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Samples Containing Silver

General Information

Occurrence – Silver (Ag) is the second most abundant of the group 11 elements (Cu, Ag, Au) which are also known as the “coinage metals”.  With an abundance in the earth’s crust of 0.08 ppm, Ag is about 20x more abundant than Au and several orders of magnitude less abundant than Cu.  Silver is found in nature as the free metal as well as bound to insoluble counter ions with sulfide being the most common.  In Peru lumps of the metal as large as 800 lbs. have been found.  Its occurrence as the free metal is the chief reason it has been known and used by humans since prehistoric times.  Ag has been used as a standard of value where its earliest use predates all historical records.  In modern times copper is added to silver coins to increase their durability (10% Cu in USA and 7.5 % in Britain).  In addition to the metal Ag is also found in minerals as a bromide, chloride, iodide, arsenide, antimonide, and sulfide (plus other thio- salts).  The more important minerals are argentite, hessite, proustite, pyrargyrite, cerargyrite, and native silver.  In addition, Ag is commonly found as a trace component in copper and lead ores from which significant Ag production occurs.

Uses – Pure Ag° is the whitest of the metals giving a beautiful luster when polished.  Like the other group 11 elements Ag° exceeds most elements in malleability, ductility and heat conductivity and exceeds all elements in electrical conductivity.  It is harder than Au but softer than Cu.  It alloys easily with Cu and Au and many other elements to form a wide variety of alloys.  Silver’s group 11 cousins (Cu, Au) are similarly known for their usefulness as coin metals.  Ag has a broad range of applications but is best known for its use in tableware (sterling silver), electronics, coinage, medicine, photography, dentistry, and jewelry.

Chemical Properties – Silver has +1, +2 and +3 oxidation states.  The most stable and common oxidation state for Ag is +1.  The trace analyst is unlikely to encounter anything other than silver in the +1 oxidation state (Ag+1), since this is the only stable oxidation state produced when acid digestions and other common sample preparation chemical operations are performed.  Ag+2 is very unstable requiring very strong oxidizing agents; i.e., the reaction Ag+2 + e = Ag+1 has a reduction potential of +1.97v vs. N.H.E. in acidic media.  Ag2+ in nitric acid is prepared from the reaction of Ag+1 with ozone (O3).  The formation of the +2 state in alkali is possible at much lower energies where the reduction potential is only +0.57v vs. N.H.E., but preparations such as caustic fusions that remain at a high pH for measurement are not common or recommended for trace Ag determinations (although if Ag+2 is present it is likely to present fewer problems than the +1 state as you are about to see).

Silver has several chemical properties that are noteworthy:

  • Ag+1 in acidic media reacts with Cl-1 at comparable concentrations to form insoluble AgCl, and with excess Cl-1 can form soluble AgCl2-1.  All silver halogen salts are reduced to the metal when exposed to light (photo-reduction).  The metal tends to deposit on the container surface.  Systematic errors due to loss and memory/contamination tend to be most problematic for the trace analyst.
  • The principal hydrolysis product is brown Ag2O which is formed when alkali is added to a silver Ag+1 solution in dilute HNO3 and water.  Surprisingly, the solid hydroxide, AgOH, is unstable existing only a short time as a transient compound.

2Ag+1   +  2OH-1   →  2AgOH →  Ag2O↓  +  H2O

  • Silver carbonate does not precipitate in air saturated solutions.
  • Silver does not form polynuclear silver-hydroxide compounds.
  • Silver is easily reduced to the metal and electroplating is a common practice on both an analytical and commercial scale.
  • Ag+1, which is the only cationic form of silver in dilute nitric acid, forms more insoluble salts than any other metal.

The following is a summary of solubilities for common silver salts:

Solubility of common silver salts at room temp. (~22 °C)

Salt

Solubility in g/100g H2O

Acetate

1.04

Arsenate

0.085

Arsenite

0.00115

Borate

0.905

Bromate

0.196

Bromide

0.014

Carbonate

0.105

Chloride (most common)

0.0154 (~1.4 ppm Ag+1 and 0.5 ppm Cl-1)

Chromate

0.00256

Cyanide and thiocyanate

0.022

Ferricyanide

0.066

Fluoride

172

Iodate

0.00503

Iodide

0.028

Nitrate

216

Oxalate

0.00378

Oxide

0.00248

Phosphate

0.064

Sulfate

0.83

Sulfide

0.0174

Tartrate

0.0201


The big problem is chloride
– The use of nitric acid and/or HF is preferred for preparation of samples for Ag analysis.  Solutions of Ag in either acid are stable for extended periods.  However, trace levels of HCl or Cl-1 must be eliminated otherwise a fixed error due to AgCl precipitation will result.  With ≥1.4 ppm Ag concentrations even 0.5 ppm levels of chloride will be a problem.  Avoiding these levels of chloride in a trace metals laboratory where HCl is a common and necessary reagent is very difficult if not impossible from a practical point of view.  Many analysts experience low Ag recoveries when working in HNO3 due to trace chloride contamination.  Although analysts are aware of the problems with chloride, they are puzzled because no AgClis observed. However, these low AgCl levels are difficult to spot and have likely already photo-reduced onto the container walls, i.e.:

 AgCl (or AgCl2-1)  +  hv  =  Ag°↓  +  Cl-1

The most common solution to this problem is to work in HCl where the following reaction occurs:

 AgCl↓  +  Cl-1  =  AgCl2-1 (soluble)

You still have the problem with photoreduction to the metal, but elimination of light exposure is considered more achievable than elimination of trace chloride.

Consequently, most analysts are forced to work in a matrix containing HCl levels of 1% v/v or greater.  If HCl must be included in your analytical sample preparation, which is often the case, then these tips should prove helpful.

  1. If the sample preparation requires the use of HCl, keep the HCl content high (2-10% v/v).
  2. Keep the Ag+1 concentration low (0.5 µg/mL).
  3. If the Ag+1 concentration must go above 10 µg/mL the HCl concentration must increase to >10% (v/v).  Add HCl generously.
  4. Keep the exposure to light at a minimum.
  5.  LDPE bottles wrapped in tape are a better option than HDPE or amber HDPE due to impurities from the amber pigment and the HDPE.  There is sufficient chloride from the catalyst residue in the HDPE to cause a problem.
  6. The solution should be kept for no more than 5 months.

When working with HCl it cannot be over emphasized that a high HCl content (>1% v/v) is needed to keep the Ag in solution as the AgCl2-1 anionic chloride complex.  In short, keep the HCl concentration high and the Ag concentration low.  And remember that solutions containing suspended AgCland/or the AgCl2-1 anionic chloride complex are photosensitive. The Ag+1 will undergo photo-reduction to the metal (Ag°).  When intentionally working in HCl minimize exposure to light.

 At Inorganic Ventures – Inorganic Ventures uses metallic Ag° 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 high purity HNO3.  Impurities for each lot (measured using ICP-MS and ICP-OES) are reported on the Certificate of Analysis.  The most common matrix for Ag CRMs manufactured at IV is HNO3.

Acids other than HNO3 require the use of an oxidizing agent such as H2O2 (trace metals grade that is unstablized) before dissolution can be effected.  On special occasions high purity AgNO3 is used which avoids certain problems arising from sub-oxide impurities left from HNO3 acid.  The solubility of AgNO3 in water is 216g / 100 g H2O at 20 ºC making it the most soluble (yields the most Ag per gram of salt) of the common Ag salts.  Complications that occur from reactions with other solution components with easily reduced metals are avoided by heating the freshly prepared Ag in HNO3 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 Ag in the sample.  If the sample type is typically low in Ag 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 Ag along with expected levels:

  • Earth’s crust – 0.08 μg/g
  • Seawater – from 0.0001 μg/L to 1 μg/L
  • Potable water – max 10 μg/L
  • Hazardous waste – up to 5 mg/L
  • Soils – commonly found range 0.01 – 5.0 μg/g
  • Biological materials – mammal flesh 4-24 μg/kg dry weight; plants 0.03-0.5 μg/g
  • Foodstuffs – up to 10 μg/g

Silver contamination risk is low from most grinding equipment, storage containers, and the analyst but it is high from previously used laboratory glassware and plasticware that has been exposed to Ag solutions.  Glassware and plasticware that has been exposed/used to contain Ag solutions will contain Ag through either chemisorption or photoreduction.  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. can used.  They can be made of stainless steel, ceramics, silica/quarts, and polymers.  Geological samples should be crushed prior to subsampling and then powdered to <150µm.  Clean this equipment between samples by grinding an uncontaminated sand sample to avoid cross contamination.  Samples can be stored in sealable plastic bags.   
  • New PFA and PTFE Teflon is low risk but used Teflon should be carefully cleaned by heating with 1:1 HNO3.
  • Water samples should be acidified to at least 1% v/v HNO3, made of LDPE (to avoid chloride contamination) and made ‘light tight’ by wrapping with tape to avoid Ag photoreduction.  Storage in a dark place is recommended.
  • All plasticware should be leached at 60 ºC with dilute 1% HNO3 and rinsed with 18 MΏ water.

Silver contamination from the air is not a problem, but contamination from HCl fumes is. As noted above, the presence of HCl in an Ag solution can create a number of problems.  At Inorganic Ventures we have conducted studies exploring the effect of HCl upon Ag CRMs from which the following conclusions were drawn. 

  1. Fumes from conc. HCl (35 wt.%) will penetrate LDPE and TCT bags.
  2. Never open a bottle of concentrated HCl in the laboratory.  Only open such containers in a fume hood.
  3. Never work with solutions of Ag in a hood with an open container of concentrated HCl, even if the Ag solution is in a closed bottle and in TCT packaging.
  4. Open solutions of diluted HCl at concentrations as high as 1:1 (conc./water) will not be a problem with Ag solutions in a closed container over a period of several days.  However, open containers of Ag are a problem, as indicated by a summary of one of our exposure experiments detailed below:

Experimental setup – A open bottle of 1:1 HCl was placed in a glove box with an open beaker containing 1000 ppm Ag, and a closed 125mL LDPE bottle also containing 1000 ppm Ag.

Upon checking the 1000 ppm Ag solutions after 1 day no precipitate was observed in either the open beaker or the sealed bottle (Photo 1).

Taken after 1 day of exposure; all is clear.

Photo 1. Taken after 1 day of exposure; all is clear.

However, after 13 days it was observed that a precipitate had formed in the open beaker (Photo 2), but the solution in the sealed bottle was still clear.

Taken after 13 days of exposure. There is no indication of a ppt in the bottle of sealed 1000 ppm Ag after sitting in the glove box with an open bottle of 50%  (v/v) HCl.

Photo 2: Taken after 13 days of exposure. There is no indication of a ppt in the bottle of sealed 1000 ppm Ag after sitting in the glove box with an open bottle of 50%  (v/v) HCl.

The risk of contamination is low for Ag.  However, trace analysts must be made aware of the high risk of contamination from used laboratory glassware and the problems from HCl contamination.  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 Ag containing solutions, as well as suggestions for ICP analyses of silver, may be found by clicking on the Ag element symbol at:

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

The Metal and Alloys

Metal – HNO3 is the best “solvent” for Ag°.  HNO3 reactions will vary depending upon dilution with water and when very dilute the HNO3 will lose its oxidizing ability.  A 2:1 dilution in water is recommended where the following reaction occurs:

 Ag°  +  2HNO3  ↔  AgNO3  +  NO2 (brown fumes)  +  2H2O

 Ag° is soluble in hot 80% (v/v) H2SO4 in the presence of air (O2).

Alloys – Many alloys can be dissolved using 2:1 HNO3 / H2O.  Alloys containing the “HF” elements will dissolve in a 1:1:1 mixture of concentrated HNO3 / HF / H2SO4

Oxides, Minerals and Ores

Oxides – The oxides are soluble in dilute acids.  Dilute HNO3 is most popular.  If the oxides are somewhat resistant to dilute acid attack, then a fusion with sodium carbonate should be used first.

Ores – An acid dissolution can be performed in HF+HNO3+HClO4.  Ores can also be fused with disulfate.

Organic Matrices

Ashing of organic materials, foodstuffs, plant, and blood and sewage sludge as a preliminary decomposition step can be expedient for samples containing Ag.  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 Ag° when organics are present and refractory metal oxides of many elements which can be very difficult to dissolve in dilute acids (Ag° 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 uses sulfuric acid as an ashing aid.  Do not use ashing temperatures above 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

Silver 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 Ag 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://ivstandards.com/tech/reliability/part12.asp