Magnesium Acetate

[rmurken]

I have a notion to brew up some magnesium acetate (precipitate Mg(OH)2 from epsom salt and NaOH, rinse, dissolve in vinegar) to use in my tank. I usually dose kalk but I have paused since running it has put Ca over 500, with alk steady to slightly falling at 8.3.

Need to keep my alk steady while the Ca comes down a bit. Using washing soda for that now, but my Mg could benefit from a modest boost (1300’s), so thought Mg acetate might kill two birds with one stone.

Welcome your thoughts.

 

[Randy Holmes-Farley]

That's a fine plan, but you could just dose the Epsom salt and sodium hydroxide. Both happen to be ingredients in my highest pH DIY two part.

 

[nereefpat]

The acetate would still make it in the tank, therefore you would be carbon dosing too, right?

I would just bolus dose the epsom salt by 50-100 ppm once a day until you reach the level you want, and stay with the washing soda for alk.

It does sound like you know what you are doing, and it would be something fun and different to try, but I would just do it the easiest way...just my opinion of course.

 

[rmurken]

That's a fine plan, but you could just dose the Epsom salt and sodium hydroxide. Both happen to be ingredients in my highest pH DIY two part.

Indeed! But out of compulsiveness (unheard of in this hobby, I know), I figured I could try an approach that avoids adding Na and SO4 as spectator ions. My tank is low demand (.6 or so dKh/day) and with weekly 10%ish WC’s, I think the imbalance issue is probably more theory than reality.

But why go for good enough when you can spend the wee hours after everyone’s in bed playing mad scientist with HS chemistry?

 

[rmurken]

The acetate would still make it in the tank, therefore you would be carbon dosing too, right?

I would just bolus dose the epsom salt by 50-100 ppm once a day until you reach the level you want, and stay with the washing soda for alk.

It does sound like you know what you are doing, and it would be something fun and different to try, but I would just do it the easiest way...just my opinion of course.

It would be C-dosing, but hopefully not so much that I’d have to supplement nitrate and phosphate...in fact if I dipped too much on those, that would be a reason by itself to stop and use Randy’s infinitely more practical approach.

As to knowing what I’m doing...lol!

 

[rmurken]

Edit: I got this wrong the first time. Below is my second try.

Here’s something my 11th grade chem nerd can’t quite settle: NaOH vs. Na2CO3 for alkalinity. If the object is to minimize sodium, they’re equal, right? Equations:

NaOH + H2CO3 -> Na + H2O + HCO3 (one sodium, one bicarb)

Na2CO3 + H2CO3 -> 2Na + 2HCO3 (two sodium, two bicarb)

Gross oversimplification since alkalinity is primarily HCO3, but followed by CO3, OH, and other things, but I am a bear of little brain.

The NaOH would have dramatic pH implications upon addition and so would have to be rather dilute and slowly added. Slower and/or more dilute than kalk.

So probably not much marginal benefit to dosing NaOH rather than washing soda unless you really want to go hard on pH.

 

[Randy Holmes-Farley]

Indeed! But out of compulsiveness (unheard of in this hobby, I know), I figured I could try an approach that avoids adding Na and SO4 as spectator ions. My tank is low demand (.6 or so dKh/day) and with weekly 10%ish WC’s, I think the imbalance issue is probably more theory than reality.

But why go for good enough when you can spend the wee hours after everyone’s in bed playing mad scientist with HS chemistry?

OK, but I'll not that if you are using calcium chloride for calcium, you need the sulfate.

 

[Randy Holmes-Farley]

Edit: I got this wrong the first time. Below is my second try.

Here’s something my 11th grade chem nerd can’t quite settle: NaOH vs. Na2CO3 for alkalinity. If the object is to minimize sodium, they’re equal, right? Equations:

NaOH + H2CO3 -> Na + H2O + HCO3 (one sodium, one bicarb)

Na2CO3 + H2CO3 -> 2Na + 2HCO3 (two sodium, two bicarb)

Gross oversimplification since alkalinity is primarily HCO3, but followed by CO3, OH, and other things, but I am a bear of little brain.

The NaOH would have dramatic pH implications upon addition and so would have to be rather dilute and slowly added. Slower and/or more dilute than kalk.

So probably not much marginal benefit to dosing NaOH rather than washing soda unless you really want to go hard on pH.

Yes, sodium hydroxide, bicarbonate, and carbonate all add 1 unit of sodium per unit of alk.

 

[Randy Holmes-Farley]

The NaOH would have dramatic pH implications upon addition and so would have to be rather dilute and slowly added. Slower and/or more dilute than kalk.

That is why I picked it for my highest pH two part recipe, but it definitely need not be more dilute than limewater (kalkwasser; why would that even be?).

Some folks like it partly because it can be highly concentrated. More concentrated than sodium carbonate solution due to solubility.

 

[rmurken]

OK, but I'll not that if you are using calcium chloride for calcium, you need the sulfate.

Noted. For the moment I am looking for a way to maintain alk and let Ca fall a bit. I have some headroom for Mg, so figured Mg acetate is theoretically perfect for Alk, a bit of Mg, and nothing else. Mg(OH)2/CO3 not soluble enough to make “Mag Kalk” (this sounds like the reefing version of “transparent aluminum” from Star Trek III, tbh).

In reality the MgOH2 I made is taking so long to settle (and I don’t have lab equipment to filter) that I’m just gonna use the washing soda 2-part that I still have and then whip up some NaOH since my pH is not so hot. I’ll dry the MgOH2 and make acetate with it next time I need to let Ca adjust downward...because why not.

 

[rmurken]

Yes, sodium hydroxide, bicarbonate, and carbonate all add 1 unit of sodium per unit of alk.

Thank you. I was hurting my brain trying to make sure of this.

 

[rmurken]

Well, realized I had a bottle of unflavored Milk of Magnesia in my “chemical bin.” So while my DIY Mg(OH)2 continues to settle (progress is mm/day), I whipped up some Mg acetate with that. Will let it sit for awhile since the MoM has a bit of sodium hypochlorite in it.

 

[rmurken]

Further note to this: unsurprisingly, Magnesium acetate solution (concentration = just enough white vinegar to dissolve all the MgOH) gets very moldy very quickly. ;Hurting

 

[Randy Holmes-Farley]

Further note to this: unsurprisingly, Magnesium acetate solution (concentration = just enough white vinegar to dissolve all the MgOH) gets very moldy very quickly. ;Hurting

lol

For this reason, when I neutralized vinegar for organic carbon dosing using calcium hydroxide, I saturated it with calcium hydroxide to keep the pH high enough to prevent growth.

 

[rmurken]

Alas, MgOH2 not as cooperative as its Ca cousin in that regard I guess. I wound up using the NaOH portion of your high pH recipe to keep alk up while my Ca came down. Ca is going right back up though, so I’m going to see if it levels off. If it just stays at 525-550 and that’s its stable level for my dosing regimen...doesn’t seem like a big deal. Maybe over time it’ll fall. Maybe my RO has Ca in it and throws off my Hanna checker. Who knows.

Will deny me the pleasure of cooking up and then fermenting mag acetate. But I’m better off without the carbon dosing anyway, since my often-zero PO4 levels need some love. <dumps fistful of food into tank.>

 

[rmurken]

Also note: plainly the sodium hypochlorite in the OTC Milk of Magnesia became a non-factor! My surmise is the acidification drove off the chlorine quickly.

 

[rmurken]

You know it just occurred to me: @Randy Holmes-Farley, I believe I’ve read posts by you elsewhere that Magnesium Hydroxide is soluble in seawater. Any reason not to dose straight MgOH2 to supplement Mg? Especially if you use kalk and don’t run into the more complicated ion balance issues that come with with two-part.