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Buffer chemistry (Aquasoil)

Cédric

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Hello everyone,
Desperately looking for an answer : If system acid carbonic/carbonate is no more (KH=0) when using aqua soil, so what does buffer the water ? Does another buffer system replace H2CO3/HCO3 ? Organic acid based buffer ? How does it work ? Any idea ?
Cheers
 

ceg4048

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Hello everyone,
Desperately looking for an answer : If system acid carbonic/carbonate is no more (KH=0) when using aqua soil, so what does buffer the water ? Does another buffer system replace H2CO3/HCO3 ? Organic acid based buffer ? How does it work ? Any idea ?
Cheers
Hello,
If KH=0 then buffering=0. Adding food combined with higher KH tap water during water changes will raise the KH over time.

Cheers,
 

Cédric

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Hello,
If KH=0 then buffering=0. Adding food combined with higher KH tap water during water changes will raise the KH over time.

Cheers,
Hi ceg, tyvm for your answer and glad to meet. I learned a lot reading your posts ("vortex" water flow with spraybar, dropchecker as pH test kit...).
No KH = No buffering. OK, simple and very clear. But, how is it that the pH is naturally fixed between 6 and 7 with aquasoil ? Is it the natural balance between KH (=0) and dissolved CO2 that determines that pH value ?
 

ceg4048

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But, how is it that the pH is naturally fixed between 6 and 7 with aquasoil ? Is it the natural balance between KH (=0) and dissolved CO2 that determines that pH value ?
Hi Cedric,
To understand my answer to this question an understanding of what "pH" actually is will be required. Many hobbyists worry about pH at the wrong times and this causes more problems than if they did not worry about it. Have a look at this post, which attempts a simplified explanation of pH:=> Unlimited nutrients using E.I.

If you are able get a better understanding of pH from that post you might now be able to understand that when other ions are added to the water, such as carbonate (CO3) or bicarbonate (HCO3), which have a negative charge, it is easier to understand that these negative charges will attract and hold (thereby neutralizing) some positively charged (H+) Hydrogen ions. The effect therefore is that neutralized positive charges of the captured H+ are effectively removed from the water and are removed from the calculation of pH, which is a calculation of the ratio of free floating (OH-) to (H+).

This is what the term "buffering" means. Any agent or procedure that neutralizes a quantity of the H+ and therefore increases the pH.
KH is a quantitative measure of the number of available negatively charged particles that have the ability to neutralize H+.
Clays soils typically have a high CEC and these clay particles have the ability to capture and hold certain ions, including H+.
When new Aquasoil is submerged it will capture and hold the acidic H+, thereby giving it the reputation of "buffering", however, the particles can only capture and hold a certain number of positively charged particles. After that limit is reached any more acid that is released into the water will simply accumulate and the "buffering" ability of the sediment will vanish.

This claim of "..pH is naturally fixed between 6 and 7 with aquasoil..." is taken out out of context and used as advertising because, as explained, this is only a temporary effect.

If you understand these concepts then you will see that there is no such thing as "natural balance between KH (=0) and dissolved CO2"
Any CO2 added to the water having no buffering capacity (KH=0) will cause the water to become acidic simply as a result of the Carbonic acid phenomenon. If CO2 is added to a tank that has both zero KH as well as having new Aquasoil, then the Aquasoil will absorb some of the H+ produced by the CO2 interaction with water, however, as CO2 is continued to be injected, the buffering will be overcome by the continued amount of H+ being produced. Any buffering agent can be overcome by adding enough acid.

There is no magical pH 6 to 7 buffering. The advertisement may simply assume that the hobbyist's water has some level of natural pH and KH that already falls within this range and that the additional initial buffering of the clay (temporarily) is enough to maintain the pH within this range regardless of any additional injected CO2.

Hope this clarifies.

CHeers,
 

Cédric

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Hi ceg4048,
Both your answer and post linked were very (x100) useful to understand things better. I started to learn about ratio H+:OH- with plasma blood pH7.4 (ratio HCO3/H2CO3 = 20:1) and acidosis at 7,2 : little drop of pH (not even acidic) with potentially a tragic outcome. This led me to understand that a pH variation for acidic solution and basic solution is not quite the same thing. But still, water chemistry is hard to understand. Fortunately there is a "tool" like UKAPS to help and people like you to enlighten us. Understanding things is of particular importance to me and I thank you for taking the time to answer me.
Cheers Cédric
 

_Maq_

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I'd like to add some minor notes to that what @ceg4048 explained so clearly.
Clay can act as a buffer and help maintaining pH within certain range. CEC is called cation exchange capacity because it means permanent exchange in response to outer conditions. Let's simplify the issue by concentrating on mere two cations - H+ and Na+. Any clay has some innate affinity to various cations, including these two named. Our hypothetical clay replaces Na+ cations with H+ cations when H+ cations are abundant, i.e. in acidic environment. Thus, the more acidic water, the more Na+ will be released and H+ bonded. That's buffering. When the pH value is raising, our clay will release H+ and bind Na+. That too is buffering.
In such a way, clays function as buffers. It is quite possible that selected (blend of) clays help keeping pH withing given range, par example between 6 and 7.
Anyway, clays - just like any other buffer - don't possess unlimited buffering capacity. If there are too many H+ cations in the water, i.e. if the water is strongly acidic, the clay will bind H+ cations up to its capacity (CEC) and no more. And vice versa - if your water is alkaline and full of Na+ cations, clay will get depleted of H+ and its buffering capacity will get exhausted.
 
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