Samples Containing Manganese

Overview

Manganese (Mn) is the third most abundant of the first row transition elements, with an estimated concentration in the earth's crust of 0.10%. The element is found in minerals mostly associated with Fe, but is seldom found at concentrations >0.5%. The chemical and physical properties of Mn are similar to Fe. This similarity to Fe, along with the close association of Mn with Fe in minerals, resulted in manganese compounds being thought to be those of Fe, and it was not until 1774 that Mn was discovered as a separate element. Even in this modern age of spectroscopy, it is easy to miss the presence of trace Mn in Fe, due in part to the fact that Mn-55 (monoisotopic) is surrounded by the two most abundant Fe isotopes at masses 54 and 56, and the fact that with the technique of ICP-OES, the Fe and Mn emission lines are close to one another. IV has found that the Mn 280.106 nm line is best for high-Fe-containing solutions, and that trace Fe in Mn can be best measured using the 261.187 nm Fe emission line. Using low-resolution ICP-MS, it is difficult at best to detect trace Mn in Fe, and neither the 54 nor 56 masses are suitable for measuring trace Fe in samples high in Mn.

Most of the ore produced is used to make ferromanganese (~80% Mn), which in turn is used in the production of steel, where it functions as an oxygen scavenger. Mn is also alloyed in steel to impart toughness. Mn is alloyed with other metals to impart greater strength and hardness (Al and Cu). Mn is also used in batteries, dyes, and many other applications.

The chemistry of Mn is different in many respects from its neighbors Cr and Fe. In general, Mn oxides are easier to dissolve, and their chemical behavior is much more predictable. Mn exists in oxidation states ranging from +7, +6, +5, +4, +3, +2, and 0. In acidic media, Mn²⁺ is thermodynamically favored and is the state that results from most sample preparations intended for ICP measurement. In acidic media, the higher oxidation states are unstable, and Mn (+3 through +7) solutions are found to be strongly oxidizing. Permanganate (MnO₄⁻) is a strong oxidizing agent in both acid and basic media, where Mn²⁺ is the redox product in acidic media and MnO₂ in basic media. Interestingly, in acidic media, MnO₂ is nearly as strong an oxidizing agent as dichromate, again forming Mn²⁺ as the redox product. MnO₂ is a commonly occurring compound of Mn, which is insoluble at a pH >7. The most common oxidation states of Mn found in nature are the +2, +3, and +4 states. The preparation chemistry of Mn-containing compounds always ends up as an acidic solution where the Mn oxidation state is +2; i.e., the +3 and +4 states will find something to "chew" upon in the preparation. The Mn²⁺ ions can be in equilibrium with Mn(OH)₂, Mn₃O₄, Mn₂O₃, and MnO₂, where Mn₃O₄ and Mn₂O₃ are rarely precipitated from solution, and apart from Mn(OH)₂, only the very stable MnO₂ is formed frequently. It should be noted that all of the Mn oxides and hydroxides are insoluble in water. Another noteworthy chemical behavior is that the metal will react with warm water to give Mn(OH)₂ and H₂. When Mn(OH)₂ (white in color) is precipitated, it will slowly air-oxidize to higher manganese oxides/hydroxides, noted visually by the precipitate slowly turning black/brown.

In considering the following reaction:

Mn²⁺ ⇌ Mn(OH)₂ (s)

Mn begins precipitating out of concentrated solutions (air-saturated) at a pH of 7.5. The addition of ligands such as acetylacetonate, citrate, triethanolamine (TEA), and EDTA will delay or prevent precipitation up to a pH of 12. TEA is particularly useful, in that it has excellent water solubility, and it is not strongly basic like the more common alkyl amines. In addition, TEA/water solutions will not require different ICP/plasma conditions.

Most Mn²⁺ compounds that would be hypothetically possible in common preparation chemistry are very soluble in water at room temperature (bromide, chloride, fluoride, fluorosilicate, nitrate, and sulfate). There are a number of compounds formed during fusions where minimal water solubility is to be expected (carbonate, hydroxide, and oxalate), but these are not a concern upon the addition of acid to the caustic fuseate. The Mn²⁺ salts that are not soluble in water, but that readily dissolve upon the addition of acid, are the sulfide, carbonate, phosphate, oxalate, borate, and sulfite. In dilute nitric acid, the Mn²⁺ ion forms very stable, colorless solutions, except at high concentrations, where the solutions appear pink. The single-element Mn CRMs that are prepared by IV are done so in such a manner as to ensure that only the nitrate ion is present (no fluoride, chloride, or sulfate). Mn²⁺ in dilute nitric acid can be mixed at working concentrations with all of the elements commonly measured by ICP, as well as all of the counter ions commonly present (NO₃⁻, SO₄²⁻, F⁻, Cl⁻, PO₄³⁻, and tartrate). In general, Mn chemistry is much less challenging with respect to preparations involving the metal or oxides than that of any of the other first row transition elements.

Sampling and Handling

The measurement of Mn is required for a wide range of samples, including ores and minerals, soils, sludges, water (both drinking and waste), biological, agricultural, metallurgical, and industrial samples. There is a great risk of contamination when any sample handling apparatus/device is made of, or has come in contact with, stainless steel. In addition, since Mn is so prevalent in the earth's crust, the air (particulate dust) must be considered as a contamination route. The following considerations apply:

• Many tools that pulverize, mix, cut, etc., contain Mn. Attempt to use devices made of ceramics, silica/quartz, and polymers where possible.
• Many polymers, such as Teflon, have been exposed to stainless steel (cutting/pulverizing) prior to molding. New plastic and Teflon containers should always be acid-leached to remove possible contamination from the stainless steel elements.
• The collection of biological samples is also at risk of contamination, due to the very low (ppb) levels of Mn sought. The use of steel needles, scalpels, or any metallic object that may contain Mn should be avoided.

The risk of contamination is great for Mn when alloys, steels, and grinding equipment are used in some part of the sample collection or preparation. For more on sample contamination risks, see chapters 8, 9, and 10 of the Inorganic Ventures Trace Analysis Guide.

For general information on sampling and subsampling, see Sampling and Subsampling.

The Metal and Alloys

Metal

The metal reacts with warm water to give Mn(OH)₂ and H₂. Mn metal dissolves readily in any dilute acid where the Mn(OH)₂ is soluble. Most preparations are performed in dilute nitric or hydrochloric acids.

Alloys

The use of nitric/HClO₄/HF is most commonly used (0.2 grams of alloy + 20 mL 1:1 nitric acid/water, 10 mL conc. HClO₄ + 1 mL HF) to dissolve the ferromanganese alloys, although concentrated nitric acid alone should be sufficient for many alloy sample types. For silicomanganese alloys, the use of concentrated nitric/HF mixtures should suffice. If difficulty is encountered, try adding the HF first, followed by the dropwise addition of nitric acid. Caution is always advised when using HClO₄ or HF. Iron and steel are dissolved in dilute nitric acid (1:1). Titanium alloys are dissolved in concentrated HCl/H₂SO₄/H₂O/HF (20:10:10:1). Tungsten and ferrotungsten alloys are dissolved in nitric/HF.

Oxides, Minerals and Ores

Oxides

All oxides and hydroxides of Mn are insoluble in water, but are soluble in warm HCl, forming the Mn²⁺ ion. The higher oxides and hydroxides are reduced to Mn²⁺ upon addition of HCl, with evolution of Cl₂. MnO₂ (or the hydrated oxide) is insoluble in HNO₃, dilute or concentrated, but the addition of a few drops of H₂O₂ causes rapid solution, with the formation of Mn²⁺.

Ores

The most common approach for Mn ores is the use of concentrated HCl and HF. A 1 gram sample of the ore is dissolved with not more than 50 mL of concentrated HCl. If silica is present, 5 to 10 mL of concentrated HF is added after the sample has already been heated to dissolve most of the sample. For ores high in sulfur, such as pyrites, etc. (sulfide ores), the sample is treated with concentrated nitric acid, followed by heating with perchloric acid until dissolution is complete. If the ore has been strongly ignited, it may be necessary to fuse the sample with sodium carbonate (0.25 to 1.0 gram sample + 20 grams sodium carbonate, fused at 1000 °C for 10 to 30 minutes in a Pt crucible), followed by dissolution with nitric acid. In this case, HF is not needed, unless silica content is high (silicates formed from the carbonate fusion are soluble, but do not heat). This approach also avoids the use of perchloric acid.

Organic Matrices

Ashing of organic materials, foodstuffs, plant, and blood and sewage sludge, as a preliminary decomposition step, is suggested for samples containing Mn. The formation of a refractory form of manganese oxide is not at all likely. If ashing is used, it is suggested to keep the temperature low (400 to 450 °C max) and to use an ashing aid, such as high-purity sodium carbonate or magnesium nitrate. 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 on ashing, please see the following paper: 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.