Samples Containing Magnesium
Magnesium (Mg) is a silvery white metal that burns with incandescence when heated in air. The metal is soluble in all acids. Analysts typically dissolve the metal in dilute (1:1) nitric acid. Caution should be observed since the reaction can become quite vigorous.
Mg Oxide, Hydroxide, Carbonate
The oxide is a refractory material and an insulator. The carbonate is used to polish silver and is the basis for certain toothpowders. The hydroxide, 'Milk of Magnesia", is a remedy for an acid stomach.
All are readily soluble in acids. Most analysts prefer dilute (1:1) nitric acid.
The various ores exist as Mg by itself (magnesite, MgCO3 ; kieserite, MgSO4-H2O), combined with Ca (dolomite, CaSO3-MgCO3) and combined with K, Si, Fe, and B to form a variety or ores of commercial interest.
Since the magnesite and dolomite still contain trace amounts of Fe, Al, Si, Mn etc., it is considered best to prepare Mg containing ores by fusion with either lithium carbonate in graphite crucibles or sodium carbonate in Pt crucibles. The fuseate is dissolved in dilute HCl. Consult the following method for the preparation of limestone for Ca, Mg, Fe, Al, Mn, and Si using a lithium carbonate fusion: Sample Preparation Procedure.
Mg-Al alloys are lightweight and have a high tensile strength per unit weight. A 1:1:1 mixture of water conc. HNO3 and conc. HCl will dissolve Mg alloys. The most common elements found in Mg alloys are Al, Mn, Cu, Si, Cr, and Ni.
These encompass a wide variety of materials including oil additives, petroleum matrices, coal, organic plant material, biological material, synthetic organics, etc. Samples can be digested with sulfuric acid and hydrogen peroxide, or sulfuric/nitric/perchloric. For more detailed information about acid digestions of organics, please see the following article: Acid Digestions of Organic Samples.
The sulfuric peroxide procedure is as follows:
- Add a 0.1 gram sample to a 125 mL Erlenmeyer flask.
- Add 5 milliliters of 98% sulfuric acid.
- Heat until dense white fumes form. Sample will be dark in color.
- Add 30% hydrogen peroxide drop-wise until solution clears.
- Continue heating until solution remains clear with dense white fumes and does not revert back to a dark color.
- Remove from heat. Cool and dilute to desired volume with 18 megohm water.
It is also very acceptable to dry ash organic samples for Mg analysis in a Pt crucible and then bring the resulting MgO into solution using a sodium carbonate fusion. For more information, see the portion of our Trace Analysis Guide that discusses Ashing.
The following is a general guide for carbonate fusions:
- Make certain that the sample is well mixed with the sodium carbonate.
- A 5-9's pure sodium carbonate is recommended and available from EM Science.
- Mix the sample with the flux at no more than a 1:20 ratio. Typical sample to flux ratios are in the 1:10 area.
- If organic matter is present, either the sample is mixed with the flux initially and heated slowly to 500 °C for ~ 2 hours before bring up to full temperature, or the sample can be pre-ashed at 500 °C and then the ash mixed with the flux.
- Use Pt as the crucible container material.
- Perform the fusion at 1000 °C in a muffle furnace. Avoid flames since this fusion is difficult to perform in a flame due to the high melting point of the sodium carbonate.
- Most fusions are complete in 15 minutes and some require up to 45 minutes.
- Dissolve the fuseate in dilute HCl (1:1).
Hydrolytic Stability and Preferred Matrices
- Mg hydrolysis is detectable in water just before precipitation of Mg (OH)2 at a pH of ~ 9.5. When diluting Mg standards, pre-acidified water is not required.
- Mg(OH)2 is one of the least soluble Mg compounds. Solubility issues should be of concern for water solutions containing Mg and fluoride, carbonate, phosphate, borate, arsenite, and arsenate. Increased acidity improves solubility of the above salts.
- The fixed alkali and other alkaline earth hydroxides precipitate Mg+2 as white gelatinous Mg(OH)2.
- The following table shows the improvements in stability of Mg+2 with different complexing agents. The pH where precipitation begins is shown for 0.1 M solutions of each complexing agent:
|Complexing Agent||pH where precipitation begins|
- Mg can be mixed with any of the elements at high concentrations (200 to 2000 µg/mL) with the exception of the fluoride containing elements (Ti, Zr, Hf, Nb, Ta, W, Si, Ge, Sn, Sb, Mo) and arsenic. Moderate to low levels (≤ 100 µg/mL) can be mixed with all of the elements.
- Mg has a relatively simple emission spectrum. A popular Mg emission line is the 280.270 nm line with a DL of ~ 0.00006 µg/mL (axial view) because of it's relative freedom from spectral interference combined with high sensitivity.
Detailed Elemental Profile
Chemical compatibility, stability, preparation, and atomic spectroscopic information is available by clicking the element below. For additional elements, visit our Interactive Periodic Table.