Do we get the same pH drop?
Well, obviously not. Adding the same amount of acid to a solution with different buffer capacity will generate different pH drop. But, I think, that is exactly what Darrel is saying here...
How much the pH falls depends upon the reserve of carbonate buffering, we usually measure this as "dKH".
Can't agree with that. kH reading has nothing to do with solid salts it states how much of hidrocarbonate ions are dissolved in the water.
No and Yes. No to the sentence where you say that kH reading has nothing to do with solid salts. Hydrocarbonate salt are composed of solid ions. HCO3-, CO3-2 are ions derived from salts like KHCO3, NaHCO3, Na2CO3, etc. They are not normally present in tap/drinking water. The only carbonated salt usually present in tab or drinking water is CaCO3. And they are all solid salts. In solution you have 1 mol K+ or Ca+2 and 1 mol of HCO3-1 and 1 mol of CO3-2 from KHCO3 and CaCO3 respectively. The salt dissociates in positive and negative ions and that is why they get dissolved or surrounded by water molecules interacting with them. If your tap water has only CaCO3 as carbonate and you have a reading from your local water company, you could calculate your máximum KH (those report give the GH value normally and not the KH) using the Ca+2 concentration (it will normally be lower as there are other compounds using Ca+2)
Er no again. Hardness (or any other salt) does not affect the solubility of the CO2. Thus if one adds say 10 grams of CO2 to 200litres volume of water to get 30ppm (for instance) you will always get 30ppm regardless of the hardness of the water.
What hardness does do is alter the pH changes for the addition of your CO2.
I agree and disagree here. The KH does not affect the solubility of CO2. If you add 10 g, then you add 10 g and the 10 g are dissolved (or not, depending on pressure and temperature, for how long they will remain dissolved will depend on the equilibrium). The pH drop will depend on the KH. Higher KH, lower pH drop. Lower KH, higher pH drop. Adding the same amount of CO2, of course. I desagree where you say that if you add 10g of CO2 and get 30 ppm, you will get that independent of the KH valeu. That is not correct. To get the same amount of CO2 in two tanks with different KH, you will need to provide different amounts of CO2. The higher the KH the higher the amount of CO2 to be provided to achieve a 1 unit pH drop. But you will have 30 ppm still. What varies is the proportions of the different components of the equilibrium equation:
CO2(gas) + H2O <---> H2CO3 (sol) <---> HCO3-(sol) + H+ <---> CO3-2(sol) + H+
Let´s build an hypothetical case. Without any addition (CO2), one can have 30 ppm H2CO3 and 300 ppm HCO3 and the ratio HCO3/H2CO3 is 10 or one can have 3 ppm H2CO3 and 30 ppm HCO3 and ratio is also 10. First case is high KH and second is low KH, and both are at equilibrium and at whatever pH. Let´s say we add 30 ppm CO2. It is more difficult to increase the proportion in solution from 30 to 60 ppm of H2CO3 (high HCO3 concentration of 300 ppm pushes equilibrium to the left, avoiding the CO2 dissolution, and that is what biochemist call
buffer effect or the resistance of a acid/base pair to allow a pH change) than from 3 to 33 (lower HCO3- concentration will be displaced to the right towards CO3-2) using CO2. When they reach equilibrium, in the first situation you will have maybe 35 ppm H2CO3 and 305 ppm HCO3 (almost no change in HCO3/H2CO3 ratio or pH as HCO3- is pushing equilibrium to the left). In the second situation you will have around 20 ppm H2CO3 and around 40 ppm HCO3 (easier to move equilibrium to the right and a big change in HCO3/H2CO3 ratio or big pH change). In first case you will have say 5 ppm (35 ppm final minus 30 ppm initial) and in the second you will have say 17 ppm CO2 (20 ppm final minus 3 ppm initial). If you add again 30 ppm CO2, you will reach say 10 ppm in the first case and close to say 30 in the second. In first case your pH drop has been low (conserved HCO3/H2CO3 ratio) and in the second your pH drop has been high (shifted HCO3/H2CO3 ratio). In the end, in the first situation, to reach 30 ppm HCO3- you will have to add a lot more CO2 but at the end the HCO3/H2CO3 ratio will have to be exactly the same as in the second situation, maybe a ratio of 1... but same pH drop (same ratio). And still 30 ppm.
And this is why ADA aquasoil decreases KH, by removing either HCO3- or CO3-2 ions by combination with a positive charged compound or ion which generates surely an insoluble product that most likely precipitates out of solution. This moves the equilibrium to the right of the equation releasing H+ ions and decreasing pH. And all of that with the purpose of making it easier for the aquarist to achieve a good CO2 value in solution without needing a lot of CO2
Sorry for the dense post and I hope I explained myself correctly
Cheers
Pedro