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Samples Containing Zinc

Group 12 – Zn


General Information

Occurrence – Zinc (Zn) is the first of the group 12 elements.  Zinc is an element that has been known for a very long time and is best known as an alloying element in the manufacturing of brass.  The abundance of Zn in the Earth’s crust is 76 ppm and in seawater averages only ~0.001 ppm.  Zinc is found in nature only in combined form, and Zn ores in order of importance are sphalerite (sulfide), smithsonite (carbonate) and willemite (silicate). 

Although Zn has been used in alloys as far back as ancient Rome, Zn was not known to be a distinct element until the late 17th century.  An additional 100 years passed before it became important in industrial uses. 

Uses – Zn has become one of the most useful elements.  Zinc is used in a number of alloys such as brass (Zn, Cu), German silver (Zn, Cu, Ni), and the die casting alloys (Zn, Cu, Al).  Zinc’s resistance to rusting makes it valuable as a protective coating on iron surfaces (galvanized iron).  Zn is also used as anodes in electrical batteries.  ZnO is a useful pigment and additive in rubber goods and tires, and is used extensively as a paint pigment.  It is used in glazes, ointments, enamels, and wood preservatives, etc.  It has been found necessary in the growth of plants.  Zn metal is used as a reducing agent (usually as an amalgam) for a number of metals such as gold (Au) on an industrial scale.  Because of its widespread use as a releasing agent and lubricant in the plastics industry (zinc stearate) it presents quite a contamination problem for the trace analyst.  Zinc presents almost as much of a problem as Ca which makes the testing of ppb levels problematic from a contamination perspective.

Chemical Properties – The group 12 elements do not form ions with incomplete d subshells (their electronic structure is (peroid -1)d10peroids2).  Zinc has only the +2 oxidation state due to its electronic structure Zn(3d104s2)  →  Zn+2 (3d10); i.e., the stability of the filled 3d10.  Although a transition element the (period -1) d orbital is full, resulting in some properties that are different than the preceding transition elements.  For example, the standard reduction potential for Zn (Zn+2  +  2e  →  Zn) is ~-0.7v whereas Cu, which is the preceding element in that period is ~0.3v (Cu+2  +  2e  →  Cu). Consequently, Zn has a chemistry that loosely resembles Ca.  Zn is more “calcium like” than the other first row transition elements, and ZnO dissolves easily (even when heated) which is not the case for any of the other first-row transition metal oxides (Sc thru Cu).  Like Ca+2, Zn+2 forms precipitates with oxalate, fluoride, sulfate, and iodate, and it forms a stable complex with many of the same complexing agents as Ca+2 (such as EDTA, tartrate and citrate).

At Inorganic Ventures – Inorganic Ventures uses metallic Zn° shot 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.  The solubility of Zn(NO3)2 in water is 138 g / 100 g H2O at 30 °C making it one of the most soluble of the common Zn salts. 

Sampling and Handling 

Sampling of metallic zinc presents difficulties due to segregation of impurities within the metal as well as contamination from sampling tools.  Tungsten carbide cutting / milling tools are recommended.  Objects may be drilled using a nonferrous drill in such a way that the sample is obtained from the surface as well as the bulk in appropriate proportions.  Thick objects may be sawed with a bandsaw without a lubricant.  Brass sieves should not be used.

In the sampling of zinc ores during the grinding or pulverization of the sample, care should be taken to avoid local heating in contact with air to prevent the conversion of the sulfide to the sulfate or oxide.  Ores containing pyrite are pyrophoric and should be dried at low temperatures to minimize oxidation.

In developing a sampling approach, it is helpful to have a ballpark idea of the concentration of Zn in the sample.  If the sample type is typically low in zinc 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 Zn along with expected levels:

  • Earth’s crust – 70 - 80 μg/g   
  • Seawater – 0.6 to 5 μg/L
  • Potable water – max. 5 μg/mL
  • Rural air – <1.0 µg/m3
  • Urban air – 0.1 to 1.7 µg/m3
  • Biological materials – 1 - 10 µg/g
  • Foodstuffs – levels vary greatly with most foods containing 1 - 700 µg/g

Zn contamination risk is 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 that may be Zn plated to minimize corrosion.  Ceramics, silica/quartz, and polymers likely contain trace amounts of zinc. 
  • New PFA and PTFE Teflon may contain Zn from the manufacturing process.
  • All plasticware should be leached at 60 °C with dilute 1% HNO3 and rinsed with 18 MΏ water.
  • Contamination risks from soil should be avoided by thoroughly rinsing all plants for analysis with DI water.
  • Zinc always appears in air particulates.  Clothing may contain Zn 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 Zn in local air particulates. 

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 Zn containing solutions, as well as suggestions for ICP analyses of zinc, may be found by clicking on the Zn element symbol at:

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

The Metal and Alloys

Metal – Zinc metal is soluble in a variety of reagents including HNO3, HCl, alkalis, acetic acid, and hot concentrated H2SO4.  Impurities in the metal will accelerate the dissolution.  Dilute 1:1 HNO3 is popular for a variety of reasons.  Dilute HCl is also popular.    

Alloys – Zn° 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 Zn.  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 reaction of the zinc with the crucible walls and results in an apparent loss.  In addition, the formation of refractory metal oxides of many elements may occur making the use of ashing aids such as Na2CO3 very expedient.  Acid digestions using nitric, perchloric and sulfuric acids are suggested.  For more on ashing please see the following paper:

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

Zinc 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 Zn 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