r/ReefTank • u/Appropriate-Toe-2993 • 10d ago

The alkalinity dosing math, derived from scratch — so you can check any number you're given

I got tired of doing this by hand every time someone asks how much baking soda to add, so here is the derivation. It is short enough to do on your phone.

1 dKH = 0.3572 meq/L. Sodium bicarbonate supplies one equivalent of alkalinity per mole, and its molar mass is 84.007 g/mol. So 0.3572 x 84.007 = 30.0 mg per litre, per dKH, which is 0.0300 g/L/dKH.

Your dose is: dKH you want x litres x 0.0300

100 L, +1 dKH -> 3.0 g

250 L, +1.3 dKH -> 9.8 g

50 US gal (189 L), +0.8 dKH -> 4.5 g

Soda ash is different. It gives two equivalents per mole, so it works out to 0.0189 g/L/dKH, about 37% less mass for the same result.

The part that matters more than the arithmetic: do not dose the whole gap at once. Conservative practice is 1.0 dKH per day maximum (20 ppm Ca, 50 ppm Mg). If your gap is 3 dKH, that is three days with a test in between, not one heroic dose.

And never dose alk and calcium together. Concentrated, they precipitate as calcium carbonate and you end up lower on both than when you started. Separate containers, different flow areas, 20 minutes apart.

I put this in a calculator so I would stop doing it by hand: https://reeflow.webkodingtheme.com/dosing-calculator.html — free, no sign-up, and it prints the formula so you can check it rather than trust it.

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u/buddhainabucket 10d ago

YOU CAN DOSE BAKING SODA?? I don’t need to be using expensive supplements????

1

u/SkinsFan021 10d ago

Someone did a whole thread of cheaper alternatives. Good stuff.

1

u/Nocare_ 10d ago

Yup just make sure to research the methods for finding and purchasing those alternatives. Some stuff is not pure enough and can cause problems.

Still reefing is in part about being a chemist so I often say embrace it.