Standard condition definition

Electrochemical cells can be constructed using an almost limitless combination of electrodes and solutions, and each combination generates a specific potential. Keeping track of the electrical potentials of all cells under all possible situations would be extremely tedious without a set of standard reference conditions. By definition, the standard electrical potential is the potential developed by a cell In which all chemical species are present under standard thermodynamic conditions. Recall that standard conditions for thermodynamic properties include concentrations of 1 M for solutes in solution and pressures of 1 bar for gases. Chemists use the same standard conditions for electrochemical properties. As in thermodynamics, standard conditions are designated with a superscript °. A standard electrical potential is designated E °. 

The simplest definition of sensitivity is the signal-to-noise ratio. One criterion for judging the sensitivity of an NMR spectrometer or an NMR experiment is to measure the height of a peak under standard conditions and to compare it with the noise level in the same spectrum. Resolution is the extent to which the line shape deviates from an ideal Lorentzian line. Resolution is generally determined by measuring the width of a signal at half-height, in hertz. 

It is very often necessary to characterize the redox properties of a given system with unknown activity coefficients in a state far from standard conditions. For this purpose, formal (solution with unit concentrations of all the species appearing in the Nernst equation its value depends on the overall composition of the solution. If the solution also contains additional species that do not appear in the Nernst equation (indifferent electrolyte, buffer components, etc.), their concentrations must be precisely specified in the formal potential data. The formal potential, denoted as E0, is best characterized by an expression in parentheses, giving both the half-cell reaction and the composition of the medium, for example E0,(Zn2+ + 2e = Zn, 10-3M H2S04). 

At standard conditions G and H are not functions of pressure, by definition. Thus, Equation 6.33 can be written at standard conditions for finite changes in G° and H° as ... 

By definition, the standard enthalpy of formation of all elements in their standard states, under standard conditions, is zero. 

Note that care must be taken to obtain the value for AG°, as the definition of standard conditions sometimes differ. 

According to the definition of the A-B bond dissociation enthalpy, reactants and products in reaction 5.1 must be in the gas phase under standard conditions. That is to say that those species are in the ideal gas phase, implying that inter-molecular interactions do not exist. DH (A-B) refers, therefore, to the isolated molecule AB, and it does not contain any contribution from intermolecular forces. Though this is obviously the correct way of defining the energetics of any bond, there are many literature examples where bond dissociation enthalpies have been reported in solution. 

By definition, the enthalpy of formation of an element in its standard state is zero. The standard state of an element is usually its most stable form under standard conditions. Recall, from section 5.1, that standard conditions are 25°C and 100 kPa (close to room temperature and pressure). Therefore, the standard state of nitrogen is N2(g). The standard state of magnesium is Mg(s). 

Fig.1 Calculated free energy diagram for hydrogen evolution at a potential U = 0 V relative to the standard hydrogen electrode at pH = 0. The free energy of H+ + e is by definition the same as that of j - i at standard conditions. The free energy of H atoms bound to different catalysts is then found by calculating the free energy with respect to molecular hydrogen including zero-point energies and entropy terms (reprinted from Ref 83 with permission).

Other common ways of expressing abundances, particularly of solid or liquid particles, is to express them as concentrations in units of micrograms per cubic meter or nanomoles per cubic meter. For purposes of consistency, concentrations expressed in these units should be normalized to standard conditions of temperature and pressure. Because there is some confusion as to what constitutes standard conditions in atmospheric chemistry (273 K and 1.013 bar are commonly used in chemistry and physics and 293 K and 1.013 bar are used in engineering), it is important to define the standard conditions that are assumed when reporting data. This explicit definition is frequently not done. Concentrations expressed in these units can be easily converted to mixing ratios by use of the ideal gas law ... 

Values of E° by definition refer to conditions under which all species are in their standard states at 298 K. For non-standard conditions the electrode potential, E, of a redox reaction is given by the familiar Nernst expression (equation 24), where... 

Standard conditions are not necessarily consistent with standards definitions. The careful researcher will always take note of file conditions stated for determining values in a tabulated list. 

Validation is an essential procedure that demonstrates that a manufacturing process operating under defined standard conditions is capable of consistently producing a product that meets the established product specifications. In its proposed guidelines, the U.S. Food and Drug Administration (FDA) has offered the following definition for process validation [1],... 

If the electrophoretic tool is called upon to bring a definite advance to medical science, only accurate techniques and reproducible results under standardized conditions can warrant an important future for these hopeful but still imperfect beginnings. 

Next, the definition of PSD is discussed based on the uncertainty regarding the size of the particle that is selected when a particle is selected. The original PSD q0 (l/l) is regarded as a probability density function, and the PSD satisfies the following standardization condition ... 

When hydrochloric acid reacts with zinc a definite volume of hydrogen is displaced by a given weight of metal. The weight of zinc dissolved can be accurately determined, and when the volume of hydrogen liberated is corrected to standard conditions the ratio between the two is obtained. 

Measurement of Gases. Since it is difficult to weigh a body of gas, but comparatively easy to find its volume, the amounts of gases are almost invariably estimated by measuring the volume. But the volume of a definite amount of gas is very dependent on the conditions, and to make a volume measurement have an accurate meaning it becomes necessary to know exactly the conditions of pressure, temperature, and dryness under which the measurement is made. To make the results of all measurements comparable it is customary to calculate what the measured volume would become if so-called standard conditions prevailed. 

However, standard conditions in biochemistry and in biology are different, since chemical reactions in cells occur at around pH 7. Therefore, standard conditions in biochemistry differ from those in chemistry, which implies that the standard Gibbs free energy within a biological system is denoted as A G0/. Standard conditions in biochemistry and biology are a pH equal to 7 and a constant water concentration that does not appear in the mathematical definition of the equilibrium constant. 

The definition of standard conditions is more difficult for nitration than for hydrogen exchange because of uncertainty in the NO,+ activity variation, and the peculiar behavior of benzene itself. The standard conditions chosen were 25°C and H0 -6.6 (i.e., 75 wt% H2S04 at 25°C) [75JCS(P2)I600]. The choice of 25°C was made because kinetics for many nitrations have been followed at this temperature, and most in the range 0-100°C. The standard acidity of H0 -6.6 minimizes the extrapolations needed for the rate profiles of many substrates and is close to the range of normal behavior found for benzene. 

In Equation (18b), the activity quotient is separated into the terms relating to the silver electrode and the hydrogen electrode. We assume that both electrodes (Ag+/Ag and H+/H2) operate under the standard condition (i.e. the H+/H2 electrode of our cell happens to constitute the SHE). This means that the equilibrium voltage of the cell of Figure 3.1.6 is identical with the half-cell equilibrium potential E°(Ag+l Ag) = 0.80 V. Furthermore, we note that the activity of the element silver is per definition unity. As the stoichiometric number of electrons transferred is one, the Nemst equation for the Ag+/Ag electrode can be formulated in the following convenient and standard way ... 

From the context of the current discussion, it should be evident that the mean activity coefficient cited above is related to molarity. On the other hand, as is to be proved in Exercise 4.2.2, the definition of S remains virtually unaltered by switching from molarity to molality in aqueous solutions at ordinary conditions of temperature and pressure. Thus, the quantity specified by Eq. (4.2.3a) may be considered to represent either 7 (T,P,c) 7 (c) or 7 (T,P,m) 7 nonaqueous solvents are employed, or whenever T and P deviate greatly from standard conditions, the two preceding quantities cannot be used interchangeably Eq. (4.2.3a) specifies 7 [Pg.392]

Since the definition of E° in Eq. (96) refers to standard conditions, that is, conditions in which no external electrical field applies on the reactant or the product, it is more convenient to consider the act of electron transfer as a succession of three individual steps as outlined in Scheme 5, which is reminiscent of that in Scheme 3 established for the homogeneous analogous situation (Sec. II.C.l). In Scheme 5, Rq or Pq relates to R or P in the closest plane to the electrode where no electrical potential applies, that is, at the end of the diffuse layer, denoted Xq in Fig. 14d. Rd, and Pelectron transfer Xd, usually considered close to or slightly within the OHP, where an electrical potential Os (i.e., the electrical potential at the electron transfer site) applies. 

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