Saturation Levels of CaCl and MgCl in RO/DI Water

[Todd]

Bump

[Aquattro]

Todd, I think you'll find the value is temperature dependant, as well as pH, which will vary due to CO2 content. I'd probably just go with Randy's guide..

[tang daddy]

Todd I don't know if this answers your question on Mg but when I read it it kinda gave me an understanding that in natural saltwater 1 Mg ion has 8 water molecules bound to it. If you were going to apply this to rodi you would first have figure out what 1g of rodi would work out to in ppm of ions and then convert that to micromolar.

What I mean to say is that it is very confusing and unless you're good with math and science there are easier roots like kalk drips, liquid Mg and or calcium carbonate which has some Mg in it however I think the first 2 has higher concetrations, dang even nori has Mg in it there is a table chart I was reading in the RH farley article on Mg.

If you were gonna be mixing dry goods to rodi why not test to see what the ppm is to a 5g mix and then you can eventually work it out so that your top off will slowly add the supps you need to achieve ideal levels.

I cant be bothered so the route I am taking is adding salifert liquid Mg, calcium carbonate powder(the kind you find at a pool store), salifert AA, 2 little fishes 2 part, coraline accel, and soon sodium chloride to my 20g top off water and then slowly add it in at a rate of 3.5g a day. As long as it goes in in little amounts I cant see why my corals wouldn't enjoy these supps as I was already dosing 2 part, AA, and coraline excell with mo ill effects 2 more usefull ingriedients to the mix can't make it that bad if anything it's a huge benifit for everything from sps to fish to rock.

Gl with that and please keep us updated on your results.



Magnesium in Seawater
In full strength seawater (S=35), magnesium is present at approximately 53 mM (mM is short for millimolar, which is a measure of the actual number of ions present, as opposed to ppm (parts per million), which is a measure of the mass of ions present). Only sodium (469 mM) and chloride (546 mM) are present at higher concentration, with sulfate (28 mM) following close behind. Magnesium is about five times more abundant than calcium (10 mM). Magnesium is significantly lighter than calcium, so when compared on a weight basis, it is only about 3 times as concentrated (1285 ppm vs. 420 ppm).
Figure 1. This sally lightfoot crab probably doesn't realize that the coralline algae that it is standing on likely has a high magnesium content. Photo by Debi Coughlin.
One other comment on seawater concentrations of magnesium. The magnesium content of seawater has not been constant since the oceans formed. Specifically, the magnesium content has often been lower, as in the late cretaceous period. As is discussed below, the amount of magnesium getting into calcium carbonate skeletons is a function of how much magnesium is in the water. Consequently, the magnesium content of ancient sediments can be significantly lower than more modern ones from similar organisms.1 In addition to being an interesting fact, this result may also play a role in the suitability of certain limestone deposits in maintaining magnesium in aquaria. For example, such limestone is sometimes used in CaCO3/CO2 reactors or as the raw material for making calcium hydroxide (lime). If it is low in magnesium, one may find additional supplements necessary to maintain modern seawater magnesium concentrations. These issues will be detailed more in future articles.
Magnesium is present in seawater as the Mg++ ion, meaning that it carries two positive charges, just as calcium does. Most of the magnesium is present as the free ion, with only water molecules attached to it. It is estimated that each magnesium ion has approximately eight water molecules tightly bound to it. That is, water molecules that are so tightly bound that they move with it as the magnesium ion moves through the bulk of the water. For comparison, singly charged ions like sodium have only 3-4 tightly bound water molecules. A small portion (about 10%) of the magnesium is present as a soluble ion pair with sulfate (MgSO4), and much smaller portions are paired with bicarbonate (MgHCO3+), carbonate (MgCO3), fluoride (MgF+), borate (MgB(OH)4+), and hydroxide (MgOH+).
While these ion pairs comprise only a small portion of the total magnesium concentration, they can dominate the chemistry of these other ions. An extended discussion of these facts is beyond the scope of this article, but is should be noted that these ion pairs can have huge impacts on seawater chemistry. In the case of carbonate, for example, the ion pairing to magnesium so stabilizes the carbonate that it is present in far higher concentrations than it would be present in the absence of magnesium. This effect, in turn, makes seawater a much better buffer in the pH range of 8.0-8.5 than it otherwise would be. Without this ion pairing, seawater pH might be significantly higher, and more susceptible to diurnal (daily) swings.