Detection of magnesium in a buffer? 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.
$\ce{Mg^2+}$ 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)