PAIRS OF MISCIBLE SOLVENTS

Acetic acid with chloroform, ethanol, ethyl acetate, acetonitrile, petroleum ether, or water. 

Aniline with acetone, benzene, carbon tetrachloride, ethyl ether, n-heptane, methanol, acetonitrile or nitrobenzene. 

Benzene with acetone, butyl alcohol, carbon tetrachloride, chloroform, cyclohexane, ethanol, acetonitrile, petroleum ether or pyridine. 

Butyl alcohol with acetone or ethyl acetate. 

Aniline with acetone, benzene, carbon tetrachloride, ethyl ether, n-heptane, methanol, acetonitrile or nitrobenzene. Benzene with acetone, butyl alcohol, carbon tetrachloride, chloroform, cyclohexane, ethanol, acetonitrile, petroleum ether or pyridine. 

Chloroform with acetic acid, acetone, benzene, ethanol, ethyl acetate, hexane, methanol or pyridine. 

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Mixed Solvents. Where a substance is too soluble in one solvent and too insoluble in another, for either to be used for recrystallisation, it is often possible (provided they are miscible) to use them as a mixed solvent. (In general, however, it is preferable to use a single solvent if this is practicable.) Table 8 contains many of the common pairs of miscible solvents. 

If the substance is found to be far too soluble in one solvent and much too insoluble in another solvent to allow of satisfactory recrystallisation, mixed solvents or solvent pairs may frequently be used with excellent results. The two solvents must, of course, be completely miscible. Recrystallisation from mixed solvents is carried out near the boiling point of the solvent. The compound is dissolved in the solvent in which it is very soluble, and the hot solvent, in which the substance is only sparingly soluble, is added cautiously until a slight turbidity is produced. The turbidity is then just cleared by the addition of a small quantity of the first solvent and the mixture is allowed to cool to room temperature crystals will separate. Pairs of liquids which may be used include alcohol and water alcohol and benzene benzene and petroleum ether acetone and petroleum ether glacial acetic acid and water. 

You could first wash the column with methanol, then trichloromethane, then heptane (or methanol, ethyl ethanoate, heptane). You cannot go directly from methanol to heptane because the two are only partly miscible. The column needs to be washed with about 20 dead volumes of each solvent (about 50 cm3 of each solvent for a 25 cm x 4.6 mm column). To get back to CH3OH/H2O 50 50 you would have to go through the sequence of solvents in reverse. If buffer solutions or ion-pairing reagents have been used in the mobile phase, very much longer equilibration times may be needed. 

According to Figure 5.2 and to chemical experience, the selection of other pairs of non-miscible organic liquids is difficult and yields mainly unusual (not to say, exotic) solvents or pairs of solvents [68] such as fluorous liquids (cf. Chapter 6). This is the reason why no other organic-organic biphasic catalytic processes have yet been commercialised. 

For the two-component, two-phase liquid system, the question arises as to how much of each of the pure liquid components dissolves in the other at equilibrium. Indeed, some pairs of liquids are so soluble in each other that they become completely miscible with each other when mixed at any proportions. Such pairs, for example, are water and 1-propanol or benzene and carbon tetrachloride. Other pairs of liquids are practically insoluble in each other, as, for example, water and carbon tetrachloride. Finally, there are pairs of liquids that are completely miscible at certain temperatures, but not at others. For example, water and triethylamine are miscible below 18°C, but not above. Such pairs of liquids are said to have a critical solution temperature, For some pairs of liquids, there is a lower (LOST), as in the water-tiiethylamine pair, but the more common behavior is for pairs of liquids to have an upper (UCST), (Fig. 2.2) and some may even have a closed mutual solubility loop [3]. Such instances are rare in solvent extraction practice, but have been exploited in some systems, where separations have been affected by changes in the temperature. 

A method similar to the above was proposed recently by Kuhn, which uses a pair of solvents immiscible at a lower temperature but becoming miscible with elevating temperature5. To the present authors this method seems more easily applicable and perhaps more promising than the counter-current distribution, since no special instrumentation is necessary for the former. At any rate, it is obvious that these three methods do not allow fractionation of copolymers only by the composition without interference of the molecular weight, because they all are based on the solubility difference among constituent species. 

TABLE 18. SOME COMMON IMMISCIBLE OR SLIGHTLY MISCIBLE PAIRS OF SOLVENTS... 

Figure 14.1. Equilibria in a ternary system, type 1, with one pair of partially miscible liquids A = 1-hexene, B = tetramethylene sulfbne, C = benzene, at 5(TC (JR.M. De Fre, thesis, Gent, 1976). (a) Equilateral triangular plot point P is at 20% A, 10% B, and 70% C. (b) Right triangular plot with delines and tieline locus, the amount of A can be read off along the perpendicular to the hypotenuse or by difference, (c) Rectangular coordinate plot with tieline correlation below, also called Janecke and solvent-free coordinates.

All pairs of solvents whose M numbers differ by 15 units or less are miscible in all proportions at 15°. 

By definition, the M number is a reliable means for predicting miscibility of any liquid with the standard solvent. When applied to other pairs of liquids selected from Table 6-3, it leads to a high percentage of valid predictions. 

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