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#16
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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/ |
#17
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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 |
#18
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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 |
#19
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water testing, and a chemistry lesson
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#22
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water testing, and a chemistry lesson
tose (LeighMo) wrote in message ...
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 Got you. I will read these when I can find a moment, thanks. |
#23
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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 |
#24
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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 |
#26
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water testing, and a chemistry lesson
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#27
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water testing, and a chemistry lesson
(David Lloyd) wrote in message . com...
) wrote in message . com... 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. Okay, understood. KH/CO2/Total Carbon is a complicated ball of wax depending on what you are specifically talking about and which environment it is in. The theory is fairly well dealt with in chemistry class. But the practical matter and applying it to plant tanks is another matter. Test the KH, go to the pH/KH/CO2 table and follow it down till you find the pH you need to have a CO2 level between 20-30ppm. Add enough CO2 gas ONLY(no acids/"buffers" etc) to get this pH. That's it. Regards, Tom Barr |
#28
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water testing, and a chemistry lesson
(David Lloyd) wrote in message . com...
) wrote in message . com... 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. Okay, understood. KH/CO2/Total Carbon is a complicated ball of wax depending on what you are specifically talking about and which environment it is in. The theory is fairly well dealt with in chemistry class. But the practical matter and applying it to plant tanks is another matter. Test the KH, go to the pH/KH/CO2 table and follow it down till you find the pH you need to have a CO2 level between 20-30ppm. Add enough CO2 gas ONLY(no acids/"buffers" etc) to get this pH. That's it. Regards, Tom Barr |
#29
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water testing, and a chemistry lesson
The way that carbonate affects pH is a tad complex and not always well
understood. It involves Bronstead acid-base theory, which 99.99% of the world doesn't need to know. Lots of factors such as temperature, general hardness, and other chemicals affect pH and KH. So I'll try to simplify and give some basic generalizations as to how CO2 and carbonate affect pH. If you want to better understand these principles find a college chemistry text or e-mail me and I'll try to help. CO2 when mixed with water becomes carbonic acid, H2CO3. Carbonic acid is in simple language one acidic proton molecule with one basic bicarbonate molecule. CO2 bubbles will slowly react with water given proper contact time and depending on the carbonate levels all ready in the water. Soda water (all carbonated drinks) have been supersaturated CO2 at high pressure to make carbonic acid. At atmospheric pressure the excess carbonic acid likes to disassociate into water and CO2 (the bubbles in your pop). Baking soda (sodium bicarbonate or NaHCO3) is one sodium molecule, one proton, and one carbonate molecule. Calcium carbonate, lime, limestone, aragonite, crushed coral, shells, and kalkwasser (which is German for calcium carbonate) is CaCO3. Adding carbonate in any of its many forms including CO2 (as carbonic acid), baking soda, or limestone will raise the amount of carbonates in water. dKH or KH stands for German degrees of carbonate hardness and is the old method for measuring carbonates. Chemists today prefer to measure it in parts per million or in molarity. Here are some basic generalizations: Injecting CO2 into water will lower the pH and raise the KH. Adding baking soda to water will raise the pH up to about 8.3 and will raise the KH. Adding shells (or limestone) to water will very slowly raise the pH (to about 8.3) and the KH. CaCO3 is slow to dissolve in most forms. Carbonates usually help stabilize pH. As plants absorb carbonates from the water the KH goes down and pH generally goes up. Water with a low KH will absorb CO2 from the air and will raise the KH. This is how it happens in nature. Water with a high KH will degas (expel) CO2 and lower the KH (the soda pop effect). Some people will see small bubbles form on the sides of a glass with fresh water if it is allowed to sit for awhile, this is the degassing of CO2 and Nitrogen. Fish tanks look better with lots of live plants. Sorry for the chemistry lesson, but I wanted to share some of the knowledge that I remembered from getting my chemistry degree. Ross F. "LeighMo" wrote in message ... Many plants require a minimal level of KH (this is the CO2 level in your water) to grow. If you want fast, beautiful growth, it is worth trying to change. Unless you are injecting CO2, there isn't much point to changing pH, GH, and you'll have no control over KH. I think you misunderstand the relationship between KH, pH, and CO2. KH is not the CO2 level in your tank. It's the amount of carbonate in the tank, which isn't the same thing. And injecting CO2 does not give you control over KH. I do agree that plants need some KH. If your KH is zero, it's definitely worth trying to increase it. But you don't do that with CO2. You do it with carbonate. Baking soda, calcium carbonate, etc. Leigh http://www.fortunecity.com/lavender/halloween/881/ |
#30
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water testing, and a chemistry lesson
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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