Thread: Determination of Mg content in rocks

I have to design an experiment that can determine the amount of Mg 2+ content in rocks by Tuesday. Does anyone have any suggestions because I'm really not sure where to start.

Thanks!

You need to start by telling us of your background. We don't know if you are a true chemical engineer whose company will actually be doing such analyses on real ores based on your recommendations, or whether you are working on some goofy assignment from a flaky history teacher who has never set foot in a laboratory. So what's the deal?

Well, I'm actually an undergraduate student enrolled in an Analytical Chemistry course and I am working on a proposal for the experiment I explained. We have to design and also perform the experiment for our final lab in the course. So no, it's not for some flaky history teacher.

OK, now we're getting somewhere.

Your first step is to obtain some rocks. Avoid granite; I recommend limestone. It's softer and crushes more easily.

Next you have to crush the rock and grind it up into a fine powder. Mining companies have special mills for this purpose. Do you have something suitable? Even for a tabletop ball mill expect to take at least 24 hours for this process.

Now you have to choose your method of analysis. Gravimetric? That's best for a small lab wthout fancy instrumentation but it will be difficult because you first have to remove the silica, iron, and other impurities from your sample.

So you had better review your lab's inventory of instruments to see what gadgets you can use. Do a Google search for various methods using your instrumentation on hand.

I have one suggestion. Instead of doing your demo on actual ground up rock, prepare a test sample of magnesium salts out of a bottle. You can proceed with your analysis of the magnesium without the attendant difficulties of using real ore. Gravimetric analysis would then work nicely.
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Okay, let me clarify some things. I was given these instructions "Determine the Mg 2+ content in a rock". I am assuming the rock will be given to me, hopefully in a ground up form. What I have to do is come up with a proposal for the experimental process I would use (inluding all chemicals along with their reactions) on how I would determine this. I also have to include the calculations I would need to perform; this is due Tuesday. We do not have access to the lab until we are able to actually perform the experiment (which once turned in Tuesday the teacher and TA will correct) so there is no demo, just the real thing.

So, I came up with an experiment. I would appreciate it if someone could tell me if this seems plausible assuming my lab has the following materials.

Here is the general scheme:
1.) rock (containing Mg 2+) + HF (able to dissolve rocks)---->MgF2 (and if Ca, Sr, and Ba are in the rock they will precipitate as well)
2.) To be sure that the above mentioned ions are not part of the MgF2 precipitate
(MgF2+ precipitate) + Sulfate ---->MgSulfate(aq) + possible Ca, Sr, Ba,sulfate precipitate
3.) I would then discard the precipitate and use the aq MgSulfate sln to determine the initial Mg content in the rock.

So far, so good. You have described the separation steps. Now follow up with the actual analysis for magnesium in the MgSO4. What will be your method?

The easiest way would be to stick a sample of the rock in an x-ray photo-electron spectroscopy or Auger electron spectroscopy machine. That will simply spit out the Mg content (as well as the content of every other element in the rock) in about 2 seconds, with no special sample preparation. If you don’s have and XPS or AES, I would dissolve the rock in acid and analyze the Mg content of the resulting solution with ICP-MS, ICP-AA, atomic emission, gravimetric analysis, or whatever.

Edit: Are you actually going to be doing this? If so, I would try to use something other than HF. It's on my short list of "If the only way to do it is with one of these, I'll just do something else" chemicals.

Originally Posted by SteveF

So far, so good. You have described the separation steps. Now follow up with the actual analysis for magnesium in the MgSO4. What will be your method?

hmmm. That is the part that I'm not sure about. Any suggestions?

Originally Posted by Scifor Refugee

The easiest way would be to stick a sample of the rock in an x-ray photo-electron spectroscopy or Auger electron spectroscopy machine. That will simply spit out the Mg content (as well as the content of every other element in the rock) in about 2 seconds, with no special sample preparation. If you don’s have and XPS or AES, I would dissolve the rock in acid and analyze the Mg content of the resulting solution with ICP-MS, ICP-AA, atomic emission, gravimetric analysis, or whatever.

Edit: Are you actually going to be doing this? If so, I would try to use something other than HF. It's on my short list of "If the only way to do it is with one of these, I'll just do something else" chemicals.

Yes I am actually going to be performing this experiment. Also, if I had access to an XPS or AES machine, I would definetly use those, but unfortunately I don't. I posted above that I'm going to dissolve the rock in acid and then use gravimmetric analysis to try and determine the Mg content, so thanks for confirming

Originally Posted by Scifor Refugee

I would try to use something other than HF. It's on my short list of "If the only way to do it is with one of these, I'll just do something else" chemicals.

Heh-heh! I felt there was no need to comment on your plan of using HF because:

• Your school lab probably doesn't stock any.

• Your instructor would likely forbid a beginner from using it.

• You are unlikely to need it. Your instructor will not give you actual crushed rock but a test sample made of magnesium salts plus other inert fillers.

So read up on a gravimetric procedure for the quantitative analysis of magnesium. That shoud do it.

I just talked to my instructor. I am receiving a real rock, not even a crushed up one; he's hardcore. But, I cant use HF even though we do have it.

Now I'm going a totally different route and using a complexation titration.