molarity of 4 deg kH water - The Planted Tank Forum
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post #1 of 16 (permalink) Old 07-05-2011, 01:20 AM Thread Starter
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molarity of 4 deg kH water

OK this may have been asked 8 zillion times but I'm new here so be kind.

What is the molarity of 4 deg kH water. I calculated that it is 0.0142 molar.

That means that 0.0142 molar NaOH, NaHCO3, 1/2 Na2CO3, KOH, 1/2Ca(OH)2 etc. will all be 4 deg kH water. ie. anything that supplies an alkalinity of 0.0142 molar in the form of OH, HCO3 or CO3 will be 4 deg kH water.

Can anyone verify this?

If you play with chemicals you should know some chemisty.
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post #2 of 16 (permalink) Old 07-05-2011, 01:38 AM
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you're crazy
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post #3 of 16 (permalink) Old 07-05-2011, 04:13 PM
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OH ions don't add to KH, only carbonate and bicarbonate ions do. I don't know what the molarity of 4 dKH water is.

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post #4 of 16 (permalink) Old 07-05-2011, 06:57 PM
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Is there some reason someone would need to know this that takes care of planted tanks? I don't recall the subject ever being discussed before, but I don't come here as much as I should.

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post #5 of 16 (permalink) Old 07-05-2011, 08:17 PM
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Quote:
Originally Posted by Hoppy View Post
OH ions don't add to KH, only carbonate and bicarbonate ions do. I don't know what the molarity of 4 dKH water is.
Actually yes, OH- counts as part of kH. kH is measured as the amount of acid to bring the pH down to a specific pH (4.5). Since hydroxides will react with acid too, they are counted as part of kH.

As to the OP question:

4dkH * (18mgCaCO3/L)/deg * 1mol/100gCaCO3 * 1g/1000mg = 0.00072M

But since CaCO3 gives 2 equivalents of kH, *2 = 0.00144M (we disagree by a factor of 10).

Why use the alternatives? NaOH is hygroscopic, Na2CO3 is also hygroscopic and converts to NaHCO3. Baking soda is the most stable form of solid for making kH solutions.

Kevin

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post #6 of 16 (permalink) Old 07-05-2011, 08:41 PM
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Quote:
Originally Posted by KevinC View Post
Actually yes, OH- counts as part of kH. kH is measured as the amount of acid to bring the pH down to a specific pH (4.5). Since hydroxides will react with acid too, they are counted as part of kH.

As to the OP question:

4dkH * (18mgCaCO3/L)/deg * 1mol/100gCaCO3 * 1g/1000mg = 0.00072M

But since CaCO3 gives 2 equivalents of kH, *2 = 0.00144M (we disagree by a factor of 10).

Why use the alternatives? NaOH is hygroscopic, Na2CO3 is also hygroscopic and converts to NaHCO3. Baking soda is the most stable form of solid for making kH solutions.
I think OH ions count for alkalinity, but not carbonate hardness (KH):
http://en.wikipedia.org/wiki/Carbonate_hardness
http://www.thekrib.com/Plants/CO2/ha...arryfrank.html
Alkaliniity and KH are not interchangeable.

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post #7 of 16 (permalink) Old 07-05-2011, 09:14 PM Thread Starter
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Quote:
Originally Posted by Hoppy View Post
I think OH ions count for alkalinity, but not carbonate hardness (KH):
Since there is lots of CO2 in your tank and even the atmosphere:

Na+ + OH- + CO2 -> Na+ + HCO2-

The amount of CO2 that will dissolve in your drop checker depends only on the molar amount of Na+ in the water. (Assuming there are no other ions present).

ie. The color of your drop checker depends on the molarity of Na+ in the solution and the concentration of CO2 in the atmosphere.

Actually it doesn't matter if the + ion is sodium. It can be K, Ca or anything that is strongly alkaline.

If you play with chemicals you should know some chemisty.
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post #8 of 16 (permalink) Old 07-05-2011, 09:21 PM Thread Starter
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Quote:
Originally Posted by KevinC View Post
Actually yes, OH- counts as part of kH. kH is measured as the amount of acid to bring the pH down to a specific pH (4.5). Since hydroxides will react with acid too, they are counted as part of kH.

As to the OP question:

4dkH * (18mgCaCO3/L)/deg * 1mol/100gCaCO3 * 1g/1000mg = 0.00072M

But since CaCO3 gives 2 equivalents of kH, *2 = 0.00144M (we disagree by a factor of 10).
You are right it is .00144. I made a typing error. Thanks!

0.2M NaOH is easily available and easy to calibrate so I use it all the time.

If you play with chemicals you should know some chemisty.
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post #9 of 16 (permalink) Old 07-06-2011, 01:50 PM
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Quote:
Originally Posted by Hoppy View Post
I think OH ions count for alkalinity, but not carbonate hardness (KH):
http://en.wikipedia.org/wiki/Carbonate_hardness
http://www.thekrib.com/Plants/CO2/ha...arryfrank.html
Alkaliniity and KH are not interchangeable.
But the test we use doesn't distinguish - or think of it as an alkalinity test measuring kH. It measures (counts) any ion that reacts with the acid we are adding as a titrant. OH- reacts with acid. In normal aquarium water (pH<8), there is very little OH-, so the significant base reacting with the acid is the bicarbonate ion. However if as the OP proposes you make a solution only using NaOH, then OH- is the main reactant. NaOH solutions do absorb CO2 from the air, making NaHCO3 and lowering the pH.

I do wonder if the solution will react with CO2 more slowly (or faster) without bicarbonate ions present. Maybe the OP can try both under the same conditions and compare?

Quote from the second reference you provided: Carbonate Hardness is a confusing term because it does (not, sic) refer to hardness, but rather to the alkalinity (the ability of a solution to resist a pH change with an addition of an acid.) from the carbonates and bicarbonates. Other anions (such as hydroxide, borates, silicates, and phosphates) can contribute to the alkalinity.

Kevin

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post #10 of 16 (permalink) Old 07-06-2011, 04:48 PM
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A KH test kit does measure alkalinity, but that is normally not a problem, since the KH of idealized aquarium water contains only carbonates/bicarbonates that affect the test kit reading. However, one of the primary reasons for measuring KH is to use that number to get an approximation of how much CO2 is dissolved in the water. Even average aquarium water does contain enough other ions that contribute to the test kit reading, to make the reading useless for determining how much CO2 is in the water. And, aquarium water can contain other ions that affect the pH, again making any CO2 concentration calculation very inaccurate.

KH means carbonate hardness, a measure of the carbonate and bicarbonate concentration. Just because a test kit, labeled as a KH test, measures something else doesn't change the definition of KH.

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post #11 of 16 (permalink) Old 07-06-2011, 11:56 PM Thread Starter
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Quote:
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KH means carbonate hardness, a measure of the carbonate and bicarbonate concentration. Just because a test kit, labeled as a KH test, measures something else doesn't change the definition of KH.
I agree but disagree. kH measures the alkalinity associated with the carbonate buffer system.

Here is an example:
If you have 3 drop checkers, one with 0.00142 molar NaOH, one with 0.00142 molar NaHCO3 and one with .000703 molar Na2CO3 and place them into your "CO2 excess" tank, they all will eventually be the same color and in fact the same pH because they are are all at the same kH. I've done this and I use NaHCO3 and NaOH interchangeably in my systems.

Actually, kH is only partially a function of the amount of HCO3- or CO3-2 present. It depends (as I said) on the alkalinity associated with the carbonate buffer system.

If your curious how to test for this here is how it is done;

First take a known amount of your sample and titrate it to pH 4.5 with a standardized HCl solution. Boil your sample to remove any residue CO2 then retitrate back to pH 4.5. This will give the total alkalinity of the sample and will eliminate any CO2 from your sample.

Now back titrate your sample with a standardized NaOH to a pH of 8.5. This will give the alkalinity of the sample not associated with CO2. On a molar basis subtract one result from the other and do the math and you have the kH of the sample. (Now who understood that? PM me and I help you with the chemistry.)

Now if your sample doesn't have much PO4-3 (say less than 100ppm) Your non specific alkalinity will be a very small part of the total alkalinity and the difference will be small and the kH will be the same as your total alkalinity.

This is what the typical kH test kit does. I test my tank water both ways and though doing the long way is tedious and more accurate, the short way using the kH test kit is OK for ordinary work.

I have found in my work using soft acid water that the standard kH test kits give reliably good results even when using phosphate fertilizers.
in my systems.

If you play with chemicals you should know some chemisty.
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post #12 of 16 (permalink) Old 07-07-2011, 09:28 PM
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For whatever reason I am surprised that equal concentrations of a strong base and a weak base will have the same pH. The solutions must at least start out at different pHs no?
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post #13 of 16 (permalink) Old 07-07-2011, 10:54 PM
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ReEducation

As someone who is interested in chemistry, could someone reiterate why knowing the above molarity calculation/value is important.

Is this this for a dosing indication solution or buffer?
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post #14 of 16 (permalink) Old 07-07-2011, 11:18 PM
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Originally Posted by AirstoND View Post
As someone who is interested in chemistry, could someone reiterate why knowing the above molarity calculation/value is important.

Is this this for a dosing indication solution or buffer?
It's so you can make a drop checker solution with sodium hydroxide instead of baking soda.
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post #15 of 16 (permalink) Old 07-15-2011, 12:14 AM
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So does the formula work?

I was interested in this also, does carbonate need to be dissolved for it to be a carbonic indicator otherwise it's a hydroxide, sodium or whatever else indicator?
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