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I have a PolyA tailing kit for putting PolyA tails on the 3' end of RNA. The kit comes with a proprietary 5X buffer and a 25mM MnCl2 solution, and in the standard set up I use 20uL of 5X buffer and 10uL of 25mM MnCl2. When trying to force the enzyme to accept an alternative substrate I did the reaction without the MnCl2 and it worked. Further research revealed that the enzyme also works with Mg++ ions, even though the rate is faster with Mn++, and some old papers on the enzyme mention using both Mg and Mn in the buffer. This leads me to believe that the buffer contains Mg++, but I don't know for sure because the manufacturer won't tell me.

I immediately thought I could use atomic absorption spectroscopy, but our department does not have one. I looked up colorimetric assays online, but the kits cost about $300, which is more than I want to spend on this idea. What I'm asking is if anyone knows a relatively simple method to detect magnesium in a solution? I have access to a UV-Vis spectrophotometer and fluorimeter.

EDIT: After digging for other phone numbers for the manufacturer I found someone who would tell me a little about the buffer, and it does contain magnesium. He couldn't me how much though. Spectro-Phone-otemotry.

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Magnesium 2+ forms an insoluble precipitate with hydroxide ion. Perhaps add a source of hydroxide ion? Mg2+ also is detectable by flame test. –  Dissenter Aug 20 at 17:51
Just tried a flame test to see what would happen, it gave me bright orange. The buffer probably contains sodium chloride. Old literature recipes I've seen use anywhere from 100 to 300mM NaCl, so a 5X buffer could have 500 - 1500mM NaCl and overwhelm the MgCl2. I tried a flame test on the MnCl2 solution as well, hoping to see yellowish green, but only saw a few orange sparks, so maybe my ethanol burner isn't good enough for flame testing. –  user137 Aug 20 at 18:10
Even trace amounts of Na+ can make the flame go yellow/orange, rendering the test rather useless. –  Dissenter Aug 20 at 18:12
Manganese hydroxide is also insoluble. I don't know if you can distinguish it from magnesium hydroxide. –  canadianer Aug 21 at 0:34
Thankfully the manganese is kept in a separate vial, so I can choose to add it or not. –  user137 Aug 21 at 15:12

2 Answers 2

$\ce{Mg^2+}$ forms complexes with 4-(2-Pyridylazo)resorcinol that can be detected spectrophotometrically: Simultaneous spectrophotometric determination of calcium and magnesium in water, described by E. Gómez, J.M. Estela, V. Cerdà in Analytica Chim. Acta 1991, 249, 513-518.

(DOI: 10.1016/S0003-2670(00)83027-5)

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Will order some from sigma and give it a try. Looks like I can get 1g for about $25. Don't know how soon I can get back with an answer. –  user137 Aug 20 at 20:16
@user137 Good luck with the experiment! –  Klaus Warzecha Aug 20 at 21:02

There is also the venerable complexometric determination of magnesium with EDTA. Determining hardness of tap water is a popular lab exercise, your chemistry department's teaching lab might even have EDTA solution and complexometric indicator in stock. It's worth a shot.

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Can you get an indicator that is specific for Mg but not Mn? –  canadianer Aug 21 at 0:34
Most probably you will have other metal ions in the solution, eg Ca++, which can make this measurement useless. –  Greg Aug 21 at 1:45

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