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#1
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Brown water!
The thing in calcium carbonate that does the buffering is the carbonate, not the calcium. Don't know anything about plaster of paris, but calcium sulfate is different from calcium carbonate. Carbonate exists in water as an equilibrium between carbonate ion (CO2, with a charge of -2) and bicarbonate ion (HCO2, with a charge of -1). It has the ability to pick up or drop a hydrogen ion (H+). It's the amount of free hydrogen ion that determines pH, I'm not a chemist, but maybe the sulfate doesn't have the same buffering capacity as carbonate. Joan ___________________ Calcium carbonate, in the form of calcium sulfate, increases the KH when introduced into the water at a pH of 7.8 . So how does increased levels of calcium carbonate cause wild pH swings? Regards, Hal |
#2
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Brown water!
On Sat, 12 May 2007 17:19:26 CST, Joan wrote:
I'm not a chemist, but maybe the sulfate doesn't have the same buffering capacity as carbonate. Let me try again. The calcium sulfate (plaster of Paris.) when dissolving into the water carries calcium carbonate into solution. Calcium carbonate in limestone stops entering solution when the pH of the water reaches 7.8. Sorry if I mislead you, but I thought Ingrid knew that. You are correct in wanting calcium carbonate to buffer KH, so do I. However calcium can also enter the water and register on a tester as GH, but I want calcium carbonate in the water and to register as KH on my test kit. I'm not a chemist either, just a dummy with soft water and a few experiences, trying to learn something. Regards, Hal |
#3
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Brown water!
calcium sulfate puts calcium and sulfate into water.
calcium carbonate puts calcium and carbonate into water. bicarbonate is the buffer system of living things. the dissociation constants are different, maybe one is affected by pH or by temp more than the other. Like I said, too old for this. Ingrid On Sun, 13 May 2007 10:20:16 CST, Hal wrote: On Sat, 12 May 2007 17:19:26 CST, Joan wrote: I'm not a chemist, but maybe the sulfate doesn't have the same buffering capacity as carbonate. Let me try again. The calcium sulfate (plaster of Paris.) when dissolving into the water carries calcium carbonate into solution. Calcium carbonate in limestone stops entering solution when the pH of the water reaches 7.8. Sorry if I mislead you, but I thought Ingrid knew that. You are correct in wanting calcium carbonate to buffer KH, so do I. However calcium can also enter the water and register on a tester as GH, but I want calcium carbonate in the water and to register as KH on my test kit. I'm not a chemist either, just a dummy with soft water and a few experiences, trying to learn something. Regards, Hal |
#4
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Brown water!
I know nothing about calcium and ponds, however calcium carbonate rock
(and to a very much lesser extent, calcium sulfate) is something I have been involved with for many years. Calcium sulfate, or gypsum, forms some caves, but is most commonly encountered as speleothems (formations) in limestone, or calcium carbonate,, largely as gypsum "flowers" or needles. Early on in cave exploration, most cavers used lamps powered by calcium carbide and water. When calcium carbide reacts with water, it gives off acetylene, which is burnt, the "spent" carbide consisting mostly of calcium hydroxide. Caves are formed largely when carbon dioxide is dissolved in surface water which forms a weak acid solution which in turn dissolves the limestone, slowly resulting in a cave. To be sure, there are other contributors to speleogenesis, but this is a very common cause. I not a chemist, and not a limestone hydrologist either, but I can't help but feel that calcium hydroxide and dissolved carbon dioxide play a role in this also. -- Galen Hekhuis I may have mispoken |
#5
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Brown water!
OK, I went back and did a search. This message was from Rod Farlee
who IS a chemist. this is what he said on 6/16/1999 Ingrid "But this "pH pill" is not the "one size fits all", "toss it in and forget it" remedy that the ******** suggests it is. It certainly works, but with some caveats. It raises GH (general hardness, calcium) more than calcium carbonate (limestone) alone does. That's because it contains calcium sulfate (gypsum), which is much more soluble than calcium carbonate, and which raises GH but not KH (alkalinity, bicarbonate). So unless one has very soft, slightly alkaline water to begin with, care should be taken. GH and KH should both be monitored. If GH is getting too high, take it out, and add baking soda if needed to raise KH. Or limit the amount to less than 1 lb/1000 gallons, to ensure GH does not continue to rise. If much larger amounts of plaster of Paris are added, it just won't work. Gypsum is 170 times more soluble than calcite. Gypsum will continue to dissolve, and push GH "off the scale". Through the common ion effect (calcium), this will suppress dissolution of the calcite (calcium carbonate), so KH will remain low. If a very large amount were used, GH and pH would go up, and KH would actually go down, and the pond might get cloudy with calcite precipitation. That would be a Bad Thing: it's the next step on the road to an alkali lake. Gotta watch for that. Happily, it takes some weeks for the "pH pill" to dissolve anyway, so these gradual changes can be monitored. And the effects of an "overdose" are ameliorated if one's water contains a substantial magnesium component in it's GH (the average eastern or midwestern river does, Ca/Mg ~ 4). (Reasons: MgCO3 is much more soluble than CaCO3, and it interferes with calcite cystallization so allows supersaturation to occur. This reduces high GH driving KH down.) Finally, if one receives regular rainfall, one might never see these overdose effects. But the potential is there, and I treaded close to it before I understood what was happening." |
#6
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Brown water!
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#7
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Brown water!
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