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Sample Preparation Guides

General Information

Occurrence – Cadmium (Cd) occurs in small quantities in practically all zinc ores.  It is found in most slab zinc and zinc materials, sheet zinc, zinc oxide, etc.  In ores it occurs usually as the sulfide, the rare mineral greenockite being CdS.  The metal cadmium is largely obtained as a by-product from zinc smelting.  Its abundance in the Earth’s crust is 0.15 ppm. 

Cadmium was discovered in zinc carbonate by F. Stromeyer and simultaneously in zinc oxide by K. Hermann in 1817.

Uses – Cd is used in alloys.  Its alloy with gold is green colored and is a popular metal in jewelry.  Its alloy with Ag resists tarnishing, and Cd is used as a metal coating for rust proofing.  When added to Cu it increases the tensile strength of this metal.  Cadmium is used in certain alloys as trial plates for silver coinage, and more recently as substitutes for tin base bearing metals.  It is used as a paint pigment as the yellow sulfide and may also be found in some dental amalgams.

Chemical Properties – The group 12 elements do not form ions with incomplete d subshells (their electronic structure is (peroid -1)d10peroids2).  Cd has only the +2 oxidation state due to its electronic structure Cd(4d105s2)  →  Cd+2 (4d10), i.e. the stability of the filled 4d10.  The metallic radii of these elements are larger than those of the metals which precede them due to the involvement of their s electrons in metallic bonding.  Their covalent radii are also large with the M+2 radii increasing substantially from Zn to Cd to Hg.  The melting points of the group 12 metals are relatively low (Cd = 320.9 °C).  Dry ashing organic carbon-based samples for trace analysis of Cd is a problem if steps are not taken to maintain an oxidizing chemical environment (please see Organic Matrices below). 

The alkali hydroxides and carbonates form white precipitates with Cd+2 that are insoluble in water unless a chelating agent like tartrate, citrate or EDTA is present; excess reagent will not bring them back into solution (unlike Zn).  Other reagents that form precipitates with Cd+2 in water are oxalate, phosphate, arsenate, arsenite, and sulfide.  All Cd salts are “white” with the exceptions of the largely covalent bonded bromide, iodide and sulfide.  All of these salts are brought into solution by adding HNO3.  If fact, HNO3 dissolves all known compounds of Cd.  In NH4OH excess reagent will bring the precipitate back into solution. 

The behavior of the halides (X) within group 12 is noteworthy.  Zinc halides (ZnX2) dissolve in water forming the Zn(H2O)4+2 ion whereas Cd forms Cd(X)+, Cd(X)3-, Cd(X)4-2 depending upon the concentration of the halogen.  Hg is even more pronounced than Cd in forming the anionic halogen complex where HgCl2 is practically nonionized in aqueous solution.  At IV we have found ≥10% v/v HCl to be an excellent solvent for stabilizing ppb Hg solutions, presumably discouraging chemi-adsorption on plastic containers/contact surfaces through the formation of the anionic halogen complex (HgClx-x+2).  We have thus far not found this approach required for stabilizing ppb Cd solutions but are keeping this option in mind if a problem should arise.

At Inorganic Ventures – Inorganic Ventures prefers to use metallic Cd° shot as the starting material with a documented purity that is confirmed to be 99.999+% using ICP-MS and ICP-OES.  Starting material information, purity and final product impurities can be found on IV’s website.  The pure metal is dissolved in dilute electronic grade HNO3.   Impurities for each lot (measured by IV using ICP-MS and ICP-OES) are reported on the Certificate of Analysis.  A recent (at time of publication) 10,000 µg/mL Cd CRM contained a non-detectable Zn value <0.002 µg/mL.  Considering that cadmium is largely obtained as a by-product from zinc smelting, this is remarkable.  This specific lot (P2-CD685077) was manufactured from Cd shot analyzed to be 99.9996% Cd, which is typical.  Axial view ICP-OES was used to measure Zn which was confirmed by ICP-MS. 

Sampling and Handling 

Cadmium is used widely in the industrial world.  It is also very toxic, and similar to As is listed as a known carcinogen by the U.S. National Toxicology Program (NTP) and the International Agency for Research on Cancer (IARC).  Industrial uses of Cd can impact levels in water, air, and soils.  Considering the acute toxicity measuring low levels of Cd in environmental and biological samples is of special interest, particularly as conventional samples containing higher levels of Cd do not present any significant sample preparation difficulty (other than when ashed which will be discussed further).

Critical to the development of a sample analysis scheme is the level of Cd to be expected.  This, along with the analytical measurement technique dictates the sample size needed which in turn influences the sample preparation approach.  The following are Cd levels expected in uncontaminated samples:
 

Environment

Concentration

(µg/g unless stated otherwise)

Soil

0.150 – 1

Dust

50 – 150

Fresh water

0.1  – 1000

Sea water

0.01 – 0.15

Salt

0.001 – 0.14

Plants

0.04 – 1.2

Tobacco (Cd concentrates is tobacco)

3 – 5

Air (Non-contaminated / Contaminated)

0.0001 – 0.3 / 1-300 (ug/m3)

Blood

0.1 – 2 µg/L

Serum

0.01 – 0.2 µg/L

Urine

0.01 – 0.6 µg/L

Milk

0.10 – 0.20 µg/L

Hair

0.02 – 3.6

Facial hair

0.09 – 4.3

Teeth

0.04 – 1.50

Lung

<0.01 – 5

Liver

0.01 – 20

Kidney

0.01 – 2

 

Reference Values from industrialized regions and where sludge-based and phosphate fertilizers are used

 

Air

Threshold limit (8 h day) 0.01 mg/m3

Drinking water

Max. permissible 5 µg/L

Wastewater

Max. acceptable 200 µg/L

Dishes

(leach from 24 hr with 4% acetic) 0.5 mg/L

Food

<70 µg/day

Blood / Urine

10 µg/L / 10 µg/L

Kidney cortex

200 µg/g (renal dysfunction at 250 µg/g)

Liver

20 µg/g

Sample handling and storage are major problems for reliable trace analysis.  The sample itself should be as representative as possible, particularly with respect to total size of the area explored. 

The main Cd contamination issue we have observed at IV is the use of yellow disposable pipette tips by some of our customers.  For more on sample contamination risks see chapters 8, 9 and 10 of the Inorganic Ventures Trace Analysis Guide: 

http://www.inorganicventures.com/tech/trace-analysis/environmental-contamination 

For general information on sampling and sub-sampling see:

http://www.inorganicventures.com/tech/reliability/part03.asp

Detailed handling information related to Cd containing solutions, as well as suggestions for ICP analyses of cadmium, may be found by clicking on the Cd element symbol at: 

https://www.inorganicventures.com/periodic-table

The Metal and Alloys

Metal – Cadmium metal is soluble in HNO3 with evolution of NOx and is the preferred solvent.  It dissolves more slowly in hot, moderately dilute HCl or dilute H2SO4 with evolution of hydrogen. 

Alloys – Cdº and its alloys are soluble in H2O / HCl / HNO3 mixtures.   

Oxides, Minerals and Ores

Oxides –The hydroxides and carbonates are soluble in dilute acids.  Dilute HNO3 and or HCl are most popular. 

Ores – Digestion with HCl and HNO3 followed by evaporation and filtration of the SiO2 is a classic approach.

The following is a fusion method that many prefer:

Fusion method – One gram of the finely divided (very important there is 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 (ash) 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 is 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 sodium carbonate is needed to prevent this).  The need for the potassium nitrate may be eliminated by extending the ashing time to overnight.  Fusion is then done at ~1000 °C for 10 to 15 minutes (do not stir unless the potassium nitrate is eliminated, i.e., a “quiet fusion”).  The fuseate is soluble in dilute nitric acid. 

Minerals – The above fusion with Na2CO3 and KNO3 is recommended.  The elimination of the KNO3 should be explored by extending the ashing time to overnight.

Organic Matrices

Ashing of organic materials, foodstuffs, plant, and blood and sewage sludge as a preliminary decomposition step can be expedient for samples containing Cd.  The melting points of the group 12 metals are relatively low (Cd = 320.9 °C).  Dry ashing organic carbon based/containing samples for the trace analysis of Cd is a problem if steps are not taken to maintain an oxidizing chemical environment.  However, with a reduction potential (Cd+2  +  2e  →  Cd) of -0.403v versus N.H.E. this problem is easily managed through the use of ashing aids such as sodium carbonate (in air at 400 to 450 °C) and/or sulfuric acid/magnesium nitrate at 550 °C (use a Pt crucible for the Na2CO3 and quartz for the H2SO4 / Mg(NO3)2).  In general, it is wise to keep the ashing temperature low (400 to 450 °C max) and to use an ashing aid such as high purity sodium carbonate.  The dry ashing of samples with no ashing aid will result in the loss of Cd.  In addition, the formation of refractory metal oxides of many other elements that may be present will not occur making the use of ashing aids such as Na2CO3 very expedient.  For more on ashing please see the following paper:

http://ivstandards.com/tech/reliability/part14.asp.

Acid digestions using nitric, perchloric and sulfuric acids are suggested. 

Cadmium 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 Cd can be digested with HNO3/HClO4.  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://ivstandards.com/tech/reliability/part12.asp