Sample Preparation Guide pt10: Samples Containing Titanium
Part 10: Samples Containing Titanium
Overview
Titanium (Ti) will typically be found in the elemental or oxide forms. In nature, it occurs only in the combined sate. It is a characteristic constituent of igneous and metamorphic rocks and of the sediments derived from them. The chief mineralogical occurrences are as oxides, titanates and silicotitanates. Titanium is frequently associated with magnetite and hematite and may constitute a considerable proportion of such deposits.
Sampling and Handling
The sampling and handling of titanium materials is not generally a problem because they are relatively inert and nontoxic. An exception is titanium tetrachloride and also some organotitanates. These compound types when exposed to air will react with any moisture present to produce a dense white cloud containing TiO2 and the corresponding acid or alcohol, i.e. HCl in the case of the tetrachloride. Precautions must be taken when handling such materials to avoid eye contact, skin contact, inhalation and ingestion.
Materials in which titanium dioxide pigments and titanium metal are used are also relatively not hazardous and do not present 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 Metal
Amorphous titanium is a dark gray powder. The fused metal resembles polished steel. When cold it is very brittle, but at a red heat may be forged and drawn into wire. Titanium is a prominent structural metal. Titanium and its alloys were developed rapidly during the early part of the 20th century and it's currently a prominent structural metal. It is strong and light (strength/weight ration is good), it does not corrode in marine environments, oxidizing solutions and chloride solutions.
Ti0 is soluble in HCl, HF and sulfuric acids. In hot dilute HCl, Ti0 dissolves to form Ti+3 if oxidizing agents are excluded. Cold, dilute H2SO4 readily dissolves Ti0 to form Ti+3; the hot concentrated acid gives Ti+4 and SO2. Ti0 is not soluble in nitric acid due to the passivity of the surface from oxide formation. HF combined with nitric acid, and water are most commonly used to dissolve Ti and its alloys:
- Ti0 - use a 1:5:10 ratio of HNO3:HF:H2O
- Ti0 alloys with Mo, Sn and Zr - use a 1:2:1 ratio of HNO3:HF:HCl
- High temperature alloys (Ti, Fe, Co, Cr, Al, Mo, Ta, Hf) - 1:1:1 ratio of HNO3:HF:H2O - 1 gram in 30 mL of acid mixture
Ti0 is also soluble by fusion with potassium pyrosulfate. The fuseate is generally dissolved in highly acidic solutions such as 30% H2SO4 where higher concentrations of acid are required for higher Ti amounts to prevent hydrolysis.
Ti Oxides
Ti2O3, which has a black or blue color, is soluble in concentrated HCl or sulfuric acid. The main oxide form of titanium is TiO2. The dioxide exists in three crystalline states namely anatase, rutile and brookite. The anatase and rutile forms are soluble in concentrated sulfuric acid, HF, and HF mineral acid combinations. My favorite combination for dissolving anatase or rutile is 1:1 HF/HNO3 where some heat is generally required to speed dissolution. When the anatase or rutile forms are heated to temperatures at or above 800 °C, they are converted to the brookite form. My experience has been that the brookite form is not soluble in any acids and requires fusion. Fusion with sodium carbonate, pyrosulfate, alkali metal hydroxides, and sodium tetraborate are common. Here, my favorite is the pyrosulfate fusion using a Pt crucible:
The finely powdered sample is mixed with 35 times its weight of potassium pyrosulfate and heated in a Pt crucible until the melt appears clear. It can be swirled or stirred with a Pt wire. The crucible is removed from the heat and tilted and turned slowly to distribute the melt over the walls of the crucible as it cools. The solidified melt in this way often cracks into small pieces, and falls off the walls of the crucible. Potassium pyrosulfate has a melting point of 419 °C. If heating is continued too long, the simple sulfate begins to precipitate out from the melt. Should this happen, cool and add a few drops of sulfuric acid to regenerate the flux. Note that potassium disulfate begins to decompose around 300 °C and sulfur trioxide is evolved rapidly at 500 - 600 °C.
If you wish to stay away from fusions altogether, then avoid ashing samples at temperatures above 500 °C to avoid formation of the brookite form, which may simply be referred to as the "ignited oxide."
Minerals and Ores
One gram of the ore is treated with 10 to 50 mL of a mixture of sulfuric and hydrofluoric acids (1:5), a few drops of HNO3 added, and the solution evaporated to fumes of sulfuric to expel the silica as the volatile H2SiF6 (excess HF is also expelled). If a residue remains upon dissolving with water containing a little HF (or dilute nitric or dilute sulfuric of HF is not tolerated easily) it is filtered and fused with K2S2O7 (potassium pyrosulfate - see above method for oxides).
The ore may be fused with a 20:1 ratio of sodium carbonate to sample in a Pt crucible. Fusion takes place at 1000 °C and should be swirled until the melt is clear. The fuseate can be dissolved in a 1:1:1 HF/HNO3/H20 mixture and then diluted with water. The other popular fusion method for ores is the pyrosulfate method described under the Oxides section above.
Alloys
In those alloys where titanium exists as the base metal, methods of solution can be employed as given in The Metal section above, usually some combination of acids including hydrofluoric. For alloys where Ti is one of the lesser constituents, the use of concentrated nitric acid, Aqua Regia or a mixture of acids suitable to dissolution of the base metals is used. After the sample is dissolved, make certain to keep the acid content of the final solution sufficiently high to avoid hydrolysis (see Hydrolytic Stability and Preferred Matrices below).
Organic Matrices
This includes a wide variety of materials including petroleum matrices, coal, organic plant material, biological material, synthetic organics, etc. Samples can be digested with nitric/perchloric. 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 for Ti analysis in a Pt crucible and then bring the resulting TiO2 into solution using one of the methods described above, making sure to use temperatures above 500 °C. For more information, see the portion of our Trace Analysis Guide that discusses Ashing.
Hydrolytic Stability and Preferred Matrices
- Ti is particularly prone to hydrolysis due to its small ionic radius of 0.64 Angstrom for Ti+4. The hydrolysis of Ti+4 to Ti(OH)2+2 is complete in 1M HClO4; no values have been reported for the relevant formation constants. The titanyl ion (Ti(OH)2+2) further hydrolyses to Ti(OH)3+ and Ti(OH)4 (aq).
- Ti begins to precipitate (as the Ti(OH)4) from solution at a pH of between -0.3 (high conc. Ti) to 2.5 ( low conc. Ti). Chelating and complexing agents that successfully retard hydrolysis are EDTA, Fluoride, oxalate, tartrate, and Tiron.
- Titanium oxide, hydroxide, and phosphate are all insoluble in water and neutral to basic media.
- The TiF6-2 ion is stable in aqueous solution and has been characterized by NMR.
- The following table shows the improvements in the hydrolytic stability of Ti+4 with different complexing agents. The pH where precipitation of Ti(OH)4 begins is shown for 0.1 M solutions of each complexing agent:
| Complexing Agent | pH where precipitation begins |
|---|---|
| Fluoride | 7.8 |
| EDTA | 6 (low Ti levels) |
| Oxalate | 4.5 |
- Ti 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. When Ti is in the presence of transition metals excess HF is needed. The transition metals and some non-metals will strip the fluoride from the Ti leaving open the possibility of hydrolysis. Moderate to low levels (≤ 100 µg/mL) can be mixed with all of the elements. Ti is more stable in relatively high (greater than 5% v/v) levels of acid.
- Stability: 2-100 ppb levels stable (alone or mixed with all other metals) as the Ti(F)6-2 for months in 1% HNO3 / LDPE container. 1-10,000 ppm single element solutions as the Ti(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.
