Distribution of a solute between two solvents

If a solute is soluble in two solvents, and the solvents are immiscible, it is found that the concentration of solute in the one layer is always bigger than the concentration of solute in the other layer by a fixed amount. This ratio (symbolized and known as the distribution or partition ratio) is a constant at a particular temperature  

Our first example is the distribution of solid iodine the solute) between water and tetrachloromethane the solvents). If we add a few crystals of iodine to some water and tetrachloromethane (CCI4) in a flask and shake the mixture well, the iodine colours the water a pale brown colour and the tetrachloromethane a purple colour. There is more iodine in the organic layer (the tetrachloromethane layer) than in the aqueous layer (the water layer). At 25°C experiments show that 

It is important to realize that the value of at one temperature is the same whether we add 0.1 g, 1 g, or 2 g (etc.) of iodine to the mixture of solvents. This is because the partitioning of a solute between two solvents is another example of a dynamic equilibrium in which the relative concentration of the iodine in the two phases is constantly being maintained  

If we add iodine to one layer and shake, the iodine concentration in both layers alters so as to maintain the value of 

At its simplest, the distribution ratio allows us to calculate the concentration of solute in one layer, knowing the concentration of solute in the other. For example, if the concentration of iodine in the aqueous layer was determined by titration with 

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Hydrogen Bonding and Internal Pressure. It is clear from what has been considered that the extent to which mixtures deviate from ideality governs the distribution of a solute between two solvents. It is now possible to predict the nature of the deviation from ideality of mixtures of substances on the basis of their hydrogen-bonding potentialities and internal pressures. 

Apart from the above considerations, the distribution of a solute between two immiscible phases is always dependent upon temperature [128-130], solubility [131,132], resistance to degradation [133-135], solvent purity [130], and phase composition [136,137] (several reviews [130,132,138] give full details about the impact of these parameters on log determination). 

The distribution of a solute between two mutually immiscible solvents can be represented by the simple equation,... 

Partition The partitioning (distribution) of an essential oil between different solvents, utilizing the different solubilities of the oil components. Usually the distribution of a solute between two immiscible solvents in contact with each other for example an essential oil in solvents pentane (a hydrocarbon) and aqueous methanol will separate into their constituents. Oxygenated compounds... 

The Partition of a Solute between Two Solvents. If a solution of iodine in water is shaken with chloroform, most of the iodine is transferred to the chloroform. The ratio of concentrations of iodine in the two phases, called the distribution ratio, is a constant in the range of small concentrations of the solute in each solution. For iodine in chloroform and water the value of this ratio at room temperature is 250 hence whenever a solution of iodine in chloroform is shaken with water or a solution in water is shaken with chloroform until equilibrium is reached the iodine concentration in the chloroform phase is 250 times that in the water phase. 

It is seen on. consideration of the various equilibria that the distribution ratio of a solute between two solvents is equal to the ratio of the solubilities of the solute (as a crystalline or liquid phase) in the two solvents, provided that the solubilities are small. Moreover, the distribution ratio of a gaseous solute between two -solvents is proportional to the ratio of its two Henry s-law constants (its solubilities in the two solvents). 

Although Peligot observed in 1842 that uranyl nitrate is soluble in ether, it was not until materials of high purity were needed for nuclear reactors that extensive applications and developments, both industrial and analytical, were made. The literature on applications of liquid-liquid extraction (solvent extraction) is extensive for details of the various procedures the reader is referred to the original papers and to compilations. " This chapter examines separations involving distribution of a solute between two immiscible phases and chemical equilibria of significance to the distribution ratio. Batch, countercurrent, and continuous liquid-liquid extractions are described in turn, followed by consideration of the factors governing the distribution ratio and finally by some illustrative applications. 

Ether extraction of an acidified solution of iron(III) chloride is an old way of removing iron. The principle of solvent extraction exemplified is extensively applied in research and industry where solutes are moved from aqueous to non-aqueous phases and vice versa to effect separation or purification. The distribution of a solute between two immiscible liquids is expressed by the partition coefficient, K [A]/[B] more precisely, when activities are substituted for concentrations in the two phases, K =... 

The distribution of a solute between two immiscible solvents depends on its solubility in each and also on the amount of each solvent present. The dissolved substance distributes itself between the two immiscible solvents so that the ratio of its concentration in each solvent remains a constant for any particular temperature. This principle, called the distribution law, as applied to aqueous solutions, can be expressed mathematically by the following equation ... 

Use the law of mass action to explain the distribution of a solute between two immiscible solvents (Section 14.8, Problems 71-74). 

Solvent extraction involves the distribution of a solute between two iimnis-cible liquid phases. This technique is extremely useful for very rapid and clean separations of both organic and inorganic substances. In this chapter, we discuss the distribution of substances between two phases and how this can be used to form analytical separations. The solvent extraction of metal ions into organic solvents is described. 

The discussion in this section has been concerned with the distribution of a solute between two liquid, phases whose equilibrium is unaffected by the added solute. This will occur if the amount of added solute is very small, or if the solvents are essentially immiscible at all conditions. However, if the amount of dissolved solute is so large as to affect the miscibility of the solvents, the solute addition can have a significant effect on the solvents, including the increase (salting in) or decrease (saltin out) of the mutual solubility of the two solvents, as was discussed in Sec. 11.2. It is important to emphasize that such situations are described by the methods in Sec. 11.2 as a multicomponent liquid-liquid equilibrium problem, in contrast to the procedures in this section, which are based on the assumption that the partial or complete immiscibility of the solvents is imaffected by the addition of the partitioning solute. 

Yet another aspect of this general question of the factors determining phase equilibria is illustrated by the partition of a solute between two solvents. In so far as the activity of a solute in a dilute solution is proportional to its concentration, the equilibrium distribution will be such that the ratio of concentrations in two immiscible solvents in contact is constant at constant temperature and pressure. The same principle will apply to the solution of a gas in a liquid. The concentration of dissolved gas is proportional to its partial pressure above the solution. These statements presuppose that the molecular complexity of the solute is the same in both phases, and that association or dissociation, ionic or otherwise, is excluded. 

EXPERIMENT 4B Distribution of a Solute between Two Immiscible Solvents 37... 

The effect of molecular interactions on the distribution coefficient of a solute has already been mentioned in Chapter 1. Molecular interactions are the direct effect of intermolecular forces between the solute and solvent molecules and the nature of these molecular forces will now be discussed in some detail. There are basically four types of molecular forces that can control the distribution coefficient of a solute between two phases. They are chemical forces, ionic forces, polar forces and dispersive forces. Hydrogen bonding is another type of molecular force that has been proposed, but for simplicity in this discussion, hydrogen bonding will be considered as the result of very strong polar forces. These four types of molecular forces that can occur between the solute and the two phases are those that the analyst must modify by choice of the phase system to achieve the necessary separation. Consequently, each type of molecular force enjoins some discussion. 

The term solvent extraction refers to the distribntion of a solute between two immiscible liquid phases in contact with each other, i.e., a two-phase distribution of a solute. It can be described as a technique, resting on a strong scientific foundation. Scientists and engineers are concerned with the extent and dynamics of the distribution of different solutes—organic or inorganic—and its use scientifically and industrially for separation of solute mixtures. 

Solvent extraction is another name for liquid-liquid distribution, that is, the distribution of a solnte between two liquids that must not be completely mutually miscible. Therefore, the liquid state of aggregation of matter and the essential forces that keep certain types of liquids from being completely miscible are proper introductory subjects in a study of solvent extraction. Furthermore, the distribution of a solute depends on its preference for one or the other liquid, which is closely related to its solubility in each one of them. Thus, the general snbject of solnbilities is highly relevant to solvent extraction. 

Lipophilicity is a molecular property experimentally determined as the logarithm of the partition coefficient (log P) of a solute between two non-miscible solvent phases, typically n-octanol and water. An experimental log P is valid for only a single chemical species, while a mixture of chemical species is defined by a distribution, log D. Because log P is a ratio of two concentrations at saturation, it is essentially the net result of all intermolecular forces between a solute and the two phases into which it partitions (1) and is generally pH-dependent. According to Testa et al. (1) lipophilicity can be represented (Fig. 1) as the difference between the hydrophobicity, which accounts for hydrophobic interactions, and dispersion forces and polarity, which account for hydrogen bonds, orientation, and induction forces ... 

The International Union of Pure and Applied Chemistry (IUPAC) recommends the use of liquid-liquid distribution rather than the traditional term, solvent extraction. However, solvent extraction is still used commonly in the literature, and that is why it is also being used here interchangeably (Chapter 7). Solvent extraction utilizes the partition of a solute between two practically immiscible liquid phases—one a solvent phase and the other an aqueous phase. Liquid-liquid partitioning methods are important separation tools in modern biotechnology. They have become increasingly popular as part of a... 

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