Samples Containing Nickel

General Information

Occurrence — Ni has an abundance of 0.016% in the lithosphere and is the twenty-second element in abundance in the earth's crust. The chief ore is pentlandite [(NiFe)₉S₈]. Ni also occurs in meteoric iron, josephinite, and awaruite, and it occurs in arsenates, antimonates, silicates, sulfides, and phosphates. Canada is the main refiner of the metal. Impurities that may be left from the refining include Fe, Cr, Cu, Co, Zn, S, P, Si, Se, Sb, and As.

Uses — Ni is a hard, silvery white metal capable of taking a high polish. It is malleable, ductile, and strong, forming wire stronger than iron. Ni is harder than pure iron but does not oxidize in dry or moist air. It is magnetic, but loses its magnetism when heated above 345 °C. Ni is used to make alloys that are used in a variety of applications. German silver (Ni, Cu, Zn) is used in cutlery; Chromel (Ni, Cr) is used in electrical wiring; Inconel (Ni, Fe) is used for gas turbine blades; Monel (Ni, Cu, Fe, Mn) is used to make machinery parts for marine equipment used in pumping and oil drilling; Nichrome (Ni, Cr, Fe) is used for electrical coils; Nisil (Ni, Si) for thermocouples; Nitinol (Ni, Ti) for shape-memory objects; Cupronickel (Ni, Cu) for spectacle frames and electronic components; and Mumetal (Ni, Fe) for soft magnets.

Chemical Properties — The most stable and common oxidation state for Ni is +2. This is the oxidation state produced when acid digestions and other common sample preparation chemical operations are performed. Inorganic Ventures produces single-element CRMs of Ni in the +2 oxidation state in dilute HNO₃ that can be mixed with HCl, HNO₃, H₂SO₄, HF, and H₃PO₄. Avoid aqueous solutions where the pH is higher than 6.0. Complexing agents that greatly extend the stable pH range (>12) include cyanide and EDTA. Ni is stable with most metals and inorganic anions in acidic media at a pH of <2. Low ppb levels of Ni are stable for months in 1% HNO₃/LDPE containers. 1-10,000 ppm solutions are chemically stable for years in 1-5% HNO₃/LDPE containers.

At Inorganic Ventures — Inorganic Ventures uses metallic Ni as the starting material, with a documented purity that is confirmed to be 5-9's using ICP-MS and ICP-OES. The pure metal is dissolved in hot, concentrated, electronic-grade HNO₃. Impurities for each lot (measured using ICP-MS and ICP-OES) are reported on the Certificate of Analysis.

Sampling and Handling

In developing a sampling approach, it is helpful to have a ballpark idea of the concentration of Ni in the sample. The following are typical levels of Ni to be expected in a variety of sample types:

• Earth's crust — 100 μg/g
• Seawater and municipal water — low to sub ng/L
• Rural air — up to 10 ng/m³
• Urban air — 100 ng/m³
• Biological materials — low ng/g to low μg/g
• Foodstuff — 0.05 to 1 μg/g

Ni contamination risk is moderate to high. The following precautions should be considered:

• Many tools that pulverize, mix, cut, etc., contain stainless steel, where Ni is commonly present. Attempt to use devices made of ceramics, silica/quartz, and polymers where possible.
• New PFA and PTFE Teflon may contain Ni from the manufacturing process (molding and cutting, using alloys containing Ni).
• The collection of biological samples is most difficult, since they are at the greatest risk of contamination, due to the very low (ppb) levels of Ni sought. The use of steel needles, scalpels, or any metallic object that may contain Ni should not be used. Ni is more concentrated in the hair, fingernails, feces, and sweat than in organs or blood (<1 ppb).
• Sweat contains Ni (~50 ng/g); therefore, plastic gloves should be worn when handling samples.
• All plasticware should be leached at 60 °C with dilute 1% HNO₃ and rinsed with 18 MΩ water.
• Contamination risk from the soil should be avoided by thoroughly rinsing all plants for analysis with DI water.
• Ni always appears in air particulate. Clothing may contain Ni from dust in the air. An evaluation of the need for a clean suit and clean-air laboratory should be made based upon the level of Ni in the local air particulate.

The risk of contamination is moderate to high for Ni. Trace analysts are very aware of all the contamination problems associated with elements like Na, Ca, Al, Si, Fe, and Zn, but Ni is often overlooked. 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 — HCl and H₂SO₄, dilute or concentrated, slowly attack Ni⁰. Dilute 1:1 HNO₃ attacks the metal, but the surface of the metal is passivated by the formation of the oxide when concentrated HNO₃ is used. Concentrated HNO₃, when heated to near boiling, will overcome the oxide layer passivation and react vigorously. Up to 0.5 grams of the pure metal can be readily dissolved with 30 mL of 1:1 HNO₃/H₂O + heat.

Alloys — All of the above-mentioned alloys can be dissolved using a 1:1:1 mixture of concentrated HNO₃/HF/H₂O (G.G. Welcher, et al., Anal. Chem., 46, 1227 [1974]). If HF must be avoided, then try substituting HCl for the HF and making the final dilutions in sufficient HCl to prevent hydrolysis of the refractory "HF" elements, if present. For example, many Ni alloys can be dissolved in 1:1:1 HNO₃/HCl/H₂O, but difficulty maintaining solution of the high-temperature alloys that may contain Ti, Mo, Ta, or Hf needs to be countered through the liberal use of HCl, i.e., many sample digestates are lost through hydrolysis when diluted with plain water rather than 30% v/v HCl/H₂O.

Oxides, Minerals and Ores

Oxides — The hydroxides and carbonates are soluble in dilute acids. Dilute HNO₃ and/or HCl are most popular. The oxides are somewhat resistant to dilute acid attack and may require, for example, opening out with sodium carbonate.

Ores — The variety of sample types in this category is considerable. Meteoric iron to mixed metal sulfides, as well as silicates, arsenates, antimonates, phosphates, etc., leave the analyst faced with a tough problem in deciding upon the best approach. Below you will be presented with four different preparations.

This first preparation, which was found in A Handbook of Decomposition Methods in Analytical Chemistry; Bock, Rudolf, Ed.; Halsted Press, Div. Wiley & Sons: New York, 1979; translated by Ian L. Marr, involves taking 2-5 grams of the ore or slag and digesting with (10 mL H₂O + 25 mL HNO₃), followed by bringing to SO₃ fumes after adding 40 mL of 1:1 H₂SO₄. Attempting to use an acid digestion would be profitable for the laboratory that has little equipment.

The second and third methods, which were found in Standard Methods of Chemical Analysis, N. Howell Furman, Ed.; D. Van Nostrand Company, Inc.: Princeton, NJ, 1962 (page 697), are billed as applicable to a broad range of Ni ores.

Second Acid Digestion — One gram of the finely powdered ore is weighed into a porcelain dish and mixed thoroughly with 3 grams of powdered KClO₃. The dish is covered with a watch glass, and 40 mL of conc. HNO₃ is slowly added. The dish is allowed to stand in a cool place for a few minutes, then placed on a water bath and digested until the sample is completely decomposed. Stir the mixture frequently with a glass stirring rod, and add a little KClO₃ from time to time until decomposition is complete. The addition of 10 mL of conc. HCl and washing the sides with water and heating to bring about solution may be required.

Third Method — Fusion Method — One gram of the finely divided (very important to be no "grainy" material) sample is placed in a large platinum crucible, together with five times its weight of sodium carbonate. The crucible and contents are placed in a muffle and heated at 450 °C to air-oxidize the combustible material. Heat the crucible at this temperature until oxidation is complete, and if uncertain, then heat for ~7 hours. The sample is cooled, and 5 grams of a mixture of sodium carbonate and potassium nitrate (10 parts sodium carbonate to 1 part potassium nitrate — please note that potassium nitrate attacks Pt, and a large excess of the sodium carbonate is needed to prevent this) is added. Fusion is done at ~1000 °C for 10 to 15 minutes (do not stir, i.e., a "quiet fusion"). The fuseate is soluble in dilute nitric acid.

Minerals — The above fusion with Na₂CO₃ and KNO₃ is recommended.

Organic Matrices

Ashing of organic materials, foodstuffs, plant, and blood and sewage sludge, as a preliminary decomposition step, can be expedient for samples containing Ni. 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. The dry ashing of samples results in the formation of NiO, which is very difficult to dissolve in dilute acids, and therefore ashing aids are strongly encouraged. Acid digestions using nitric, perchloric, and sulfuric acids are 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 °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 on ashing, please see the following paper: Ashing

Nickel 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 to low ppm levels of Ni can be digested with nitric/perchloric acid. 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

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