Triple-point

How does the iodine fingerprint technique work The basic principle is that solid iodine is one of the few substances that has a significant vapor pressure from the solid and actually sublimes directly from the solid into the gas phase. Although it is common in organic laboratory procedures to purify volatile compounds by sublimation in a vacuum pistol at very low pressure, carbon dioxide and iodine are among only a very few substances that sublime easily at room temperature and pressure. Thus, the technique of sweeping iodine vapor over a surface is a dry procedure without any liquid and yet some data is available for the liquid [4]. 

Selected Values of Alpha and Beta for Liquids at Temperatures in C 

Compound Temperature (°C) Alpha (10V°C) Beta (107MPa) Density (g/mL) 

While we are discussing sofids, hquids, and gases we can consider the difference in heat capacities for solids and liquids. We will now need to use some of the information from the HUGA set of equations. Along the way we will repeat the case for an ideal gas and show where the derivation changes for the general case. We start from the definitions of C/ and Cy. 

FIGURE 6.6 An iodine enhanced fingerprint. (Photo provided courtesy of Forensics Source 2010.) Thanks to Eric Schellhom, director of marketing, and Floyd Wilson who developed a print on an outside rough surface as a severe demonstration as requested. Close examination reveals clear print lines suitable for computer analysis. 

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Equations (2) and (3) are physically meaningful only in the temperature range bounded by the triple-point temperature and the critical temperature. Nevertheless, it is often useful to extrapolate these equations either to lower or, more often, to higher temperatures. In this monograph we have extrapolated the function F [Equation (3)] to a reduced temperature of nearly 2. We do not recommend further extrapolation. For highly supercritical components it is better to use the unsymmetric normalization for activity coefficients as indicated in Chapter 2 and as discussed further in a later section of this chapter. 

Thermal conductivity is expressed in W/(m K) and measures the ease in which heat is transmitted through a thin layer of material. Conductivity of liquids, written as A, decreases in an essentially linear manner between the triple point and the boiling point temperatures. Beyond a reduced temperature of 0.8, the relationship is not at all linear. For estimation of conductivity we will distinguish two cases < )... 

Usually, Tj is taken as the triple point and T2 as the normal boiling point. The average error is about 5%. 

At a given temperature and pressure, a pure compound can exist in one, two or three states. The compound exists at three different states at the triple point and at two different states along the curves of vaporization, freezing and sublimation. Refer to Figure 4.6. 

At the triple point, the free energies of each phase are equal ... 

Triple point temperature K Heat of fusion kJ/lc Heat of vaporization kJ/kg Liquid conductivity atr, W / (m-K) Liquid conductivity AtT W/(m-I0 Temperature Ti K Temperature h K... 

Triple point temperature Heat of fusion Heat of vaporization Liquid conductivity at r, Liquid conductivity at Temperature Tx Temperature Tz... 

Triple point Heat of Heat of Liquid liquid Temperature Temperature... 

Molecular dynamics and density functional theory studies (see Section IX-2) of the Lennard-Jones 6-12 system determine the interfacial tension for the solid-liquid and solid-vapor interfaces [47-49]. The dimensionless interfacial tension ya /kT, where a is the Lennard-Jones molecular size, increases from about 0.83 for the solid-liquid interface to 2.38 for the solid-vapor at the triple point [49], reflecting the large energy associated with a solid-vapor interface. 

Referring to Fig. XVII-17, use handbook data to calculate the vapor pressure of O2 ordinary liquid at the melting point of the 6 phase. Comment on the result. Locate the 2D S-L-V triple point. 

At equilibrium, in order to achieve equality of chemical potentials, not only tire colloid but also tire polymer concentrations in tire different phases are different. We focus here on a theory tliat allows for tliis polymer partitioning [99]. Predictions for two polymer/colloid size ratios are shown in figure C2.6.10. A liquid phase is predicted to occur only when tire range of attractions is not too small compared to tire particle size, 5/a > 0.3. Under tliese conditions a phase behaviour is obtained tliat is similar to tliat of simple liquids, such as argon. Because of tire polymer partitioning, however, tliere is a tliree-phase triangle (ratlier tlian a triple point). For smaller polymer (narrower attractions), tire gas-liquid transition becomes metastable witli respect to tire fluid-crystal transition. These predictions were confinned experimentally [100]. The phase boundaries were predicted semi-quantitatively. 

To understand the conditions which control sublimation, it is necessary to study the solid - liquid - vapour equilibria. In Fig. 1,19, 1 (compare Fig. 1,10, 1) the curve T IF is the vapour pressure curve of the liquid (i.e., it represents the conditions of equilibrium, temperature and pressure, for a system of liquid and vapour), and TS is the vapour pressure curve of the solid (i.e., the conditions under which the vapour and solid are in equili-hrium). The two curves intersect at T at this point, known as the triple point, solid, liquid and vapour coexist. The curve TV represents the... 

The normal melting point of a substance is the temperature at which solid and hquid are in equilibrium at atmospheric pressure. At the triple point, the pressure is the equilibrium vapour pressure of the system (solid liquid - vapour) and the temperature differs from the melting point. The difference is, however, quite small—usually only a fraction of a degree—since the line TV departs only slightly from the vertical within reasonable ranges of pressure. 

It is clear that if the vapour at a pressure below the triple point is reduced sufficiently in temperature, it will condense directly to the solid form, or, sublimation will ensue. In order that a solid may pass directly... 

Condensation takes place on the cold surface directly as the solid because the pressure is below that at the triple point. 

If the triple point pressure of a solid is below one atmosphere, it will melt if the heating is conducted rapidly so that the vajiour pressure can exceed that at the triple point. If camphor is heated in a closed space, the vapour pressure increases and when the value of 360 mm. is reached, the solid will melt further heating results in an increase in the vapour pressure and the camphor will boil when the vapour pressure is 760 mm. 

For substances with a moderate triple point pressure e.g., benzoic acid, 6 mm., m.p. 122° naphthalene, 7 mm., m.p. 80° ), the simple process described above for camphor will not give a satisfactory yield of a sublimed product. Thus, for example, if naphthalene is heated it will melt at T (80°), and will boil when the vapour pressure is 760 mm. (218°) ... 

For most practical purposes the temperature and pressure at the triple point may be regarded as not differing appreciably from the melting point and the vapour pressure at the melting point respectively. 

Temperature kelvin K Defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. 

Table 5.27 Compressibility of Water Table 5.28 Mass of Water Vapor In Saturated Air Table 5.29 Van der Waals Constants for Gases Table 5.30 Triple Points of Various M aterlals 5.9.1 Some Physical Chemistry Equations for Gases...

TABLE 5.30 Triple Points of Various Materials Continued)... 

The ultimate definition of thermodynamic temperature is in terms of pV (pressure X volume) in a gas thermometer extrapolated to low pressure. The kelvin (K), the unit of thermodynamic temperature, is defined by specifying the temperature of one fixed point on the scale—the triple point... 

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