Sublimation
Sublimation is the direct transition from the solid state to vapor, and the heat absorbed by it is equal to the sum of the latent heats of fusion and of vaporization.
Introduction
Sublimation is not a procedure that is generally regarded as an analytical technique. It is a process, however, by which compounds can be purified or mixtures separated and as such can be of value as a single step or as an integral part of a more complex analytical method. It is applicable to a range of solids of inorganic or organic origin in a variety of different matrices and can be particularly useful when heat-labile materials are involved.
As a method of sample purification sublimation has been used to produce high-purity materials as analytical standards. A specific and common example of sublimation used as a means of purification is the removal of water from heat-labile materials in the process known as freeze-drying. The technique is described more fully below.
As a separation technique fractional sublimation has been used either to purify samples for analysis by removing undesirable components of the matrix or to remove the analyte from the matrix for subsequent analysis.
4.8 Sublimation
Sublimation is the transition of a substance directly from the solid phase to the gas phase without passing through the intermediate liquid phase (Table 4.8, Fig. 4.2). Sublimation is an endothermic phase transition that occurs at temperatures and pressures below the triple point of a chemical in the phase diagram. The reverse process of sublimation is the process of deposition in which some chemicals pass directly from the gas phase to the solid phase, again without passing though the intermediate liquid phase.
Table 4.8. Phase Transformations From Gas to Liquid to Solid and the Reverse
To: Gas Liquid Solid
From: Gas N/A Condensation Deposition
Liquid Evaporationa N/A Freezing
Solid Sublimation Melting Transformationb
a
Also, boiling.
b
For example, change in crystal structure.
Figure 4.2. Representation of phase changes.
The term sublimation refers to a physical change of state and is not used to describe transformation of a solid to gas in a chemical reaction. For example, the dissociation on heating of solid ammonium chloride (NH4Cl) into ammonia (NH3) and hydrogen chloride (HCl) is not sublimation but a chemical reaction:
NH4Cl → NH3 + HCl
Sublimation requires additional energy and is an endothermic change and the enthalpy of sublimation (also referred to as the heat of sublimation) can be calculated by adding the enthalpy of fusion and the enthalpy of vaporization.
Stone Pavements, Lag Deposits, and Contemporary Landscape Evolution
13.7.4 Sublimation Lag
Sublimation lag is widely occurring in association with the latitude-dependent mantle on Mars (Hauber et al., 2011). This lag forms as a result of the sublimation of the ice-rich latitude-dependent mantle. Ice is removed by sublimation from the uppermost zone of the meters—thick ice and dust layer leaving behind a thick, rocky, lag deposit at the surface (Head et al., 2003; Schorghofer and Aharonson, 2005; Schorghofer, 2007; Levy et al., 2011). Additionally, formation of sublimation lag may be associated with the sublimation of newly exposed ice-rich regolith on the floors of new impact craters, especially in the middle latitudes (Byrne et al., 2009; Dundas and Byrne, 2010; Kossacki et al., 2011)
Glacial and Periglacial Geomorphology
J.B. Murton, in Treatise on Geomorphology, 2013
8.14.3.7 Sublimation Ice
Sublimation ice forms by the direct transformation of water vapor into ice within cavities in frozen ground. Some such cavities are open to the atmosphere, such as ice caves, mine workings and artificial ice cellars. Where water vapor sublimates into ice in open thermal contraction cracks, sublimation ice can constitute a significant component of wedge ice. Other cavities are completely enclosed in permafrost, such as those formed by the doming action of pressurized methane (Mackay, 1965). Water vapor probably enters such cavities through diffusion, allowing delicate sublimation ice crystals to form inside.
Ice Surface Morphology
Pavel Bella, in Ice Caves, 2018
4.2.4.1.5 Subglacial Ablation Forms
Forms generated by the ice sublimation due to air flow
Sublimation cavities at the contact of ice block with bedrock. In DobÅ¡iná Ice Cave, the subglacial cavity with sublimation ice crystals under the Great Curtain at the Ground Floor (Fig. 4.2.8), as well as the narrow slit-shaped cavity in the RuffÃny’s Corridor (several centimeters to max. 1 m wide) was formed and enlarged by ice sublimation between the ice block and bedrock (Bella, 2003, 2007). The air flow from lower unknown cave parts (supposed to exist below the ice block and glaciated parts) or the adjacent Dry Chamber is indicated also by the coating of sublimation ice crystals formed on the rock wall and ice ceiling surface of these subglacial cavities (air flows through underlying rock blocks and debris). Periodically, the upper parts of the narrow slit-shaped cavity in the RuffÃny’s Corridor are filled by new ice, mostly formed by the freezing of sheet wash water flow. The basal melting of the ice block in DobÅ¡iná Ice Cave was not studied in detail, however, studies performed in ScăriÈ™oara Ice Cave have shown that the ablation of the base of ice block is a result of the ice melting generated by geothermal heat flux and of the ice sublimation due to the circulation of cold and dry air under the ice block. Subglacial tunnels are enlarged mostly by ice sublimation (PerÈ™oiu, 2004, 2005).
3.5 Sublimation
Sublimation can play a significant role in the surface energy and mass balance of glaciers, particularly in the tropics. Sublimation occurs during most of the dry season, decreasing available melt energy drastically (Wagnon et al., 1999) since the latent heat of sublimation (LS = 2848 kJ kg− 1) is 8.5-times greater than the latent heat of fusion (melting) (LM = 334 kJ kg− 1) (Winkler et al., 2009). In the Peruvian Andes, the dry season (May–September) features consistent low specific humidity, a vertical water vapor pressure gradient above the surface that is positive downward and high surface roughness (Winkler et al., 2009) which favors the direct transition from ice to vapor reducing the energy available for melting (Wagnon et al., 1999).
An automatic weather station (AWS) operating at 5794 m on Mount Kilimanjaro has been used to quantify the energy balance and ablation processes. Mölg and Hardy (2004) used an energy balance model that incorporated radiative, turbulent heat, and subsurface energy flux. They found that radiative heat dominates the systems and that the turbulent latent heat flux, which is always negative, leading to sublimation, is the second important energy flux.
Stigter et al. (2018) examined the energy balance and sublimation on Yala Glacier, Nepal at 5350 m to quantify the role snow sublimation plays in glacier mass budget. They used a bulk-aerodynamic method to estimate cumulative sublimation and evaporation for the 2016–2017 winter season, yielding 125 and 9 mm, respectively. These observations found that cumulative sublimation is 32 mm for a 32-day period from October to November 2016, during the dry post-summer monsoon season. This is equivalent to 21% of the annual snowfall, and highlights the importance of sublimation for glacier loss during this region's cold dry winter monsoon period. If the TSL remains high or increases during the dry winter monsoon, it indicates ongoing surface ablation and/or sublimation. In recent years, high transient snow lines have persisted into mid-winter on many Himalayan glaciers, such as in the Mount Everest region and Gangotri Glacier (Figs. 14 and 15). This leads to an increase in winter and spring ablation on the lower part of Himalayan glaciers (Salerno et al., 2015).
ANALYTICAL REAGENTS | Purification
E.J. Newman, in Encyclopedia of Analytical Science (Second Edition), 2005
Sublimation
In sublimation, a solid substance is volatilized by heating and the vapor is condensed back to the solid at a cooled surface. The distance between the surface of the vaporized solid and the collecting surface is short compared with distances used in distillation. Sublimation may be conducted at atmospheric pressure but reduced pressure is often employed to enhance sublimation and to speed up the process. An atmosphere of inert gas at low pressure is advisable for sensitive compounds.
Purification through sublimation is applicable to a number of organic and inorganic substances and is useful for the purification of many simple inorganic compounds used as working standards in analysis, including ammonium halides, arsenic(III) oxide, phosphorus(V) oxide, and iodine.
Cryostratigraphy
Julian B. Murton, in Reference Module in Earth Systems and Environmental Sciences, 2021
3.3.2.2 Sublimation contacts
Sublimation contacts develop as ground ice is lost by sublimation (i.e., as water changes state directly from solid to vapor without an intermediate liquid phase). Water vapor diffuses from a region with a high frost point (high vapor density) to a region of lower frost point (lower vapor density) (McKay et al., 1998). An example of a sublimation contact is the ice table, which represents an abrupt boundary between dry (ice-free) near-surface mineral soils and underlying ice-bearing permafrost (Mellon et al., 2009). The ice table represents the equilibrium depth determined by seasonal changes in sublimation and vapor deposition (Fisher et al., 2016). Unlike the permafrost table (a thermal boundary at 0 °C), the ice table denotes a boundary between frozen and unfrozen ground, and generally occurs at a mean annual ground temperature below 0 °C.
Sublimation contacts within centimeters of the ground surface occur in Antarctica and on the planet Mars. The depths of the ice table measured in parts of the McMurdo Dry Valleys, Antarctica, range from 0 to 98 cm (Marinova et al., 2013; Fisher et al., 2016). Where the ice table is at 0 cm depth in University Valley, the permafrost table is also at the ground surface (Lacelle et al., 2016; i.e., there is no thermal active layer, and the permafrost extends to the ground surface). For another site elsewhere at 1600 m elevation in the McMurdo Dry Valleys, where a sandy active layer 12.5 cm deep overlies dry permafrost at a depth of 12.5–25 cm above ice-cemented permafrost, McKay et al. (1998) determined that the annual mean frost point at the top of the ice-cemented ground (i.e., ice table at 25 cm depth) was − 21.7 ± 0.2 °C during 1994, compared to an annual mean frost point in the atmosphere above it of − 27.5 ± 1.0 °C. The upward drop in temperature implies a net flux of water vapor from the ground ice to the atmosphere, with a recession of the surface of the ice-cemented ground of ~ 0.4–0.6 mm a−1. Consequently, the ground ice there is either sublimating over time or is episodically recharged by other transport processes during different environmental conditions. In either case, the ice table at the top of ice-cemented permafrost represents a sublimation contact, though Fisher et al. (2016) concluded, based on modelling, that the ice-table depth in the area was probably in equilibrium with the temperature and humidity at the present-day ground surface rather than in the atmosphere. At the Phoenix Lander site on Mars, the ice table between dry, weakly cohesive ice-free soil and underlying ice-rich soil ranged in depth between 1.3 and 11.2 cm, with a mean depth of 4.6 cm (Mellon et al., 2009).
Volatile evolution and atmospheres of Trans-Neptunian objects
Leslie A. Young, ... Robert E. Johnson, in The Trans-Neptunian Solar System, 2020
6.5 Variation of atmospheres over an orbit
Because the sublimation pressures depend exponentially on the temperatures of the volatile ices, the gases surrounding volatile-bearing TNOs vary with heliocentric distance and subsolar latitude, and possibly time of day and latitude. This was initially modeled for Triton and Pluto (see reviews by Spencer et al., 1997; Yelle et al., 1995). Since those reviews, trends of increasing atmospheric pressure for both Triton and Pluto were observed using the technique of stellar occultation, with an increase by factors of 2 and 3, respectively (Elliot et al., 1998, 2000a, b, 2003a; Olkin et al., 1997, 2015; Meza et al., 2019; see Section 6.6). The new time base of atmospheric observations and the New Horizons flyby of Pluto inspired new models of seasonal variation (e.g., Young, 2012, 2013, 2017; Hansen et al., 2015; Olkin et al., 2015), including general circulation models (e.g., Forget et al., 2017) and evolution of atmospheres on the timescale of millions of years (e.g., Bertrand and Forget, 2016; Bertrand et al., 2018).
When N2 was discovered on the surface of Eris, authors speculated that volatiles on TNOs, especially N2, could raise temporary atmospheres near perihelion (e.g., Dumas et al., 2007). This was generalized in Stern and Trafton (2008), and applied numerically to the known or suspected volatile-bearing TNOs by Young and McKinnon (2013). When thinking about atmospheres on TNOs, it is useful to distinguish three types: global, collisional, and ballistic. For global sublimation-supported atmospheres, such as Mars or current-day Pluto and Triton, volatiles sublime from areas of higher insolation, and recondense on areas of lower insolation, transporting latent heat as well as mass (Trafton, 1984; Ingersoll, 1990; also see reviews by Spencer et al., 1997; Yelle et al., 1995; Stern and Trafton, 2008). As long as the volatiles can be effectively transported, the surface pressures and the volatile ice temperatures will be nearly constant across the surface. Sublimation winds transport mass from latitudes of high insolation to low insolation. Trafton (1984) showed that pressures stay within 10% across the surface if the sublimation winds (v) are less than 7.2% of the sound speed (vs). The sublimation wind speeds can be found by conservation of mass; the mass per time crossing a given latitude equals the integral of the net deposition from that latitude to the pole. The wind speeds depend on the subsolar latitude (Trafton, 1984), if we consider diurnally averaged insolation; higher wind speeds are needed to transport volatiles pole to pole (high subsolar latitudes) than equator to pole (low subsolar latitudes). For an “ice ball” uniformly covered in volatiles, the maximum sublimation wind speed, v, can be expressed as
(6.8)
where S = S1AU(1 − A)/R2 = 4εσTavg4 is the absorbed normal insolation, and L is the latent heat of sublimation, in energy per mass. ξ in Eq. (6.8) is a numerical factor accounting for the subsolar latitude, λSun. We calculated ξ numerically, following the prescription of Young (1993). From these calculations, ξ is well approximated by a cubic expression
(6.9)
For a 400–1400-km radius body uniformly covered with CH4 ice to have a global atmosphere, the pressure needs to be greater than ∼17–295 nbar for polar illumination (3.1 × 1019 to 1.6 × 1020 cm−2, 41.0–45.6 K; Fig. 6.5), or 1.9–33 nbar for equatorial illumination (3.6 × 1018 to 1.8 × 1019 cm−2, 38.1–42.0 K). For N2, the pressures are similar, so the temperatures are lower: 14–244 nbar for polar (1.5 × 1019 to 7.4 × 1019 cm−2, 29.1–31.9 K) or 2–28 nbar for equatorial (1.8 × 1018 to 68.5 × 1018 cm−2, 27.2–29.7 K). N0 increases slightly faster than r because both S and N0 increase with temperature; p0 increases even faster, slightly faster than r2, because of its dependence on g0 (Eq. 6.2). The temperatures in Fig. 6.5 are highly simplified. Seasonal thermal inertia can be important, even at the long timescales in the outer solar system. More significantly, bodies are unlikely to be uniformly covered in volatiles. For example, much of the N2 on Pluto is located in the basin known as Sputnik Planitia (Moore et al., 2016), and Triton’s N2 may be perennially confined to the southern hemisphere (Moore and Spencer, 1990).
Nonglobal atmospheres will vary with location and time of day, but may still be collisional, if the column density is greater than ∼1014 cm−2 for either N2 or CH4. Io is a classic example of a local atmosphere that is collisional around the subsolar point, and demonstrates some of the processes that are active in even these thin atmospheres. Atmospheric chemistry can occur even in these local, tenuous atmospheres (Wong and Smyth, 2000). Supersonic winds certainly flow and transport volatiles, even if they are not effective at equalizing pressures and temperatures (e.g., Walker et al., 2012). Recently, Hofgartner et al. (2018) used the Ingersoll et al. (1985) meteorological model developed for Io study the transport of N2 on Eris at aphelion, when it is a local, collisional atmosphere, and found significant transport of N2 ice. Even for more tenuous “surface-bounded exospheres,” the loss of volatiles can modify landforms (see review by Mangold, 2011). For example, sublimation erosion may lead to the narrow divides between craters on Hyperion (Howard et al., 2012) or redeposition on the crater rims on Callisto, where the convex summits see less of the warm surface than do concave crater interiors, and are therefore local cold traps (Howard and Moore, 2008).
Ground Ice
Julian B. Murton, in Reference Module in Earth Systems and Environmental Sciences, 2021
4.3.5 Vapor deposition and sublimation
Vapor deposition and sublimation may increase and reduce, respectively, the amount of ground ice in near-surface permafrost, especially in porous substrates in cryotic and ultraxerous polar deserts, where liquid water is rare to absent. Deposition occurs when water vapor changes directly to solid (ice) without an intermediate liquid phase. Vapor deposition forms ground ice in the perennially cryotic zone and intermediate cryotic zone of University Valley, in the McMurdo Dry Valleys of Antarctica (Lapalme et al., 2017a). The occurrence of near-surface ground ice in sediments dominated by homogeneous, medium to coarse sand discounts frost-susceptibility as a cause of ice enrichment (Lapalme et al., 2017a,b). Instead, temperature gradients between the ground surface and underlying substrate drive vapor deposition in the substrate when it is colder than the surface in summer. The source of the water vapor is probably either (1) direct condensation of atmospheric moisture into the ground and/or (2) the remains of transient frost that forms at or beneath the ground surface in winter and which is transported down into the ground along a negative temperature gradient in summer. Other factors that may affect the stability and depth of near-surface ground ice in this ultraxerous region include episodic snow cover, surface albedo, insolation, clouds, terrain shadowing, surface orientation and katabatic winds (Marinova et al., 2013).
Sublimation can remove ground ice from the active layer and near-surface permafrost, for example, when the ground surface is cooler than the substrate, which is common in winter. In the McMurdo Dry Valleys, mean annual water-equivalent precipitation of 10–100 mm combined with strong winds exacerbate evaporation and sublimation losses (Bockheim et al., 2007). Both factors probably contribute to the widespread occurrence of dry-frozen permafrost, likely to maximum depths of a few meters in sediments beneath surfaces older than 115 ka along the floors and lower sidewalls of larger ice-free valleys. Dry-frozen permafrost—which commonly underlies the active layer and overlies ice-cemented permafrost—forms by sublimation of water in ice-cemented permafrost through time. Net rates of ice loss are sensitive to air temperature: decreasing air temperature leads to decreasing sublimation losses (Pollard et al., 2012). In general, the rate of pore-ice sublimation increases as subzero temperature increases toward the freezing point and as relative humidity decreases, though the relationship is complicated by the influence of sediment moisture content and wind speed (Law and van Dijk, 1994).