Sample Preparation Guides
Occurrence – Copper (Cu) is the first of the group ll elements (Cu, Ag, Au) which are also known as the “coinage metals”. Cu is much more abundant than the other group 11 elements with an abundance of 0.01%, i.e., a natural abundance that is roughly equivalent to that of Ni and Cr in the earth’s crust. Copper is found in nature as the free metal which is the chief reason it has been known and used by humans for over 5000 years. Its earliest use predates all historical records. Copper was known as the metal from the island of Cyprus. The Romans called the metal aes cyprium (the metal from Cyprus) later simplified to cyprium and then to cuprum.
It is also found in minerals as a sulfide, oxide and carbonate and less commonly in antimonides, arsenates, phosphates, silicates and sulfates. Copper ores are widely distributed throughout the world. Copper is found in meteorites, numerous rocks, soil, seawater, mineral water, plants and animal organisms. Cu is present as either an impurity or a desired constituent in nearly all metallurgical products. It does present contamination issues especially when accurate Cu determinations are required at the ppm level.
Uses – Pure Cu is a salmon–pink metal that is exceeded only by Ag and Au when compared to the “common use” metals in malleability, ductility and heat conductivity and is exceeded only by Ag in electrical conductivity. It is malleable, ductile and strong forming wire roughly ½ the strength of iron. Cu mixes well with other metals to form a wide variety of alloys. Copper’s combined alloy ability, malleability and strength explains why it is used in alloys as a coin metal. The group 11 cousins (Ag, Au) of copper are similarly known for their usefulness as coin metals.
Cu has a broad range of uses but is best known for its use in cookware and electrical wiring and coinage. Copper is also used in inorganic pigments for paints and plastics, in pharmaceuticals, and fungicides.
Chemical Properties – Copper has +1, +2 and +3 oxidation states. The most stable and common oxidation state for Cu is +2 and the principal hydrolysis product is Cu2(OH)22+. The +2 is the oxidation state produced when acid digestions and other common sample preparation chemical operations are performed, and this is the oxidation state for all of Inorganic Ventures Cu containing products. Common inorganic anions forming relatively insoluble Cu compounds are S2-, CO32-, OH1-, C2O42- and SeO32-. Inorganic Venture’s produces single element CRMs of Cu in in dilute HNO3 that can be mixed with HCl, HNO3, H2SO4, HF, and H3PO4. Avoid aqueous solutions where the pH is higher than 4.0 (highest pH that avoids hydrolysis). Complexing agents and ligands that extend the pH range of hydrolytic stability (extended pH range in parentheses after the ligand) include citrate (pH 6.0), DCTA, DPTA, and EDTA (pH 12). Cu is stable with all metals at ICP working levels in acidic media at a pH of <2. Low ppb levels of Cu are stable for years in 1% HNO3 / LDPE containers (though possible contamination from the container should considered). 1-10,000 ppm solutions are chemically stable for years in 1-5% HNO3 / LDPE containers.
At Inorganic Ventures – Inorganic Ventures uses metallic Cuº as the starting material with a documented purity that is confirmed to be 99.999+% using ICP-MS and ICP-OES. The pure metal is dissolved in dilute electronic grade HNO3. Impurities for each lot (measured using ICP-MS and ICP-OES) are reported on the Certificate of Analysis. Acids other than HNO3 require the use of an oxidizing agent such as H2O2 (trace metals grade that is unstabilized) before dissolution can be effected. The solubility of Cu(NO3)2 in water is 125g / 100g H2O at 20º C making it the most soluble (yields the most Cu per gram of salt) of the common Cu salts. Complications that occur from reactions with other solution components with easily reduced metals are avoided by heating the freshly prepared Cu solutions to rid them of the nitrogen sub-oxides.
Sampling and Handling
In developing a sampling approach it is helpful to have a ballpark idea of the concentration of Cu in the sample. If the sample type is typically low in copper it is necessary to be aware of contamination risks and take steps to minimize them. The following are sample types expected to be low in Cu along with expected levels:
- Earth’s crust - 70 μg/g
- Seawater - 0.1 to 0.8 μg/L
- Potable water – max 1μg/ml
- Rural air - up to 10 ng/m3
- Urban air – 100 ng/m3
- Biological materials – low ng/g to low μg/g (healthy adult daily intake of 199-150 mg Cu giving blood plasma of 200-700 μg/L; liver up to 7 μg/g fresh tissue; urine is low with poor agreement between sources (likely contamination issues).
- Foodstuffs – up to 10 μg/g
(Please note that the Cu levels shown above will tend to drop as the analytical community continues to improve in its ability to avoid Cu contamination. Do not discount data that fall below the ranges given above for this reason only.)
Cu contamination risk is moderate to high. When working with samples expected to be at trace levels (≤1 μg/g) the following precautions should be considered:
- Many tools that pulverize, mix, cut, etc., contain stainless steel where Cu is commonly present (0.1-0.4%). Attempt to use devices made of ceramics, silica/quartz, and polymers where possible.
- New PFA and PTFE Teflon may contain Cu from the manufacturing process (molding and cutting using alloys containing).
- 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 Cu sought. The use of steel needles and scalpels, or any metallic object that may contain Cu should not be used. Cu is more concentrated in the hair, fingernails and feces and sweat than in organs or blood. (Concern has been expressed that contamination errors have negated most published information on trace-element determinations in biological matrices. J. Versleck, et.al., Biol. Trace Elem. Res., 12 ,45 (1987).)
- All plasticware should be leached at 60 ºC with dilute 1% HNO3 and rinsed with 18 MΏ water.
- Contamination risk from the soil should be avoided by thoroughly rinsing all plants for analysis with DI water.
- Cu always appears in air particulates. Clothing may contain Cu from dust from the air. An evaluation of the need for a clean suit and clean air laboratory should be made based upon the level of Cu in the local air particulate.
The risk of contamination is moderate to high for Cu. Trace analysts are very aware of all the contamination problems associated with elements like Na, Ca, Al, Si, Fe and Zn but Cu 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 sub-sampling see:
The Metal and Alloys
Metal – Dilute HNO3 is the best “solvent” for Cuº. HNO3 reactions will vary depending upon dilution with water and when very dilute the HNO3 will lose its oxidizing ability. This is an important consideration depending upon the intended use of the dissolute. The reaction that predominates depends upon the amount of water present with the HNO3:
1. 6 H+ + 3 Cuº + 2 HNO3 (dilute) ↔ 3 Cu2+ + 2 NO ↑ (clear gas) + 4 H2O
2. 2 H+ + Cuº + 2 HNO3 (conc.) ↔ Cu2+ + 2 NO2 ↑ (brown fumes) + 2 H2O
Reaction 1 predominates when the acid is diluted to 1:1 or lower with water and concentrated (69.2 wt.%) HNO3 reacts with Cuº to give off brown fumes according to reaction 2. Thermodynamically and kinetically these two reactions differ resulting in nitrogen sub-oxides that may be reactive with other chemicals added later in the preparation. For example, if HCl is to be added to the preparation the HNO3 dissolute needs to be heated to remove the NOx followed by a significant dilution with water to render the HNO3 unreactive; otherwise gas evolution will slowly occur.
Cuº and its alloys are soluble in HCl / H2O2 mixtures. The reduction potential for Cuº is 0.337 v, explaining the requirement for an oxidizing agent. Hot 15% HCl alone in air will slowly dissolve Cuº with the formation of Cu2Cl2 and H2. Alloys are most easily dissolved in HNO3 / HCl. A mixture of HNO3 / HCl / HF (1:1:1) is a very powerful combination when “HF” elements like Si are present.
Alloys – Many 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 Cu alloys can be dissolved in 1:1:1 HNO3 / HCl / H2O but difficulty maintaining solution of high temperature alloys containing 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.
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, fusion 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, described in A Handbook of Decomposition Methods in Analytical Chemistry (R. Bock, Ed.; Halsted Press, 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 appropriate for the laboratory that has little equipment.
The second and third methods are presented in Standard Methods Of Chemical Analysis (N. Howell Furman, Ed.: D. Van Nostrand Company, Inc.: Princeton, NJ 1962, p. 697), and described as applicable to a broad range of Cu 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) – One gram of the finely powdered (very important that no “grainy” material is present) 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 ºC 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 added 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). Fusion is done at ~1000 deg °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.
Ashing of organic materials, foodstuffs, plant, and blood and sewage sludge as a preliminary decomposition step can be expedient for samples containing Cu. 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 Cu° when organics are present and refractory metal oxides of many elements which can be very difficult to dissolve in dilute acids (Cu° requires an oxidizing agent) and therefore ashing aids such as Na2CO3 are strongly encouraged. Acid digestions using nitric, perchloric and sulfuric acids are suggested. Wet ashing is the use sulfuric acid as an ashing aid. Do not use ashing temperatures above 450 °C. If sulfuric acid is added to a 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:
Copper 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 Cu 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: