Bentley's Textbook of Pharmaceutics

Chapter 16

Distillation

Distillation is a method of separating substances which differ appreciably in their vapour pressures. It is used in pharmacy either to extract volatile active principles from vegetable drugs or to separate volatile substances from their less volatile impurities; it also provides a method of recovering volatile solvents, notably alcohol, for further use. With the exception of ‘destructive distillation’, the term is normally applied to liquid–solid systems; the following qualifications are used.

Simple distillation is the process of converting a liquid into its vapour, transferring the vapour to another place, and recovering the liquid by condensing the vapour, usually by leading it into contact with a cold surface. The apparatus used consists essentially of three parts; the still in which the volatile material is vapourized; the condenser in which the vapours are condensed; and the receiver in which the distillate is collected. Simple distillation can produce partial separation of components with different boiling points in a liquid mixture, the more volatile component being obtained in increased concentration in the vapour. The process is generally used for the separation of liquids from nonvolatile solids, e.g. preparation of distilled water and recovery of alcohol in the preparation of dry extracts.

Fractional distillation (or rectification) is the process employed to separate miscible volatile liquids having different boiling points. It differs from simple distillation in that partial condensation of the vapour is allowed to occur in a fractionating column, through which the vapour must pass before reaching the condenser. This column enables the ascending vapour from the still to come in contact with the condensing vapour returning to the still, and this results in the enrichment of the vapour in the more volatile component.

Steam distillation is used for the distillation of water-immiscible liquids of high boiling points, e.g. turpentine, aniline. By bubbling steam through the liquid, the mixture boils at below the normal boiling point of either component. The distillate consists of the two liquids in the same proportions as in the vapour.

Destructive distillation is the term used to describe the decomposition of a substance, usually a natural product, by heat followed by the condensation and collection of the volatile products of decomposition. It is not a pharmaceutical process but is used in the manufacture of some substances used in medicine, examples being the destructive distillation of wood and of coal to produce tars.

Simple Distillation Under Atmospheric Pressure

Fig. 16.1 Steam-jacketed still.

Operations which are in frequent or continuous use often have specially designed stills suited to one product or group of products. In pharmacy the best-known example is the continuous water still for producing distilled water for various purposes including Water for Injection (WFI).

On a manufacturing scale, distilled water is required in large quantities as a component of many preparations, and for the aqueous extraction of drugs, where the accumulated residues from the use of tap water would be objectionable. The steam condensate from the heating jackets and coils of the plant is quite unsuitable, because it may contain traces of chlorine, derived from the public supply, and will certainly be contaminated with iron from the surfaces of the pipes, etc. The distilled water required should be specially prepared.

Manesty Automatic Water Still

A steam heated form of this continuous action water still, of the size often used in manufacturing laboratories, is shown in section in Fig. 16.2. Ordinary water from the public supply or condensed steam from the plant enters at the base of the still and surrounds the condenser tubes, which are vertical and open at both ends. As the water rises in the jacket around the condenser tubes, it condenses the steam descending the tubes. By a suitable adjustment of the rate of flow, the condensing water is heated almost to boiling point at the top of the jacket where dissolved gases are released and are allowed to escape into the air. Much of the heated water from the jacket flows to waste, but the remainder passes into the bowl-shaped still head, where it is boiled by steam circulating under pressure through a copper coil. The steam from the boiling water is unable to escape except through the condenser tubes, the upper ends of which protrude into the still head. The descending steam is condensed into distilled water, which flows from the lower ends of the tubes.

Fig. 16.2 Manesty steam-heated automatic water still: internal structure.

The heat generated by the condensation of the steam is therefore used to preheat the water entering the head, not only driving off dissolved gases but also economizing in the amount of steam used in the coil.

Fig. 16.3 shows a smaller, electrically heated form of the Manesty automatic water still, the principle of which is the same as that of the steam heated type.

Fig. 16.3 Manesty electrically heated automatic water still: internal structure.

Simple Distillation Under Reduced Pressure

Small Scale

The effect of pressure on boiling point has already been discussed. From this it is clear that liquids, which are unstable at their boiling point under atmospheric pressure, may be distilled at a much lower temperature under reduced pressure with less likelihood of decomposition. Similar considerations apply in the concentration of solutions of thermolabile substances. Distillation under reduced pressure is very commonly used for the evaporation of the menstruum in the preparation of extracts (Chapter 13). Evaporative distillation in high vacuums is of great importance in the purification of vitamins.

Vacuum distillation is most conveniently carried out in a Claisen flask (Fig. 16.4). The second neck prevents splashing of the violently agitated liquid. Bumping occurs very readily during vacuum distillation, but is easily prevented by means of a stream of air bubbles from a tube drawn out to a very fine capillary dipping in the boiling liquid. The capillary should be sufficiently fine to permit only a slow stream of bubbles to be blown by mouth through a little ether in a test tube. It is usual to use apparatus with interchangeable ground glass joints. Heating with a naked flame requires considerable practice, so it is advisable to use a water bath or oil bath maintained at about 20°C higher than the boiling point of the liquid under reduced pressure. In all vacuum distillations, a small pressure gauge (manometer) should be inserted between the pump and the receiver. In carrying out the distillation, heating is not commenced until the required vacuum is attained; evacuation of all apparatus. Containing hot liquid will almost invariably result in the liquid frothing over into the receiver. Thin walled glass apparatus, such as that bottomed flasks and conical flasks, should never be used for vacuum distillation.

Fig. 16.4 Vacuum distillation apparatus.

For the evaporation of extracts under reduced pressure at below 60°C, the apparatus shown in Fig. 16.5 is very convenient for laboratory scale work. Two water pumps, one of which is used intermittently, are required. The apparatus is evacuated and a suitable quantity of liquid is allowed to flow into the still by opening screw clip A. Screw clip B is closed and the second pump is turned off. The liquid is heated by means of a water bath up to 50–60°C and the distillate is collected in the Buchner flask. During distillation, screw clip A is adjusted so that the extract enters the still at the same rate as it distils. When the Buchner flask is nearly full, stopcocks C and D are closed, screw clip B is opened and the receiver may be detached and emptied; in the meantime the distillate is collected in E. The receiver is replaced and then evacuated by the second pump, clip B is closed, cock D opened and the distillate from E is run into the Buchner flask by opening C. The whole process is then repeated.

Fig. 16.5 Evaporation under reduced pressure.

In some instances persistent foaming occurs during vacuum distillation. This may be overcome by adding capryl alcohol to the liquid to be distilled, or by inserting a second air capillary in the thermometer neck of a Claisen flask; the stream of air drawn through breaks the rising foam. Antifoaming still heads have been devised and are very effective for dealing with this problem.

Vacuum Stills

These are employed for distilling substances that have a high boiling point at atmospheric pressure, or for substances that are damaged by a high temperature, or for removing the last traces of a volatile solvent. Fig. 16.6 shows the arrangement of a vacuum still. To facilitate the collection and removal of the distillate, without stopping the distillation, two receivers are fitted. By a suitable arrangement of cocks they may be used alternately, the distillate being run off from one while the other is connected to the still under vacuum. Stills vary in design more than any other plant; usually they are specially made to do a particular class of work. A vacuum still with column is shown in Fig. 16.7.

Fig. 16.6 Vacuum still.

Fig. 16.7 Vacuum still with column.

The vacuum used for distilling should be the best possible, especially if fractional distillation is to be done. For pressures of 3–5mmHg with a large-vacuum fractionating still, a double-stage dry pump of good capacity, is necessary. To produce pressures of 1mmHg and less on anything larger than laboratory scale plant requires special design of plant and is only attempted for very special purposes. The wet type of pump and the type of dry pump giving a vacuum of about 68cm are quite unsuitable for use with vacuum stills, although the dry pump may be used for the removal of the last traces of volatile solvents from nonvolatile extracts. It is possible to use more than one oven or evaporator with one pump, provided that the capacity of the pump is big enough, but when working vacuum stills the aim should be to use a separate pump to each still. If two or more stills are working at the same time with only one pump, then the vapour pressure of a substance which is being distilled in one still will probably hinder obtaining a good vacuum in the others. The volume occupied by the vapour of a substance in the high vacuum used in distilling is very great. At a pressure of 5mmHg the vapour is expanded 152 times. It is important that the swan neck of the condenser and the fractionating column if present, are of adequate size to prevent back pressure caused by the friction due to the high velocity of the vapour in restricted tubes.

With the high vacuum required for distilling, the prevention of leaks assumes great importance. They frequently arise in the packing of the glands of valves and the bottoms and tops of plug cocks. Leaks in the bottoms of plug cocks can be eliminated by using those specially designed for vacuum work; the bottoms of the cocks are closed and glands are fitted to the spindles. Another common source of leak is in the bottom outlet of the still. This valve or cock fitted to a pipe which must pass through the steam jacket, gets hard wear and strain due to the quick and extreme changes of temperature. If other means such as a dip pipe can be used for emptying the still, it is a good plan to dispense with this outlet, but very often the final residue is too viscous to be easily removed except through a bottom outlet. It will be economical to use only the very best quality of valves and cocks on vacuum stills.

Fractional Distillation

Theoretical Considerations and Small-Scale Methods

Fractional distillation is the process employed to separate miscible volatile liquids having different boiling points. In a mixture of two liquids, each may be regarded as dissolved in the other, and the possibility of separating the two liquids by fractional distillation depends on the effect each has on the vapour pressure of the other.

Vapour Pressure of Miscible Liquids

When the two components of a binary mixture are completely miscible, the vapour pressure of the mixture is a function of the composition as well as the vapour pressures of the two pure components. In an ideal solution where the relation of vapour pressure and composition is given by Raoult’s law, the partial vapour pressure of each volatile component is equal to the vapour pressure of the pure component multiplied by its mole fraction.

Thus for a mixture of A and B:

where P_A and P_B are the partial vapour pressures of the components when the mole fractions are X_A and X_B, respectively. The vapour pressures of the pure components are

and

The total vapour pressure P of the system is then P_A + P_B (Fig. 16.8).

Fig. 16.8 Vapour pressure and composition of miscible liquids.

Binary mixtures that follow Raoult’s law are those where the attraction between A and B molecules is the same as those for the pure components, e.g. benzene/toluene and paraffin mixtures. When the interaction of A and B molecules is less than that between the molecules of the pure constituents, the presence of B molecules reduces the A–A interaction, and similarly the A molecules reduce the B–B interaction. The partial vapour pressure is now greater than expected from Raoult’s ideal solution law and the system is said to exhibit positive deviation, e.g. benzene/ethyl alcohol, chloroform/ethyl alcohol. Negative deviation occurs when the A–B attraction is greater than the A–A or B–B attraction and the vapour pressure is less than expected, e.g. chloroform/acetone.

Referring to the ideal system (Fig. 16.8), the partial vapour pressure of the more volatile constituent B is higher so that the vapour phase will contain more of this component. By removing and condensing this vapour a liquid richer in B is obtained. This aspect is more readily seen from the curves relating to the boiling points of the/various compositions rather than the vapour pressures.

Boiling Point Diagrams and Fractional Distillation

The boiling point diagram for an ideal mixture corresponding to Fig. 16.8 is shown in Fig. 16.9. The lower curve shows the manner in which the boiling point of the mixture changes with composition; the upper curve relates the composition of the vapour in equilibrium with the liquid at the same temperature. Hence the boiling point of the mixture X is T. The vapour at T will have a composition fixed by point Y, which corresponds to a mixture richer in B than in A. If this vapour is condensed, a liquid having the same composition (X₁) will be obtained. The boiling point of the mixture X₁ is T₁ and when it is boiled, vapour having the composition Y₁, which yields a liquid X₂ on condensation, is obtained. X₂ is nearly pure B. Hence in this example, extensive fractionation has been achieved by boiling the mixture and condensing the vapour in equilibrium with the liquid and repeating the process with the condensed vapour. The volume of distillate (composition X₂), obtained will be small, since as the vapour is drawn off, the liquid remaining in the still gradually becomes poorer in component B and its boiling point rises. Fractional distillation is based on these principles.

Fig. 16.9 Boiling point and composition of miscible liquids.

The difference in composition between X and X₂ is equal to that produced in a fractionating column of two theoretical plates. Many laboratory columns bring about separations equivalent to ten or more theoretical plates so that efficient separation of the two components is possible. The change from X to X₁ is the composition difference corresponding to one theoretical plate and under ideal conditions this would occur in simple distillation.

Azeotropic Mixtures

An azeotropic mixture or constant boiling mixture is one in which the composition of the liquid and the vapour in equilibrium with it is the same. Thus the mixture behaves like a pure liquid in so far as it distils without change in composition or boiling point. Miscible binary liquids form azeotropic mixtures when the vapour pressure curve of the mixture exhibits a maximum or minimum. Such mixtures cannot be separated into their pure components by distillation. It is possible to separate them by distillation only into one component and a constant boiling mixture. Their behaviour on distillation is most easily followed by referring to the boiling point versus composition curves as shown in Fig. 16.10; (A) represents a mixture which possesses a maximum vapour pressure, i.e. low boiling point azeotrope; (B) represents a mixture with a minimum vapour pressure, i.e. high boiling point azeotrope. In this figure the concentration changes arising during distillation using a fractionating column equivalent to two theoretical plates are shown, i.e. X to Z. From (A) it can be seen that repeated fractionation will produce a distillate tending to the composition of the azeotropic mixture represented by point C. The material left in the still will be richer in B than it was originally; eventually pure B will be left after the whole of A has been distilled off in the form of the azeotropic mixture. If the original composition lies to the right of C, then by similar reasoning A will be left in the still. Mixtures of minimum boiling point are more common than mixtures exhibiting a maximum boiling point, e.g. alcohol and water, alcohol and benzene, alcohol and chloroform. Alcohol, boiling point 78.3°C, and water boiling point 100°C form a mixture of minimum boiling point 78.15°C, containing 95.57 per cent w/w alcohol. The composition of mixtures of minimum boiling point varies with pressure as, for example, water and alcohol can be completely separated by distilling at 28°C under 7cm pressure.

Fig. 16.10 Boiling point composition graphs; (A) Minimum boiling azeotrope; (B) Maximum boiling azeotrope. (C) is the composition of the constant boiling mixture.

On distilling a mixture of maximum boiling point, the distillate will be richer in the component A or B depending on whether the original concentration lies to the right or left of C. Fig. 16.10 (B) shows the original concentration X lying to the left of C and the distillate thus becomes richer in B.

Ternary Mixtures

Mixtures of three components, which do not form azeotropes, may be separated by fractional distillation in the same way as binary mixtures. Azeotropic ternary mixtures of maximum vapour pressure (minimum boiling point) are important. Water, boiling point 100°C, alcohol, boiling point 78.3°C and benzene, boiling point 80.2°C, form a ternary azeotropic mixture, boiling point 64.85°C, containing 18.5 per cent of alcohol, 7.4 per cent of water and 74.1 per cent of benzene; the boiling point of this mixture is lower than the boiling point of any binary mixture of any of the components. Fractionation of this ternary mixture is used on the large scale for the production of absolute alcohol. Absolute alcohol cannot be obtained by normal fractionation of dilute alcohol since a constant boiling mixture of 95.57 per cent w/w is formed. Benzene is added to the alcohol–water azeotrope and when distilled the mixture yields first the ternary water–alcohol–benzene azeotrope, boiling point 65.85°C, until all the water is removed from the system. Next a binary alcohol–benzene azeotrope, boiling point 68.2°C distils over and finally absolute alcohol, boiling point 78.3°C. Trichloroethylene may be used instead of benzene in this process. Solutions in a solvent such as benzene may be dried by azeotropic distillation; benzene forms a constant boiling mixture with water, boiling point 69.3°C. All the water present in the benzene is removed when about 10 per cent of the solvent has been distilled.

Fractionating Columns

If a mixture of chloroform, boiling point 61.2°C and benzene, boiling point 80.2°C, is distilled, the vapours first evolved will be richer in the more volatile component, chloroform; as this is distilled off, the vapours will become gradually richer in benzene, and the temperature of distillation will gradually rise. By changing the receiver when the temperature recorded on the thermometer in the head of the still has risen from 61 to 63°C, a fraction is obtained which is richer in chloroform than in benzene. Other fractions, which are increasingly rich in benzene, may be collected over the ranges 63–68°C, 68–73°C, 73–78°C, 78–80°C. Repeated separate distillations of the intermediate fractions, fractions of the same boiling range being combined, will ultimately result in a separation of the liquid into two main fractions, boiling point 61–63°C and 78–80°C, which represent a rough separation of the two constituents of the mixture. This process is very tedious and the same or a better effect can be achieved in a single distillation through a fractionating column. It should be remembered that complete fractionation by distillation is possible only with liquids which do not form azeotropic mixtures.

A fractionating column, which is inserted between the still and the condenser, acts by bringing about repeated distillations throughout the length of the column. The action of the column is partially to condense the vapours rising from the boiling liquid; this condensate will be richer in the more volatile component than the original liquid and it is vapourized again by the condensation of more ascending vapours; the vapours so produced will be still richer in the more volatile component, and when condensation and vapourization takes place further up the column, further enrichment in the more volatile component will be effected. Under ideal conditions this will result in the lower boiling point component arriving at the top of the column and the higher boiling point component being left at the bottom of the column. Thus a temperature gradient will be established along the column when distillation is in progress, and ultimately the vapour passing out of the column will be very rich in the low boiling point component. This series of events may be easily visualized by considering the action of a bubble-cap column which is used in large-scale distillation plant. The column consists of a number of plates mounted above one another, over which flows the condensed liquid (reflux) and through which the ascending vapour is made to bubble. Fig. 16.11 shows three bubble-cap plates in section. If we assume that at each plate the vapour and liquid reach equilibrium conditions, i.e. each plate is theoretically perfect the change in composition of liquid on passing from plate A to plate C will be equivalent to that produced by three theoretical plates. Ascending vapour from the still passes through the bubble caps on plate A and the vapour rising from it will be richer in the more volatile component. This vapour passes through the liquid on B, condenses, and the heat of condensation partially vapourizes the liquid. The process of condensation and vapourization will be repeated at C and so on, all the way up the column. In this simplified consideration of column action, each bubble-cap plate has the same effect as a separate still. The changes in composition at each plate can be estimated from the boiling point versus composition curves.

Fig. 16.11 Section of distillation column (bubble-cap type).

Design of fractionating columns: The purpose of a fractionating column is to achieve an extensive liquid–vapour interface so that equilibrium between ascending vapour and reflux can be rapidly attained. In the laboratory, packed columns are frequently used; the packing may consist of single turn helices (spirals) of wire or glass, glass rings, cylindrical glass beads, stainless steel rings, etc. The Vigreux column which in the best types has indentations in the walls, spirally arranged and occupying most of the interior, acts as a packed column. In the Widmer column (Fig. 16.12) fractionation occurs in the central spiral which is kept warm by the vapour around it. Condensed liquid is returned to the still by a trap.

Fig. 16.12 Widmer column.

The separation of a mixture in a column depends upon the proportion of vapours traversing the column which are condensed and returned to the still—the reflux ratio. A column operating under total reflux will not yield distillate; a state of dynamic equilibrium is reached and a maximum degree of separation of the components is obtained along the length of the column. If the quantity of distillate removed at a time is small, this will minimize disturbance of equilibrium conditions and the concentration gradient along the column will remain reasonably stable. For columns operating above about 60°C, heat loss should be prevented by insulation, e.g. asbestos cord, silver vacuum jacket. For temperatures above 100°C the column is surrounded by a heating jacket which is generally adjusted to the temperature of the vapour that emerges from the top of the column. Heat loss will cause excessive condensation within the column which may result in flooding and also will disturb the steady temperature gradient along the column. Under adiabatic conditions the temperature gradient is determined by the vapour–liquid equilibria in the column. For efficient separation it is essential to use a high reflux ratio and collect distillate slowly so that the column is operating under equilibrium conditions the reflux ratio is con veniently controlled by means of a suitable still head. Fig. 16.13 shows a total condensation variable take-off still head. In operation the vapours are refluxed with the tap closed until equilibrium and the resulting optimum separation are attained; the tap is opened and a small quantity of distillate is taken off; the tap is closed, the column again operated under total reflux and more distillate is then run off. By this means very sharp separations are achieved both at atmospheric and under reduced pressure. Heat input to the still should be controlled—if too little, the packing is insufficiently wetted by reflux and if too much, the vapour velocity may be too great for equilibrium to be attained.

Fig. 16.13 Total condensation variable take-of still head.

Efficiency of fractionating columns: The efficiency of a column is commonly measured by the height equivalent to a theoretical plate (HETP). At a theoretically perfect plate, the vapours are brought into equilibrium with the liquid through which they pass; the length of a column required to achieve this is defined as the HETP, and vapour drawn off from the top of such a length of column is in equilibrium with the liquid at the bottom. The HETP value is obtained by analysis of the liquid in the still, the vapour at the head of the column and from data for the composition of liquid and vapour in equilibrium with one another.

Fractional Distillation Under Reduced Pressure

Any of the apparatus described for fractionation at atmospheric pressure may be used for fractionation in vacuo. In some instances a short fractionating column is made to fit into the neck of the still. The most convenient receiver for vacuum fractionation is one of the forms of the Perkin triangle (Fig. 16.14). By suitable manipulation of the taps, the receiver may be changed without interrupting the distillation. Azeotropic mixtures may be separated by vacuum fractionation. Vacuum fractionation has found extensive application in the separation of the mixed fatty acids derived from oils and fats.

Fig. 16.14 Simple form of Perkin triangle.

Molecular Distillation

In molecular distillation or evaporative distillation the pressure of the vapour above the liquid is much lower than the pressure of a saturated vapour in equilibrium with the liquid. The distance between the evaporating surface and the condenser is about equal to the mean free path of the vapour molecules at that very low pressure. Molecules leaving the surface of the liquid are therefore more likely to hit the condenser surface than to collide with other molecules and little or no recondensation at the surface of the liquid takes place. In ordinary distillation at atmospheric pressure and under reduced pressure, conditions approaching equilibrium are maintained and a considerable amount of recondensation (refluxing) occurs. Molecular distillation is usually carried out at 0.001–0.00001mm pressure. There is no well-defined temperature at which distillation commences, unlike ordinary distillation procedures. An extensive review of the principles and techniques has been given by Hickman (1944). This technique has been applied with great success to the purification of vitamin A. For this purpose the fish liver oil is fed continuously as a thin film on to heated discs rotating in a high vacuum.

Distillation in Steam

Theoretical Considerations and Small-Scale Methods

Vapour Pressure of Immiscible Liquids

In a pair of immiscible liquids each liquid exerts its own vapour pressure and neither liquid has any appreciable effect on the vapour pressure of the other. The vapour pressure of the mixture at a given temperature is equal to the sum of the vapour pressures of the two pure components at that temperature. Completely immiscible liquids are not known, although the miscibility of mercury and water, for instance, is very low indeed. For practical purposes, the miscibility of many organic liquids, e.g. turpentine or toluene, and water is negligible and calculations of the relative volatilities of such mixtures from their vapour pressures yield approximately correct results.

Distillation of Immiscible Liquids

A mixture of immiscible liquids begins to boil when the sum of their vapour pressures is equal to the atmospheric pressure. Thus in the case of water and a liquid which boils at a much higher temperature than water, the mixture boils below the boiling point of pure water. The boiling point of turpentine is about 160°C, but when it is mixed with water and heated, the mixture boils at about 95.6°C. At this temperature the vapour pressure of water is 647mm and that of turpentine, 113mm; the sum 647 + 113 = 760mm which is the normal atmospheric pressure. From these facts, it will be seen that a high boiling substance may be distilled with water at a temperature much below its boiling point. For substances which are insoluble in water and not decomposed by it, this provides an alternative to distillation under reduced pressure. Certain volatile solids, e.g. camphor, may be distilled in the same way.

For volatile substances which are miscible with water, distillation in steam would involve the same principles as fractional distillation.

The composition of the vapour over a pair of immiscible liquids may be calculated approximately from the vapour pressures and molecular weights of the components. Since in distillation this vapour is condensed, this also represents the composition other distillate. If the vapour pressures of the two components at the boiling point of the mixture are p₁ and p₂, by Avogadro’s hypothesis, the number of molecules present in the vapour is proportional to the vapour pressures, p₁ and p₂. The weights of the components in the vapour will therefore be in the ratio p₁M₁: p₂M₂, where M_I and M₂ are their respective molecular weights. Hence the proportion by weight, w₁, w₂, of the two components in the distillate is given by the equation:

For turpentine, which contains a high proportion of pinene, C₁₀H₁₆, molecular weight 136, and water, molecular weight 18, the vapour pressures at the boiling point of the mixture, 95.6°C, are 113 and 647mm, respectively and the distillate will contain turpentine and water in the ratio, 113×136:18×647 or 1.32:1. For camphor, C₁₀H₁₆O, molecular weight 152, boiling point at 760mm 206°C, the vapour pressures at the boiling point of the mixture, 99°C, are 24.6 and 735mm and the distillate will contain camphor and water in the ratio 24.6×152:735×18 or 1:3.54.

From these calculations, it can be seen that a substance of low volatility can be satisfactorily distilled in steam provided its molecular weight is considerably higher than that of water. Water is particularly suitable for the special case of codistillation, termed steam distillation on account of its low molecular weight and high boiling point. Other compounds of similar molecular weight to water, e.g. hydrogen sulphide, ammonia, methane, are gases at ordinary temperatures. Liquids other than water have been used for codistillation, e.g. paraffins for the codistillation of indigo. Substances which are only very slowly distilled with steam at about 100°C may be distilled with superheated steam.

Small-Scale Apparatus

A suitable apparatus for steam distillation on the laboratory scale is shown in Fig. 16.15. The safety tube in the steam generator permits the expulsion of some water if excessive pressure is developed. The distillate separates into two layers, water and the other component; these are separated in a separating funnel. The actual yield is often somewhat greater than the calculated yield since minute particles of the substance are carried over mechanically by the steam.

Fig. 16.15 Steam distillation apparatus.

Steam distillation is used for the determination of volatile oils in drugs and, conversely, distillation with toluene is used for the determination of moisture in drugs.

Large-Scale Apparatus

Steam distillation is used to extract most of the volatile oils, such as clove, aniseed, eucalyptus and so on. When the distillation is done in the locality where the material grows, special forms of stilt axe used often quite primitive, and arranged to meet local conditions. A common form of water still is shown diagrammatically in Fig. 16.16. A still of this kind has a jacket and, in addition, live steam is sometimes injected below the material, which is supported on a perforated false bottom. Means of charging and discharging the still are provided by manholes in the top and side.

Fig. 16.16 Still for steam distillation of volatile oils.

Most volatile oils are lighter than water and will separate from the distillate as an upper layer. If a Florentine receiver, shown between the still and condenser in Fig. 16.16, is used the water can be run off from the spout on the left and can be returned to the still, as shown in the diagram, or run to waste. The oil which collects on the surface is run off from the upper spout. Some volatile oils are heavier than water, in which case the operation is reversed. Where the specific gravity of the oil is so near 1.0 that separation does not take place, it may be necessary to collect the whole of the distillate and extract it with a volatile solvent, subsequently distilling off the solvent from the oil.