Salifert Magnesium test

[tribble222]

I've made the switch to all-grain and am getting ready to brew my first batch. I've read I should now care more about my water. To help prepare I bought 3 Salifert tests: Alkalinity, Calcium, and Magnesium.

Alkalinity came out to 4.1 dKH
Calcium: 10 ppm

The magnesium test has me a bit stumped. It's supposed to change from pink to gray or blue but before I start titrating it is already a very dark deep blue, almost black. Does this mean little to no magnesium in my water? I guess this wouldn't be surprising given the low calcium.

One part of the instruction manual that concerns me is "The magnesium test can be used for testing Marine water only" and "Calcium and strontium will not interfere when their total concentration is between 300 and 500 ppm."

My calcium is not between 300 and 500ppm and I'm testing tap water, not marine water. Is this why the test is not working??

Thanks for any advice!!

P.S. the city water report lists Mg as 1-14ppm and the lowest this kit can read is 30ppm.

 

[ajdelange]

Yes, that's probably what it means. I'm speculating here but they could, for example dose the sample with EGTA which would chelate the calcium, and then add an indicator that is pink in the presence of calcium and/or magnesium but turns blue in their abscence (Eriochrome Black T ?). In a sample with appreciable magnesium T would be red (remember you took the Ca++ out with EGTA. Now you would add EDTA titrometrically and when it complexed the magnesium the solution would turn blue.

Why not get a test that is designed for general water rather than reef aquarium water such as one of the kits offered by Hach or LaMotte? These have the additional advantage that they read in ppm as CaCO3. They are usually set up so that one kit does both total and magnesium hardness. First the Mg++ is removed by buffering to high pH (Mg(OH)2 precipitates) and then titrating with EDTA. That give calcium hardness. A second test is done at pH below where Mg(OH)2 precipitates and the result is the total hardness. The difference is the magnesium hardness.

 

[mabrungard]

AJ, I'm pretty sure that Ca precipitates at a lower pH than Mg. It should be the first one dropping out. Its the same with lime softening, Ca is relatively easy to drop, but you have to push the pH higher if you want the Mg to drop too.

 

[ajdelange]

Yes, calcium carbonate does precipitate at lower pH than magnesium hydroxide (if the saturation pH is lower than the pH at which magnesium hydroxide precipitation starts which it almost always will be unless the water is close to DI) but calcium hydroxide precipitates at higher pH than magnesium hydroxide. This is relied upon both in total and calcium hardness tests I described above and is the basis for 'split treatment' lime softening. Enough lime (calcium hydroxide) is added to the water to raise its pH to the point where Mg(OH)2 precipitates (the calcium hydroxide stays in solution). The reduced magnesium water is decanted off the precipitate and its high pH neutralized by acid bicarbonate ions from untreated water. Obviously you can only get rid of a portion of the magnesium that way but that's not the point here.

 

[tribble222]

Excellent, thanks for the info!

 

[mabrungard]

Um, lime softening does not precipitate calcium hydroxide from hard water. It precipitates calcium carbonate. Calcium carbonate is much less soluble than calcium hydroxide and magnesium hydroxide.

In my duties as a professional engineer which includes all forms of municipal water treatment, I see the operators of the lime-softening plants that I've consulted for, dredging and disposing of tons of chalk sludge, not calcium hydroxide.

 

[ajdelange]

Um, lime softening does not precipitate calcium hydroxide from hard water.

Never said it did:

Enough lime (calcium hydroxide) is added to the water to raise its pH to the point where Mg(OH)2 precipitates (the calcium hydroxide stays in solution). The reduced magnesium water is decanted off the precipitate and its high pH neutralized by acid bicarbonate ions from untreated water. Obviously you can only get rid of a portion of the magnesium that way but that's not the point here.

It precipitates calcium carbonate.

That's what happens when the bicarbonate neutralizes the high pH:
Ca++ + 2HCO3- + Ca++ + 2(OH)- ---> 2CaCO3 + 2H2O

Calcium carbonate is much less soluble than calcium hydroxide and magnesium hydroxide.

That depends. As I noted in an earlier post in a sample with only tiny amounts of alkalinity and calcium hardness the saturation pH for CaCO3 could be higher than the pH at which Mg(OH)2 precipitates. In the Hach test I referenced earlier pH 13 is considered sufficient to remove all the magnesium. In split treatment we don't go that high. I'm going to guess about 12.

In my duties as a professional engineer which includes all forms of municipal water treatment, I see the operators of the lime-softening plants that I've consulted for, dredging and disposing of tons of chalk sludge, not calcium hydroxide.

That's right:

Enough lime (calcium hydroxide) is added to the water to raise its pH to the point where Mg(OH)2 precipitates (the calcium hydroxide stays in solution).

Well, enough of it, anyway, to precipitate the magnesium.

Surprised you never ran into 'split treatment' before. I doubt you'd run into it in your professional life but from what I understand it was quite common in breweries with waters that contained more magnesium than desired. Maybe there weren't many of those. See DeClerck.