# Alkalinity and Carbonate Hardness (KH)



## Hoppy (Dec 24, 2005)

While doing research and thinking about a post in this forum I decided to write down what I know about alkalinity and KH. That helps me remember it. Now, to supplement my memory I'm posting it here so I can look it up later if I want to.
......................................................

KH is not a measure of hardness, but a measure of alkalinity. Alkalinity is the capacity of an aqueous solution to neutralize an acid. So, it is a measure of the concentration of negative ions in the water. Those negative ions are carbonate (CO3), bicarbonate (HCO3), borate (BO3), phosphate (PO4), etc. When the pH is below about 8.4, more than 90% of the alkalinity in natural water is made up of bicarbonates. For that reason KH in our aquarium water is almost always a very close measurement of the concentration of bicarbonate in the water.

“The main sources for natural alkalinity are rocks which contain carbonate, bicarbonate, and hydroxide compounds. Borates, silicates, and phosphates also may contribute to alkalinity.” (Alkalinity and Stream Water Quality )

KH is not a buffer against changes in pH. But, the combination of carbonic acid, a minor part of dissolved CO2 in water, and bicarbonates, is a buffer against changes in pH for small additions of weak acids. Buffers maintain the pH at a fixed value, and that value is a function of the relative concentrations of CO2 and bicarbonates in the water - more CO2 lowers the pH being maintained, and more carbonate raises the pH.

The concentration of bicarbonates in water is not affected by the amount of CO2 in the water. (See The Principle of Conservation of Alkalinity by Pankow - http://tinyurl.com/j7vrj7t ) Or, “The Principle of Conservation of Alkalinity by Pankow … shows mathematically that the total alkalinity of a sample CANNOT be changed by adding or subtracting CO2. …” (Chemistry and the Aquarium: What is Alkalinity? ? Advanced Aquarist | Aquarist Magazine and Blog ).

In the unusual case of water which has a very small concentration of bicarbonates in it, usually surface water, the water company that supplies that water will add chemicals to the water to raise the pH enough to prevent erosion of copper and lead in the plumbing where the water is used. Some of these chemicals may raise the alkalinity - hydroxide or phosphate compounds, for example. It may be possible to have a relatively high KH, but very low concentration of bicarbonates, in that water.


----------



## mcdanielnc89 (Jun 15, 2009)

If this is the case, how does one keep these stable? Injecting CO2, drops my pH <=6.0. I can add baking soda, and it raises, but once i put that CO2 back in, it's gone.


----------



## Hoppy (Dec 24, 2005)

Adding CO2 will always drop the pH, and the pH will go back up as the added CO2 outgasses. A drop of in pH of 1.0 means you increased the CO2 in the water by a factor of 10 to the change in pH power. This has no effect on KH.

Are you adding baking soda or baking powder? The latter will not work at all.


----------



## mcdanielnc89 (Jun 15, 2009)

Hoppy said:


> Adding CO2 will always drop the pH, and the pH will go back up as the added CO2 outgasses. A drop of in pH means you increased the CO2 in the water by a factor of 10 to the change in pH power. This has no effect on KH.
> 
> Are you adding baking soda or baking powder? The latter will not work at all.


I am using baking soda. So why does my kh go down to 0 then 

Sent from my SAMSUNG-SM-G930A using Tapatalk


----------



## Hoppy (Dec 24, 2005)

Unfortunately, it is usually much easier to eliminate possible causes than it is to find the real cause. We know that some substrates, specifically Eco Complete (as I recall) will reduce the KH. But, unless your play sand is not primarily silicate, it shouldn't drop the KH by any method I can remember reading about. If it was dolomite, which is pretty common in Missouri, it should raise the KH, not lower it. Does ADA Aquasoil lower the KH? I think it might. Also, peat lowers the KH, I think. I hope some of our chemistry majors will look into this too.


----------



## latchdan (Sep 7, 2007)

Safe-T-Sorb will also lower KH and GH considerably for several weeks/couple months.


----------



## Hoppy (Dec 24, 2005)

latchdan said:


> Safe-T-Sorb will also lower KH and GH considerably for several weeks/couple months.


Do you know how it lowers the KH? It does have a good CEC, but that lowers cations in the water, not anions, like carbonate and bicarbonate.


----------



## mistergreen (Dec 9, 2006)

mcdanielnc89 said:


> I am using baking soda. So why does my kh go down to 0 then
> 
> Sent from my SAMSUNG-SM-G930A using Tapatalk


Try adding baking soda but don't add CO2. See if the kH still go down to 0.


----------



## mcdanielnc89 (Jun 15, 2009)

mistergreen said:


> Try adding baking soda but don't add CO2. See if the kH still go down to 0.


No, the kh stays above 3 if I don't use co2

Sent from my SAMSUNG-SM-G930A using Tapatalk


----------



## Vohlk (Apr 8, 2016)

I might be crazy but I would be curious to see what would happen:
Day1: Add baking soda an hour or so before CO2 starts (to allow proper distribution) and measure the Kh right before CO2 starts, then again right as CO2 is ending.
Day2: measure Kh right before CO2 starts again
Make sure you arent changing anything these two days

Also I am not sure but I have some ideas, what is the pH of your tank normal, and are you dosing any ferts, if so what?

Sorry hopefully this isn't thread jacking, if it is op let me know and if mcdanielnc89 wants we can continue our conversation elsewhere


----------



## Hoppy (Dec 24, 2005)

Don't leave us all in suspense by taking the discussion private!


----------



## latchdan (Sep 7, 2007)

Hoppy said:


> Do you know how it lowers the KH? It does have a good CEC, but that lowers cations in the water, not anions, like carbonate and bicarbonate.



Not sure hoppy, just using api test kits.


----------



## WaterLife (Jul 1, 2015)

Hoppy said:


> Do you know how it lowers the KH? It does have a good CEC, but that lowers cations in the water, not anions, like carbonate and bicarbonate.


I think I read that while Safe T Sorb does have a High Cation Exchange Capacity (negative charged sites attracting cations), it also has a fair amount of "_Anion_ Exchange Capacity" (positive charged sites attracting anions).

Once the positive charged sites are "filled" to equilibrium/capacity, it won't lower the KH anymore (well, it keeps it at filled capacity/equilibrium. Same for negative charged sites, equilibrium.)


----------



## Vohlk (Apr 8, 2016)

*Ready for chemistry? maybe? not sure...? I hope........*



I guess I should explain my hypothesis because it ties into safe T Sorb and whatnot. Safe T Sorb has good CEC right?
Well when you bubble CO2 through H20 it reacts to form "carbonic acid". (air quotes because this is an equilibrium which means the CO2 moves back and forth between carbonic acid and aqueous CO2, anyways though....) You can represent this as an equilibrium equation:

CO2 + H2O <--> H2CO3

This can be better repressented if you dissociate the H+ and instead show this as:

CO2 + H2O <--> H+ + HCO3- 

Here is where adding sodium bicarbonate comes in (baking soda)

NaHCO3 <--> Na+ + HCO3-

So overall you have:

*CO2 + H2O + NaHCO3 <--> H+ + Na+ + 2(HCO3-)* right?

Well HCO3- reacts with water (H2O):

HCO3- + H20 <--> H2CO3 + OH-

So lets look at that bolded equation again:

CO2 + H2O + NaHCO3 <--> *H+ + Na+ + 2(HCO3-) + H2O (excess it's a fishtank remember) <--> H+ + 2Na+ + H2CO3 +OH- + + HCO3-* (yes there is still some left here just wait for it)

now the H+ and the OH- form water
Leaving
2Na+ + H2CO3 + HCO3-

Those Na+ ions will get absorbed by your safe T sorb because of the good CEC

so thus:

H2CO3 +HCO3- + H2O(xs) <--> H2O + 2CO2(aq) + H+

This shows why your pH is still dropping and your Kh is not rising
Essentially your sodium bicarbonate is actually creating more CO2 instead of dissolving and raising your kH if you dont use safe T sorb then what you should see is that your kH will rise when you turn off your CO2 and let it sit for a long while

Also you might have some pretty large pH swings.

Let me know if any of this made any sense sorry this post is 10 miles long.....


----------



## Hoppy (Dec 24, 2005)

That's certainly interesting! I am far from being a chemist, so I can't really comment on whether or not this can actually occur, but I also haven't seen any other explanation.

Wouldn't this process only continue until the substrate adsorbs its full capacity of Na+? If so, then repeated additions of sodium bicarbonate, or addition of a big dose of sodium bicarbonate, would eventually stop this from happening.


----------



## Vohlk (Apr 8, 2016)

Yes but all of the bicarbonate would have been absorbed

Sent from my SM-N900V using Tapatalk


----------



## Vohlk (Apr 8, 2016)

So they wouldn't see a rise in their kh at all
Also the reaction with co2 will continue to happen as it is added

I was just trying to show how the bicarbonate could be processed via the water
And this reaction does naturally occur readily

Sent from my SM-N900V using Tapatalk


----------



## Vohlk (Apr 8, 2016)

I guess what the reaction shows is that bicarbonate in a fish tank with a media that absorbs cations likely will 1 erode the media more quickly
And 2 the bicarbonate will essentially off gas as co2

Sent from my SM-N900V using Tapatalk


----------



## Vohlk (Apr 8, 2016)

It's similar to the reaction people do with diy co2 systems
Citric acid is substituted by carbonic acid instead though

Sent from my SM-N900V using Tapatalk


----------



## Maryland Guppy (Dec 6, 2014)

Hoppy said:


> Wouldn't this process only continue until the substrate adsorbs its full capacity of Na+? If so, then repeated additions of sodium bicarbonate, or addition of a big dose of sodium bicarbonate, would eventually stop this from happening.


SateTSorb will reach a point of saturation and stop removing hardness from the water column.
If not pre-charged it can take a while though.


----------



## mistergreen (Dec 9, 2006)

Some plants consume bi-carbs don't they?


----------



## Vohlk (Apr 8, 2016)

From what I understand not really
Baking soda has a use in the gardening world as a sort of fungal cider I believe but this is used to change the osmotic pressure and kill the bacteria that way
Or it is used to raise the pH
The problem with aquariums is that there is so much water you would need to add an incredible amount of baking soda to see this affect
As in use it as a substrate....

I know some terrestrial plants can't handle baking soda well though for the same resons the bacteria can't
The osmotic pressure "burns" the leaves or roots you spray you soda water on
Lucky for us I think most aquatic plants are resilient enough to maintain osmotic pressure and imo I wouldn't worry about it

Sent from my SM-N900V using Tapatalk


----------



## mcdanielnc89 (Jun 15, 2009)

Vohlk said:


> I might be crazy but I would be curious to see what would happen:
> Day1: Add baking soda an hour or so before CO2 starts (to allow proper distribution) and measure the Kh right before CO2 starts, then again right as CO2 is ending.
> Day2: measure Kh right before CO2 starts again
> Make sure you arent changing anything these two days
> ...


Pm me

Sent from my SAMSUNG-SM-G930A using Tapatalk


----------



## Vohlk (Apr 8, 2016)

Just for the record I am sure plants use sodium (I believe to help transmit signals similar to the way our nerves function)
But the amount would be tiny
Probably less than iron
And most water already has some bicarbonate in them
I think I saw a post earlier of people having co2 in there water naturally out of the tap and I think sodium bicarbonate would be the cause
Here in Michigan we have a lot of limestone that contributes bicarbonate naturally (Na)
And most water has at least a little bit in it from various other minor mineral deposits

Sent from my SM-N900V using Tapatalk


----------



## WaterLife (Jul 1, 2015)

mistergreen said:


> Some plants consume bi-carbs don't they?


Yeah, they can.
Quite a few articles/studies out there (which I believe many more plants, besides the ones studied, also can utilize bicarbonates/KH from the water as their carbon source)
https://www.researchgate.net/publication/257129840_Bicarbonate_use_in_three_aquatic_plants

Plants can even strip out the carbonates from calcium carbonate (you can look up Biogenic Decalcification).

I had a heavily planted low tech tank, inert substrate, no supplemented co2 (DIY nor pressurized), inert sand sub, "non-Walstad", and the KH would constantly drop to zero (out the tap at 4 dKH). I believe Diana even experienced the same thing.


----------



## Vohlk (Apr 8, 2016)

So what that study showed I believe (correct me if I'm wrong)
But as pH dropped carbonate usage efficiency increased? Correct?
If so then it is not the plants using the bicarbonate for say but the plants are using the co2 from the equilibrium of the carbonate reacting with the acidic water
As co2 is used up the carbonates in the water are also used up because the reaction shifts in favor of the co2 side

So they kinda use the catbonates... not directly but indirectly

Sent from my SM-N900V using Tapatalk


----------



## Vohlk (Apr 8, 2016)

Maybe I should say that the lower pH is to begin with the farther the equilibrium will lie towards the co2 side (more free h+ ions to react with the carbonate)

Sent from my SM-N900V using Tapatalk


----------



## Hoppy (Dec 24, 2005)

I did some simple calculations to see how much sodium would be adsorbed by a SafeTSorb substrate in a typical 10 gallon tank.

CEC is measured in miliequavalents per 100 grams of material. For SafeTSorb type materials the CEC is 30 mEq per 100 grams. One mg of Na+ equals .043 mEq, so 30 mEq equals 30 divided by .043, or 700 mg of Na+, or .7 grams of Na+ (also Na). That means 100 grams of SafeTSorb will adsorb .7 grams of sodium ions.

The density of SafeTSorb is 2.2 gm/ml. 100 grams of SafeTSorb is 45 ml in volume. For a standard 10 gallon tank, with 2 inches of SafeTSorb substrate, there will be 20 x 10 x 2 = 400 cubic inches of SafeTSorb as substrate. That is 6500 ml. And, it is 6500/45=144 hundreds of grams, or 14400 grams of SafeTSorb. Since 100 grams of SafeTSorb will adsorb .7 grams of sodium ions, a total of 100 grams of sodium ions can be adsorbed in a typical 10 gallon tank.

One gram of sodium bicarbonate will raise the KH of 10 gallons of water by 1 degree KH. One gram of sodium bicarbonate contains .27 grams of sodium. If you add enough baking soda to raise the KH by 1 degree, you have added .27 gram of sodium ions. The SafeTSorb can adsorb 100 grams of sodium ions, so it would take about 400 grams of baking soda to saturate the SafeTSorb with sodium ions. That is almost a pound of baking soda!!

Unless I made a big error in decimal point, or other big errors, the SafeTSorb substrate could certainly strip out sodium ions from baking soda dosed aquariums pretty quickly!


----------



## Vohlk (Apr 8, 2016)

Exactly thank you for running through all the math I was to lazy to but what you wrote up looks right to me,
So basically using safe t sorb in conjunction with sodium bicarbonate is a pretty bad idea unless you are looking for saturated safe t sorb and an empty box of baking soda


Basically safe t sorb further pushes the equation to the right* (towards sodium bicarbonate turning into co2)



Sent from my SM-N900V using Tapatalk


----------



## Hoppy (Dec 24, 2005)

My brain is numb right now! Why wouldn't potassium carbonate or calcium carbonate used to raise the KH also experience this same problem? Why is sodium bicarbonate a unique problem?


----------



## Vohlk (Apr 8, 2016)

All of those will have the same problem potassium cations or calcium cations, the reason we are talking about sodium bicarbonate is that that is what baking soda is
But the same principle holds true for all cation bicarbonates

Sent from my SM-N900V using Tapatalk


----------



## Audionut (Apr 24, 2015)

The thing about cations and anions in water, is that they (cations/anions) must maintain an equal charge balance.

A substrate can't just strip cations (Ca++, Mg++, K+, H+) without releasing cations into the water to maintain an equal charge balance.

If the substrate (especially new) is stripping cations from the water, it's likely releasing H+ cations into the water to maintain charge balance.

H+ + HCO3 = H2CO3 = KH drop = pH drop.

The substrate isn't stripping anions from the water (HCO3- (KH)), it's the release of H+ ions causing the transformation of HCO3 to H2CO3. Hence why as the substrate begins to age, it's effect on KH reduces, as all of the cation exchange sites are now full with cations such as Ca++, Mg++ and K+. No more free H+ ions to affect sources such as bicarbonate.

CO2 does not have any significant effect on alkalinity, as per the OP.


----------



## Vohlk (Apr 8, 2016)

H2CO3 has an equilibrium with CO2 and H2O when dissolved in water though
So if you are removing the cation and replacing it with a hydrogen
And the CO2 is offgasing then the equilibrium shifts to the right towards CO2 and water
It won't be fast but it will happen I would think

Sent from my SM-N900V using Tapatalk


----------



## Audionut (Apr 24, 2015)

That all happens within the left hand side of the Carbonate buffer.

(CO2 + H2O <> H2CO3) <> (H+ + HCO3- <> H+ + CO3--)

To adjust alkalinity (KH (Carbonate buffer)), HCO3- + CO3-- concentration _must_ change. The addition of CO2 increases the concentration of H2CO3, which has a direct effect on pH due to the change in equilibrium between H2CO3 (the acid), and HCO3- + CO3-- (the base).

The interesting point is that the change in equilibrium was not due to a reduction in HCO3- + CO3+ (alkalinity).

Soil also needs an equal charge balance. Fancy new substrates come pre-loaded with lots of H+ ions attached to the cation exchange sites. As these substrates get loaded with other cations (Ca++, Mg++, whatever+), they release these H+ ions into the water. Some of these H+ ions then react with the Carbonates. This forces a change in the equilibrium of the carbonate buffer _via the transformation of the bases_ (right hand side of the equation) _to acids_ (left hand side of the equation).

Again, pH drops. This time there was a reduction in alkalinity.

Alkalinity isn't a measure of the equilibrium between the substances (that's pH). Alkalinity is a measure of the concentration of HCO3- and CO3-- ions. Fluffing around in the left hand side of the above equation doesn't _significantly_ affect the right hand side of the equation.


----------



## Hoppy (Dec 24, 2005)

Vohlk said:


> All of those will have the same problem potassium cations or calcium cations, the reason we are talking about sodium bicarbonate is that that is what baking soda is
> But the same principle holds true for all cation bicarbonates
> 
> Sent from my SM-N900V using Tapatalk


You just shot down your theory, didn't you? If the same principle holds true for other cations, then it wouldn't be possible to hold the KH of our water constant no matter what we use to raise the KH. Doesn't that mean that NaHCO3 is not the reason for the drop in KH?

Since there is wide acceptance by science that alkalinity is not affected by additions of CO2, aren't we just agreeing that this is true? (I am tempted to enroll in some chemistry classes!)


----------



## Vohlk (Apr 8, 2016)

What I was talking about was when you remove the cations via safe t sorb or purigen or whatever substance you wish to use, like you said H+ is then released in its place. That H+ (again like you said) will shift the equilibrium left towards the acid and subsiquently, left towards CO2 + H20. CO2 here is free to leave the system in the form of gas (seeing as if you are adding CO2 then you are already past the normal atmospheric equilibrium with water). So by removing the cations and the reaction proceeds to the left bicarbonate ions are turned into H2CO3, then CO2 + H2O. This would reduce your overall concentration of HCO3- and CO3 (2-). 

Would this be fair to say that adding sodium bicarbonate to CO2 enriched water in which the cations are removed from would not be a good way of increasing kh?
Again if kh is measured as bicarbonate and carbonate, adding bicarbonates in this sense will only temporarily increase kh because the sodium is being replaced by H+ which eventually will become a component in H2O (via equilibrium).

In response to you hoppy basically what I am saying is that I would think that using safe t sorb in conjunction with sodium bicarbonate would not increase your kh, but that if you where to not use safe t sorb (assuming inert substrate) it would then be possible to increase your kh.


Let me know if I am just being dumb or what I am running on little to no sleep because work has been crazy so maybe I have been aimlessly ranting about nothingness this whole time.......
Also to note, the OP is not using safe t sorb. (he pm'd me earlier)


----------



## Hoppy (Dec 24, 2005)

What is OP using for a substrate? Is it even a high CEC material?

One factor I didn't even consider in my little calculation is that the cations that have the highest priority in being adsorbed are those with 2 or 3 charges. So, ahead of sodium for being adsorbed are Al+++, Ca++, and Mg++, all of which are in the tank water, with lots of Ca++ and Mg++ if the GH is high. And, those two take up two seats on the bus, leaving even less room for Na+, which is also competing with K+. That just means there is less room for adsorbing sodium than I calculated, maybe by an order of magnitude, still leaving a lot of room.

I don't think you are being dumb at all, just exercising logic. I do think you are missing something, or we could not maintain our KH whatever it comes from, and we do maintain it in almost all of our tanks. But, I'm not competent enough to guess what is missing.


----------



## Vohlk (Apr 8, 2016)

I wonder if during cycling most peoples water contains enough ions to saturate (or mostly so) their substrate. Not sure, but yes i would agree with you obviously I am missing something pretty major, because I would think that the concentration of ions in peoples water would not be enough to saturate high CEC substrates so quickly but I do not know, my water here is hard as rocks, local saying is "between a rock and water..." (just kidding... man I need sleep I think but it's quiet hard) pH is really high too, mine comes out of the tap somewhere at 8.6ish 8.8 maybe.


----------



## Maryland Guppy (Dec 6, 2014)

Hoppy said:


> What is OP using for a substrate? Is it even a high CEC material?


http://www.plantedtank.net/forums/9508001-post8.html

This was my first question. In a provided pic it almost looked like SafeTSorb.
Turned out to be playsand.


----------



## Audionut (Apr 24, 2015)

Vohlk said:


> Would this be fair to say that adding sodium bicarbonate to CO2 enriched water in which the cations are removed from would not be a good way of increasing kh?


Well, if you want to increase carbonate hardness...........

You need to disassociate what CO2 is doing in the water, and what increased H+ ions are doing in the water. A substrate releasing H+ ions into the water doesn't care one iota what the CO2 concentration is.



Vohlk said:


> Again if kh is measured as bicarbonate and carbonate, adding bicarbonates in this sense will only temporarily increase kh because the sodium is being replaced by H+ which eventually will become a component in H2O (via equilibrium).


If you have a constant supply of H+ ions being added to the water, then you need a constant source of KH to maintain KH. And yes, if you constantly add H+ and KH, you constantly produce CO2 + H2O. Again though, it doesn't matter what the CO2 concentration of the water is. The chemistry law that states H+ + HCO3- > H2CO3 isn't affected by CO2 concentration in the water.


----------



## Hoppy (Dec 24, 2005)

WaterLife said:


> I think I read that while Safe T Sorb does have a High Cation Exchange Capacity (negative charged sites attracting cations), it also has a fair amount of "_Anion_ Exchange Capacity" (positive charged sites attracting anions).
> 
> Once the positive charged sites are "filled" to equilibrium/capacity, it won't lower the KH anymore (well, it keeps it at filled capacity/equilibrium. Same for negative charged sites, equilibrium.)


Is the above the reason why some substrates can drop the KH?



Vohlk said:


> *Ready for chemistry? maybe? not sure...? I hope........*
> 
> 
> 
> ...


Is this now shown to be a dead end alley??

I suppose I should start a poll on this??:grin2:


----------



## Vohlk (Apr 8, 2016)

I would assume that yes this is why some substrates drop kh
You can start a poll but this weekend I will try some experiments with this (will try to)
Up to you or both works too I do think lots of input would be good but I fear that people's personal experiences have too many variables and the results from the poll might not be accurate (how many people know if there substrate has high cec value... very few I imagine)
But there still might be value in it let me check my Chem real quick and make sure I didn't do something dumb though 

Sent from my SM-N900V using Tapatalk


----------



## Audionut (Apr 24, 2015)

If HCO3 reacted with water, the forum would be full of a million, "why does my KH constantly drop" threads. Even in bare bottom tanks.

You can't dissociate H2CO3 and write the equation as CO2 + H2O <> H+ + HCO3-. That's rewriting chemistry law.


I'd rather be wrong that not even bother to try.


----------



## Vohlk (Apr 8, 2016)

I would think that overall it would. If the CO2 concentration was higher wouldn't that make the reaction resist moving to the left? It might eventually get there but at a much slower rate. (thinking kinetics)
Again might be crazy talk but I was wondering if you had a link to the law that you are talking about. (I believe you, it makes sense that overall it might not matter but maybe in a short period of time it might (hours as opposed to days)) I am just curious so I can read from the same resources you are, if it is just something you remember that's okay too.


P.S. I have no idea how to type any of the above message in a manner which is not passive aggressive but my intention is to learn myself and I am most certainly not trying to be passive aggressive. No offense intended.
"Live, Love, and Learn"


----------



## Vohlk (Apr 8, 2016)

Audionut said:


> If HCO3 reacted with water, the forum would be full of a million, "why does my KH constantly drop" threads. Even in bare bottom tanks.
> 
> You can't dissociate H2CO3 and write the equation as CO2 + H2O <> H+ + HCO3-. That's rewriting chemistry law.
> 
> ...



Could it not be CO2 + H2O <> *H2CO3 (carbonic acid)* <> H+ + HCO3-

maybe this article explains it better:
http://www.acidbase.org/index2.php?show=sb&action=explode&id=22&sid=66


----------



## Audionut (Apr 24, 2015)

Use your common sense.

CO2 + H2O <> H2CO3 <> H+ + HCO3-. Right?

What makes you think you can just take <> and H2CO3 and just throw them away so that _you_ can rewrite the equation as CO2 + H2O <> H+ + HCO3-?

The entire equation is written as CO2 + H2O <> H2CO3 <> H+ + HCO3- <> H+ + CO3--.

As you well know, the <> describe the ability of the substances to move back and forth between each other. <> _does not_ mean you can skip out bits and rewrite the equation so that the very right hand side of the equation can move back and forth to the very left hand side of the equation. It can, and it does, but it actually has to pass through all of those other bits you want to just throw away. It can't (and doesn't) go from H+ + CO3-- to CO2 + H2O in one fowl swoop.

H2CO3 <> *H+* + HCO3-

There's a reason why the right hand side of the above equation contains H+. Because it actually takes the _addition_ of H+ to transform HCO3- to H2CO3. Dumping CO2 into water does not add H+.

Nevermind the passive aggressiveness. As long as we stick to attacking each others posts, and not each other.


----------



## Vohlk (Apr 8, 2016)

I understand this (I believe I corrected myself in my last post but I will here again in case I did not earlier, yes it is true that:
CO2 + H2O <> H2CO3 <> H+ + HCO3-
Is what happens (I oversimplified earlier)
I am not sure though how this changes the conversation? (although if you are concerned about kinetics it will slow the reaction further)

Though on a technicality:
CO2 + H2O <> H2CO3 <> H+ + HCO3- <> H+ + CO3--
should be:
CO2 + H2O <> H2CO3 <> H+ + HCO3- + *H2O*<> *2*H+ + CO3-- (or better yet H3O+ + H+ + CO3(2-)) better better yet (2H3O+ + CO3(2-))
The problem with this is that H+ isn't just sitting there
Its forming H3O+
This might happen at reasonable speeds from H2CO3 to H+ + HCO3-, but now that you have already created H3O+ it will slow down the reaction to CO3(2-), it will for sure still happen but it will move left much faster than it will move right (at least to H2CO3)
Now in the particular situation that I am talking about (CO2 enriched tank) that CO2 is only temporarily in excess, after it is shut off (night cycle) the reaction would proceed further left as the concentration of CO2 drops.


----------



## mcdanielnc89 (Jun 15, 2009)

Maryland Guppy said:


> http://www.plantedtank.net/forums/9508001-post8.html
> 
> This was my first question. In a provided pic it almost looked like SafeTSorb.
> Turned out to be playsand.


Yes, Playsand is the substrate..


P.S. My brain hurts with all of this chemistry talk. I'm almost not able to keep up! lol I'm doing parameter readings today so I'll post those in my original thread later.


----------



## Audionut (Apr 24, 2015)

Vohlk said:


> should be:
> H+ + HCO3- + *H2O*<> *2*H+ + CO3--


No. Because on the left hand side of the equation you have H4 + C + O4, and on the right hand side of the equation you only have H2 + C + O3. Both sides of chemical equations _*must*_ equal. The only role water plays (in the transformation of these chemical species) is as a solution. ie: It allows the cations and anions to dissolve to free cations and free anions.

To convert one portion of (dissolved/free) CO3-- to HCO3-, add two H+ ions. Period. That's what the chemistry says before you try adding and removing things. That's also why CO3-- has a larger contribution to alkalinity then HCO3-. Because CO3-- must combine with two H+ ions before transforming from a base to an acid, whereas HCO3- transforms to an acid with only one H+ ion.




Vohlk said:


> Now in the particular situation that I am talking about (CO2 enriched tank) that CO2 is only temporarily in excess, after it is shut off (night cycle) the reaction would proceed further left as the concentration of CO2 drops.


Correct. So during enrichment, CO2 reacts with H2O to form H2CO3. pH drops as H2CO3 concentration is increased, shifting the equilibrium (relative differences between concentrations) to the left.

During off-gasing, CO2 escapes to the atmosphere causing H2CO3 concentration to decrease, shifting the equilibrium to the right.

Effectively, CO2 and H2O and H2CO3 concentrations increase and decrease. When CO2 concentration is _increased_, H2O concentration is _decreased_ due to the formation of H2CO3, and vice-versa.

The part I fail to understand, is how HCO3 or CO3 concentrations are affected? As far as I'm concerned, their concentrations are not affected in any significant manner.


----------



## mistergreen (Dec 9, 2006)

If you guys have any chemistry questions, check out Chemistry Stack Exchange where chemists hang out.


----------



## Vohlk (Apr 8, 2016)

Other way around on your equilibrium shifts (shifts opposite of what is increase aka if CO2 increases in concentration the reaction shifts right and vice versa) As a reaction shifts right or left is subsequently affects the shifting of the next reaction in series (aka if CO2 increases, reaction shifts right (favoring right movement) and produces more H2CO3, thus the next set of reactions shifts right again further favoring H3O+ and HCO3-, and then again... I think you get this by now) Concentration is *directly* related to the direction a reaction shifts basic kinetics. period.

About your first comment now, you are ignoring what I actually wrote:

CO2 + H2O <> H2CO3 <> H+ + HCO3- + H2O<> 2H+ + CO3-- (or better yet H3O+ + H+ + CO3(2-)) better better yet (*2H3O+ + CO3(2-)*)

Remember H2O is in excess (xs), this eliminates the need to show the stoichiometric coefficient on the left hand side (middle) of the reaction.
If you look carefully the reaction is balanced water is in escess so you can ignore its contributions on the right hand side and the left.

And as you said CO3(2-) contributes to alkalinity moreso than HCO3- yes, but it is also more reactive (2 bonding sites as opposed to one), this is why it is also "unstable" (not nuclear unstable but stable in the presence of ions), this means that it very quickly reacts to form HCO3-, but HCO3- doesn't react very quickly to form CO3(2-).

Remember HCO3- is acting as both an acid and a base because this is equilibrium...... thats the whole point it is a stronger base than it is an acid, thats why under stable conditions (no added CO2) the right is favored.


----------



## Audionut (Apr 24, 2015)

Excellent, now please allow me to play dumb.

AcidBase.org


> so that Equations (6.2.1) and (6.3.1) tell us that [CO2(dissolved)] is usually on the order of several hundred times larger than [H2CO3]. It is therefore common practice to neglect [H2CO3}


Given that we are within the bounds of an aquarium forum, and thus CO2 concentration discussion should remain within the bounds of aquarium CO2 concentrations (lets say under 100 ppm CO2 (aq)), how do you think the above quote applies to alkalinity in aquariums.

Do you think the injection of CO2 (within the bounds of an aquarium) (that is always "several hundred times larger then H2CO3") has _any *significant*_ effect on alkalinity?


----------



## Vohlk (Apr 8, 2016)

> All these CO2 reactions are reversible, and this reversibility has two interesting consequences. The first is that *if the partial pressure of CO2 is zero in the gas above a solution containing any C02(dissolver), H2CO3, HCO3- or CO3 2- molecules, all the reactions will be driven to the left, and all these species will disappear.* Limestone rocks, coral reefs, mollusk shells, and similar CaCO3-based solid structures are stable in sea water only because Pc in the atmosphere, and therefore in the ocean, is not zero, but about 0.3 mm Hg, or 40 Pa.


In real life the partial pressure of CO2 in the atmosphere is rather small (.3mm Hg as it says in the next sentence), what the bolded sentence is essentially saying is that if the amount of CO2 +carbonates in H2O is higher than that of atmospheric pressure it will essentially "push" the carbonates into the atmosphere (in the form of dissociating to CO2 gas first as has been previously discussed). 100ppm of CO2 may not sound like a lot but remember that that is 100 ppm in water, water is much more dense than air is, I will have to see if I can find the post but there was one on the forum here earlier that talked about how ppm in aqueous solutions is not equivalent to that of gaseous and how 100ppm aqueous was a fairly large amount higher (magnitudes maybe) than what the equivalent equilibrium ppm in air would have to be. It's the same reason for why there isn't 400ppm CO2 in our tank (thats atmospheric concentration). Basically, we are pushing the partial pressure of CO2 up past the partial pressure of CO2 in the atmosphere this is what causes the out-gasing (life is all about equilibrium) yes I know that ppm isn't exactly the same as partial pressure but they are related.

So yes I would hazard to say there is an affect of CO2 on alkalinity, but again this is just theory, it very well could be wrong, I am not sure I believe it yet (the most important part of a "theory" is to not believe it till you can prove it), without experimentation it would be impossible for me to say given the current state of what we have presumably concluded upon.
*Significance?* not sure if it will be, if anything.



Also remember that by adding sodium bicarbonate, that further contributes to the bolded statement above.

I have enjoyed our conversation I must say, a calm debate is quiet refreshing compared to some of the other "discussions" I have seen around here on the forums more recently, thank you for keeping it civilized... (am actually gratefull... not trying to sound like a stuck up snob)


----------



## mistergreen (Dec 9, 2006)

Yes, co2 ppm in water is measured by weight, ppm in air is measured by volume.


Sent from my iPad using Tapatalk HD


----------



## Audionut (Apr 24, 2015)

Fair enough. I'm probably a fraction more trusting of theory then you appear to be (in this particular case I've put the theory into practice), but your point stands.

FWIW, I dose H+ and HCO3- into my aquarium, because................it just seems more appropriate to generate CO2 + H2O directly in the aquarium, rather then to generate CO2 outside of the aquarium, and suffer all of the cons associated with diffusing CO2 into water. Mind you, I'm not trying to generate three gazillion ppm of CO2 either. I'm happy to increase the CO2 concentration by some margin, while considering it's affects on other life forms in the water, rather then creating a non-limiting CO2 concentration (without respect).

Cheers.


----------



## Vohlk (Apr 8, 2016)

No I respect that, a much more homogeneous approach, but I do have a question, how are you dosing H+? Hydrochloric acid?

Do you do this to remineralize aswell? What are your tap conditions? Sorry if I am being nosey just curious.


----------



## Audionut (Apr 24, 2015)

Citric acid. RO water. Then I don't need to account for anything in the water except my own stupidity.

There's more details here: http://www.plantedtank.net/forums/12-tank-journals/933937-audionuts-chemistry-lesson.html

Although that thread hasn't been updated in some time, but I've still been playing during that time. Feel free to poke for more details in that thread, and I can find some time for an update if desired.


----------



## Hilde (May 19, 2008)

Hoppy said:


> In the unusual case of water which has a very small concentration of bicarbonates in it, usually surface water, the water company that supplies that water will add chemicals to the water to raise the pH enough to prevent erosion of copper and lead in the plumbing where the water is used. Some of these chemicals may raise the alkalinity - hydroxide or phosphate compounds, for example. It may be possible to have a relatively high KH, but very low concentration of bicarbonates, in that water.


My tap water has a ph of 8.6 and it is very soft. Read that this is due to chemicals the city puts in the water.

So then you can get a false kh reading?

Bump:


mistergreen said:


> Try adding baking soda but don't add CO2. See if the kH still go down to 0.


I was dosing baking soda (Sodium Bicarb). I found that Calcium carbonate works better. My Kh Gh and ph in the tank are very low.


----------



## Hoppy (Dec 24, 2005)

Hilde said:


> My tap water has a ph of 8.6 and it is very soft. Read that this is due to chemicals the city puts in the water.
> 
> So then you can get a false kh reading?


 I don't think so. KH, despite its name, carbonate hardness, is really alkalinity, the ability of the water to neutralize acids. It does mean that the charts showing ppm of CO2 vs KH and pH will be way off.


> Bump:
> 
> I was dosing baking soda (Sodium Bicarb). I found that Calcium carbonate works better. My Kh Gh and ph in the tank are very low.


How did calcium carbonate work better? It should be better for the plants, since calcium is used by plants while sodium isn't, or at least isn't in any quantity.


----------



## mistergreen (Dec 9, 2006)

Hilde said:


> Bump:
> 
> I was dosing baking soda (Sodium Bicarb). I found that Calcium carbonate works better. My Kh Gh and ph in the tank are very low.


I don't think Calcium Carbonate dissolves with your pH. It is poorly soluble in regular water/pure water.


----------



## mcdanielnc89 (Jun 15, 2009)

Hilde said:


> My tap water has a ph of 8.6 and it is very soft. Read that this is due to chemicals the city puts in the water.
> 
> So then you can get a false kh reading?
> 
> ...


I believe this is what happened with me though I don't know



mistergreen said:


> I don't think Calcium Carbonate dissolves with your pH. It is poorly soluble in regular water/pure water.


In my case I had so low of a ph that is dissolved just fine


Sent from my SAMSUNG-SM-G930A using Tapatalk


----------



## Audionut (Apr 24, 2015)

Hoppy said:


> KH, despite its name, carbonate hardness, is really alkalinity


A subset of alkalinity anyway. Alkalinity is a measure of _all of the things that neutralize acids_. Carbonate hardness being a measure of the Carbonate portion of alkalinity.

Hobby KH test kits should really be labeled as alkalinity test kits, because they measure alkalinity, not just the carbonate hardness portion.


----------



## Hoppy (Dec 24, 2005)

Audionut said:


> A subset of alkalinity anyway. Alkalinity is a measure of _all of the things that neutralize acids_. Carbonate hardness being a measure of the Carbonate portion of alkalinity.
> 
> Hobby KH test kits should really be labeled as alkalinity test kits, because they measure alkalinity, not just the carbonate hardness portion.


I don't think there is a hobbyist test kit that measures just the carbonate contribution to alkalinity. Fortunately, something like 90% of the alkalinity in naturally occurring water is from carbonates. I wonder when we use distilled or RO/DI water, and reconstitute it, how much of the alkalinity is from carbonates.


----------



## Audionut (Apr 24, 2015)

Hoppy said:


> Fortunately, something like 90% of the alkalinity in naturally occurring water is from carbonates.


From the OP. Chemistry and the Aquarium: What is Alkalinity? ? Advanced Aquarist | Aquarist Magazine and Blog

Carbonate and Bicarbonate have a relative contribution to alkalinity of 96.5% in seawater. In freshwater this will almost all be bicarbonate. Unless the (fresh) water is polluted, you can rule out Borate, which gives another 2.9%, bringing the total to 99.4%.

In freshwater you are likely to have some contribution to alkalinity from Phosphate. Of course, this depends on how much phosphate is in the water, and the pH of the water. 









Alkalinity test kits titrate samples to pH 4.2 (or thereabouts). If the water pH is under 7.0, then most of the phosphate won't contribute to alkalinity since it's already in the form of H2PO4.



Hoppy said:


> I wonder when we use distilled or RO/DI water, and reconstitute it, how much of the alkalinity is from carbonates.


Subtract the phosphate contribution and the rest is likely (bi)carbonate.


----------

