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Drying of Solvents and Laboratory Chemicals

Purification of Laboratory Chemicals, Pergamon Press 1980 (2nd Ed.), pp 20-25

by DD Perrin, WLF Armarego, DR Perrin -

Removal of solvents

Where substances are sufficiently stable, removal of solvents from recrystallized materials presents no problems. The crystals, after filtering at the pump (and perhaps air-drying by suction), are heated in an oven above the boiling point of the solvent (but below their melting point), followed by cooling in a desiccator. Where this treatment is inadvisable, it is still often possible to heat to a lower temperature under reduced pressure, for example in an Abderhalden pistol. This device consists of a small chamber which is heated externally by the vapour of a boiling solvent. Inside this chamber, which can be evacuated by a water pump or some other vacuum pump, is placed a small boat containing the sample to be dried and also a receptacle with a suitable drying agent. Convenient liquids for use as boiling liquids in an Abderhalden pistol, and their temperatures, are given in Table 8. In cases where heating above room temperature cannot be used, drying must be carried out in a vacuum desiccator containing suitable absorbents. For example, hydrocarbons, such as benzene, cyclohexane and petroleum ether, can be removed by using shredded paraffin wax, and acetic acid and other acids are absorbed by pellets of sodium hydroxide or potassium hydroxide. However, in general, solvent removal is less of a problem than ensuring that the water content of solids and liquids is reduced below an acceptable level.

Removal of water

Methods for removing water from solids depend on the thermal stability of the solids or the time available. The safest method is to dry in a vacuum desiccator over concentrated sulphuric acid, phosphorus pentoxide, silica gel, calcium chloride, or some other desiccant. Where substances are stable in air and melt above 100C, drying in an air oven may be adequate. In other cases, use of an Abderhalden pistol may be satisfactory.

Often, in drying inorganic salts, the final material that is required is a hydrate. In such cases, the purified substance is left in a dessicator to equilibrate above an aqueous solution having a suitable water-vapour pressure.

The choice of desiccants for drying liquids is more restricted because of the need to avoid all substances likely to react with the liquids themselves. In some cases, direct distillation of an organic liquid is a suitable method of drying both solids and liquids, especially if low-boiling azeotropes are formed. Examples include acetone, aniline, benzene, chloroform, carbon tetrachloride, ethylene dichloride, heptane, hexane, methanol, nitrobenzene, petroleum ether, toluene and xylene. Addition of benzene can be used for drying ethanol by distillation. In carrying out distillations intended to yield anhydrous products, the apparatus should be fitted with guard-tubes containing calcium chloride or silica gel to prevent entry of moist air into the system. (Many anhydrous organic liquids are appreciably hydroscopic.)

Removal of water from gases may be by physical or chemical means, and is commonly by adsorption on to a drying agent in a low temperature trap. The effectiveness of drying agents depends on the vapour pressure of the hydrated compound - the lower the vapour pressure the less the remaining moisture in the gas.

The most usually applicable of the specific methods for detecting and determining water in organic liquids is due to Karl Fischer. (cf. J Mitchell and DM Smith, Aquametry, Interscience NY, 1948) Other techniques include electrical conductivity measurements and observation of the temperature at which the first cloudiness appears as the liquid is cooled (applicable to liquids in which water is only slightly soluble.) Addition of anhydrous cobalt(II)iodide (blue) provides a convenient method (colour change to pink on hydration) for detecting water in alcohols, ketones, nitriles and some esters. Infrared absorption measurements of the broad band for water near 3500 cm-1 can also sometimes be used fro detecting water in non-hydroxylic substances.

Intensity and capacity of common desiccants

Drying agents can conveniently be grouped into three classes, depending on whether they combine with water reversibly, they react chemically (irreversibly) with water, or they are molecular sieves. The first group vary in their drying intensity with the temperature at which theya re used, depending on the vapour pressure of the hydrate that is formed. This is why, for example, drying agents such as anhydrous sodium sulphate, magnesium sulphate or calcium chloride should be filtered off from the liquids before the latter are heated. The intensities of drying agents belonging to this group fall in the sequence:

P2O5 >> BaO > Mg(ClO4)2, CaO, MgO, KOH (fused), conc H2SO4, CaSO4, Al2O3 > KOH (sticks),
silica gel, Mg(ClO4)2·3 H2O > NaOH (fused), 95% H2SO4, CaBr2, CaCl2 (fused) > NaOH (sticks),
Ba(ClO4)2, ZnCl2 (sticks), ZnBr2 > CaCl2 (technical) > CuSO4 > Na2SO4, K2CO3

Where large amounts of water have to be removed, a preliminary drying of liquids is often possible by shaking with concentrated solutions of calcium chloride or potassium carbonated, or by adding sodium chloride to salt out the organic phase (for example, in the drying of lower alcohols).

Drying agents that combine irreversibly with water include the alkali metals, the metal hydrides, and calcium carbide.

Suitability of individual desiccants

Alumina

(Preheated to 175C for about 7h). Mainly as a drying agent in a desiccator or as a column through which liquid is percolated.

Aluminium amalgam

Mainly used for removing traces of water from alcohols, which are distilled from it after refluxing.

Barium oxide

Suitable for drying organic bases.

Barium perchlorate

Expensive. Used in desiccators. Unsuitable for drying solvents or any organic material where contact is necessary, because of the danger of explosion.

Boric anhydride

(Prepared by melting boric acid in an air oven at a high temperature, cooling in a desiccator, and powdering.) Mainly used for drying formic acid.

Calcium chloride (anhydrous)

Cheap. Large capacity for absorption of water, giving the hexahydrate below 30C, but is fairly slow in action and not very efficient. Its main use is for preliminary drying of alkyl and aryl halides, most esters, saturated and aromatic hydrocarbons, and ethers. Unsuitable for drying alcohols and amines (which form addition compounds), fatty acids, amides, aminoacids, ketones, phenols, or some aldehydes and esters. Calcium chloride is suitable for drying the following gases-hydrogen, hydrogen chloride, carbon monoxide, carbon dioxide, sulphur dioxide, nitrogen, methane, oxygen, paraffins, ethers, olefins and alkyl chlorides.

Calcium oxide

(Preheated to 700-900C before use.) Suitable for alcohols and amines (but does not dry them completely). Need not be removed before distillation, but in that case the head of the distillation column should be packed with glass wool to trap any calcium oxide powder that might be carried over. Unsuitable for acidic compounds or esters. Suitable for drying gaseous amines and ammonia.

Calcium sulphate (anhydrous)

(Prepared by heating the dihydrate or the hemihydrate in an oven at 235C for 2-3 hours; it can be regenerated.) Available commercially as Drierite. It forms the hemihdrate, CaSO4· ½ H2O, so that its capacity is fairly low (6.6% of its weight of water), and hence is best used on partially dried substances. It is very rapid and efficient (being comparable with phosphorus pentoxide and concentrated sulphuric acid). Suitable for most organic compounds. Solvents boiling below 100C can be dried by direct distillation from calcium sulphate.

Copper(II) sulphate (anhydrous)

Suitable for esters and alcohol. Preferable to sodium sulphate in cases where solvents are sparingly soluble in water (for example, benzene or toluene).

Magnesium amalgam

Mainly used for removing traces of water from alcohols, which are distilled from it after refluxing.

Magnesium perchlorate (anhydrous)

(Available commercially as Dehydrite. Expensive) Used in desiccators. Unsuitable fro drying solvents or any organic material where contact is necessary, because of the danger of explosion.

Magnesium sulphate (anhydrous)

(Prepared from the heptahydrate by drying at 300C under reduced pressure.) More rapid and effective than sodium sulphate. It has a large capacity, forming MgSO4·7 H2O below 48C. Suitable for the preliminary drying of most organic compounds.

Phosphorus pentoxide

Very rapid and efficient, but difficult to handle and should only be used after the organic material has been partially dried, for example with magnesium sulphate. Suitable for acid anhydrides, alkyl and aryl halides, esters, ethers, hydrocarbons and nitriles, and for use in desiccators. Not suitable with acids, alcohols, amines or ketones, or with organic molecules from which a molecule of water can fairly readily be abstracted by an elimination reaction. Suitable for drying the following gases-hydrogen, oxygen, carbon dioxide, carbon monoxide, sulphur dioxide, nitrogen, methane, ethylene and paraffins.

Potassium (metal)

Properties and application are similar to those for sodium, and it is a correspondingly hazardous substance.

Potassium carbonate (anhydrous)

Has a moderate efficiency and capacity, forming the dihydrate. Suitable for an initial drying of alcohols, bases, esters, ketones and nitriles by shaking with them, then filtering off. Also suitable for salting out water-soluble alcohols, amines and ketones. Unsuitable for acids, phenols and other acidic substances.

Potassium hydroxide

Solid potassium hydroxide is very rapid and efficient. Its use is limited almost entirely to the initial drying of organic bases. Alternatively, sometimes the base is shaken first with a concentrated solution of potassium hydroxide to remove most of the water present. Unsuitable for acids, aldehydes, amides, esters, ketones, or phenols. Also used for drying gaseous amines and ammonia.

Silica gel

Granulated silica gel is a commercially available drying agent for use with gases, in desiccators, and (because of its chemical inertness) in physical instruments (pH meters, spectrophotometers, balances). Its drying action depends on physical adsorption, so that silica gel must be used at room temperature or below. By incorporating cobalt chloride into the material it can be made self-indicating, redrying in an oven at 110C being necessary when the colour changes from blue to pink.

Sodium (metal)

Used as fine wire or as chips, for more completely drying ethers, saturated hydrocarbons and aromatic hydrocarbons which have been partically dried (for example with calcium chloride or magnesium sulphate). Unsuitable for acids, alcohols, aldehydes, amines, esters, organic halides or ketones. Reacts violently if much water is present.

Sodium hydroxide

Properties and applications are similar to those for potassium hydroxide.

Sodium-potassium alloy

Used as lumps. Lower metling than sodium, so that its surface is readily renewed by shaking. Properties and applications are similar to those for sodium.

Sodium sulphate (anhydrous)

Has a large capacity for the absorption of water, forming the decahydrate below 33C, but drying is slow and inefficient, especially for solvents that are sparingly soluble in water. It is suitable for the preliminary drying of most types of organic compounds.

Sulphuric acid (concentrated)

Widely used in desiccators. Suitable for drying bromine, saturated hydrocarbons, alkyl and aryl halides. Also suitable for drying the following gases - hydrogen, nitrogen, carbon dioxide, carbon monoxide, chlorine, methane and paraffins. Unsuitable for alcohols, bases, ketones or phenols.