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Old 01/08/2009, 10:47 PM   #1
Elliott
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mixing Kalk with sea water

I am considering doing water changes by running my skimmer wet and using a small amount of Kalk mixed with sea water to replace the skimmate loss. I have been using Kalk and RO water as my top off, just wondering if there is a problem mixing Kalk powder with sea water?


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Old 01/09/2009, 12:03 AM   #2
bertoni
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Kalk will raise the pH enough to cause calcium carbonate precipitation, most likely, so that approach is unlikely to work. I don't know of anyone who's made it work in a practical way.


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Old 01/09/2009, 06:33 AM   #3
Elliott
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ahh, ok thanks Jonathan


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Current Tank Info: tank video: https://www.youtube.com/watch?v=va1dI7mdnGU ,900g in wall mixed reef, another 500g sumps, 19 AI Sol LED's, 2 CL's w/VFD's controlled pumps to 24 eductors, 2 Tunze WB's, 2 Barr 5220's and RK2 25PE and BK500, etc
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Old 01/09/2009, 06:52 AM   #4
Randy Holmes-Farley
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You'll also instantly precipitate magnesium hydroxide, making mud.

I discuss that here:

What is that Precipitate in My Reef Aquarium?
http://reefkeeping.com/issues/2005-07/rhf/index.htm

from it:

Experimental Measurements of Magnesium Hydroxide Formation
In order to substantiate the theoretical calculations described above for magnesium hydroxide, I also tested the precipitation of magnesium hydroxide at elevated pH. Starting with fresh Instant Ocean artificial seawater, I added dropwise a concentrated solution of sodium hydroxide in fresh water. As each drop entered, a cloudy/white glob formed that settled to the bottom. At first, each drop could be fully dissolved with sufficient swirling. By the time the pH of the solution reached the upper 9's, the glob was only very slowly dissolving. By pH 10.4, the solution was very cloudy, even after extensive swirling.

Taking this cloudy solution at pH 10.4, I added hydrochloric acid to reduce the pH to 10.0. All of the cloudiness disappeared within a few minutes. When set aside for 90 minutes, however, a solid precipitate formed. Upon reducing the pH to 8.2 with hydrochloric acid, this precipitate remained. It remained undissolved for many days.

My conclusion is that the initial cloudiness is magnesium hydroxide that rapidly forms and dissolves (not magnesium carbonate, as described below). It appears somewhat more soluble than the theoretical calculations above would suggest. That is, it does not precipitate out until the pH is above 10, and seems fully soluble at pH 10, while the calculations suggest that the initial point of precipitation should be lower (mid 9's). This result appears to be because the magnesium hydroxide that precipitates first is not the most stable form, and it is the most stable form (the crystalline form brucite) that is typically reported in Ksp values. The initial precipitate is faster to form, but is more soluble.

The notion that there are different forms of magnesium hydroxide that can form is supported in the chemical literature. One group has reported, for example, that the initial precipitate that forms when raising seawater's pH is not the most stable form, that only more slowly is made as the initial precipitate slowly converts into a less soluble crystal. They note: "In the case of Mg(OH)2, a considerable difference between the solubility of freshly precipitated hydroxide and that of aged hydroxide was revealed."2 A second group reported similar findings: "Precipitated Mg(OH)2 is at first more soluble than brucite, but in time its solubility decreases to that of brucite."3

When sitting at high pH for more than a few minutes, calcium carbonate can precipitate from these high pH solutions, as was observed. Since seawater is supersaturated with respect to calcium carbonate, even at pH 8.2, this material does not dissolve on reducing the pH to 8.2, even after many days of exposure.


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Old 01/09/2009, 07:48 AM   #5
Elliott
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excellent as always, thanks Randy


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Rodney Dangerfield - "I worked in a pet store and people would ask how big I would get."

TOTM, March 2015

Current Tank Info: tank video: https://www.youtube.com/watch?v=va1dI7mdnGU ,900g in wall mixed reef, another 500g sumps, 19 AI Sol LED's, 2 CL's w/VFD's controlled pumps to 24 eductors, 2 Tunze WB's, 2 Barr 5220's and RK2 25PE and BK500, etc
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Old 01/09/2009, 11:59 AM   #6
Randy Holmes-Farley
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Happy Reefing.


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Old 01/10/2009, 08:09 AM   #7
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Marine water softening.


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