Sample Preparation Guide pt17: Samples Containing Manganese
Part 17: 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 for Mn is best for high Fe containing solutions and that trace Fe in Mn can be best measured using the 261.187nm Fe emission line. Using low resolution ICP-MS it is difficult at best to detect trace Mn in Fe and that 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 Mn+2 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 thru +7) solutions are found to be strongly oxidizing. Permanganate (MnO4-) is a strong oxidizing agent in both acid and basic media where Mn+2 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 Mn+2 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 Mn+2 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 Mn(OH)3&4 noted visually by the precipitate slowly turning black/brown.
In considering the following reaction:
- Mn+2 <—> 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 Mn+2 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 are not a concern upon the addition of acid to the caustic fuseate. The Mn+2 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 Mn+2 ion forms very stable colorless solutions except at high concentrations where the solutions appears pink. The single element Mn CRMs that are prepared by IV are done so in such a manner to ensure that only the nitrate ion is present (no fluoride, chloride or sulfate). Mn+2 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-, SO4=, F-, Cl-, PO4-3 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 form the stainless steel elements.
- The collection of biological samples are also at risk of contamination due to the very low (ppb) levels of Mn sought. The use of steel needles, and 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 additional details, see the following information on 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 drop wise 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 Mn+2 ion. The higher oxides and hydroxides are reduced to the Mn+2 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 Mn+2.
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 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.
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