General Information

Occurrence – Mercury is not abundant, nor widely distributed. It’s abundance in the earth’s crust is 0.085 ppm. It is found in the upper portions of cinnabar deposits, a mineral HgS, the chief source of the element. Hg has been found in quartz, sandstone, schists, iron pyrites, bituminous substances, eruptive, and sedimentary rocks of all ages. It occurs as the chloride in horn silver. It is occasionally associated with Zn ores and is generally found locally concentrated. The minerals that are more common are cinnabar, HgS; calomel Hg₂Cl₂; coloradoite, HgTe; amalgam, Ag·Hg; livingstonite, HgSb₄O₇; and tiemannite, HgSe.

Uses - Metallic mercury dates back to 1600 B.C. Aristotle called it “fluid silver”. Hg was used in the application of Gold leaf (gilding). The sulfide was used as a pigment. Current uses of Hg include thermometers and barometers, gas containment, vacuum pumps, Hg vapor lamps, in amalgams, with metals like Ag, Sn, and Na (dentistry, reducing agents, Au extraction from ores), treatment of skin diseases, antiseptics, fungicides, and related agents. Metallic mercury is an excellent electrode material in electrochemistry. The overpotential for H₂↑ evolution is high extending the potential usable range by over 1.0 volts vs. SCE over a Pt electrode. It is used as a coating on solid electrodes such as Pt or as a Dropping Mercury Electrode (DME) first used in controlled potential voltammetry in the 1920s.

Hg Chemistry as Practiced & Observed at IV

The group 12 elements do not form ions with incomplete d shells (their electronic structure is (p_eroid-1) d¹⁰ p_eroids²). Unlike Zn and Cd which only have the +2 oxidation state due to the stability of the filled d orbital, Hg has been assigned the +1 as well as the +2 oxidation state. However, due to its electronic structure [ Hg (5d¹⁰ 6s²) → Hg⁺² (5d¹⁰)] and the stability of the filled 5d¹⁰ , it appears that Hg⁺¹ either does not exist or only exists as a diatom (Hg:Hg)⁺² which contains :Hg° and Hg⁺² rather than two ·Hg⁺¹ atoms (here the 6s² electrons are represented as a colon “:” and the 6s¹ electron as “·”) .

An understanding of the chemistry of Hg is important before attempting to make accurate analytical measurements:

It is accepted that Hg⁺² reacts with atomic Hg as shown:

(1) Hg₂^{+2 =} Hg⁺² + :Hg° K_eq1 = [Hg⁺²]/ [Hg₂⁺²]

If the diatom contains two Hg⁺¹ atoms, then they would have a spin free electron and be paramagnetic. Each of these electrons (5d¹⁰ 6s¹) are in fact diamagnetic indicating no spin free electron (no 6s¹). In addition to the lack of paramagnetic behavior, Raman spectroscopy has confirmed the existence of a covalent Hg-Hg bond. Consequently, the existence of monovalent Hg does not appear to be the case. This is critical to understanding the chemistry of aqueous Hg solutions.

The stability of the Hg₂⁺² toward disproportionation i.e. the K_eq1, can be calculated as follows using the standard potentials found in any table of half-cell redox reactions:

(2) Hg₂⁺² + 2e = 2Hg ° E = 0.789 v vs. N.H.E

(3) Hg₂⁺² = 2Hg⁺² + 2e E = - 0.920 v vs. N.H.E

(4) = (2) + (3) 2 Hg₂⁺² + 2e = 2Hg⁺² + 2Hg ° + 2e ΔE = - 0.131 v vs. N.H.E

Equation (4) is simply the sum of the half-cells (2) and (3) where (3) is written as an oxidation half-cell thereby requiring a reverse in sign from positive 0.92 to – 0.92 volts. Dividing through both sides by 2 and cancelling out the electrons gives equation (1):

(1) Hg₂⁺² = Hg⁺² + Hg ° ΔE = - 0.131 v vs. N.H.E

K_eq1 = [Hg⁺²]/ [Hg₂⁺²] (The activity of the elemental Hg° is assumed to be = 1. )

The equilibrium constant (K_eq1) is calculated using fundamental thermodynamics as follows:

ΔG = - nFΔE = -RTlnK_eq1

Where:

G = Gibbs free energy

n = 1 ( one gram equivalent of Hg₂⁺² is converted to one gram atomic weight of Hg⁺² and one gram atomic weight of Hg°)

F = Faraday

ΔE = -0.131 v vs. N.H.E

R = Rydberg constant

T = temperature in degrees K

Substituting in values for above

ΔE = RT/nF 2.303 logK_eq1 = 0.0591 log K_eq1

-0.131 / 0.0591= logK_eq1

-2.21 = log K_eq1

K_eq1= [Hg⁺²]/ [Hg₂⁺²] = 6.3 x 10^-3

This value of K_eq1 shows that Hg⁺² (aq.) should react with Hg° to form Hg₂⁺² (aq.), which has been shown to be the case as stated above. Below it will be shown how this equilibrium is used to explain aqueous/acidic Hg chemistry.

Manufacturing Hg Aqueous/Acidic CRMs Hg CRMs in aqueous nitric acid are manufactured at IV from the metal (Hg⁰) in a borosilicate glass reactor. The reaction is as follows:

(5) Hg⁰ + HNO₃ → Hg⁺² + NO₂↑ (brown gas) + xs HNO₃ + heat to expel NO₂ → Hg⁺² + xs HNO₃ → Dilute with H₂O to make stock products/concentrates

Reaction (5) is heated until no more brown fumes are observed, and the solution appears water white. This Hg solution is assayed by two methods namely EDTA titration and ICP. The concentration of Hg is also calculated from the weight of the metal and weight of final solution. When the ‘brown’ fumes are not removed from the solution the EDTA value is low relative to the ICP value due to the reduction of the Hg⁺² to Hg⁺¹ which is shown as reaction (6) below:

(6) 2H₂O + 2NO₂ (dissolved/brown) + 2Hg⁺² → Hg₂⁺² + 2NO₃^-1 + 4H⁺¹

2Hg⁺² + 2e = Hg₂⁺² +0.920 (v vs. NHE )

2[NO₃^-1 + 2H⁺¹ + e = NO₂ + H₂O] + 0.80 (v vs. NHE )

2H₂O + 2NO₂ (dissolved) + 2Hg⁺² → Hg₂⁺² + 2NO₃^-1 + 4H⁺¹ (∆E = +0.12 v)

The positive ∆E shows that this reaction is thermodynamically favorable i.e. the Gibbs free energy (∆G = - nF∆E) is negative.

The Mercury (I) Ion As stated in inorganic text books such as Cotton and Wilkinson , the mercurous Hg(I) ion exists in a binuclear state and is expressed in chemical reactions as Hg₂⁺². The binuclear ion is not stable toward disproportionation according to the following reaction (1) which was also discussed above:

(1) L + Hg₂⁺² = HgL⁺² + Hg⁰

Reaction (1) occurs when a complexing or precipitation agent (L) comes in contact with the divalent-diatomic ion forming a complex or precipitate more readily with Hg⁺² than with Hg₂⁺² (which is most of the time). Chloride, which forms Hg₂Cl₂, is one of the few examples where disproportionation of the Hg₂⁺² may not occur when it comes in contact with the chloride ion but the extent is dependent upon HCl concentration and solution acidity.

QC As discussed above, the bulk solution of Hg⁺² (made from the metal…20K, 10K, 1K) is titrated with EDTA and is assayed by ICP.

The EDTA (L^-4) titration:

(7) Hg⁺² + L^-4 = HgL^-2 (Log K₁ = 21.8)

If Hg₂⁺² is present the EDTA will react with the mercury (II) diatomic divalent ion in such a way that it will disproportionate to divalent mercury and elemental mercury which is shown below as follows:

(8) Hg₂⁺² + L^-4 = HgL^-2 + Hg⁰

The ICP assay:

The ICP assay value will not be in error so long as it does not disproportionate giving elemental Hg. Elemental Hg which will give false high results by ICP due to what we refer to as a vaporization interference. Therefore, if the diatomic divalent mercury ion has already disproportionated then the Hg° will be present causing a high result for the ICP assay. This high ICP result paired with a low result for the EDTA can be used to determine if disproportionation has occurred or not. If the ICP assay is equal to the gravimetric value then we know that the divalent diatomic ion is present but has not undergone disproportionation. If there is no difference between the assay values obtained from EDTA titrimetry and ICP and both equal the gravimetric Hg value, then we know that the lot is free of the divalent diatomic ion.

Issues with Hg

Issues with Hg are very common. For example, our QC department experienced an interference issue where an unexpected impurity in a lot of trace-metals grade HCl reacted as follows:

(9) Hg₂⁺² + ?L = HgL^+? + Hg⁰

The elemental Hg⁰ gives a vaporization interference with the ICP-OES measurement making the intensity appear high (this is caused by the high vapor pressure of the elemental mercury causing more to reach the plasma through vaporization that just from nebulization alone). The impurity was later identified as H₃PO₄. These are the types of unexpected problems we need to be prepared for when attempting to measure trace Hg by ICP.

The following are suggestions for avoiding problems with Hg:

• Avoid neutral/basic media  especially amines and alcohol amines.
• Avoid mixing with tartrate  will be reduced to the metallic form.
• Reducing environments can convert Hg⁺² to the Hg₂⁺² dimer or the metallic form.
• Hg₂⁺² dissociates into metallic mercury Hg° and Hg⁺² when a ligand is added.
• Hg⁺² can reduce to Hg₂⁺² if NO₂ gases remain in the container from reaction/production processes.
• Adding HNO₃ and boiling will convert Hg₂⁺² back to the Hg⁺²
• Alternative solution: add 1ppm AuCl₃ to stabilize Hg to avoid adsorption to plastic (EPA bulletin) IV has found this approach to be very effective resulting in ppb Hg solutions stable in excess of one year.
• Hg- is stable in higher HCl concentrations* (~ 10%) where is exists as HgCl₄^-2.

*Concerns with an HCl matrix:

• Cl interference issues for As and Se on ICP-MS  use collision cell (H₂ or He mode)
• If there is any Hg₂⁺² form present it will disproportionate giving elemental Hg° and causing false high results when HCl is added. We recommend boiling with excess HNO₃ prior to adding HCl to make sure dimer is not present
• If Ag is present, excess chloride will need to be added to keep Ag in solution. The solution will be photosensitive toward the photoreduction to Ag° metal.

Sampling and Handling

Sample handling and storage are major problems for reliable trace analysis. The sample itself should be as representative as possible. Hg has been used widely in the industrial world since the early 20^th century. It has been rumored that Michael Faraday 1791-1867) died of Hg poisoning and that Hg metal could been seen in the cracks of his laboratory in England well into the 20^th century. Hg has earned the status of being very toxic and extremely difficult to sample and to store without chemical change of some sort. Common problems include contamination of the sample not only by conventional means such as apparatus and impure reagents but by vapor transfer existing in laboratories where Hg is in the air (an unfortunate problem in many laboratories). Broken thermometers are an all too common source of Hg contamination.

There is more information in the literature for trace Hg analysis than any other element and the literature should be consulted before attempting to collect and store a sample. For more on sample contamination risks see chapters 8, 9, and 10 of the Inorganic Ventures ‘Trace analysis Guide’.

For general information on sampling and sub-sampling refer to Part 3 of our Trace Analysis Guide.

The Metal, Alloys, Oxides and Organic Matrices

Metal - Hg° metal is soluble in HNO₃ with evolution of NO_x and is the preferred solvent.

Alloys - Hgº and its alloys are soluble in H₂O / HCl / HNO₃ mixtures.

Oxides - The hydroxides and carbonates are soluble in dilute HNO₃ .

Organic Matrices - Ashing of organic materials, foodstuffs, plant, and blood and sewage sludge as a preliminary decomposition step that is not recommended for Hg. Dry ashing organic carbon based/containing samples for the trace analysis of Hg is never recommended.

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

Hg is listed in the scope for EPA Methods 3050A / 3050B (Open Vessel Acid Digestion..) and 3051 / 3052 (Microwave Assisted Acid Digestion); these methods are suggested for environmental samples (sediments, sludges, soils and oils).

Samples containing mid to low ppm levels of Hg 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 refer to Part 12 of our Trace Analysis Guide.

Detailed handling information related to Mercury containing solutions, as well as suggestions for ICP analyses of Mercury, may be found by clicking on the Hg element symbol on our interactive periodic table.