Samples Containing Strontium or Barium

The Metals

Note: Ba and Sr will be treated collectively due to very similar chemistries.

Both barium (Ba) and strontium (Sr) are silvery white metals resembling aluminum. The metals are unstable in air and burn with incandescence in moist air or when heated. They decompose water, evolving H₂ and forming Sr(OH)₂ or Ba(OH)₂. The same reaction occurs more readily in acids — exercise caution.

Oxides, Hydroxides, Carbonates

The oxides are white and are obtained by ignition of the hydroxide, carbonate, nitrate, oxalate, and all of their organic salts. The oxides, hydroxides, and carbonates are all readily soluble in dilute acids, combining to form the ions of the corresponding salt. They absorb CO₂ from the air, becoming carbonates. In moist air, oxides become hydroxides, with the generation of heat if sufficient water is present. The hydroxides dissolve readily in dilute acids and are much more soluble in water than the corresponding Ca hydroxide.

The oxide, hydroxide, and carbonate are all readily soluble in dilute acids. Most analysts prefer dilute (1:1) nitric acid.

Minerals and Ores

Since the ores contain such a wide assortment of other elements, it is considered best to prepare them by fusion with either lithium carbonate in graphite crucibles or sodium carbonate in Pt crucibles. If barium sulfate is present, the fusion converts it to the barium carbonate and the corresponding sodium sulfate, which can be leached out from the fuseate with water prior to dissolution of the fuseate in dilute HCl. The following method is for the preparation of limestone for Ca, Mg, Fe, Al, Mn, and Si using a lithium carbonate fusion, but can be applied to samples high in Sr and Ba: Sample Preparation Procedure.

The following is a general guide for sodium carbonate fusions:

1. Make certain that the sample is well mixed with the sodium carbonate.
2. A 5-9's pure sodium carbonate is recommended and available from E.M. Science.
3. 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.
4. If organic matter is present, either the sample is mixed with the flux initially and heated slowly to 500 °C for ~2 hours before bringing up to full temperature, or the sample can be pre-ashed at 500 °C and then the ash mixed with the flux.
5. Use Pt as the crucible container material.
6. 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.
7. Most fusions are complete in 15 minutes, and some require up to 45 minutes.
8. Dissolve the fuseate in dilute HCl (1:1).

When Ba is associated with sulfate as BaSO₄ or the fluoride, it is suggested that the analyst fuse the sample with H₃BO₃ or B₂O₃. This fusion is carried out in Pt crucibles at 1000 °C, where the sample-to-flux ratio is at least 1:8 and can be as high as 1:30. The SO₃ is expelled due to the high temperature of the acidic melt. The BaF₂ is opened out by this fusion procedure, and the fluoride is tied up with the boron, making the dissolution of the fuseate in water or dilute acid possible without reformation of the BaF₂.

Lithium tetraborate (Li₂B₄O₇) has also proven to be a very useful way of opening out many of the ores associated with Ba and Sr. This fusion is carried out in Pt for ~10 minutes at 1000 °C, and the fuseate is dissolved at room temperature by stirring with dilute (5% v/v) nitric acid. When metal ions that are readily hydrolyzed are present, dissolution in the presence of EDTA in 0.01 M HCl is advantageous.

Alloys

The Ba and Sr metals are not used to a great extent, nor are their alloys. Although commercial alloys are relatively rare, their dissolution can be effected using dilute nitric or nitric/HCl mixtures.

Organic Matrices

These include a wide variety of materials, including oil additives (Ba), petroleum matrices, coal, organic plant material, biological material, synthetic organics, etc. Samples can be digested with nitric/perchloric acid. For more detailed information about acid digestions of organics, please see the following article: Acid Digestions of Organic Samples.

It is also very acceptable to dry-ash organic samples for Ba or Sr analysis in a Pt crucible, and then bring the resulting oxides into solution using a sodium carbonate fusion, or, if Ba or Sr alone are sought, dissolution in dilute nitric or HCl. For more information, see the portion of our Trace Analysis Guide that discusses Ashing.

Hydrolytic Stability and Preferred Matrices

• Ba and Sr begin to precipitate (as the carbonate, due to dissolved CO₂) from solution at a pH of between 6.2 (high conc.) to 10 (low conc.).
• Following are solubility data for Sr and Ba as compared to Ca (g/100 mL H₂O):

• The following table shows the improvements in the hydrolytic stability of Ba+2 and Sr+2 with different complexing agents. The pH where precipitation of BaCO3 and SrCO3 begins is shown for 0.1 M solutions of each complexing agent: