Preparation for Samples Containing Magnesium
How to prepare for samples containing Magnesium
- iCAP 6000
- iCAP 7000
- iCAP Pro
- iCAP Q
- iCAP RQ
- iCAP TQ
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.
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.
- 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:
- 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) 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 of 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.
- 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.
|Image - Mg - pH where precipitation begins.png||August 02, 2022|