Does injecting CO2 into the water change TDS? - The Planted Tank Forum
 
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post #1 of 7 (permalink) Old 01-14-2014, 11:01 PM Thread Starter
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Does injecting CO2 into the water change TDS?

I'm not sure about the chemistry of the situation here. I know that co2 when injected forms HCO3- which would be detected by a TDS meter. However, is HCO3- the compound that plants use as carbon?
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post #2 of 7 (permalink) Old 01-14-2014, 11:47 PM
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No, it does not change the TDS, try and it and see 1st. See if you can soften the water and measure a KH difference.

CO2 can convert a TINY bit of HCO3 into carbonic acid, about 1 out of 400 molecules or 0.25%.

But this is insignificant, otherwise we could simply add CO2 and not need RO systems...........to soften tap water.

This is the part most often overlooked in these discussions which often are filled with a lot of misunderstanding and myth.

Plants can use bicarbonate if there's enough light and there's not much cO2, but generally this is an indirect method where they pump H+'s and acidify the HCO3 to CO2 at the cell surface. This is not the same as adding CO2 gas.

If you add say acetic acid or HCl to the water, this will react with the HCO3 and form water and CO2, but virtually none of the CO2 added will do this.




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post #3 of 7 (permalink) Old 01-14-2014, 11:58 PM Thread Starter
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Thanks for the reply. So essentially the formation of HCO3- doesn't correlate to the amount of co2 in the water. This would be especially true since only .25% of the co2 turns into hco3.

So plants directly use the gaseous form CO2(g) ?

Also I thought the point of an RO system was to remove TDS. The difference between softening water and using something like RO is a very large difference. Ion exchange vs actual removal of charged particles.

Last edited by Darkblade48; 01-15-2014 at 01:25 AM. Reason: Back to back posts
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post #4 of 7 (permalink) Old 01-15-2014, 01:27 AM
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Also I thought the point of an RO system was to remove TDS. The difference between softening water and using something like RO is a very large difference. Ion exchange vs actual removal of charged particles.
This is due to the misuse/misunderstanding of the word "soften."

In general, we use it to mean removal of magnesium and calcium cations. Whether this is achieved by ion exchange (exchanged for sodium cations) or completely removed (e.g. via reverse osmosis) is ambiguous.

In most cases, "soft water" refers to water that does not form (calciferous) lime scale.

Anthony


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post #5 of 7 (permalink) Old 01-15-2014, 07:17 PM
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So plants directly use the gaseous form CO2(g) ?
A gas dissolved in water is no longer a gas, it is a solute in the water. (Just as salt dissolved in the water is no longer a solid.) That is what the plants use, when we dissolve CO2 into the water. That dissolved CO2 is always coming out of solution at the water surface to the air, which has a lower concentration of CO2 than would be in equilibrium with the amount in the water. In fact, even if the CO2 in the water is in equilibrium with that in the air, the CO2 still comes out of solution into the air, but at the same rate that it is dissolving into the water from the air. CO2 moves very freely from the air to and from the water.

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post #6 of 7 (permalink) Old 01-15-2014, 07:35 PM
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No it does not change tds but will change your ph balance


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post #7 of 7 (permalink) Old 01-16-2014, 05:25 PM Thread Starter
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Okay thanks guys. I guess I was just trying to wrap my head around exactly what the plants use as carbon when you inject co2. I will conclude then that plants use co2(aq) as per what hoppy said.

All of this started because I noticed my TDS was about 20 higher during the peak of the day when the co2 was on for 4+ hours. Don't know what might be causing it.

On second thought the light could be causing charged matter to split off from substances during the day, then recombine to a neutral form at night.

Anyway, thanks again.
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