Sample Preparation Guide pt21: Samples Containing Cobalt

Part 21: Samples Containing Cobalt

General Information

Cobalt (Co) is found with minerals containing a possible variety of elements (Ni, As, S, Fe, Cr, Cu, Pb, and Mn) as the oxide, sulfide or arsenide but is not found naturally occurring as the metal. Co is not mined directly but is produced as a byproduct (mostly from Ni and Cu mining) of mining operations in Africa, the former Soviet Union and North America.

Cobalt is an element that is low to moderately abundant in the Earth’s crust. Its abundance is roughly that equal to scandium. Cobalt and scandium are the less abundant of the first row transition elements, making them both candidates as internal standard (IS) elements. Cobalt is not favored for its use as an IS for its complex spectrum, but rather for its compatibility with all of the common mineral acids and fluxes used in sample preparation.

The chemistry of cobalt allows for the use of a wide variety of preparation techniques and is typically included in methods intended for a large number and variety of elements. For example, it is easily included in EPA method 3052 “Microwave Assisted Acid Digestion of Siliceous and Organically Based Matrices” which uses combinations of nitric, hydrochloric and hydrofluoric acids heated in a Teflon bomb up to temperatures of around 180 C, acid digestions in quartz or borosilicate glass using combinations of HNO₃, HCl. H₂SO₄ and H₂O₂ as well as in dry ashing and fusions using a wide variety of crucible materials, ashing aids and fluxes. The main, and possibly only, thing to avoid with sample preparations is formation of the oxide in dry ashing procedures.

Cobalt is one of the ferrous metals (Fe, Co, Ni from old table Group VIII of the extended periodic table), having properties that are very similar to Fe and Ni. Co can have oxidation states of -1, 0, +1, +2, +3, and +4. The -1 oxidation state is encountered, for example in “oxo process” catalysts [Co (CO)₄^-] where Co is in a high temperature reducing atmosphere. The 0 oxidation state will be found in a variety of alloys, such as high speed steels (for example steels used for cutting and drilling), cemented carbides (WC is cemented with either Co or Ni metal to make it less brittle) and Co based super alloys. Cobalt metal is rapidly dissolved at room temperature by nitric acid, which is not the case for Fe and Ni which both require heat. The +1 and + 4 oxidation states are uncommon. Cobalt is most commonly found in the +2 oxidation state. The +2 state is the final oxidation state after sample preparation and acidification of samples containing Co in any of the possible oxidation states using conventional sample methods. The +3 oxidation state is more stable when complexed but uncomplexed is very unstable with a high (1.4V) reduction potential making it a very powerful oxidizing agent. In general, sample preparations for Co are similar to those for Fe and Ni and are relatively simple compared to the rest of the Group VIII elements (i.e. the Precious Metals). Sample preparations for ICP measurements are typically taking Co in the 0 or +2 oxidation states to solutions of Co ⁺² in dilute nitric acid matrices. Further discussion will be confined to these oxidation states.

The Oxides

Cobalt has two oxides that are more commonly encountered in conventional sample preparation schemes, namely CoO and Co₃O₄. CoO, which is grey-green, can be obtained by heating the sample to temperatures in excess of 1000 C. This temperature is generally considered unnecessarily high in sample dry ashing preparations. More commonly, samples are ashed at temperatures in the 450 to 500 C range typically described as a dull red heat. Under these conditions the black Co₃O₄, which is the most stable of the oxides, is formed. The cobalt (II, III) oxide or cobalt tetra oxide is refractory toward dissolution in dilute acids at room temperature or with heating.

Compounds and Salts of Co (II)

Co (II) arsenate, carbonate, hydroxide, iodate, phosphate, sulfide, borate, oxalate and cyanide are insoluble in water, whereas the chloride, nitrate, fluoride and sulfate are all quite soluble. Consequently, typical acid digestions are not hampered by samples containing large amounts of cobalt in moderate to small volumes.

The hydroxide is commonly encountered in sample preparations of Co. The Co⁺² ion has been shown to form mononuclear hydrolysis products ranging from Co(OH)+ to Co(OH)₄^-2. The solubility product of Co(OH)₂↓ is ~ 10^-15. Precipitation doesn’t begin until a pH of 6.4. The use of EDTA is most effective in keeping Co⁺² in solution at pHs of 6.5 to 13 (Log K_CoEDTA = 16.3). Other common ligands and chelates that extend the solubility pH range, although not as effectively as EDTA, are acetylacetonate, ammonia, citrate, ethylenediamine and tartrate.

Sampling and Handling

Co is used in alloys to impart wear resistance and high-temperature hardness, but there is not a great risk of contamination when using stainless steel. However, it is always advised that the analyst know the composition of the steel or alloy coming in contact with the sample. For example, grinding equipment and WC mills may use Co as a binding agent. For the most part, clean rooms or special handling equipment is not required. Biological materials with ultra-trace amounts of cobalt may be handled with stainless steel or other metallic devices such as spatulas and scapulas. The composition of any biological materials should be known before approval for use in the sample handling/preparation apparatus/device. The following considerations apply:

If the tools that you plan to use to pulverize, mix, cut, etc., contain Co or have an unknown composition, then attempt to use devices made of ceramics, silica/quartz, and polymers where possible.
Many polymers, such as Teflon have been exposed to alloys (cutting/pulverizing) prior to molding. As a precaution, new plastic and Teflon containers should always be acid leached to remove possible contamination from a variety of elements.
The collection of biological samples are also at risk of contamination due to the very low (ppt) levels of Co sought. The use of steel needles, scalpels or any metallic object that may contain Co should be avoided.

For more on sample contamination risks, see chapters eight, nine and ten of the Inorganic Ventures ‘Trace Analysis Guide’.

The Metal and Alloys

Metal

Unlike Fe and Ni, Co metal dissolves readily at room temperature using 1:1 nitric acid. Heating is never required and, in fact, may cause the reaction to get out of control. If the metal is a powder, exercise caution since the reaction will be quite vigorous.

Alloys

The use of dilute 1:1 nitric acid is commonly used for cobalt alloys. Heating is typically required and the use of a microwave can greatly speed the reaction. An impressive 1:1:1 mixture of HNO₃ + HF + H₂O is used to dissolve high-temperature alloys of Fe/Co/Cr/Ti/AL/Mo/Ta/Hf (use 10 mL mixture/ 0.2 grams sample).

Oxides and Ores

Oxides

As a general rule, both grey and black cobalt oxides are soluble in hot HCl if they have not been ignited. If the oxides are insoluble in HCl, then fusion with potassium bisulfate or sodium carbonate in a Pt crucible is required. The following is a general guide for carbonate fusions:

Make certain that the sample is well mixed with the sodium carbonate.
A 5-9’s pure sodium carbonate is recommended and available from E.M. Science.
Mix the sample with the flux at no more than a 1:20 ratio. Typical sample to flux ratios are in the 1:10 area.
If organic matter is present, either initially mix the sample with the flux and heat slowly to 500 C for ~ 2 hours before bringing up to full temperature or pre-ash the sample at 500 C and then mix the ash with the flux.
Use Pt as the crucible container material.
Perform the fusion at 1000 C in a muffle furnace. Avoid flames since this fusion is difficult to perform in a flame due to the high melting point of the sodium carbonate.
Most fusions are complete in 15 minutes but some require up to 45 minutes.
Dissolve the fuseate in dilute HCl (1:1)

Ores

The ores containing cobalt vary widely in their chemical nature, making it difficult to present a method for all types. All sample types should be pulverized to pass an 80 to 100 mesh sieve. Sulfides and ores containing organic matter can be mixed with sodium carbonate and ashed in a Pt crucible at 500C for one hour prior to bringing to a temperature of 1000 C as described below. If the ore has already been strongly ignited, you can go directly to 1000 C with sodium carbonate (see above procedure). When a sodium carbonate fusion is performed, HF is not needed unless silica content is high (silicates formed from the carbonate fusion are soluble but do not heat the fuseate to dissolve since this will bring about the formation of polysilicic acid, which is a gelatinous precipitate). This approach is applicable to many types of ores, including sulfides, ores containing organic matter, oxides, and ores containing silica.

Organic Matrices

Ashing of organic materials, foodstuffs, plant, and blood and sewage sludge as a preliminary decomposition step is not suggested for samples containing Co, unless an ashing aid such as sodium carbonate is used and the crucible material is Pt. If magnesium nitrate is used as the ashing aid then porcelain or quartz crucibles are acceptable. Conventional dry ashing procedures will result in the formation of a refractory form of cobalt oxide, unless there is a significant amount of one or more elements from groups I, II or III. 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 (see above for procedure) or heating of the ash to fumes with sulfuric and hydrofluoric acids is suggested.

The acid digestion of organic samples containing Co is also very common. Co does not offer any unusual challenges over the other elements. There are numerous oxidative acid digestion procedures available that are tailored to the matrix.

For more on acid digestion, please see the following paper: Acid Digestions of Organic Samples

Ashing

Overview

Ashing in analytical chemistry is defined as the heating of a substance to leave only noncombustible ash, which is analyzed for its elemental composition.

Ashing Techniques

The sample preparation techniques incorporating some form of ‘ashing’ are as follows:

Dry Ashing

Dry Ashing is usually performed by placing the sample in an open inert vessel and destroying the combustible (organic) portion of the sample by thermal decomposition using a muffle furnace. Typical ashing temperatures are 450 to 550 C. Magnesium nitrate is commonly used as an ashing aid. Charring the sample prior to muffling is preferred. Charring is accomplished using an open flame.

Sulfated Ashing

Sulfated Ashing involves treatment of the sample after charring using an open flame with sulfuric acid (the char is wetted using the minimum amount of sulfuric acid and then brought to dryness before muffling) and then placing in a muffle furnace.

Wet Ashing

Wet Ashing is treatment of the sample with a moderate amount of sulfuric acid before charring. Charring is performed using an open flame. Liquid samples tend to foam. After the excess sulfuric acid is driven off, the sample is muffled as above.

Low-temperature Ashing

Low-temperature Ashing involves treatment of the sample at ~ 120 C using activated (singlet state) oxygen.

Closed System Ashing

Closed System Ashing involves thermal decomposition in oxygen in a closed system, such as a Schöniger flask or an oxygen Parr bomb.

Advantages of Ashing

Ashing techniques are understandably used only for samples containing a significant amount of combustible or organic material as the matrix. With this in mind, let’s look at the major advantages of ashing:

The ability to decompose large sample sizes.
The need for little or no reagents.
The technique is relatively safe.
The ability to prepare samples containing volatile combustion elements, such as sulfur, fluorine and chlorine (the Schöniger oxygen flask combustion technique is very popular in this case).
The technique lends itself to mass production.

The technique of graphite furnace atomic absorption spectrometry (GFAA) incorporates sample ashing as part of an automatic measurement cycle. Trace analysts have learned a great deal about the loss of volatile components during ashing due to the ease with which the analyte signals can be compared to different ashing temperatures, times, addition of ashing aids and other conditions. Typically, graphite carbon is the container material that the sample comes in contact with when using GFAA.

Disadvantages of Ashing

The trace analyst should be very familiar with their sample type before performing an ash. Some of the problems that have been encountered are listed as follows:

Losses due to retention of the ashing container.
Losses due to volatilization.
Contamination from the ashing container.
Contamination from the muffle furnace.
Physical loss of ‘low density’ ashes when the muffle door is opened (air currents).
Difficulty in dissolving certain metal oxides.
Formation of toxic gases in poorly ventilated areas (Note that all charring should take place in a hood and the muffle furnace must have a hood canopy for proper ventilation).

Common Problems

If the sample type is unknown (with respect to the matrix) then a % ash, EDXRF scan, IR scan, and C, H, and N analysis will provide sufficient information in most cases to make informed decisions. The following are common problems that can be adverted with one or more of the above preliminary analyses:

Protect your Pt° ware by looking for P (high levels will attack and attach to the Pt°) and elements that alloy with Pt° which include the precious metals, Cu, and Hg.
When using ‘silica’ containing crucibles (porcelain, Vycor, quartz, glass, and fused silica) look for elements that form basic oxides such as the alkali earth elements. Na is commonly found and its oxide will form (unless the char is sulfated) and attack the silica.
Look for volatile elements (Cd, B, Hg, Pb, Se, Zn, As, Sn, Sb, S, and halogens), especially if moderate to large amounts of F or Cl are present.
Si is a common element that is typically determined by dissolution of an ash performed in Pt°. Methyl silicones are widely used and very common. If Si is present as a silicone oil then it will be partially lost as the hexamethycyclotrisiloxane and the hexamethydisiloxane.
Retention and physical loss of analyte(s). The use of high purity Mg(NO₃)₂ as an ashing aid will help prevent losses of ‘low density’ ashes, and will help in preventing retention losses.
Difficult to dissolve oxides. Use as low of an ashing temperature as possible (400 to 550 C maximum). Look for Ti, Zr, Nb, Hf, Ta, W, Ni, Co, Fe, Cr, Sb, and Mo. The type of crucible material will determine the treatment that the ash can undergo. Pt° is not attacked by HF, which will dissolve several of the above oxides.
Loss due to reduction to the metal. Look for easily reduced elements, such as Cu and the precious metals. Use the appropriate crucible material to allow for the necessary dissolution reagents for the metal.

Examples of Ashing Procedures

Consult the elemental profiles found in our Interactive Periodic Table for additional ashing procedures.

General Dry Ashing Procedure

This procedure is used for a wide variety of sample types, which include organic polymers, natural products such as agricultural materials, biological materials, petroleum products and synthetic organic research materials. The laboratory supervisor should be consulted with new or unfamiliar sample types to determine if this method is appropriate.

Procedure: Dry ashing procedures are typically and preferentially performed in Pt° crucibles. Glassy carbon can be used, but some attack may occur. Nickel and iron can also be used, but the metal from the crucible can cause significant spectral interference. A sample size ranging from a few milligrams to 100 grams is weighed into the crucible. Due to highly toxic fumes, the crucible is placed on a hot plate and set on the highest setting. Do this in a Class-A hood.. The use of a propane torch is helpful in speeding up the process and is necessary for the ignition of certain sample types such as polyethylene. As soon as fumes cease to evolve, the sample is placed in a muffle furnace at 450 - 500 C for one hour or until all of the carbon has been oxidized.

General Sulfated Ashing Procedure

This procedure is used for a wide variety of sample types which include organic polymers, natural products such as agricultural materials, biological materials, petroleum products and synthetic organic research materials. The sulfated ash is used over dry ashing when the analyst needs to fix a material, such as the sulfate to prevent volatilization, otherwise it has no real advantages over dry ashing. The laboratory supervisor should be consulted with new or unfamiliar sample types to determine if this method is appropriate.

Procedure: Sulfated ashing procedures are typically and preferentially performed in Pt crucibles. Glassy carbon can be used, but some attack may occur. Nickel and iron can also be used but the metal from the crucible can cause significant spectral interference. A sample size ranging from a few milligrams to 100 grams is weighed into the crucible. The crucible is placed on a hot plate and set on the highest setting. Do this in a Class-A hood due to highly toxic fumes. The use of a propane torch is helpful in speeding up the process and is necessary for the ignition of certain sample types such as polyethylene. As soon as fumes cease to evolve, wet the char with concentrated sulfuric acid. Typically a few drops are required. Continue to heat the sample until the white dense sulfur trioxide fumes cease to evolve. The sample is placed in a muffle furnace at 450 - 500 C for one hour or until all of the carbon has been oxidized.