Sample Preparation Guide pt16: Samples Containing Tungsten

Overview

Tungsten (W) is the most abundant of the group 6 elements (Cr, Mo, W) with an average concentration in the earth's crust of 190 ppm (140 ppm for Cr and 1.2 ppm for Mo). The element is found in minerals high in silica and is commonly found as the tungstate (WO₄= ) and associated with Ca⁺², Cu⁺², Fe⁺² and Mn⁺². Tungsten also occurs as the sulfide (WS₂, tungstenite). In addition to the analysis of W ores analytical chemists are asked to measure W in alloys, steels, ferrotungsten, silico-tungsten, tungstic oxide, tungsten powder, alkali tungstates and high purity metals such as niobium, tantalum, titanium, zirconium and their alloys. The analytical techniques used include ICP-MS (dl~ .000005 mg/L), cathodic stripping voltammetry ( dl~.001 mg/L), neutron activation analysis (dl ~ 0.005 mg/L), UV-Vis spectrophotometry (dl ~0.01 mg/L) and ICP-OES (dl~ 0.01 mg/L). Tungsten has the highest melting point (3370 C°) of the metals which has led to its use in incandescent light filaments. It is also used in cutting tools, steel, springs, valves, axles, contact points in spark plugs, and many other products where strength, hardness, durability, and resistance to corrosion are important in addition to a high melting point.

The chemistry of W is much more similar to that of Mo and bears little resemblance to the chemistry of Cr. Tungsten exists in oxidation states ranging from +6,+5,+4,+3, +2, 0 -1,-2 and -4. Tungsten's isotopes and relative natural abundances are (amu- atomic %) 180-0.13, 182-26.3, 183-14.3, 184-30.7, and 186-28.6. ICP-MS spectroscopists favor the 182 and 184 isotopes but all of the masses are subject to MO interferences from the appropriate Rare Earth elements. Sample preparations of ores and alloys typically involve the combined use of HNO₃ and HF making +6 the most common oxidation state. In neutral and acidic media tungstates (+6) have a great tendency to hydrolyze out of solution as WO₃ (yellow powder), making HF an almost essential co-acid in sample preparations where higher W concentrations may require a considerable excess amount of HF ( ~ 3% v/v HF for concentrations of ~ 1 to 10 g/L W). The tungstate is stable in basic media making alkali fusions followed by dissolution in water possible when the use of HF is not acceptable. Inorganic Ventures prepares single element 1000 µg/mL and 10,000 µg/mL W CRMs from the metal (W°) using HNO₃ and HF where HF is in excess (~ 3% v/v) to prevent hydrolysis. At Inorganic Ventures we prepare multi elemental blends of W matrices that favor dilute nitric with trace HF, and we have found solutions down to the low ppb level to be chemically stable for months to years. Tungsten should not be mixed with elements that form insoluble fluorides (group IIA and IIIA) unless the concentrations are = 1 mg/L of each and the acid content is at least 1 % v/v and preferably = 5% v/v.

Sampling and Handling

Like Mo, W is determined in a wide range of environmental, biological, agricultural, metallurgical, and industrial (chemical industry) samples. There is a great risk of contamination when grinding, cutting and mixing equipment is used in the sampling/sample preparation. The following precautions should be considered:

• Many tools that pulverize, mix, cut, pulverize, etc. contain W. Attempt to use devices made of ceramics, silica/quartz, and polymers where possible.
• The collection of biological samples are also at risk of contamination due to the very low (ppb) levels of W typically present. The use of steel needles, and scalpels or any metallic object that may contain W should be avoided.

The risk of contamination is great for the group VI B elements (Cr, Mo, W) when alloys, steels and grinding equipment are used in some part of the sample collection or preparation. For more information on sample contamination risks see chapters 8, 9 and 10 of the Inorganic Ventures 'Trace Analysis Guide'

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

The Metal and Alloys

Metal

The metal is not soluble to any degree in HCl, H₂SO₄ , HNO₃, Aqua Regia or NaOH. The metal is very readily soluble in a combination of HNO₃ and HF. 1 mL of HNO₃ + 2 mL of HF will dissolve 0.4 grams of W°. When dissolving sponge or powder exercise caution due to the exothermic nature of the reaction by adding the W very slowly or by keeping sample sizes to a minimum.

Alloys

The use of nitric/HCl, HClO₄/H₃PO₄ and H₂SO₄/H₃PO₄ have all been used but again the most popular is the nitric/HF combination. 10 mL of nitric + 10 mL HF is sufficient to dissolve 1 gram of a W alloy. Tungsten steels can be dissolved with a 1:1 mixture of H₂SO₄ + H₃PO₄ acids where the phosphate complexes with the tungsten thereby keeping it in solution.

Oxides, Minerals and Ores

Oxides

The amorphous dioxide, WO₂, is readily soluble in warm HCl and H₂SO₄. It is also soluble in the alkali hydroxides to form tungstates. The crystalline dioxide is not soluble in hot concentrated acids or alkalis. WO₃ is insoluble in water and most acids but soluble in HF. Oxides that have been strongly ignited are insoluble in acids and require fusion. Although a number of fusions have been reported it is recommended here that the sodium carbonate fusion be used which is performed in Pt at 1000 C° with a 20:1 flux to sample ratio followed by dissolution of the fuseate with a nitric/HF combination.

Ores

The most common approach for ores is the use of nitric acid and HF. 0.25 grams of the ore is dissolved with not more than 10 mL of concentrated nitric + 10 mL of concentrated HF. If the ore has been strongly ignited it may be necessary to first fuse the sample with sodium carbonate ( 0.25 up to 1 gram sample + 20 grams sodium carbonate fused at 1000 C° for 10 to 30 minutes in a Pt crucible followed by dissolution with nitric + HF).

Organic Matrices

Ashing of organic materials, foodstuffs, plant, and blood and sewage sludge as a preliminary decomposition step is suggested for samples containing W but formation of a refractory form of WO₃ is likely. If ashing is used it is suggested to keep the temperature low (400 to 450 deg C max) and to use an ashing aid such as high purity sodium carbonate. If the sample is high in silica subsequent fusion of the ash with sodium carbonate is suggested or heating of the ash to fumes with sulfuric and hydrofluoric acids.

For more informationon, 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.

Tungsten