Partition or Distribution Coefficient

A given substance X is partially soluble in each of two immiscible solvents. If X is placed in a mixture of these two solvents and shaken, an equilibrium will be established between the two phases. That is, substance X will partition (distribute) itself in a manner that is a function of its relative solubility in the two solvents  

The equilibrium constant, Kp, for this equilibrium expression is known as the partition or distribution coefficient  

The equilibrium constant is thus the ratio of the concentrations of the species, X, in each solvent for a given system at a given temperature. The partition coefficient can be conveniently estimated as the ratio of the solubility of X in solvent 1 vs. solvent 2  

When solvent 1 is water and solvent 2 is an organic solvent such as diethyl ether, the basic equation used to express the coefficient Kp is 

This expression uses grams per 100 mL for the concentration units. Note that the partition coefficient is dimensionless, so any concentration units may be used if the units are the same for both phases. For example, grams per liter (g/L), parts per million (ppm), and molarity (M) can aU be used. If equal volumes of both solvents are used, the equation reduces to the ratio of the weights (gorganic/gwater) of the gjven species in the two solvents  

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Data collected with a simple RPLC procedure has been found to be in good agreement with 1-octanol shake-flask partition or distribution coefficients over a 3.5 log range [64], A chemically bonded octadecylsilane support is coated with 1-octanol. With 1-octanol-saturated buffers as mobile phases, a stable baseline (compared to 1-octanol absorbed on silica) is... 

The first of these environmentally-important parameters can be expressed as a partition coefficient. In aqueous solution many, but not all pesticide compounds exhibit strong affinity for soil organic matter or concentrate in the lipid phase of soil organisms. Some, notably the cationic group, also exhibit marked affinity for clay or other mineral surfaces. An overall partition (or distribution) coefficient (kD) can be defined ... 

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... 

A solute can be extracted from one phase into another in which it is more soluble. The ratio of solute concentrations in each phase at equilibrium is called the partition coefficient. If more than one form of the solute exists, we use a distribution coefficient instead of a partition coefficient. We derived equations relating the fraction of solute extracted to the partition or distribution coefficient, volumes, and pH. Many small extractions are more effective than a few large extractions. 

The partition or distribution coefficient, 1 is equal to the ratio of the concentration of the solute in the solvent to that in the water, i.e.,... 

The constant Kd is known as the partition, or distribution, coefficient. Some experimental results are collected in Table 1.19. It is important to note that the ratio c2/Cj is constant only when the dissolved substance has the same relative molecular mass in both solvents. The distribution or partition law may be formulated thus when a solute distributes itself between two immiscible solvents there exists for each molecular species, at a given temperature, a constant ratio of distribution between the two solvents, and this distribution ratio is independent of any other molecular species which may be present. The value of the ratio varies with the nature of the two solvents, the nature of the solute, and the temperature. 

To test this hypothesis further, a partition or distribution coefficient was calculated from the sorption at pH 2 by a number of soils. The same experimental technique was used as previously described. A sorption index is calculated by the following expression ... 

Partition or distribution coefficients Environmental phenomena frequently involve the transfer of a substance, i (or solute) from one medium (phase 1) into another (phase 2). The amount transferred can be quantified by measuring the equilibrium concentration of i in both media (phases). If the overall attraction between i and phase l is weaker than that between i and phase 2, the substance will prefer to be in phase 2. The resulting distribution or partitioning of i can be estimated quantitatively by defining a partition or distribution coefficient (K np) as follows ... 

Hydrophobicity The preference of a solute to dissolve in apolar solvents or molecules over polar solvents. A common measure of hydrophobicity is the partition or distribution coefficient in the -octanol/water system, often used to predict pharmacokinetics and pharmacodynamics in QSAR and Hansch analysis. 

The distribution of a solute between two immiscible phases expresses the degree of separation performance. Liquid-liquid extraction relies on the uneven distribution of solutes between the two immiscible phases. It is usually expressed in terms of a partition or distribution coefficient ... 

Toxicity of many substances, their biological uptake and hpophilic storage in the body, their bioconcentrations (movements through the food chain resulting in higher concentrations), and so forth, as well as their fate in the environment can be predicted from their octanol/water partition coefficients, Poet-The partition or distribution coefficient of a substance is the ratio of its solubility in two immiscible solvents. It is a constant without dimensions, and depends on the temperature and pressure. Thus,... 

Extraction is seldom the sole method used to purify a compound, but it is a rapid and versatile technique that can be used to achieve a preliminary separation prior to a final purification step. Separation of components by extraction depends upon the difference in solubility of a compound in two mutually insoluble phases. Mathematical aspects of extraction are formulated in terms of a simple distribution law, K = CJC, which states that at equilibrium a solute will distribute itself between two immiscible phases, a and 6, such that the ratio of concentrations in the two phases is a constant at a given temperature. The constant K is called the partition or distribution coefficient. If a substance dissolved in solvent b is to be extracted into a second solvent a), it is obviously advantageous to choose solvent a such that the value of K will be as large as possible. Unfortunately, there is no sure way of predicting K, and the organic chemist relies on the rule that like dissolves like and his previous experience in selecting the best solvent system for an efficient extraction. 

Single-stage, batch or continuous separation steps in multiphase systems can only lead to near-complete separations when the partition (or distribution) coefficients of the components over the various phases are sufficiently different. Because of the common structural similarity of main products and contaminants, this is usually not the case. The key parameter is the so-called separation factor S (Fig. 3). Assuming thermodynamic equilibrium between outlet flows of a single equilibrium stage, the separation factor S relates performance to the ratio of auxiliary flow (V) and feed flow (L) and the distribution coefficient of the... 

Relating the formation constant to chromatographic retention requires that it be expressed in terms of a partition or distribution coefficient (Af,). The partition coefficient is defined as the ratio of solute concentration in the stationary phase to that in the mobile phase... 

The film surface, x = 0, is assumed to be at equilibrium with the gas phase, expressed through a dimensionless partition or distribution coefficient m (g/m )/(g/m ). m can be extracted from Henry s constants (see Table 6.1) or from activity coefficients (see Illustration 6.14). Some typical values appear in Table 4.7. 

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