Sample Preparation Guide pt13: Containing Niobium or Tantalum
Part 13: Samples Containing Niobium or Tantalum
Overview
Niobium is named from Niobe the daughter of Tantalus in Greek Mythology. It was named Columbium (Cb) at the time of its discovery in 1801. This name was used until IUPAC adopted the name "niobium" in 1950 after more than a century of controversy.
Niobium (Nb) and Tantalum (Ta), much like Zr and Hf, have nearly identical atomic radii. The radii of the ions (Nb+5 and Hf+5) are also almost identical which explains in part the fact that the chemistry of these two elements is so similar.
Nb and Ta are always found together in natural mineral deposits and both are relatively quite rare, with abundance estimates in the earths crust ranging from 0.01 to 0.0001 % m/m. The most important ores are Tantalite (Mn, Fe)·(Ta, Nb)2O6 containing 38-86 % m/m Ta2O5 and Columbite (Mn, Fe)·(Nb, Ta)2O6 containing 40-78% m/m Nb2O5. These formulas are written in such a way as to accommodate the fact that niobium and tantalum replace each other freely in these minerals in addition to the fact that Fe and Mn substitute for each other as well. Tantalum has a better corrosion resistance than Nb and is used for plating chemical apparatus which is in turn used for handling highly corrosive acid liquors and for radiation shields and crucibles in high-temperature vacuum furnaces. Niobium has a small thermal neutron capture cross-section and good resistance to several liquid-metal coolants. It is therefore of major interest in nuclear technology. Niobium is also found in a variety of alloys used for a variety of applications ranging from jet engines to bone implants.
Sampling and Handling
The most common matrices where Nb and Ta are found are minerals, ores, metals, and alloys. The sampling of Ta/Nb bearing ores and minerals requires special attention due to the fact that these elements vary widely even among samples from neighboring locations due to the fact that they are easily substituted for one another as discussed above. Consequently, samples must be obtained from multiple containers where they are crushed to about 10 mesh, coning and quartering are applied and the material is further pulverized to pass a 20 mesh screen. The material is further mixed where it is reduced (coned/quarted) to about 300 grams and then further ground so that the analytical sample will pass an 80 mesh screen.
Metals and alloys are sampled by standard procedures. For additional details, see the following information on sampling and subsampling.
The Metals and Alloys
Niobium and Tantalum metals resemble Platinum in appearance when polished and they are somewhat darker and bluer when unpolished. The metals are attacked by HCl, HNO3 or Aqua Regia, hot or cold, dilute or concentrated. There is no reaction with hot dilute H2SO4, but boiling, concentrated H2SO4 slowly dissolves them. The metals are soluble in HF and are readily attacked by a mixture of HF and HNO3. In short, the metals are best dissolved in HF in combination with other acids such as nitric, perchloric and HCl.
The following are some suggestions:
- Nb0 or Ta0 (0.1 gram) - use a 5 mL 15M HNO3 + 3 mL HF + 3 mL H2O. (There are as many combinations of nitric/HF/water that will work. At a minimum, you need the same number of moles of nitric acid as Nb0 or Ta0 plus a minimum of 8 times the number of moles of HF. Water helps solubility but too much can destroy the nitric acid oxidizing ability, so keep the water to no more than the volume of nitric, i.e.,1:1 nitric/water. The use of concentrated sulfuric can speed the reaction because of its action upon the nitric acid. Some specific combinations follow.)
- Nb/Ta steel (0.2 g) - use 5 mL HNO3 + 2 mL (40%) HF + 5 mL H2O.
- Alloys containing Zr + some combination of Ti, W, Nb, Ta, and Hf (0.5g) -5 mL HNO3 + 5 mL HF + 15 mL 1:1 H2SO4.
- Ta steel (1 g) - 30 mL Aqua Regia + 6 mL conc. HF.
It is interesting to note that in the numerous acid digestions reviewed, all of them included the use of HF.
Oxides, Minerals and Ores
The most important ores are Tantalite (Mn, Fe)·(Ta, Nb)2O6 containing 38-86 % m/m Ta2O5 and Columbite (Mn, Fe)·(Nb, Ta)2O6 containing 40-78% m/m Nb2O5. The minerals and ores in addition to the oxides are generally not soluble in acids. Fusions are generally required and the following are the most commonly used:
Sodium Peroxide
Fusion of up to 0.5 grams of sample with 10 grams of sodium peroxide (mix well). Use an Ni crucible and fuse for 5 to 10 minutes at a dull red heat over a small flame. Exercise caution if organic material is present by performing a dry ash first. Peroxide fusions containing organic materials are done but can be very reactive to explosive.
There is a technique for protecting the Ni crucible from attack by the peroxide which involves lining the crucible with sodium carbonate by melting (1000 °C)) and then swirling to line the walls while cooling. Several grams of sodium peroxide are then melted in the crucible and allowed to solidify on the on the bottom after lining with the sodium carbonate to prevent particles of the ore from being caught in the carbonate lining and remain unfused. This is a great technique for protecting the Ni and eliminating this very spectrally rich element from entering your sample solution, but some practice is required to develop the technique.
When cool (after fusion with the peroxide), the crucible is placed in a large Pt or porcelain dish and covered with warm water. The vessel is covered with a watch glass and the solution is boiled until the carbonate lining has dissolved. The crucible is then removed. The solution is next made acid (excess of 10% or more) with HCl and boiled until carbon dioxide is expelled. This should give a clear solution containing the Zr and all other constituents of the ore.
Potassium Carbonate
The potassium salt is preferred over sodium carbonate for Nb and Ta. Fusion is done in a Pt crucible at 1050 °C. Fuse/heat for at least 30 minutes (Nb/Ta materials may take up to one hour depending upon the mesh of the sample and mixing with flux). Make certain that the sample is finely ground and well mixed with at least a 20:1 ratio of flux to sample. Be careful not to attack the Pt when dissolving the fuseate. Some analysts prefer to dissolve the fuseate/flux with dilute nitric acid first (5%) and then transfer to a plastic beaker where HF is added to stabilize the Nb and Ta. If elements such as Ca are present then a more concentrated solution of HCl may be used directly on the fuseate where the final solution would contain no less than 10% HCl. The use of sulfuric acid is also common if Ba and Pb are not present.
Sodium Tetraborate
The addition of potassium carbonate (1:1 wt ratio) is helpful for samples high in silica but borax alone is generally sufficient even if the silica content is high. Some points to remember:
- Use Pt crucibles. Graphite and silver crucibles have been reported, but are not recommended.
- Use a temperature of 1050 to 1200 °C (use a temperature at which melt appears clear, i.e., check for clarity to confirm adequate temperature control).
- Samples generally require 30 minutes to fuse completely. A fine mesh / powdered sample will help speed up the fusion.
- The melted flux is viscous and mixing during heating by swirling or use of a Pt wire is recommended.
- When removed from heat, it is best to let the flux cool while swirling to get a layer upon the walls, making the dissolution of the flux easier.
- Dissolution of the flux using a nitric/water/HF mixture is recommended, unless fluoride insolubles are present. In such cases, HCl/water is suggested.
- For sample sizes up to 0.3 grams use 4 grams of borax.
- Add the flux to the crucible first, then melt and swirl to coat the walls. After cooling, add the sample, cover the crucible and fuse for up to 30 minutes or until fusion is complete as indicated by a clear melt.
Organic Matrices
This includes a wide variety of materials including petroleum matrices, coal, organic plant material, biological material, synthetic organics, etc. Samples containing mid to low ppm levels of Nb and/or Ta can be digested with nitric/perchloric. These elements will be stable at ppm levels so long as the acid content of the final sample solution is ~1M or greater.
For more detailed information about acid digestions of organics, please see the following article: Acid Digestions of Organic Samples.
For samples containing higher (high ppm to %) levels of Nb and Ta it is also very acceptable and preferable to dry ash organic samples for Nb and Ta analysis in a Pt crucible and then bring the resulting MO2 oxides into solution using one of the methods described above.
For more information, see the portion of our Trace Analysis Guide that discusses Ashing.
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.
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