Leonid Kozin is a leading Ukrainian electrochemist specialising in chemical technology, physical chemistry, electrochemistry and electro-hydrometallurgical treatment methods to achieve high-purity metals. He studied chemistry at the Kiev Polytechnic University and graduated as an Electrochemical Manufacturing Process Engineer. From 1961-1980 he worked at the Academy of Sciences Kazakhstan where he gained his PhD. He has been Professor of Electrochemistry since 1971 and achieved many prizes for his work on high-purity metals. Author of several books, papers and patents, he is now Head of Department at the Institute of General and Inorganic chemistry at the Ukrainian National Academy of Sciences in Kiev. Steve Hansen completed his BSc in Physical Metallurgy at Washington State University in 1980 and went on to do an MSc at the University of Illinois and a PhD in Materials Science at the University of Wisconsin-Madison. He is now a Senior Research and Development Engineer for APL Engineered Materials, Inc and has several books, patents and journal papers published on amalgam design and metallurgy and metal halide systems in his name.

Mercury Handbook

Chemistry, Applications and Environmental Impact

By Leonid F Kozin, Steve Hansen

The Royal Society of Chemistry

Copyright © 2013 L F Kozin and S C Hansen
All rights reserved.
ISBN: 978-1-84973-409-7

Contents

Chapter 1 Physicochemical Properties of Metallic Mercury, 1,
Chapter 2 Amalgam Solubility, 36,
Chapter 3 Diffusion of Metals in Mercury, 50,
Chapter 4 Purification of Mercury Using Chemical and Electrochemical Methods, 61,
Chapter 5 Chemical Properties of Mercury, 80,
Chapter 6 Electrochemical Properties of Mercury, 128,
Chapter 7 Lighting, 143,
Chapter 8 Synthesis of Semiconducting Compounds, 163,
Chapter 9 Chlor-Alkali Process, 180,
Chapter 10 Use of Mercury in Small-scale Gold Mining, 193,
Chapter 11 Mercury Legislation in the United States, 199,
Chapter 12 Environmental Aspects of the Industrial Application of Mercury Leonid F. Kozin, Steve C. Hansen, Nikolai F. Zakharchenko and Jason Gray, 209,
Chapter 13 Demercurization Processes in Different Sectors of Industry Leonid F. Kozin, Steve C. Hansen and Nikolai F. Zakharchenko, 228,
Chapter 14 Safety and Health Practices for Working with Metallic Mercury Woodhall Stopford, 241,
Appendix I Phase Diagrams and Intermetallic Compounds in Binary Amalgam Systems, 248,
Appendix II Density and Surface Tension of Binary Amalgams, 272,
Appendix III Inorganic and Organic Mercury Compounds, 282,
Appendix IV Selected Organometallic Compounds of Mercury, 297,
Appendix V Solubility of Common Metals in Mercury, 300,
Subject Index, 312,

CHAPTER 1

Physicochemical Properties of Metallic Mercury

1.1 Atomic Properties

High-purity mercury is a dense, silvery white liquid with an extraordinarily low melting point of 234.321 K or -38.829 °C. Mercury is a metal from Subgroup IIB of the Periodic Table and is related to zinc and cadmium. Mercury has the atomic number 80, atomic mass 200.59 and atomic volume 14.26×10-6 m3 mol-1 at 298 K. Its electronic configuration

[MATHEMATICAL EXPRESSION OMITTED]

(or more simply, Xe shell 4f145d106s2) qualifies it as a non-transition metal. It has valence states of +1 and +2. Natural mercury consists of seven stable isotopes3 and 17 synthetic and radioactive isotopes with mass numbers 185–206. The natural mercury isotopes have the following mass numbers and abundances:

Isotope Abundance (%)
196 0.146
198 10.02
199 16.84
200 23.13
201 13.22
202 29.80
204 6.85

The isotope with mass number 194 (194Hg) has a half-life of 130 days, 203Hg 47 days and 199Hg 2.4×10-9 s. Isotopes of mercury can be obtained through the following reactions:

[MATHEMATICAL EXPRESSION OMITTED] (1.1)

[MATHEMATICAL EXPRESSION OMITTED] (1.2)

[MATHEMATICAL EXPRESSION OMITTED] (1.3)

[MATHEMATICAL EXPRESSION OMITTED] (1.4)

The thermal neutron capture cross-section for natural mercury is 380 [+ or -] 20 barn. Atomic and ionic ionization potentials (φ) and their radii are as follows:

[MATHEMATICAL EXPRESSION OMITTED]

The energy required for electron shell transfer from the basic state 6s2 (i.e. transfers 6s2 -> 6s1p1) is fairly large (524.26 kJ mol-1) and demonstrates the chemical inertness of metallic mercury. Moreover, the high 6s->6p transfer energy gives evidence that mercury tends to form two covalent bonds (and, as a result, further bonding of ligands is difficult). In contrast with the high energy of s,p transfer, 5d10->5d9s1 and 5d10-5d9p1 transfers in Hg2+ ions require a very low energy of 5.3 and 14.7 eV, respectively. The electron affinity for α-mercury (α-Hg) is 1.54 eV, for β-Hg it is 1.37 eV and the electron work function is 4.52 eV.5 The electronegativity of mercury, according to different authors, is given in Table 1.1.

The atomic, covalent and ionic radii of mercury are given in Table 1.2.

1.2 Crystallography

Solid mercury has a rhombohedral structure (α-Hg) with the lattice parameters a = 0.29925 nm (2.9925 Å), β = 70° 44′ 60′. Each atom of mercury is surrounded by six neighboring atoms at a distance of 0.300 nm and six other atoms at a distance of 0.347 nm.

1.2.1 P–T Diagram

The P–T diagram for mercury is given in Figure 1.1.

α-Hg: Mercury is a liquid under ambient conditions but crystallizes into the α-Hg structure at room temperature upon compression to 1.2 GPa.

β-Hg: The α-Hg structure transforms to the β-Hg structure at 3.4 GPa at room temperature. The β-Hg lattice, formed at temperatures below 79 K, is a body-centered tetragonal structure with lattice parameters a = 0.3995 nm, c = 0.2825 nm. The space group of β-Hg is I4/mmm. Swenson determined the enthalpy of the solid-state transition α-Hg->β-Hg to be ΔHα->β = 122 J mol-1, the volume change to be ΔVα->β = -0.21 cm3 mol-1 and the entropy of transformation to be ΔSα->β = -1.54 J K-1 mol-1 at -194 °C and 0.101 MPa.

γ-Hg: According to Takemura et al., the structure of γ-Hg is monoclinic, C2/m. It forms at 12 GPa, with six atoms in the unit cell. Each mercury atom is coordinated by 10-11 atoms.

δ-Hg: The structure of hexagonal close-packed (hcp) δ-Hg forms at pressures above 37 GPa and is reported to be stable up to 193 GPa. At 193 GPa, the lattice parameters are a = 0.2612 nm and c = 0.4284 nm, which give c/a = 1.640. The c/a ratio of mercury under high pressure decreases from 1.75 at 50 GPa to 1.64 at about 200 GPa. Yan et al. also reported results on the high-pressure behavior of mercury.

Abell and King performed mechanical tests on solid mercury below 77 K. Solid mercury turns white and becomes ductile at low temperatures and was found to recrystallize at ~160 K. Slip in single crystals of mercury was studied by Rider and Heckscher. At 90 K, solid mercury deforms by slip and twinning. Plastic deformation of Hg single crystals has also been studied.

At the melting temperature, the structure of liquid mercury is close to that of solid mercury. Each atom is surrounded by six other atoms at a distance of 0.307 nm. Thus, at the melting temperature, the coordination number in the liquid is 6, the same as for solid mercury. With increase in temperature, the mercury coordination number increases as follows:

T (K) Coordination No.
234.9 6.0
296 8.2-8.3
301 10-11

1.3 Melting Point

Mercury is the lowest melting point metal. Its melting point, measured by different groups, is given in Table 1.3. The data indicate the high purity of the samples studied. With increase in pressure, the melting point of mercury shifts towards higher temperature, dT/dP = 42.44-49.84 K GPa-1. Between 1.01325 and 6.0795 GPa, the melting point of mercury increases from 286.9 [+ or -] 0.3 to 515 K.

1.4 Heat of Fusion

The heat of fusion (ΔHfusion) of mercury, according to different sources, is given in Table 1.3. The heat of fusion increases with increase in pressure. At a pressure of 1.01325 MPa the heat of fusion is 2623 [+ or -] 83.7 J mol-1 and at 2.0265 GPa it is 2958 [+ or -] 83.7 J mol-1.

1.5 Heat Capacity

The heat capacity of mercury has been studied over a broad range of temperatures. The dependence of the specific heat capacity of mercury on temperature is shown in Figure 1.2. The heat capacity of solid mercury was determined by Regel and Glazov with 257 experimental points and by Busey and Giaque. In the temperature range 150.90–233.79 K, the heat capacity curve is represented by two temperature ranges:

171.03

213.20

In the first range, the molar heat capacity of solid mercury is described by the equation

Cp = Cvibr + Cel + Can + Cvac (1.5)

where Cvibr is the lattice vibration contribution, Cel is the electronic contribution, Can is the anharmonic contribution and Cvac is the vacancy contribution. The sum of the lattice vibration contribution is calculated using the equation

Cvibr = 3R[1 – 0.05(ΘD/T)2] (1.6)

where ΘD is the Debye temperature, which for α-Hg is 79 K. Cel is the molar electronic contribution:

Cel = γT (1.7)

where γ is a constant equal to 1.81 mJ mol-1 K-2.39Can is the anharmonic component of Cp:

Cel + Can = BT + DT2 (1.8)

where B and D are constants. Cvac is the vacancy contribution:

Cvac = (LU20/RT2)exp(-U0/RT) (1.9)

where U0 is the vacancy formation energy. The constants B, D and L in eqns (1.8) and (1.9) are found through the least-squares analysis of the U0-T relationship in a given range of values. Experimentally obtained values of molar heat capacity of solid and liquid mercury are given in Tables 1.4 and 1.5. Constant-pressure heat capacity values at very low temperatures, below 20 K, were measured by van der Hoeven and Keesom and others. Van der Hoeven and Keesom measured an electronic specific heat coefficient of 1.79 [+ or -] 0.02 mJ mol-1 K-2.

Analysis of the data in Table 1.5 reveals that in the temperature range 140–234 K, when approaching the melting temperature, the heat capacity of mercury increases non-linearly with increase in temperature. The heat capacity of mercury at high temperatures [Figure 1.3] does not differ much from the classical value (Cp/3R = 1.13), which is due to the small effect of the anharmonic and electronic contributions.

1.6 Thermal Conductivity

The thermal conductivity of solid mercury is anisotropic. The thermal conductivity of mercury single crystals on the trigonal axis (λ[parallel]) and perpendicular to it (λ[perpendicular to]), in the temperature range 80–234.288 K, is described by eqns (1.10) and (1.11), respectively.

λ[parallel] = (44.8 – 0.0237T)Wm-1K-1 (1.10)

λ[perpendicular to] = (31.4 – 0.0279T Wm-1K-1 (1.11)

The thermal conductivity of liquid mercury, shown in Figure 1.4, has been extensively studied. The main contribution to the thermal conductivity of liquid mercury is made by conduction electrons. Therefore, the main heat flux in metallic mercury is transmitted, as in other metals, by conduction electrons.

The ratio of thermal conductivity, λ, to electrical conductivity, σ, at a given temperature is called the Lorentz number, L:

L = λ/σT (1.12)

Lorentz numbers calculated for the main axes of mercury single crystals agree within 3%. Table 1.6 gives the values of the Lorentz number at different temperatures.

1.7 Emissivity

The emission coefficient, ελ, of mercury from a smooth non-oxidized surface is 0.10–0.12. However, the reflectivity of polished solid mercury and a liquid surface, χ, for light flux of wavelength λ is as follows:

Form Wavelength, λ (µm) Reflectivity, χ (%)
Solid 0.45–0.70 72.3–72.8
Liquid 0.75–1.00 77.3–77.9

1.8 Boiling Point, Heat and Entropy of Vaporization

The boiling point of mercury (Tboil) has been reported in the literature with an accuracy of 0.01–0.08 °C. Experimental results along with the heat of vaporization are given in Table 1.7.

According to Hultgren et al., mercury vapor is best described as a non-ideal monomer.

Values for the heat of evaporation (ΔHevap) and entropy of vaporization (ΔSevap) also depend on pressure. Table 1.8 summarizes ΔHevap and ΔSevap values at different pressures. Thermodynamic values for the sublimation of mercury at 234.288 K are ΔHsubl = 64.1784 [+ or -] 0.06276 kJ mol-1 and ΔSsubl = 273.926 J mol-1 K-1.

1.9 Vapor Pressure

Studies of the temperature dependence of mercury vapor pressure were summarized by Huber et al. Diatomic molecules of Hg2 were found in mercury vapor. Hg2 molecules oscillate at ~36 cm-1, their internuclear distance is 3.34×10-10 m and their dissociation energy is 7.53 [+ or -] 2.09 kJ mol-1. Values of the heat of sublimation of monatomic and diatomic molecules are 61.304 [+ or -] 0.063 and 103.64 kJ mol-1, respectively. The heat of dimerization of mercury is 8.008 kJ mol-1. A small energy of dissociation of Hg2 molecules in the vapor causes gaseous mercury to be virtually monatomic and to have significant vapor pressure even at low temperatures. The thermodynamic properties of Hg2 molecules were also studied by Hilpert.

1.9.1 Solid Mercury

Measurements of the vapor pressure over solid mercury are relatively scarce. Values obtained by Poindexter are given in Table 1.9.

The vapor pressure over solid mercury is given by

[MATHEMATICAL EXPRESSION OMITTED] (1.13)

The saturated vapor pressures over a broad range of temperatures for solid and liquid mercury are fairly consistent. Analysis of experimental data in coordinates of lnPHg–1/T demonstrated good agreement between data from different authors. The most accurate results are shown in Figure 1.5. The lnPHg–T curve in Figure 1.5 also shows the triple point, boiling point and critical temperature.

1.9.2 Liquid Mercury

Based on a great amount of data, Nesmeyanov suggested the following relationship between vapor pressure and temperature for liquid mercury up to 673 K (400 °C):

[MATHEMATICAL EXPRESSION OMITTED] (1.14)

The vapor pressure of mercury according to the literature in the temperature range 298–629.810 K (Tboil) can be determined using the equation

(logP (Pa) = 5.00572 + (10.355 – 0.795 × logT – 3305/T) (1.15)

A more elaborate vapor pressure equation for mercury was suggested by Huber et al. Very accurate experimental measurements of the vapor pressure of mercury were performed by Beattie et al. from 623 to 636 K and by Spedding and Dye from 534 to 630 K. The normal boiling point of mercury was determined by Beattie et al. as 629.7653 [+ or -] 0.0016 K on the ITS-90 international temperature scale. Table 1.10 gives the vapor pressure of mercury up to its normal boiling point and Table 1.11 above the normal boiling point.

At very high temperatures, a small but noticeable change in slope on the vapor pressure curve occurs. A metal-non-metal transition occurs at ~1360 K (1087 °C).

1.9.3 Triple Point

When analyzing the thermodynamic parameters of mercury, it was found that the triple point is located at 234.3156 K with a vapor pressure of PHg = 0.157 Pa (1.55×10-6 Torr). This value is close to the values calculated by means of eqns (1.18) and (1.19), namely 3.33×10-4 and 3.35×10-4 Pa (2.510×10-6 and 2.514×10-6 Torr), respectively. Table 1.12 gives experimentally determined values of the triple point of mercury.

1.9.4 Critical Temperature and Pressure

Table 1.13 gives measurements of the critical temperature and pressure of mercury. The coexistence curve of liquid and gaseous mercury is shown in Figure 1.6.

1.10 Density

The density of mercury has been extensively studied. Literature values of the density of solid and liquid mercury are listed in Table 1.14 and plotted in Figure 1.7.

Solid mercury at 234.25 K has a density of 14.193 g cm-3, and liquid mercury at 234.288 K has a density of 13.691 g cm-3. The change in density at the liquid-solid transition is +3.5-3.7% (compared with the density of solid mercury). The volume change upon solidification is given in Table 1.15. Further studies on the density of mercury have been reported.

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