Saturation Levels of CaCl and MgCl in RO/DI Water

[Todd]

I am going to start a dosing regime using a dosing pump and am working through the logistics of it. I would like to make my solutions soon and get my levels up to a point where I can move to a maintenance mode.

I would like to know what is the saturation level of CaCl and MgCl / MgSulfate in RO / DI water. I know that Randy Holmes Farley suggests 500 g / 1 gallon of water for CaCl (Dihydrate) and 8 cups of Magnesium Sulfate into 1 gallon of water. I would like to know what is the saturation point (ie at what point will these two precipitate out), of these two solutions (CaCl / water and MgCl and Mg Sulfate in a 1:1 ratio / water) so that I can make a strong solution, thus be able to dose less of it.

I hope this makes sense. I went through the chemistry section of the reference library but in no place did I find saturation points. The chemicals I am going to be using are: Magnesium Sulfate (MgSO4)*7H20 and Magnesium Chloride HexaHydrate (MgCl2)*6H20 - AKA Dead Sea Works; I am planning on using these in a 1:1 ratio. The Calcium Choride is Anhydrous Cacium Chloride 94-97%.

I could experiment with it to figure it out, but if someone has already done this it would save me the effort, or I could just use the recommended solutions, but then it would involve a little more work - which I why I am moving to the doser.

Thanks!

[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.

[Todd]

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.

This is what I am planning on doing, but I would like to make a strong solution, so that the solution will last longer - thus I was curious to how much Mg or CaCl I could add to a solution. Thanks for all of your info, very useful.

there are easier roots like kalk drips, liquid Mg and or calcium carbonate which has some Mg in it

I am going to stick to my plan with mixing dry chemicals with RO/DI water, as it will be very cost effective in the long term and will give me exact control over my Mg, Ca and buffer levels, and will also be very automated once I get it set up.

I may make a thread with my starting levels, what I am dosing, equipment ect. I will be using pharmaceutical grade chemicals (similar to bulk reef supply, or little silvermax's chemicals). I am having a few issues with my Mg test kit (salfart) that I will need to resolve before I start dosing.

Do you have the link to the rest of the article you posted? I would be interested in reading more about particular ions ' dominating the chemistry' as this may change my feeling about dosing Mg in a 1:1 ratio (MgCl to MgSO4).

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..

Ahh, I did not think about this, very good point. However, should the pH and C02 of RO/DI water not be constant though? My understanding of pH is that it is the proportion of free hydrogen ions present in the solution, but this should be constant (ie stable) in 'pure' RO / DI water, should it not?

I think that I will start with Randy's guide and then increase it with the next batch if I feel that I need to get more milage out of the containers.

[fencer]

Look up the KsP equilbrium for your answer....

[littlesilvermax]

FWIW calcium get used up at a much higher rate in your tank then magnesium.

What salt are you using, what is the magnesium and calcium level of that salt?

[PoonTang]

I think Randys recipies are near saturation because it sure takes alot of mixing, and a little heat, to get them fully disolved.

[Todd]

I think Randys recipies are near saturation because it sure takes alot of mixing, and a little heat, to get them fully disolved.

Thanks! That was the info that I was looking for. I am sure it wouldnt have taken me long to figure it out by trial and error, but I figured why bother if someone else had already figured it out.

Originally Posted by littlesilvermax

FWIW calcium get used up at a much higher rate in your tank then magnesium.

What salt are you using, what is the magnesium and calcium level of that salt?

Thanks for the info. I am aware that the uptake levels will be different, and I am planning on using different dosing schedules for CaCl and Mg to address that.

I am using IO salt, the Ca level is usually 380 and the Mg level usually tests around 1100 using a Salfart test kit.

Here is a problem I am having though, perhaps someone will have some insight into: I have tested my wc water and found a Mg level of 1080, I then proceed to add 85g of MgCl and 85g of MgSO4 to bring my Mg levels up. I noticed no precipitate. After letting my heated water change water sit mixing, I tested my Mg levels again, only to find them unchanged and still at 1080.

I tried to do the same thing with my next batch of water change water only to have the same thing happen (levels the second time were 1020 starting, added 56g MgSO4 and 66g MgCl along with 17g CaCl).

Is this a problem with my test kit? Do the test kits work in a way that they will not measure Mg added in this way? Could it have precipitated out and I not noticed? Anything else I am not thinking of?

[littlesilvermax]

Originally Posted by Todd

Thanks for the info. I am aware that the uptake levels will be different, and I am planning on using different dosing schedules for CaCl and Mg to address that.

I am using IO salt, the Ca level is usually 380 and the Mg level usually tests around 1100 using a Salfart test kit.

Here is a problem I am having though, perhaps someone will have some insight into: I have tested my wc water and found a Mg level of 1080, I then proceed to add 85g of MgCl and 85g of MgSO4 to bring my Mg levels up. I noticed no precipitate. After letting my heated water change water sit mixing, I tested my Mg levels again, only to find them unchanged and still at 1080.

I tried to do the same thing with my next batch of water change water only to have the same thing happen (levels the second time were 1020 starting, added 56g MgSO4 and 66g MgCl along with 17g CaCl).

Is this a problem with my test kit? Do the test kits work in a way that they will not measure Mg added in this way? Could it have precipitated out and I not noticed? Anything else I am not thinking of?

I have never noticed any magnesium to prec out when I mix my IO water, and I have mixed A LOT of IO water.

Salifert test kits as of late are not that great.

I would add about 9 times the amount of MgCl vs MgSO4. Otherwise you get too high a sulfate in the water. Not a big deal though.

Try (what I always do) mixing the calcium and magnesium in the water BEFORE you add the salt.

FWIW if you get the IO water up to about 1380 ppm or so, it is unlikely that you will have to dose magnesium to the main tank. Unless you use a lot of kalk.