Hi all,
cheers Darrel
Thank you. I thought that was the answer.You can math it up, but essentially that means at pH 3 or higher there is no capacity for the sulphate ion to accept a proton, which means at the type of aquarium pH we are considering the sulphate ion does not affect pH at all.
Perfect, we have a bit of discussion of <"phosphate buffers">, but not the explanation of why (until today). This is a potassium phosphate buffer calculator <"Potassium Phosphate (pH 5.8 to 8.0) Preparation and Recipe | AAT Bioquest">That means at our usual pH range, the first proton is always lost (pKa 2.16) , the third proton is always still on the phosphate (pKa 12.32), but the second proton can come on and off, so you'll have a mix of HPO3-- and H2PO3-. You get the most "buffering capacity" at the pKa, so phosphate will be an excellent buffer around pH 7 (best at pH 7.21). If you were to add a purely phosphate salt like K3PO4 to aquarium water, the K3 will all immediately come off, and the loose PO3--- will immediately grab (on average) 1.5 protons which means the pH will go up because the concentration of free H+ will go down (some got stuck on the phosphate). People usually don't want this to happen, which is why they use "hydrated" versions of potassium phosphate, typically KH2PO4.
cheers Darrel