Sample Preparation Guide pt15: Samples Containing Molybdenum

Overview

Molybdenum (Mo) is the least abundant of the group 6 elements (Cr, Mo, W) with an average concentration in the earth's crust of 1.2 ppm and existing predominately as molybdenite (MoS₂) and wulfenite (PbMoO₄) (Minerals yearbook 2009, USGS Volume1. Metals and Minerals).

Molybdenum is most commonly used in alloy steels where relatively small amounts result in great increases in strength and hardness. It is also used in catalysts, lubricants, corrosion inhibitors and pigments.

Mo exists in oxidation states ranging from +6 to +2 but the +5, +4, +3 and +2 states are typically air sensitive. Consequently, for the analyst the most common oxidation state for Mo is +6. Molybdenum has a chemistry that is somewhat unusual for the transition elements in that it is stable in a wide variety of matrices ranging from strongly basic solutions to water (less soluble at pH7), to acidic solutions containing, sulfuric acid, nitric acid, HF, HCl, and various acid combinations. Inorganic Ventures prepares single element 1000 µg/mL and 10,000 µg/mL Mo CRMs from MoO₃ (Mo VI) in dilute ammonium hydroxide/water giving the analyst the option of which matrix to use for the final calibration or QC solution. At Inorganic Ventures we prepare multi elemental blends of Molybdenum in all of these matrices, but tend to favor dilute nitric with trace HF where we have found solutions down to the low ppb level to be chemically stable for months to years.

Sampling and Handling

Molybdenum is determined in a wide range of environmental, biological, agricultural, metallurgical, and industrial (chemical industry) samples. Samples containing molybdenum are not particularly hazardous. The greatest risk with sampling and handling of samples is contamination from grinding and mixing equipment. The following precautions should be considered:

• Many tools that pulverize, mix, cut, pulverize, etc., contain stainless steel where Mo is commonly present. 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 Mo typically present in these samples. The use of steel needles, and scalpels, or any metallic object that may contain molybdenum, should be avoided.

The risk of contamination is greater for Mo than most analysts are aware. Trace analysts are very aware of all the contamination problems associated with elements like Na, Ca, Al, Si, Fe and Zn, but Mo is often overlooked. 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

Molybdenum is soluble in hot dilute HCl. It is also soluble in concentrated H₂SO₄ heated to 200 - 250 degrees C. The metal reacts readily with HNO₃ with oxidation to MoO₃ which passivates the surface. The addition of HF to HNO₃ eliminates passivation and results in rapid dissolution of the metal. Similar behavior is seen with the addition of HCl or with the use of Aqua Regia at room temperature, and reacts more rapidly upon heating. The lack of passivation using HCl or HF in combination with HNO₃ and oxidation to the +6 oxidation state should be noted. HClO4 can also be used by itself or in combination with HF, or HNO₃.

Alloys

Due to the 'friendly' chemical behavior of Mo a variety of methods are available. Alloys can be dissolved in Aqua Regia (and other nitric/hydrochloric acid mixtures), perchloric acid or perchloric acid + phosphoric acid, nitric acid, or HF. For example, for steel or iron take one gram of drillings and dissolve in 20 mL of 1:1 HNO3 and 15 mL HClO₄. Add a little HF for high-silicon alloys (evaporate to perchloric acid fumes only if you want to rid the sample of Si). For stainless steels dissolve 0.2 to 0.5 grams in 30 mL HClO₄, 10 mL of H3PO4 and 0.5 mL of HF.

Oxides, Minerals and Ores

Oxides

Mo₂O₃ is insoluble in acids. Mo(OH)₃ dissolves with difficulty. MoO₂ is insoluble in HCl. MoO₃ is soluble in water (about 1 g/L) and is more soluble when cold that when hot. MoO₃ is readily soluble in dilute ammonium hydroxide, alkali oxide/carbonate solutions, and in concentrated mineral acids becoming much less soluble on ignition of MoO₃.

Ores

The most common approach for ores is the use of sulfuric/nitric acid decompositions where the ore sample is heated with the acids until dissolved. Transfer about 0.5 grams of sample to a 400 mL glass beaker and moisten with a few drops of DI water. Add 10 mL of concentrated HNO₃ and 10 mL of 1:1 H₂SO₄. Cover and heat until dissolved.

Organic Matrices

Ashing of organic materials, foodstuffs, plant, and blood and sewage sludge as a preliminary decomposition step is suggested for samples containing Mo, but formation of a refractory form of MoO₃ is likely. If ashing is used it is suggested to keep the temperature low (400 to 450 deg. C) and to use an ashing aid such as high purity sodium carbonate. If the sample is high in silica the subsequent fusion of the ash with sodium carbonate is suggested, or heating of the ash to fumes with sulfuric and hydrofluoric acids. Acid digestions using nitric, perchloric and sulfuric acids are also suggested. Wet ashing is suggested for oil and petroleum products using sulfuric acid in combination with magnesium nitrate as an ashing aid. Do not use ashing temperatures above 450 deg. C. If sulfuric acid is added to the petroleum sample then 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 a common practice in the petroleum industry. For more information, see the portion of our Trace Analysis Guide that discusses Ashing.

Molybdenum is listed in the scope for EPA Methods 3050A and 3050B (Open Vessel Acid Digestion) and 3051 and 3052 (Microwave Assisted Acid Digestion) and these methods are suggested for environmental samples (sediments, sludges, soils and oils).

Samples containing mid 1-100 ppm levels of Mo can be digested with nitric/perchloric. Only use trace metals grade acids due to contamination issues. For more detailed information about acid digestions of organics, please see the following article: 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.

Molybdenum