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I know Activated Charcoal and Purigen both absorb tannins from the water, but I am not sure if they negate the acidic contributions/pH lowering abilities that are associated with the tannins. Anyone know?
I mean the tannins are still in the body of water, just concentrated into little beads. But maybe they are binded/locked in a way that prevents the tannins from influencing the water chemistry.

How about high CEC substrates (Turface, Safe T Sorb, Eco Complete, etc), do they too absorb tannins or no?
Was pondering if one could create a "DIY" active acidic buffering substrate for Caridina shrimp (high CEC to remove KH, then add some acidic sources to lower pH below neutral to desired a level/pH, but unsure if high CEC would cancel out those added acids, which I doubt it would, but my chemistry is still very limited.)
 

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I know Activated Charcoal and Purigen both absorb tannins from the water, but I am not sure if they negate the acidic contributions/pH lowering abilities that are associated with the tannins. Anyone know?
I mean the tannins are still in the body of water, just concentrated into little beads. But maybe they are binded/locked in a way that prevents the tannins from influencing the water chemistry.

How about high CEC substrates (Turface, Safe T Sorb, Eco Complete, etc), do they too absorb tannins or no?
Was pondering if one could create a "DIY" active acidic buffering substrate for Caridina shrimp (high CEC to remove KH, then add some acidic sources to lower pH below neutral to desired a level/pH, but unsure if high CEC would cancel out those added acids, which I doubt it would, but my chemistry is still very limited.)

I started a 75G planted tank about 10 weeks ago. My water was extremely yellow from the tannins. After many water changes and no results I finally added purigen to my filter. Within 24 hours was crystal clear. I am also using eco complete as a substrate.

I don't know if this helps you at all but just my experience with the product.
 

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I'm not sure that tannins are positively charged and if they are not I do not believe that the substrate will absorb them but I may be wrong. I know that high cec substrates work because cations by definition are positively charged. This is one of my favorite part of aquariums and I look forward to input from someone more knowledgeable and able to explain the chemistry. Great question and thanks for posting it.
 

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I can only speak to Safe T Sorb.
It will not absorb tannin from the wood.
Purigen will remove the yellow/brown stained water and not affect hardness.
Safe T Sorb can become saturated with all things related to water hardness as with all high CEC substrates.
 

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Both activated charcoal and Purigen essentially negate the acidifying effects. Tannins are weak acids. However, to raise the PH of water the tannins have to react with water releasing hydrogen ions. If the tannins are removed prior to this reaction then the PH remains unchanged.

Once tannins become adsorbed by AC the reaction will not be possible. In fact, the tannins (phenols) are so strongly bound that they are very difficult to remove using various techniques. Terry Grant wrote a PHd thesis on this phenomenon, Irreversible Adsorption of Phenolic Compounds by Activated Carbons

Purigen works somewhat differently than AC. Purigen is a synthetic resin. I'd previously explained how ion exchange resins worked in another thread quite some time ago. So I pasted the text below.

Ion exchange resins (Purigen) can be manufactured to remove targeted contaminants. I'll try and explain the basics without chemistry jargon. It's really not necessary to understand the basic concept.

These resins are long synthetic chains of molecules that can bind with various substances. Imagine these chains as a ball of blue yarn, see below. The yarn can hold many molecules. When fresh, the ball of yarn contains huge amounts of hydrogen ions (the orange dots on the yarn).


Now imagine the molecules as having Velcro hooks on them, see below. The small orange molecules with one hook (hydrogen) won't hold very well. The big green molecules can knock these smaller ones free and stick quite well. So essentially bonding to the yarn and leaving these small orange molecules in the water. When the number of hydrogen ions in the water increases the PH goes down. The small amount of Hydrogen released typically doesn't make any significant impact on PH. This is essentially all there is to it. The larger molecules are the things we don't want and the smaller molecules are what we leave behind.


Besides designing long polymers that attract specific molecules, manufacturers can adjust the "tightness" of the ball of yarn. This can prevent molecules that are to big from bonding to the majority of the chain by physically blocking them. The image below shows two strands of yarn side by side. Notice how the larger red molecules won't fit between the strands? Essentially, by making a "tight" ball the molecules can't reach the entire surface area of the chain inside.

 

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Both activated charcoal and Purigen essentially negate the acidifying effects. Tannins are weak acids. However, to raise the PH of water the tannins have to react with water releasing hydrogen ions. If the tannins are removed prior to this reaction then the PH remains unchanged.
So you can have 'tannined' water from wood or leaves, but depending on your buffer(hardness) the pH may not change. So once you remove the tannins, (in low GH tanks and the assuming the pH has already dropped a degree), your pH should begin to rise, because you are removing the acid substance.
 

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So you can have 'tannined' water from wood or leaves, but depending on your buffer(hardness) the pH may not change. So once you remove the tannins, (in low GH tanks and the assuming the pH has already dropped a degree), your pH should begin to rise, because you are removing the acid substance.
Depending on the harness and the amount of buffers! That's the important part to grasp here. Tannins WILL lower PH. How much will X amount lower PH? Depends on the buffers in the water.

The amount of PH change will always be a relationship between the buffers and acids. Think of it as a teeter totter. If they're equal on both sides nothing will happen and the water will be neutral, 7.0. Add more to either side and it will tip the scales, hence the term "acid base balance system".

PH measurement is simply a measure of the amount of Hydrogen ions. The more you have the more acidic the water is. Those extra Hydrogens typically come from chemical reactions which is nothing more than molecules reacting to each other. They can combine or break apart into other molecules (or atoms).

A buffer will accept (bind) the hydrogen atoms increasing PH where an acid will react and produce extra hydrogen increasing PH. So when we say something is an acid it does not mean it's pure hydrogen. It simply means that when it reacts with something, typically water, it results in excess hydrogen.

So what this means is that if you remove the tannin molecules before they react there will be no change in PH. Once they're bound in AC they won't react releasing hydrogen ions hence lowering the PH.

So basically your interpretation is correct. However, it's dependent on the amount of buffer versus acids.
 
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