Alk and Mg relationship

[Cment]

i understand that when you add Mg to your system it will have an effect on raising Alk levels but is there a standard ratio that shows this? So for "X" amount of Mg added your Alk will raise "X" amount? Does it vary depending on which brand/product used?

 

[red_sea_reef]

in order to answer that question, keep in mind the basic understanding of the calcium and carbonate systems in reef tank. Calcium ions will interact with carbonate ions and precipitate as calcium carbonate. If you push the concentration of either too high, CaCO3 will start to precipitate. Magnesium interferes with this process, permitting both calcium and carbonate to be elevated above where they would be in the absence of magnesium. alkalinity is a good indicator of the concentration of carbonate. So higher alkalinity equates to higher carbonate. i do not really know how much Magnesium will raise Alk but for sure you must lower precipitation and avoid raising magnesium levels more than 100ppm per day. hope this help a little .

 

[Downbeach]

I think the only way you will see an increase in your Alk when adding Mg, would be because the addition of Mg may be helping to prevent your alk and Ca from precipitation out of solution.

 

[Cment]

I ask this question bc currently I have a bucket of IO that has a little lower Mg level than usual. I keep my tanks parameters at: Alk-7.8-8.2/Ca-440/Mg-1350. When I do a WC these are much lower so I try to match them up before doing the WC. I typically will have to raise the Mg 200ppm and by doing so my Alk jumps up to 11.5. I was just wondering if there was a way to prevent this from happening. Now its not a big deal bc I only do a 10% water change a week (8g) and it only raises my alk by less than .5dkh but was just curious if I ever had to do say a 50% WC in case of an emergency. This would drastically raise my Alk and tick off all my corals specifically my SPS and chalices....

 

[Cment]

I do typically add all the Mg at once to my brute and use tech m for my Mg source. If I pour it in slowly over the course of a day would this have any impact on how high it raises the Alk levels?

 

[Randy Holmes-Farley]

i understand that when you add Mg to your system it will have an effect on raising Alk levels but is there a standard ratio that shows this? So for "X" amount of Mg added your Alk will raise "X" amount? Does it vary depending on which brand/product used?

Adding magnesium does not itself impact alkalinity, as folks have noted. So the direct relationship is 0.

What does happen is that magnesium helps prevent the abiotic precipiation of calcium carbonate on pumps, heaters, etc. If magnesium is very low, more of that precipitation can take place, and you observe some extra demand for calcium and alkalinity.

Other factors also have a big impact. A rise of about 0.3 pH units about doubles the likelihood of precipitation.

Also, doubling either calcium or alkalinity values also doubles this likelihood.

I discuss these issues in detail here:

A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH
http://reefkeeping.com/issues/2006-06/rhf/index.php

 

[Randy Holmes-Farley]

I do typically add all the Mg at once to my brute and use tech m for my Mg source. If I pour it in slowly over the course of a day would this have any impact on how high it raises the Alk levels?

Nope. Always zero. :)