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Sample Preparation Guides

Group 11 – Au


General Information

Occurrence – Gold (Au) is the least abundant of the group ll elements (Cu, Ag, Au) which are also known as the “coinage metals”.  Au has a crustal abundance of 0.004 ppm which is roughly equivalent to that of Pt.  Gold is typically included in analytical discussions of the Platinum Group Metals (PGM).  Gold has been mined as the metal since the stone age and is mostly found as the free metal, often in association with quartz (most Au ores have a high silica content).  The native metal is never found chemically pure being alloyed with Ag, and Cu and other metals.  A purity of 60% to 98% is common.  Au is also obtained as a by-product in the refining of Cu, Ni, Pb and Zn.  Stages of the mining production of Au may include a gravity separation, a reaction with NaOH/NACN to form Au(CN)2-1, the addition of Zn° dropping out Au°, and/or electroplating out Au° from the Au(CN)2-1 solution. All of the group 11 elements have in common their ability to be electroplated from aqueous solutions as the metal and their relative ease in alloying with each other and other metals.

Uses – Pure Au° is the only metal with a very distinctive yellow color.  It has a high malleability, ductility, heat conductivity and electrical conductivity.  It is the most malleable and ductile of all the metals.  One gram can be drawn into a wire 2000 meters long, and it can be rolled into sheets 0.0001 mm thick. Because pure Au is so soft it is often alloyed with other metals to harden it and increase its durability in coinage and jewelry.  The uses of Au, initially rather limited to uses as either money or jewelry and decoration, have expanded to electronics, coating for satellite panels, photography, medicine and nanotechnology.  A significant use of Au in the analytical community is in the stabilization of ppb Hg solutions prepared for trace Hg analysis.

Chemical Properties – From an analyst’s perspective the notable chemical properties of Au are:

  • Au+3 is an oxidizing agent.  As expected, it has a high reduction potential, E° of ~1.5v vs. N.H.E. [ Au+3  +  3e  →  Au° ; E° =  + 1.50v and/or Au+3  +  2e  →  Au+1 ; E° =  + 1.41v ].  Gold compounds tend to be unstable, forming the metal when mixed or heated.  During analytical ashing preparations the metal is formed unless alkali mixed with an oxidizing agent such as NaNO3 is present.  The oxidizing property of Au+3 in dilute nitric acid is used to stabilize ppb Hg solutions. These solutions have been measured at IV to be stable for >1 year which is longer than expected for the addition of such a small amount of an oxidizing agent (1 ppm Au as AuCl3OH-1). The fact that Au and Hg have very similar ionic and metallic radii in addition to identical electronic configurations (Au = Hg+1 and Au+1 = Hg+2), as well as mercury’s tendency to form binuclear complexes, suggests something more is occurring than just Au+3 acting as an oxidizing agent.  Is the formation of polynuclear complexes such as AuHg+1, AuHg2+2, or AuHg+3 occurring?
  • Gold’s resistance to air oxidation is unique.  When Au° is heated in air no oxide is formed.  In fact, the reverse occurs.  When gold oxides are heated in air Au° and O2 are formed. 
  • When Au° is dissolved using HNO3/HCl mixtures the resulting aqueous gold chloride solutions are heated to get rid of trace nitrogen oxides.  However, Au metal forms when heated for too long.  This can be a problem in preparations of HAuCl4 from aqua regia dissolutions when heated after dissolution of the gold.  At least an equal molar amount of HCl = AuCl3 is needed to maintain stability.
  • Au is noble. The low reactivity of Au° to mineral acids is notable.   Unless mixtures of an oxidizer and ligand are used Au° will not go into solution.
  • From an analytical perspective gold’s ability to be dissolved using aqua regia makes its preparation chemistry simpler than most of the platinum metals.  The trick is keeping it in solution.
  • Although gold has +1, +2, and +3 oxidation states the +2 is rarely encountered.
  • The Au+1 and Au+3 cations do not exist in aqueous solutions but must be complexed to remain in solution.  For the analytical community the most common ligands are Cl-1 and CN-1.
  • Aurous bromide treated with dilute KOH forms aurous hydroxide which goes to purple aurous oxide upon boiling.  The aurous oxide decomposes to the metal and oxygen when heated above 205 °C. [ AuBr  +  dilute KOH  →  AuOH  +  boil  →  Au2O  +  heat  >205 °C  →  Au°↓  +  O2↑ ]
  • Au(OH)3 (brown) is formed by treating AuCl3 with KOH.  Au2O3 (dark brown) is formed by drying Au(OH)3 over CaCl2 at 100 °C which decomposes to the metal and oxygen by heating to 250 °C. [ AuCl3  +  dilute KOH  →  Au(OH)3  +  heat 100 °C over a desiccant  →  Au2O3 (auric oxide)  +  heat >250 °C  →  Au°  +  O2 ] 
  • The ease of reduction of Au+3 compounds in aqueous solution to the metal is of great concern to the analyst.  Some common laboratory chemicals that must be avoided in addition to sunlight (photoreduction) are: the metals Pb, Ag, Hg, Sn, As, Sb, Bi, Cu, Pd, Pt, Te, Fe, Al, Co, Ni, Zn, Mg, P; the ions Fe+2, Mn+2, Sn+2, Ti+3, Hg2+2, As+3, Sb+3, Cu2+2, Cr+3, SO3-2, C2O4-2, NO2-1, PO2-3, PO3-3, O2-2 and many organic compounds such as aldehydes (formaldehyde), hydrazine, hydroxylamine diacetyldioxime, ascorbic acid, sugar, etc.

At Inorganic Ventures - Inorganic Ventures uses HAuCl4 as the starting material with a documented purity that is confirmed to be >99.995+% using ICP-MS and ICP-OES.  High purity Au metal is typically ≤99.95% pure.  Impurities for each lot (measured using ICP-MS and ICP-OES) are reported on the Certificate of Analysis.  The solubility and stability of HAuCl4 in 10% HCl/water (v/v) is excellent.  By using the chloride salt complications that occur from heating freshly prepared solutions of Au dissolved in HCl/HNO3 to rid them of the nitrogen sub-oxides are avoided.

Inorganic Ventures has and is continuing to study the chemistry of Au, its stability in various matrices, mixtures with other ions, container materials, and concentrations.  Gold’s ability to stabilize ppb Hg solutions is of particular interest.

 

Sampling and Handling 

Gold is not abundant in the environment.  Contamination is not typically a risk and is associated mainly with earlier activity.  The following are typical Au levels:

  • Earth’s crust – 0.004 μg/g
  • Seawater – 1 to 100 pg/L
  • Biological materials – 0 to 5 ng/g in plants.  Normal levels of Au in clinical samples are <1 ng/g but may be 1000 times greater in patients undergoing therapy with gold compounds.

Au contamination risk is low.  However, the following precautions should be considered due to the very low levels typically found in samples:

  • Tools that pulverize, mix, cut, etc. should be cleaned to avoid contamination from sample carryover.
  • PFA and PTFE Teflon may contain Au from prior use and should be cleaned by heating with HCl/HNO3.
  • The collection of biological samples is most difficult since they are at the greatest risk of contamination due to the very low sub-ppb levels of Au sought.  (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 plastic ware 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.

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 


The Metal and Alloys

Metal – In the analytical laboratory the most commonly used reagent for dissolving Au° is freshly prepared aqua regia (3:1 v/v HCl/HNO3).  Au° is not attacked by any of the mineral acids alone but is attacked in combination with an oxidizing agent.  Gold metal is not tarnished or affected in any way by H2O or H2S, and neither HCl nor HNO3 alone attack it.  Chlorine (Cl2) as a gas or in aqueous solution converts the metal to AuCl3.  Bromine water forms AuBr3.  When finely divided it dissolves in HI if aided by O2 and KI forming KAuI4.  NaCN in the presence of air reacts with Au° to form Au(CN)2-1.  Warm concentrated H2SO4 in the presence of KMNO4 dissolves Au°.  Au can be fused with alkali nitrates, forming Au(OH)3 which is soluble in dilute HCl.  In an attempt to remove contamination from crucibles made from either Ni, Fe, or Ag during alkali fusions of Au and platinum group metals, Inorganic Ventures is currently researching a NaOH/NaNO3 fusion using glassy carbon crucibles (details of which will published on the IV website when available).

Alloys – Mixtures of HCl and HNO3 where the HCl is in excess is recommended along with the use of Teflon lined bombs and elevated temperatures.  For example, 0.2 to 0.5 grams of sample + 17 mL conc. HCl + 1 mL conc. HNO3 in 32 mL Teflon lined bomb kept at 250 °C for 24 hours.

 

Oxides, Minerals and Ores

Oxides – The oxides and hydroxides are soluble in dilute HCl.

Ores and Minerals The are many techniques that have been reported.  The following link to “Sample Digestion Methods for the Determination of Traces of Precious Metals

by Spectrometric Techniques is recommendedThis is an excellent review paper discussing the platinum group metals and Au sample preparation methods: 

Balcerzak, M. (2002) Analytical Sciences, 18, pp 737 - 750

This review compares the following sample preparation methods that are commonly used for the PGMs and Au:

1.     fire assay

2.     wet chemical acid digestion

3.     oxidizing fusion

4.     chlorination

The reader is encouraged to consult this paper for detailed information on these sample preparation methods.

 

Organic Matrices

Ashing of organic materials, foodstuffs, plant, and blood and sewage sludge as a preliminary decomposition step can be expedient for samples containing Au.  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.  Acid digestions using nitric, perchloric and sulfuric acids are also suggested.  Wet ashing is suggested for oil and petroleum products using sulfuric acid in combination with magnesium nitrate as an ashing aid (also at temperatures below 450 °C).  If sulfuric acid is added to a petroleum sample then slowly heat on a hot plate 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:

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

Although Au is not listed in the scope for EPA Methods 3050A and 3050B (Open Vessel Acid Digestion) and 3051 and 3052 (Microwave Assisted Acid Digestion) it is suggested that these methods should be explored for environmental samples (sediments, sludges, soils and oils). 

Samples containing mid to low ppm levels of Au can be digested with nitric/HCl.  For more detailed information about acid digestions of organics please see the following article: 

http://ivstandards.com/tech/reliability/part12.asp