Pure-substance reference

It must not be forgotten that the concept of pure substance, referred to earlier, is very rigorous and must take into account, not just the constitution and relative configuration of a molecule, but also the absolute configuration of each chiral center that may present. For example, again in relation to quinine (i), quinidine (2) is also known and the only difference between the two molecules is the disposition in space of the groups bonded to C(8). Nevertheless 2 is a different molecule and shows no antimalarial activity. In addition, only one enantiomer of quinine (1), the laevorotatory, corresponds to the natural compound and manifests the specific physiological properties associated with this substance. 

In physical measurement, calibration standards are of prime importance, but in chemistry, standards such as mass standards and pure substance reference materials are necessary but not sufficient and often not the most problematic aspect of establishing traceability. As every analytical chemist knows, issues such as sampling, sample stability, contamination, interferences, and incomplete recovery of the analyte are usually the major contributors to measurement uncertainty. It is being increasingly recognized that if we wish to improve the traceability of chemical measurements, then we need to put the effort where the chemical problems are, and not where the problems are in physical measurement. It is a sign of maturity that this is now happening. 

Calibrate the chemical measuring instrument using standards prepared from pure substance reference materials of known identity and purity Calibrate physical measurement processes with traceable physical standards... 

R, the NARL reference analytical value (accompanied by measurement uncertainty estimate) traceable to SI through the validated test method used and the pure substance reference standards used to calibrate the measuring instrument... 

In the method of classification of matter based on composition, a given specimen of material is regarded as either a pure substance or a mixture. The term pure substance refers to a material all parts of which have the same composition and that has a definite and unique set of properties. 

Reference materials for chemical composition are of two types pure substance reference materials, intended to realize a specified chemical species, and composite substances reference materials, intended to realize a specified content of a specified chemical species in a specified matrix. One of the analytical fields where reference materials have great importance is clinical analysis.250... 

So, when we choose the pure-substance reference state to be at the same temperature and pressure as the mixture, then the activity of component i is simply related to the reversible isothermal-isobaric work involved in adding a small amount of pure i to the mixture. This provides a physical interpretation for the activity. 

If we adopt the pure-substance reference (I), and if the solution is perfect, the activity coefficient is equal to 1 and the law of mass action takes the form of relation [3.3a], replacing the activities in that formula with the molar fractions, as follows ... 

This gives us the relation between the equilibrium constant relative to the pure-substance reference (I) and that relative to the infinitely-dilute solution reference (II) ... 

The pole diagram is a helpful tool to represent the progression of a reaction and compare reactions within the same family. It is generally plotted in convention (I), (pure-substance reference). 

Convention (I) takes as a reference the components in the pure state in the same state of aggregation as the solution (this is known as the pure-substance reference). In these conditions, the chemical potential of the reference state is the molar Gibbs energy of the pure component and the chemical potential is then written as ... 

Hence, the constant links the activity coefficients expressed, for a solute, in the two conventions pure-substance reference and dilute-solution reference. This constant does not depend on the composition of tiie solution, but instead depends on the values of the intensive variables (pressure, temperature, etc.), by way of the chemical potentials of the reference states, amongst other things. 

Figure 15.32 illustrates this process by words, a graph, and the appropriate equation for each step. The shape of the temperature-versus-heat graph is typical for any pure substance. Refer to this illusfiation as you work through the next example. It will help you to see clearly what is being done in each of the five steps. The general procedure for this kind of problem is... 

The term phase for a pure substance refers to a state of matter that is gas, liquid, or solid. Latent enthalpy (heat) effects are associated with phase changes. These phase changes involve no change in temperature but there is a transfer of energy to/from the substance. There are three possible latent effects, as detailed below ... 

Note - Certain authors propose to determine the activity coefficient of carbon in the pure-substance reference (I) using the relation ... 

Still with the aim of having mathematical expressions for the representation of the solution, Redlich and Kister offered a representation that provides an expansion of the excess Gibbs energy, a pure-substance reference in the same state of segregation as the solution (reference (I)), the equivalent of ihe Margules expansion for the activity coefficients. For a two-component solution, the pol5momial expansion up to order m is written ... 

Activity coefficients of a component with a pure-substance reference... 

With regard to the solvent, we already know its activity coefficient in convention (1) (pure-substance reference), through expression [2.21], Similarly as above, by applying relation [A.2.23] (see Appendix 2), we deduce that for all solutes, the activity coefficient in reference (I) is a constant. This constant is independent of the composition, but its value is not 1 because the reference state does not overlap the domain of definition of reference (I). However, given that, in reference (II), the activity coefficient for the solute is 1, we can apply relation [A.2.21] (see Appendix 2). From this, we deduce that the eonstant is Henry s constant ... 

Normally, the UNIQUAC model leads to the activity coefficients with the pure-substance reference In Jq switch to the activity coefficient for... 

For a real solution, the activity, in the pure-substance reference, of a component tends toward 1 when its molar fraction tends toward 1. It tends toward 0, along Heiuy s straight line, when its molar fraction tends toward 0. Thus, the curve showing its activity, in the pure-substance reference, as a function of its molar fraction, is tangential to Raoult s straight line at the point X = 0. It is tangential to Henry s straight line at the point x = 1. 

A solution is said to show positive deviation if its activity coefficient in the pure-substance reference is greater than >1), the curve is above Raoult s straight line. This solution is said to show negative deviation if its activity coefficient in the pure-substance reference is less than 1(/ <1), with the curve below Raoult s straight line. 

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