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Old 20-04-2003, 06:23 AM
rnj
 
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Default water testing, and a chemistry lesson

Please describe what your definition of KH is.

"LeighMo" wrote in message
...
Injecting CO2 into water will lower the pH and raise the KH.


You're half right.

Injecting CO2 will lower the pH.

It *might* raise the KH if you have something in the tank that will

dissolve
with more acidic water. But most of us don't. Most people don't put

anything
in their tanks that will dissolve in the water and increase its hardness.

For the vast majority of planted tank keepers, adding CO2 will not affect

KH.


Leigh

http://www.fortunecity.com/lavender/halloween/881/




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Old 20-04-2003, 06:23 AM
LeighMo
 
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Default water testing, and a chemistry lesson

Please describe what your definition of KH is.

KH is the carbonate hardness.

Yes, I know, the test kits measure alkalinity, not the actual carbonate
hardness. But in order to use the KH-pH-CO2 tables, KH is carbonate hardness.


Leigh

http://www.fortunecity.com/lavender/halloween/881/
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Old 20-04-2003, 06:23 AM
David Lloyd
 
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Default water testing, and a chemistry lesson

tose (LeighMo) wrote in message ...
Please describe what your definition of KH is.


KH is the carbonate hardness.

Yes, I know, the test kits measure alkalinity, not the actual carbonate
hardness. But in order to use the KH-pH-CO2 tables, KH is carbonate hardness.


Leigh

http://www.fortunecity.com/lavender/halloween/881/

So introducing bicarbonate (CO2 dissolved in water at this pH mostly
takes the form of bicarbonic acid) into water will not increase
"carbonate hardness"? I'm not sure I understand.
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Old 20-04-2003, 06:23 AM
LeighMo
 
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Default water testing, and a chemistry lesson

So introducing bicarbonate (CO2 dissolved in water at this pH mostly
takes the form of bicarbonic acid) into water will not increase
"carbonate hardness"? I'm not sure I understand.


Baking soda will increaase the KH. That's what I said. I was arguing with
someone who insisted you could increase your KH by injecting CO2 gas into the
water.


Leigh

http://www.fortunecity.com/lavender/halloween/881/
  #36   Report Post  
Old 20-04-2003, 06:23 AM
LeighMo
 
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Default water testing, and a chemistry lesson

Dissolving CO2 in water increases bicarbonate concentration, so why
would it not increase KH?


Okay, I think I understand what you're asking. Sorry, I think we hit one of
those "two nations separated by a common language" potholes in the earlier
post. g When you said "bicarbonate," I thought you meant baking soda, since
bicarbonate means baking soda in many parts of the U.S.

CO2 added to water mostly stays CO2. A fraction of a percent of it becomes
carbonic acid, and that's what causes the pH drop. It doesn't affect KH, as we
measure it, anyway.

For chemistry meeps:

http://www.thekrib.com/Plants/CO2/alkalinity.html

http://www.thekrib.com/Plants/CO2/kh-ph-co2.html



Leigh

http://www.fortunecity.com/lavender/halloween/881/
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Old 20-04-2003, 06:23 AM
[email protected]
 
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Default water testing, and a chemistry lesson

Okay, baking soda is sodium bicarbonate (Na^+ HCO3^-), so the addition
of that will increase KH as bicarbonate concentration has increased.
CO2 dissolves in water to form hydrogen bicarbonate (bicarbonic acid):


CO2 + H2O --- H^+ HCO3^-


Okay so you mean H2CO3.
Not the bicarbonate ion -HCO3?

I think there's some confusion there on those two terms.

Adding CO2(acid) will never form the KH (-HCO3), a buffer in this
case.
The above has _no buffer_ to start with, you assume pure water. A tiny
tiny amount will form H2CO3 but about 1:400 will be CO2 so most folks
ignore the H2CO3.

But H2CO3 is not KH. There is no alkalinity in pure water. There is no
acid base buffering system either.

Dissolving CO2 in water increases bicarbonate concentration, so why
would it not increase KH?


It does not change the bicarbonate at all.

I leave a glass of water out, it has a KH of 5. I add CO2 to it, it
still has a KH of 5. You are welcomed to try this yourself.

The total carbon has increased when you add the gas and the pH will
drop, but the KH is the same, it does not gas off or evaporate. The
gas will and the buffered solution will equilibrate and return to
starting pH if you stop adding the CO2 gas.

Regards,
Tom Barr
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Old 20-04-2003, 06:23 AM
David Lloyd
 
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Default water testing, and a chemistry lesson

) wrote in message . com...
Okay, baking soda is sodium bicarbonate (Na^+ HCO3^-), so the addition
of that will increase KH as bicarbonate concentration has increased.
CO2 dissolves in water to form hydrogen bicarbonate (bicarbonic acid):


CO2 + H2O --- H^+ HCO3^-


Okay so you mean H2CO3.
Not the bicarbonate ion -HCO3?

I think there's some confusion there on those two terms.


Maybe a little (it's a while since I had to think about these sorts of
things), but I did mean the bicarbonate anion.

I'll just explain that I'm a newbie when it comes to aquaria. I have a
scientific background but am slightly baffled by the use of "GH" and
"KH" as terms, so I may be barking up some very wrong trees here!

Here's what I understand...

CO2 dissolves in water to form carbonic acid (my terminology above was
flavoured a little by forgetfulness and therefore a little misleading,
I agree):

CO2(g) + H2O --- CO2(aq) + H2O --- H2CO3

But it doesn't stop there...

H2CO3 --- (H^+) + (HCO3^-) --- 2(H^+) + (CO3^2-)

I don't use pure water in my relatively new aquarium. I use water that
comes from my kitchen sink. We have

pH 7.6
18 dGH
11 dKH
0 ppm NH3/NH4^+
0 ppm NO2^-
20 ppm NO3^-

At pH 7.6 more than 90% of the CO2 that forms H2CO3 will wind up in
the HCO3^- state. HCO3^- is the dominant state between around pH 6.5
and pH 10.

I'm not sure I know what "KH" is. My gut feeling is that it is
supposed to indicate bicarbonate and carbonate concentrations in
solution, but I'm also pretty sure that I'm not measuring this when I
use my test kit
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Old 20-04-2003, 06:23 AM
[email protected]
 
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Default water testing, and a chemistry lesson

CO2 + H2O --- H^+ HCO3^-

Adding CO2(acid) will never form the KH (-HCO3), a buffer in this
case.


Boboo. Not right. Some is formed, but _extremely little_ will be
formed in pure water. Enough to ignore it's contribution in any
practical case in a plant tank.
Absense of Trace metals will cause less bicarbonate to form.

Regards,
Tom Barr

Regards,
Tom Barr



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