Standard condensed-phase reference

The success of spectral identification depends on the appropriate reference spectra for comparison. IR measurement of eluates that are at slightly subambient temperature is advantageous considering that the large databases of condensed-state spectra may be searched. Spectra measured by matrix-isolation GC-FTIR have characteristically narrow bandwidths compared with the spectra of samples in the condensed phase near ambient temperature or in the gas phase. In addition, the relative intensities of bands in the spectra of matrix-isolated samples often change compared with either gas- or condensed-phase spectra [195]. GC-FTIR spectra obtained by direct deposition match well with the corresponding reference spectra in standard phase... 

To our knowledge, the question of the standard state corrections in DSC experiments has never been addressed. These corrections may in general be negligible, because most studies only involve condensed phases and are performed at pressures not too far from atmospheric. This may not be the case if, for example, a decomposition reaction of a solid compound that generates a gas is studied in a hermetically closed crucible, or high pressures are applied to the sample and reference cells. The strategies for the calculation of standard state corrections in calorimetric experiments have been illustrated in chapter 7 for combustion calorimetry. 

E. S. Domalski, E. D. Hearing. Condensed Phase Heat Capacity Data. In NIST Chemistry WebBook NIST Standard Reference Database Number 69 P. J. Linstrom, W. G. Mallard, Eds. National Institute of Standards and Technology Gaithersburg, June 2005 (webbook.nist.gov). 

In electrochemistry, we deal with the energy level of charged particles such as electrons and ions in condensed phases. The electrochemical potential, Pi,of a charged particle i in a condensed phase is defined by the differential work done for the charged particle to transfer from the standard reference level (e.g. the standard gaseous state) at infinity = 0) to the interior of the condensed phase. The electrochemical potential may be conventionally divided into two terms the chemical potential Pi and the electrostatic energy Zi e as shown in Eqn. 1-21 ... 

The ion level in condensed phases has been represented by the real potential, a, referred to the standard gaseous state of the ion at the outer potential of the condensed phases. The reference level, then, is not common to all ions but differs with different ions. In chemical thermodynamics, the conventional energy scale is based on the assumption that all atoms in the stable form in the standard state are at the zero energy level, which is the thermodynamic reference level of energy for chemical substances. In the following, we discuss the relationship between the scale of the ion level represented by the real potential of ions and the conventional energy scale of particles in chemical thermodynamics. 

Domalski, E.S. and Hearing, E.D. Condensed phase heat capacity data, in NIST Standard Reference Database Number 69, Mallard, W.G. and Linstrom, P.J., Eds. (Gaithersburg, MD National Institute of Standards and Technology, 1998) (httD //webbook.nist.gov). 

As we have already seen, the standard potentials are relative to standard reference conditions—i.e., one-molal solutions at 2) = 25 °C and /) = 1 bar, in equilibrium with pure metals or pure gases. Applying the Nernst relation to a redox equilibrium such as reaction 8.163 and assuming unitary activity for the condensed phases (i.e., pure metals), we have... 

For the convenience of tabulation and compulation of thermodynamic data, it is essential lo present them in a commonly accepted form relative to a single standard slate of reference. At all lemperatures, the standard stale for a pure liquid or solid is the condensed phase under a pressure of I atmosphere. The standard stale for a gas is the hypothetical ideal gas at anil fugarity (equivalent til a perfect gas" stale), in which state the enthalpy is that of the real gas at the same temperature when the pressure approaches aero. Values of thermodynamic quantities for standard-state conditions are identified by a superscriptQ. and Hn. for instance, is the enthalpy change of a reaction when reactants and products are in the standard state. 

Although the standard state could be based on any reference behavior, for simplicity the choices are conventionally limited to one of two main types (Figure 2-1). One is the limiting behavior of a substance as it approaches zero mole fraction (condensed phase) or zero partial pressure (gas phase) this is called henryan reference behavior. The other is the limiting behavior of a substance as it approaches unit... 

Thermodynamic properties taken from Robie, Hemingway, and Fisher are based on a reference state of the elements in their standard states at 1 bar (10 P = 0.987 atm). This change in reference pressure has a negligible effect on the tabulated values for the condensed phases. [For gas phases only data from NBS (reference state = 1 atm) are given.]... 

SO small that the number of atoms at the phase boundary can be neglected compared to those in the volume of the phases. This means that the phases are completely spatially separated, and consequently the Gibbs function is additive in terms of the pure components (see line g in Figure A. la). The term p refers to the standard chemical potenhal of a condensed phase. 

The 673 K temperature was selected as the reference point for calculating the 1° KDIE at standard temperature because it is the maximum condensed phase temperature experimentally measured during combustion of a nitramine propellant formulation (see ref 24). 

NIST Chemistry WebBook http //webbook.nist.gov/chonistry/ (accessed November 10, 2010). The NIST Chemistry WebBook provides Internet access to chemical and physical property data for nearly 50,000 chonical species (compounds, ions, radicals, etc.). The data are derived from collections maintained by both the NIST Standard Reference Data Program and outside contributors. The available data include thermodynamic, gas phase, IR spectrum, condensed phase, mass spectrum, phase change, UV/Vis spectrum, reaction, vibrational and electronic energy levels, ion enogetics, constants of diatomic molecules, ion cluster, and Henry s Law. 

Here the enthalpy of the substance is referred to that of a reference standard for that material chosen at any convenient reference temperature T and pressure Pi. This formulation is particularly useful for condensed phases. In the approximation of Dulong and Petit, the reference value for atomic solids at sufficiently high temperatures is approximated by 3nR. The partial derivative for volume in the integrand may be replaced by aV, where a is the isobaric coefficient of thermal expansion, which is fairly insensitive to pressure changes. For such materials, we cast the above in the form... 

Evaluation of the energy balance of such chemical reactions requires specification of standard and reference states. For our purpose, the standard states of the condensed phases will be chosen as the pure solid at 1 bar pressure and a given temperature T (°K), that of the fluid phase as the pure gas species at a fugacity of 1 bar and stated temperature T ( K). The reference... 

The trends for the absolute shielding are thus reversed to those for the chemical shift - that is, 8k < Si means that the nucleus K is more shielded than nucleus L. The chemical shifts are taken relative to the shielding in a selected molecule (see O Table ITU)-, for instance, liquid tetramethylsilane (TMS) is used as a standard reference for. Unfortunately, large molecules in solution or in condensed phase are chosen for many nuclei to define experimental chemical shifts. It is not yet possible to perform benchmark calculations for these standard systems moreover, it is often not even possible to compute the shielding in these reference compounds at the same level of accuracy as for smaller molecules that are of interest... 

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