Sample Preparation Guide pt11 - Samples Containing Zirconium or Hafnium

Part 11: Samples Containing Zirconium or Hafnium

Overview
Sampling and Handling
The Metals and Alloys
Oxides, Minerals and Ores
Organic Matrices
Hydrolytic Stability and Preferred Matrice
Detailed Elemental Profiles
Further Reading

Overview

The atomic radii of zirconium (Zr) and hafnium (Hf) as well as the radii of the ions (Zr⁺⁴ and Hf ⁺⁴) are almost identical. This explains, in part, the fact that the chemistry of these two elements is more similar than any other elemental pair.

Sampling and Handling

The sampling and handling of Zr and Hf materials is not generally a problem because they are relatively inert and nontoxic. No toxicity is known and no industrial health hazards are reported. This is likely due to the very limited solubility of their compounds. The toxicity of Zr compounds is generally classified with a maximum working concentration (MAK) of 5 (mg m^-3), and a threshold limit value/time weighted average of 5 (mg m^-3). Hf compounds are classified a factor of ten lower. Zirconium tetrachloride is not as reactive as the TiCl₄ where hydrolysis results in the formation of derivatives such as the oxychloride, ZrOCl₂, instead of the oxide as is the case with TiCl₄.

Materials containing Zirconium and Hafnium compounds are also relatively non-hazardous and do not present unique sampling and handling problems. However, many analytical measurement techniques require a solution of the sample and HF is most often required. Safety considerations appropriate to the use of HF will apply. In materials where titanium is present in minor or trace amounts sampling and handling considerations relative to the sample matrix will apply.

For additional details, see the following information on sampling and subsampling.

The Metals and Alloys

Zirconium and hafnium metals, like titanium, are hard and resistant to corrosion. These metals resemble stainless steel in appearance with melting points of 1855 and 2222 °C respectively. Heating zircon sand in the presence of graphite in an electric furnace forms the carbides. The ZrC (zirconium carbide) that's produced is reacted with chlorine to produce ZrCl₄. The tetrachloride is reduced to elemental zirconium in the presence of metallic magnesium. Differences in the solubility of the tetrachlorides in ethanol have been used for the separation of zirconium and hafnium. Both metals are passivated by a protective oxide film, which makes them almost inert towards any chemical attack (exceptions are hydrofluoric acid and alkali melts containing fluoride that dissolve both elements). Zirconium is used in high temperature applications (furnace lining, crucible materials) but is not used extensively due to its high cost.

The metals are best dissolved in HF in combination with other acids such as nitric, perchloric and HCl. The following are some suggestions:

Hf⁰ or Zr⁰ (0.1 gram)- use a 10mL HNO₃ + 1-2 mL HF + 10 mL H₂O
Zr steel (0.2 g) - use 10 mL HNO₃ + 10 mL HCl + 5 mL (40%) HF
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

If you wish to avoid the use of hydrofluoric acid, Zr or Hf metals can be dissolved in a perchloric acid/nitric acid/sulfuric acid/water mixture (2:5:5:20 ratio respectively).

Oxides, Minerals and Ores

The two most common zirconium-containing minerals are baddeleyite (ZrO₂) and zircon (ZrSiO₄). Many Zr ores contain up to 1-2 % Hf. All but one very rare mineral contains more Zr than Hf (hafnon, HfSiO₄). The oxides are generally not soluble in acids. Fusions are typically required. 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 a 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. This technique 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 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 it will take some practice to perfect 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. Next, the solution is 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.

Sodium Carbonate

Fusion is done in a Pt crucible at 1050 °C. Fuse/heat for 40 minutes (Zr/Hf materials may take up to one hour depending upon the mesh of the sample and mixing with flux) and make certain that the sample is finely ground and well mixed with at least a 20:1 ratio of flux to sample. When dissolving the fuseate, be careful not to attack the Pt. 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 Zr and Hf. 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

Although the author prefers carbonate over borate fusions, the sodium tetraborate or Borax (Na₂B₄O₇) is the most popular fusion method for ores, oxides and minerals containing Zr/Hf. The addition of sodium 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).
Zr containing samples generally require 30 minutes to fuse completely.
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

Organic matrices include a wide variety of materials: petroleum matrices, coal, organic plant material, biological material, synthetic organics, etc. Samples containing mid to low ppm levels of Zr and/or Hf 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 Zr and Hf, it's very acceptable and even preferable to dry ash organic samples for Zr and Hf 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.

Hydrolytic Stability and Preferred Matrices

Hf and Zr are prone to hydrolysis because of their high positive valence and atomic radii that are relative small (Zr⁺⁴ = 0.87 Å; Hf⁺⁴ = 0.84 Å). The stability constant of Hf(OH)⁺³ is known but reliable information about Hf(OH)₂⁺² thru Hf(OH)_5- is not available and only rough estimates based on measured solubilities of ill-defined oxide forms. The aqueous chemistry of Zr⁺⁴ is similar to that of Hf⁺⁴, but Zr⁺⁴ has a greater tendency to form polycomplexes and more information is available.
Hf begins to precipitate (as the Hf(OH)₄) from solution at a pH of between 0.1 (high conc. Hf) to 2.7 ( low conc. Hf). Zr begins to precipitate (as the Zr(OH)₄) from solution at a pH of between 0.8 (high conc. Zr) to 2.8 ( low conc. Zr). Chelating and complexing agents that successfully retard hydrolysis are EDTA, fluoride, oxalate, tartrate, and Tiron.
The metal oxides, hydroxides, and phosphates are all insoluble in water and neutral to basic media.
The MCl₆-2 ion is unstable in even 12M HCl where some cationic hyroxo species and oxochlorides have been isolated. The form of the fluoride species is somewhat unclear but appears to be much more stable to hydrolysis than the chloride form.
The following table shows the improvements in the hydrolytic stability of Ti⁺⁴ with different complexing agents. The pH where precipitation of M(OH)₄ begins is shown for 0.1 M solutions of each complexing agent and low ppm levels of Zr and Hf:

Complexing Agent	pH where precipitation begins
Fluoride	8
EDTA	10
Oxalate	6

Zr and Hf as the fluoride complex in dilute nitric acid can be mixed with many of the elements at high concentrations (200 to 2000 µg/mL) with the exception of the Alkaline and Rare Earths. Excess HF is needed when Zr and Hf are in the presence of transition metals. The transition metals and some non-metals will strip the fluoride from the Zr and Hf leaving open the possibility of hydrolysis. Moderate to low levels of the fluoride complexes(≤10 µg/mL) can be mixed with all of the elements. Both Zr and Hf are more stable in relatively high (>5% v/v) levels of acid.
Stability: 2-100 ppb levels stable (alone or mixed with all other metals) as the M(F)₆^-2 for months in 1% HNO₃ / LDPE container. 1-10,000 ppm single element solutions as the M(F)₆^-2 chemically stable for years in 2-5% HNO₃ / trace HF in an LDPE container.

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.

Zirconium

Hafnium