 pH, then I guarantee you it will be swinging upward above 7.0, possibly to 7.4~7.6, at some point during the day even if it is only for a hour.

If your AP system is significantly agitated, either by degassing unit such as a fluidized bio-filter or vertical wet/dry grow bed, or significantly aerated DWC (i.e., raft) grow bed, then most of the carbon dioxide will be expelled before it has a chance to convert into a weak carbonic acid (H2CO3). So, then, the pH swing is ultimately left to feeding volume/frequency and number of fish/plants and the accumulation of plant decay.

Water with 6dKH, carbonate buffed, with little phosphate and little other weak acid interference, then the pH would be around 8.0~8.2.

Water with 5dKH, carbonate buffed, with little phosphate and little other weak acid interference, then the pH would be around 7.8~8.0.

Water with 4dKH, carbonate buffed, with little phosphate and little other weak acid interference, then the pH would be around 7.6~7.8.

Water with 3dKH, carbonate buffed, with little phosphate and little other weak acid interference, then the pH would be around 7.4~7.6.

[Joe Fisher]

Interesting topic. Thanks for all the info by posters in this thread!

I did a small sample test over the past 24 hours in a 5 gallon bucket with tap water. Which comes out here with a pH near 8.2.

So in this 1 5 gallon bucket I added 1/2 TBS of vinegar. Tested the pH it was around 7.2 this was last evening. Tested it again in the morning. Still around 7.2. Worked all day. Then came home and tested it again. Still around 7.2.

I then decided to test the kH. Interesting that kH ended up at near 6 dkH.

Just find it curious as to what vinegar does do to water. As this test doesn't seem to make sence.

Just adding this to the thread.

-Joe

[Scott Roberts]

Vlad - why do you think pH will be in the 10.2 range with a dKH of3 or 4? 

Vlad Jovanovic said:

You framed the question perfectly Richard  I've been wondering the very same thing!

It is very unlikely (read impossible, as far as I can tell) to use your potassium carbonate and calcium carbonate to obtain a KH of dH3-4 while having anything resembling even a remotely sane pH. Your pH will be in the pH10.2 or so range if you do that.

[Vlad Jovanovic]

When I raised the KH to dH4, in one 300 gallon system (using well water from source 1) the pH tested out out 9.6-9.7

When I did the same with a 50 gallon system (using well water from a different source) the pH tested out at 10.2 (at dH4)

Demineralized (R/O) water was also near pH 10 at dH4...

All of the above was done with soluable potassium carbonate.

I understand that, like Charles says, predicting pH using KH as a parameter is pretty tricky and will largely depend on specific source water chemistry...that said, I do not believe it would be possible to use soluable carbonates to obtain a KH of dH4 and be anywhere NEAR pH 6.4 to 6.8 regardless of you particular source water composition...It's just too far off the range...(bicarbonates however seem to be a different story...pegging the pH in the low 8's at 'higher' KH ranges...but even using bicarbs in order to be in the pH6.4 to 6.8 I'm at KH dH<1)

[Scott Roberts]

Vlad - the high pH you experience when using potassium CARBONATE is a temporary effect. If you had aerated the water, the carbonate-bicarbonate equilibrium would have established itself, and the pH would have dropped to a 7sometime number. 

The important thing is to stop using Potassium Carbonate. I can't recall one reference that says "use Potassium Carbonate".