Partition coefficients conditional constants

This distinction between Kd and D is important. The partition coefficient is an equilibrium constant and has a fixed value for the solute s partitioning between the two phases. The value of the distribution ratio, however, changes with solution conditions if the relative amounts of forms A and B change. If we know the equilibrium reactions taking place within each phase and between the phases, we can derive an algebraic relationship between Kd and D. 

Chemioai and physical properties appearance, coiour, odour, moleouiar weight, melting point, refractive index, ionisation constants, partition coefficients, solubility under different conditions of pH or solvent... 

Solution The phase in which reaction occurs will be denoted by the subscript /, and the other phase wiU be denoted by the subscript g. Henry s law constant will be replaced by a liquid-liquid partition coefficient, but will still be denoted by Kh- Then the system is governed by Equations (11.29) and (11.30) with = —kai and ( ) = 0. The initial conditions are... 

It should finally be noted that the amount of the neutral and zweitterionic forms of a compound in solution is determined by its tatuomeric equilibrium constant, defined as Kz = cz/cn. Therefore, the neutral species and the zwitterion coexist around the isoelectric pH, and membrane permeation is conditioned by Kx and by the partition coefficient values of both tautomers. 

Chen and Gross [48] derived equations to calculate partition coefficients for blood flow-limited compartments from either constant rate infusion (i.e., steady-state conditions) or intravenous bolus regimens. For a noneliminating organ under steady-state conditions,... 

It is noteworthy that the partition coefficient is included in the apparent rate constants and may be ignored only when VcellK is much smaller than VR and VD. Under these conditions, the above equations reduce to Eqs. (8) and (9) for the sink situation. 

Vapor pressure The vapor pressure of a substance is defined as the pressure exerted by the vapor (gas) of a substance when it is under equihbrium conditions. It provides a semi-quantitative rate at which it will volatilize from soil and/or water. The vapor pressure of a substance is a required input parameter for calculating the air-water partition coefficient (see Henry s law constant), which in turn is used to estimate the volatilization rate of compounds from groundwater to the unsaturated zone and from surface waterbodies to the atmosphere. 

The partition or distribution coefficient between a gas and a liquid is constant at a given temperature and pressure. The relative volatility is used in defining the equilibrium between a volatile liquid mixture and the atmosphere. The partition coefficient expresses the relative volatility of a species A distributed between a vapor phase (Al) and a liquid phase (A2). Henry s law applies to the distribution of dilute solutions of chemicals in a gas, liquid, or solid at a specific ambient condition. Equilibrium is defined by... 

Both the mass transfer kinetic parameters (diffusion in the phases, D, D j, surface renewal frequency, s) and chemical reaction rate constants (kg, kj) strongly influence enhancement of the absorption rate. The particle size, dp, the dispersed liquid holdup, e and the partition coefficient, H can also strongly alter the absorption rate [42-44,46,48]. Similarly, the distance of the first particle from the gas-liquid interface, 6q is an essential factor. Because the diffusion conditions are much better in the dispersed phase (larger solubility and, in most cases, larger diffusivity, as well) the absorption rate should increase with the decrease of the (5g value. 

Figure 7.6a gives the response surface of the partition coefficient of sulphacetamide. It can be seen that optimal extraction conditions of sulphacetamide are binary compositions of methylene chloride and methyl tert.-butyl ether. It can also be observed that the partition coefficient is nearly constant at the binary axis methylene chloride/chloroform. Therefore, small variations in the binary compositions of methylene chloride and chloroform will not significantly change the partition coefficient. In other words binary compositions with methylene and chloroform yield robust extractions for sulphacetamide. This conclusion is confirmed by the robustness plot of the partition coefficient of sulphacetamide (Figure 7.6b). This plot also shows that under conditions where the partition coefficient is optimal (binary mixtures of methylene chloride and methyl tert.-butyl ether), the robustness of the partition coefficient reaches a maximum value.

In such systems, biotransformations are generally carried out in a reaction medium composed of an aqueous phase containing the biocatalyst and a water-immiscible organic solvent which may be the substrate itself to be converted [21] or may serve as a reservoir for substrates and products [22] (Fig. 24.2). In these conditions, a constant substrate feeding in the aqueous phase is obtained owing to the partition coefficient. The substrate is used by the biocatalyst to be converted into the product of interest, which is then continuously extracted into the organic phase. 

The constant /3 contains a partitioning coefficient of the analyte between the solution and the modifying layer, as well as the constants related to the bulk electrolysis in a small volume (i.e., thin) cell (Bard and Faulkner, 2001). If the electroactive species are confined to the electrode, if the couple is perfectly reversible, and if the extraction is fast on the time scale of the experiment, the peaks in the cyclic voltammogram occur at the same potential and the areas (charge) below the cathodic and anodic branches are equal, as is the case in Fig. 7.12. Obviously, any deviations from these conditions are reflected in the shape of the CV curve. Nevertheless, even then the relationship between the peak current iv and the bulk concentration of the electroactive species can be reproducible. In the determination of Fe2+ using the above procedure, the linear calibration between 5 x 10 8 and 5 x 10-6 M concentration has been obtained. 

In most cases we can assume that the equilibrium distribution and partitioning of organic chemicals in both mammalian and nonmammalian systems is a function of lipid content in the animal and that the lipid-water partition coefficient (AW) is equal to K0w Instances where this is not the case include specific binding sites (e.g., kepone in the liver) and nonequilibrium conditions caused by slow elimination rates of higher level organisms or structured lipid phases that sterically hinder accumulation of very hydrophobic chemicals. For aquatic organisms in constant contact with water, the bioconcentration factor or fish-water partition coefficient (AW) s simply ... 

The distribution (or partition) coefficient, Ka, of a metal cation between an aqueous (aq) and organic (org) phase may also be used to assess the selectivity of a given host for a range of metal cations under standard conditions, using the equilibrium constants (K) for the following processes (Equations 1.17-1.20) (for metal picrate (Pic) salt, water (aq) and water-saturated chloroform (org) phases, 25 °C). 

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