Alkalinity and Carbonate Hardness (KH) - The Planted Tank Forum
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post #1 of 64 (permalink) Old 09-19-2016, 07:53 PM Thread Starter
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Alkalinity and Carbonate Hardness (KH)

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.
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post #2 of 64 (permalink) Old 09-19-2016, 10:24 PM
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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.
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post #3 of 64 (permalink) Old 09-19-2016, 11:05 PM Thread Starter
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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.

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Last edited by Hoppy; 09-21-2016 at 09:50 PM. Reason: corrected type
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post #4 of 64 (permalink) Old 09-20-2016, 01:53 AM
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Quote:
Originally Posted by Hoppy View Post
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

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post #5 of 64 (permalink) Old 09-20-2016, 03:28 AM Thread Starter
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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.

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post #6 of 64 (permalink) Old 09-20-2016, 03:38 AM
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Safe-T-Sorb will also lower KH and GH considerably for several weeks/couple months.
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post #7 of 64 (permalink) Old 09-20-2016, 03:27 PM Thread Starter
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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.

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post #8 of 64 (permalink) Old 09-20-2016, 04:14 PM
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Quote:
Originally Posted by mcdanielnc89 View Post
I am using baking soda. So why does my kh go down to 0 then

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


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post #9 of 64 (permalink) Old 09-20-2016, 04:15 PM
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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

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post #10 of 64 (permalink) Old 09-20-2016, 04:28 PM
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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
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Last edited by Vohlk; 09-20-2016 at 04:29 PM. Reason: possible threadjacking....
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post #11 of 64 (permalink) Old 09-20-2016, 04:47 PM Thread Starter
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Don't leave us all in suspense by taking the discussion private!
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post #12 of 64 (permalink) Old 09-20-2016, 04:58 PM
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Quote:
Originally Posted by Hoppy View Post
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.
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post #13 of 64 (permalink) Old 09-20-2016, 05:12 PM
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Originally Posted by Hoppy View Post
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.)
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post #14 of 64 (permalink) Old 09-20-2016, 05:56 PM
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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.....
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post #15 of 64 (permalink) Old 09-20-2016, 07:56 PM Thread Starter
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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.

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