Sample Preparation Guide pt12: Samples Containing Vanadium
Part 12: Samples Containing Vanadium
Overview
Vanadium (V) is widely distributed in nature. At least 135 different minerals of vanadium have been identified, but deposits containing more than 1 or 2% are rare. An important fact to take into account when handling and preparing samples is that vanadium is found generally in five types of compounds namely sulfides, oxides, sulfates, vanadates and silicates. Vanadium normally substitutes for iron (III) in rocks, concentrating especially in the iron oxide minerals magnetite and ilmenite. Weathering of vanadium (IV) bearing rocks changes it to vanadium (V) with higher solubility and this ultimate reduction and precipitation leads to its location in sandstones and other minerals.
Vanadium bearing porphyrins are found in heavy oil crudes and fractions, which is why it's used as a tracer for estimating air pollution caused by fossil-fuel combustion. Vanadium is present in almost all living organisms (including man) where certain marine invertebrates have the ability to accumulate vanadium from seawater in their blood (concentration factors of 280000 are common). Its biological role is not yet well understood.
Vanadium is used in a wide variety of ways. For example, vanadium is used in the production of many special steels, indelible inks, photography, in glass to absorb the UV, for coloring glass and ceramic glazes, and as a catalyst for various oxidation reactions such as the oxidation of sulfur dioxide in sulfuric acid production.
Sampling and Handling
The ductility of high-purity metallic vanadium makes sampling difficult unless chilling and embrittlement is used with subsequent milling or crushing of the samples. Crushing methods for vanadium alloys can be different if an appropriate technique is sought for each case. For instance, ferrovanadium alloys are more friable the lower the vanadium content and the higher the silicon content. Use of an air gun had been reported for vanadium-aluminum alloys to obtain a 48-mesh alloy powder. Sampling of vanadium halides and oxohalides should be performed in a dry box with inert atmosphere. The dissolution of samples may be performed in several ways as described below.
The sampling and handling of V in minerals and ores is typical of any ore/mineral sample and does not require special handling. For additional details, see the following information on sampling and subsampling.
The Metals and Alloys
Metal
It is insoluble in HCl, slowly dissolves in HF, hot concentrated sulfuric acid, nitric acid, or Aqua Regia. Fusion of the metal is another option. Although vanadium metal is not attacked by solutions of the alkalis it is soluble by fusion with potassium or sodium hydroxide and sodium carbonate containing potassium nitrate.
Alloys
Alloys are typically dissolved in nitric acid or Aqua Regia. The following acid combinations are suggested for some more specific types of alloys:
- Stainless steel use HNO3 + HCl.
- Ferrovanadium alloys use HNO3 + HF.
- Ti-V alloys use H2SO4 + HNO3 + HF.
- V-Al alloys treat first with NaOH dissolve Al and then use H2SO4 + HNO3 + HF.
- Ferrovanadium slag use H3PO4 + HCl.
Oxides, Minerals and Ores
Oxides
V2O2 is easily soluble in dilute acids giving a lavender-colored solution. V2O3 is insoluble in HCl, H2SO4 and alkali solutions but it dissolves in HF + HNO3. V2O4 is readily soluble in both acids and alkali giving blue-colored solutions. V2O5 is soluble in acids, alkali hydroxide and carbonate solutions. Avoid high temperature ignition of the oxides during ashing/sample preparation keeping ashing temperatures between 400 and 475 °C. Ignited oxides are more difficult to dissolve as the ignition temperature increases.
For electric-furnace slags, and flue dust a sodium peroxide fusion may be used.
Sodium Peroxide Fusion
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 which involves lining the crucible with sodium carbonate by melting (1000 deg 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.
Ores General Procedure
One gram (or more) of the finely divided (very important to be no "grainy" material) sample is placed in a large platinum crucible together with five times its weight of a mixture of sodium carbonate and potassium nitrate (10 parts sodium carbonate to 1 part potassium carbonate -please note that potassium nitrate attacks Pt and a large excess of the sodium carbonate is needed to prevent this). Fusion is done in at 1050 °C for 10 to 15 minutes (do not stir, i.e., a "quiet fusion"). The fuseate is soluble in dilute nitric acid.
Ores High in Silica
One gram of sample is treated in a Pt dish with about 10 mL of HF and 2-5 mL of concentrated H2SO4. The silica is expelled as the volatile fluoride and the HF is driven off by taking the solution to dense white SO3 fumes. The residue is extracted with hot water containing a little sulfuric acid. Any undissolved residue may be brought into solution by fusion with KHSO4. Minerals. The above fusion with Na2CO3 and KNO3 is recommended.
Organic Matrices
Ashing of organic materials, foodstuffs, plant, and blood and sewage sludge as a preliminary de composition step is suggested. Wet Ashing us suggested for oil and petroleum products using sulfuric acid. Do not use ashing temperatures above 500 °C. If sulfuric acid is added to the petroleum sample than heat on a hot plate slowly until foaming stops and a char is produced i.e. wet ashing is very time consuming but it is very common practice in the petroleum industry. For more information, see the portion of our Trace Analysis Guide that discusses Ashing.
EPA Methods 3051 and 3052 (Microwave Assisted Acid Digestion) are suggested for many environmental samples (sediments, sludges, soils and oils). Samples containing mid to low ppm levels of V can be digested with nitric/perchloric. For more information, see the portion of our Trace Analysis Guide that covers Acid Digestions of Organic Samples.
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.
|
