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So this one does a better job of explaining. And this was my argument for not measuring alkalinity when I ran freshwater. My understanding of alkalinity was that it was the pOH (percent hydroxide), which is the opposite of pH (percent hydronium). If you knew ph, you indirectly know alkalinity. And if all your alkalinity comes only From hydroxide ions, that’s true. But we necessarily have a high concentration of carbonated and bicarbonate in our water, which directly impacts alkalinity. At pH’s below 4.2, there is no hydroxide present, meaning all alkalinity or buffering capacity of the water, comes from carbonates and bicarbonates. If you’re ph is that low, there’s an issue. We want to keep our tanks much closer to a neutral ph, which means nearly equal hydronium and hydroxide ions. Even at the more desired 8.1-8.3 range, the difference in hydronium and hydroxide ions are near negligible. To repeat, if all your alkalinity came from hydroxide, there really is no reason to measure alk, if you know ph, but that’s not the case. Still, the hydroxide ion concentration gives us very little information about the buffering ability of water at our desired levels Of ph. The carbonate and bicarbonate concentrations are what will truly affect the buffering ability of the water.
Hydroxide plays very little role in telling us much about the buffering ability of water until Ph’s Start pushing beyond 8.4, which again, is not desired in our tanks. So, by taking out an extraneous value (hydroxide) we can get a much more precise and (more importantly) controllable value of alkalinity. When you want to increase alkalinity, you add Carbonate, not hydroxide ions. Corals are also uptaking carbonates, not hydroxide ions. So carbonates are really the thing shifting into the alkalinity equation.
Now here’s where I’m not certain if im coming to the correct conclusion. Because total alkalinity includes hydroxide ions, I would anticipate the buffering capacity to be greater with hydroxide ions present. In order to account for that, I’d imagine the buffering curve for only measuring alkalinity would be at an inflection point around ph of 7. Meaning,
I saw that as well. Not very helpful for us. Mindstream posted a response in a thread on the forum somewhere asking about the differences (don’t know how to link to it). I’m thinking that conversion will not be possible and I’ll just have to be content now tracking both total Alk and carbonate Alk. (No, I’m not getting rid of my Alkatronic anytime soon!)
If you can simply calculate it from pH and total Alk it should be very easy to add this field to the application.
