Yi yi yi! Correct that man.... (Not trying to be argumentative Dano, just what you're saying can be misleading...) stability IS key to PH.... but to get stability your water should be running at an alkalinity of 8 - 11 dKH... (2.5 - 4 mq/L) Same values... just different ways of expressing it.... When salinity is at 1.021 - 1.026, Alkalinity is at the aforementioned levels and Calcium is between 380 and 420 ppm then PH should be around 8.2... which is perfect... About the only thing that makes this drop is too much C02 in the system.... or a rotting animal carcass...
FYi... to make PERFECT mixed Seawater with IO salt.... get some Kent Osmo-Prep Marine.... follow the directions and your mix will be (at SG of 1.024) Calcium 410ppm, dKh 10, and PH 8.2... been doing it that way since Janary and it works every time.... but check with test kits... this is assuming your RO/DI water has a TDS <10ppm.
You won't be able to get any kind of accurate reading for PH in pure water as there are no available ions to buffer the water. The water will take on the PH of the salt mix... When you add buffer of any kind...(including Kalk) you are adding bicarbonates very high in PH... until these pair off with carbonates.. (C02), you will definitely get a spike in PH... try aerating the water and test again in 24 hours and I think you'll see a PH of 8.3 to 8.4...
This should also explain why C02 drops the PH... carbonates bind with available bicarbonates... (buffer) to reduce the overall buffer capacity of the water leaving the remaining C02 to form other ionic bonds with the end product being carbonic acid... thus dropping PH... when alkalinity is maintained, this happens at a rate that keeps PH at proper levels... Likewise too high an alkalinity will cause bicarbonate precipitation resulting in too high a PH...
I hope this gives a little insight.... some of my terminology may not be correct but the concept is...
Now maybe even I can understand this!!!
FYI - I monitor my PH with a monitor.... check Alk almost every day... and Calcium at least twice a week...
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