Kalkwasser only for Alkalinity

Randy Holmes-Farley · Nov 29, 2016

dragon99 said:
http://reefkeeping.com/issues/2005-01/rhf/index.php

The calcium ions in the solution obviously supply calcium to the aquarium, and the hydroxide ions supply alkalinity. Hydroxide itself provides alkalinity (both by definition and as measured with an alkalinity test), but corals consume alkalinity as bicarbonate, not hydroxide. Fortunately, when limewater is used in a reef aquarium, it quickly combines with atmospheric and dissolved carbon dioxide and bicarbonate to form bicarbonate and carbonate:

4. OH- + CO2 > HCO3-

5. OH- + HCO3- > CO3-- + H2O

Thanks, I didn't see this before my identical post. lol

AllSignsPointToFish · Nov 29, 2016

Randy Holmes-Farley said:
Hydroxide reacts into the carbonate/bicarbonate/CO2 system as soon as it is added. The reaction itself is measured in very tiny fractions of a second.

But it takes time to mix throughout the tank and even longer for CO2 to be pulled in from the air (which is why the pH drops over a period of hours after a bolus dose of hydroxide).

Here's a blurb from one of my articles:

The calcium ions in the solution obviously supply calcium to the tank, and the hydroxide ions supply alkalinity. Hydroxide (OH–) itself provides alkalinity (both by definition and as measured with an alkalinity test), but corals consume alkalinity as bicarbonate, not hydroxide. Fortunately, when limewater is used in a reef tank, it quickly combines with atmospheric and in- tank carbon dioxide (CO2) and bicarbonate (HCO3–) to form bicarbonate and carbonate (CO3—):

OH– + CO2 → HCO3–

OH– + HCO3– → CO3— + H2O

Once in the aquarium at an acceptable pH, there is no concern that the alkalinity provided by limewater is any different than any other carbonate alkalinity supplement. The hydroxide immediately disappears into the bicarbonate/carbonate system. In other words, the amount of hydroxide present in aquarium water is really only a function of pH (regardless of what has been added), and at any pH below 9, it is an insignificant factor in alkalinity tests (much less than 0.1 dKH). Consequently, the fact that alkalinity is initially supplied as hydroxide is not to be viewed as problematic, except as it impacts pH (see below).

The fact that limewater is very basic (the pH is typically above 12) demands that the limewater be added slowly to an aquarium unless very small additions are made. The reason for slow addition is two-fold: to prevent the local pH in the area of the addition from rising too high (slow addition permits more rapid mixing with tank water to reduce the pH), and to prevent the overall tank pH from rising too high (slow addition allows the tank to pull in CO2 from the atmosphere during the slow addition, mitigating the pH rise). Some aquarists advocate rapid addition, and that is acceptable for additions that would add significantly less than 0.5 dKH of alkalinity to the tank, but an addition of 1.4 dKH (0.5 meq/L; the equivalent of adding 1.2% of the tank volume in saturated limewater or 14 grams of solid calcium hydroxide into a 100-gallon tank) drives the pH of the whole tank too high (up by about 0.6 pH units from where ever it started).

Consequently, limewater is most often added slowly, by dripping or slow pumping. Often it is added as the top off water, replacing most or all of the evaporated water. The pumps add cost and complexity to the system, especially if combined with a float valve or switch (I use the latter and a Reef Filler pump).

Thanks again, Randy. I had already pieced most of that together between my own observations and yesterday's discussion. It appears to me that, for now, I can supply enough alkalinity with just limewater and minor carbonate dosing.

Randy Holmes-Farley · Nov 29, 2016

Sounds good.

Happy Reefing.

Dav2996 · Sep 14, 2022

Randy Holmes-Farley said:
Sounds good.

Happy Reefing.

I am having trouble raising alkalinity with Kalkwasser I am using kalk with a doser pump then dosing alkalinity to raise alkalinity. Alk is at 7. Magnesium 1400, calcium 530