Sample Preparation Guides
Chromium (Cr) is the seventh most abundant element, but minerals containing chromium are not wide spread, with most of the high Cr ores deposits being found in Africa. Chromite is the most common ore containing Chromium, with an empirical formula of FeCr2O4. The Cr and Fe are present in Chromite in the +3 and +2 oxidation states respectively, which is why it is best written and thought of as Cr2O3FeO. Cr is present in soil at concentrations ranging from 5 to 1000 ppm.
Chromium is most commonly used in alloys, steels, pigments, and chrome plating. Chromium is often described as having slow kinetics. Many of its forms are difficult to dissolve and it reacts slowly with ligands like EDTA making it's wet analytical chemistry challenging.
The most common oxidation states for Cr are 0, +3 and +6. Chromium is most commonly found in the +3 state, which is fortunate since the +3 state is considered benign as opposed to the +6 that is toxic.
At Inorganic Ventures we note the oxidation state of chromium in a given CRM by writing Cr+3 or Cr+6. We are not quite sure how this got started since it appears to be most confusing to the user that wants the "form of chromium present after sample preparation". The most common form after most sample preparations is +3, much the same as it is +3 for Fe, with the difference that we do not write Fe as Fe+3 , but just Fe when describing aqueous CRMs containing the element. Unless the analyst is in need of a CRM containing Cr+6 it is best if the Cr+3 is used, however, there are specific methods for the determination of 'hexavalent chrome' where the Cr+6 state is need.
Sampling and Handling
Chromium is determined in a wide range of environmental, biological, agricultural, metallurgical, and industrial (chemical industry) samples. Samples containing chromium are not particularly hazardous unless they contain high levels of Cr+6. The greatest risk with sampling and handling of these samples is a very real risk of contamination. The following precautions should be considered:
- Many tools that pulverize, mix cut, etc., contain stainless steel where Cr is commonly present. Attempt to use devices made of ceramics, silica/quartz, and polymers where possible.
- 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 Cr sought. The use of steel needles, scalpels or any metallic object that may contain chromium should not be used.
- The use of chromic acid/sulfuric acid cleaning solutions should be stopped.
- Cr is incorporated in some plastics from their production and processing/handling. For example, some polymerization catalysts contain chromium salts and many polymers are cut/extruded/milled, etc., with steel devices, prior to their final molding into beakers/digestion vessels/auto sampler tubes/tubing, etc. In addition, Cr may be contained in pigments. It is therefore common to find trace levels in material thought to be trace metals free such as PFA and PTFE.
The risk of contamination is greater for Cr than most analysts are aware. Trace analysts are very aware of all the contamination problems associated with elements like Na, Ca, Al, Si, Fe and Zn, but Cr is often overlooked. For more on sample contamination risks see chapters 8, 9 and 10 of the Inorganic Ventures 'Trace Analysis Guide':
The Metal and Alloys
It is isoluble in dilute HCl and dilute sulfuric acid but it is not soluble in oxidizing acids such as nitric acid, Aqua Regia or in hot, concentrated sulfuric acid. Oxidizing acids and concentrated sulfuric acid form oxides on the surface of Cr0 which are insoluble - this process is known as inducing passivity. Chromium metal is resistant to oxidative attack, which is why it serves as a protective coating (chrome plating).
Alloys are typically dissolved in Aqua Regia, perchloric acid or perchloric acid + phosphoric acid. For example, chromium steels or stainless steels (up to 1 gram) can be dissolved by heating with 40 mL of 1:1 perchloric/phosphoric acids.
Oxides, Minerals and Ores
Cr2O3 is insoluble in water and slowly soluble in dilute acid (HCl works best) if it has not been ignited. Ignited oxides require fusion. CrO3 is very soluble in water.
Fusion methods that have been reported include:
- Sodium and Potassium hydrogen sulfate in a Pt crucible.
- 2:1 NaHSO4 and NaF in a Pt crucible.
- 4:1 MgO and Na2CO3 in a Pt crucible.
Ores General Procedure
One gram (or more) 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 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 1050 deg C for 10 to 15 minutes (do not stir, i.e. a "quiet fusion"). The fuseate is soluble in dilute nitric acid.
Ores High in Silica
One gram of sample is treated in a Pt dish with about 10 mL of HF and 2-5 mL of concentrated H2SO4. The silica is expelled as the volatile fluoride and the HF is driven off by taking the solution to dense white SO3 fumes. The residue is extracted with hot water containing a little sulfuric acid. Any undissolved residue may be brought into solution by fusion with NaHSO4.
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, is not suggested for samples containing Cr due to the refractory nature of the oxides. If ashing is used, it is suggested to keep the temperature low (400 to 450 deg C max) and to use an ashing aid such as high purity sodium carbonate or magnesium nitrate. 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 deg 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, 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: http://inorganicventures.com/ashing-sample-preparation-procedures
Chromium 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 Cr 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: http://inorganicventures.com/acid-digestions-of-organic-samples
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.