The which one and how to about DRYING substances-->
mrr pyrex posted 11-12-98 05:05 AM CT (US)This is brought to you via Mr Pyrex, due to the absence of info that i have found around here and just plain being tired of seeing people not knowing how to or which one of these drying agents to use. Let me first start by saying that 99% of the credit goes to the book- Purification of Laboratory Chemicals 4th edition Armarego/Perrin (a very informative book and one i would highly recommend to anyone thinking of dreaming $75, but well worth it). All Mr. Pyrex did was to type in what they had already written in this book.
REMOVAL OF SOLVENTS- Where substances are sufficiently stable, removal of solvent from recrystallised 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 a 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 is a receptacle with a suitable drying agent. In cases where heating above room temperature cannot be used, drying must be carried out in a vacuum desiccator containing suitable absorbants. For example, hydrocarbons, such as benzene, cyclohexane and petroluem ether, can be removed by using shredded paraffin wax, and acetic acid and other acids can be absorbed by pellets of sodium, 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 depends on the thermal stability of the solids or the timie available. The safest way 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 100'c drying in an air oven may be adequate. In other cases, use of an Abderhalden pistol may be satisfactory. 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 for drying both solids and liquids, especially if low-boiling point azeotropes are formed. Examples include acetone, aniline, benzene, chloroform, carbon tetrachloride, ethylene dichloride, heptane, hexane, methanol, notrobenzene, 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 (Damp Rid) or silica gel to prevent entry of moist air into the system. (Many anhydrous organic liquids are appreciably hygroscopic)...meaning they will absorb moisture if given the chance Removal of water from gases may be by physical means, and is commonly by adsorptionon to a drying agent in a low-temperature trap. The effectiveness of the drying agent depends on the vapour pressure of the hydrated compound-the lower the vapour pressure the less the remaining moisture in the gas. Addition of anhydrous cobalt (II) iodide(blue)provides a convenient method(colour changes to pink on hydration) for detecting water in alcohols, ketones, nitriles and some esters.
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 they are used, depending on the vapour pressure of the hydrate that is formed. That 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 the 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)2.3H2O> NaOH (fused), 95% H2SO4, CaBr2, Ca Cl (fused) > NaOH (sticks), Ba(ClO4)2, ZnBr2 (sticks), ZnBr2>CaCl2 (technical) > CuSO4 > NaSO4, K2CO3. Where large amounts of water are to be removed, a preliminary drying of liquids is often possible by shaking with concentrated solutions of calcium chloride or potassium carbonate, 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 175'c for about 7hrs). Mainly as drying agent in a desiccator or as a column through which liquid is percolated.
ALUMINUM 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(covered with a metal guard). Unsuitable for drying solvents or organic materials where contact is necessary, because of the danger of EXPLOSION
BORIC ANHYDRIDE. (prepared by melting boric acid in an air oven at 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 30'c, but is fairly slow in action and not very efficient. Its main use is for preliminay dring of alkyl and aryl halides, most esters, saturated and aromatic hydrocarbons and ethers. Unsuitable for drying alcohols and amines (which form additional compounds), fatty acids, amides, amino acids, 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, also paraffins, ethers, olefins and alkyl chlorides.
CALCIUM OXIDE. (Preheated to 700'-900'c before use.) Suitable for alcohols and amines (but does not dry them completely). Need to remove before distillation, but in that case the head distillationcolumn should be packed with glass wool to trap any calcium oxide powder that might be carried over. Unsuitable for acidic compounds and esters. Suitable for drying gaseous amines and ammonia.
CALCIUM SULPHATE (anhydrous). (Prepared by heating the dihydrate or hemihydrate in an oven at 235'c for 2-3 hrs; it can be regenerated.) Available commercially as Drierite. It forms the hemihydrate, 2CaSO4.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 efficient (being comparable with phosphorus pentoxide and conc sulphuric acid). Suitable for most organic compounds. Solvents boiling below 100'c can be dried by direct distillation from calcium sulphate.
COPPER (II) SULPHATE (anhydrous). Suitable for esters and alcohols. 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 for drying solvents or any any organic material where contact is necessary, because of the danger of EXPLOSION.
MAGNESIUM SULPHATE (anhydrous). (Prepared from the heptahydrate by drying at 300'c under reduced pressure.) More rapid and effective than sodium sulphate. It has a large capacity, forming MgSO4.7H2O below 48'c. 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, ethers, esters, 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 be fairly readily abstracted by an elimination reaction. Suitable for drying hydrogen, oxygen, carbon dioxide, carbon monoxide, sulphur dioxide, nitrogen, methane, ethylene and paraffins. It is available with an indicator (cobalt salt, blue when dry and pink when wet) under the name Sicapent(from Merck).
POTASSIUM (metal). Properties and applications are similar to those of 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 inital drying of organic bases. Alternatively, sometimes the base is shaken with concentratedsolution of potassium hydroxide to remove most of the water present. Unsuitable for acids, aldehydes, ketones, phenols, amides and esters. Also used for drying gaseous amines and ammonia.
SILICA GEL. Granulated silica gel is commercially available drying agent for use with gases, in desiccators, and (because of its chemical inertness) in physical instruments. 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, re-drying in an oven at 110'c being necessary when colour changes from blue to pink.
SODIUM (metal). Used as a fine wire or as chips, for more complete drying ethers, saturated hydrocarbons and aromatic which have been partially dried (for example with calcium chloride or magnesium sulphate). Unsuitable for acids, alcohols, alkyl halides, aldehydes, ketones, amines and esters. Reacts violently if much water ism present and can cause a fire with highly flammable liquids.
SODIUM HYDROXIDE. Properties and applications are similars to those for potassium hydroxide.
SODIUM-POTASSIUM ALLOY. Used as lumps. Lower melting then sodium, so that its surface is readily renewed by shaking. Properties and applications are similar to those of sodium.
SODIUM SULPHATE (anhydrous). Has a large capacity for absorption of water, forming the decahydrate below 33'c, but drying is slow and inefficient, especially for solvent 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, hydrogen chloride, nitrogen, carbon dioxide, carbon monoxide, chlorine, methane, and paraffins. Unsuitable for alcohols, bases, ketones or phenols. Also available with an indicator (a cobalt salt, blue when dry and pink when wet) under the name Sicacide (from Merck) for desiccators.
WHHEWWWW boy is that one heck of alot of typing, but hopefully this can be of great help to a great many people.
The following is only my opinion and should only be regarded as such- personally i have found that sulphuric acid for drying HCl gas is adequate, i have tried using drierite as a secondary dryer for the HCl gas(following the bubbling through the sulphuric acid) thinking that dryer is better, but what seems to happen is the drierite absorbs alot of the HCl gas and it takes alot longer to get the finished gas out the end of the line with no appreciable difference so i feel that as long as the H2SO4 is dry then thats all you'll need. As for drying solvents such as toluene, ether(starting fliud) and acetone the drierite does just fine, i use it as follows- toluene use the non-indicating type and put about 1 ounce in the new 1 gallon container when i purchase it, then for drying after it has been used to absorb lets say an oil or when say you have a little bit of the emulsion mixed in with the recovered solventand don't feel like waiting for it to separate fully, you put an amount(which you'll have to judge for yourself) in a coffee filter paper in a funnel, and then slowly pour the solvent through the paper letting any suspected moisture run over the drierite and the drierite will absorb ONLY H2O the rest will pass right past, if necessary rinse off with a little extra fresh solvent and there ya go dry and filtered solvent, and if by chance you don't have a perfectly transparent(color doesn't matter)liquid then it might need to have more drierite added and filtered again.
Ether just shake with water first then discard water, this along with only using the first 75% of the fluid coming out of the can while being held upright and NO shaking, you can avoid the awful smell of "diesel fuel" that is sometimes associatied with this fluid. I believe this smell is from the other additives that are in the starting fluid, due to using this repeatedly and finding that once i had used this and the first batch smelled great and dried out perfect, then the second and the end of the can stinking terribly and not drying out completely and having little droplets of oil separate out that had the smell the worst. One note of CAUTION with ether it is probably the most dangerous solvent to work with when it is dry it can EXPLODE without warning due to the formation of peroxides which can be formed from exposure to light, aeration or just plain evaporating it, DO NOT work with large quantities of it. In addition it can also EXPLODE due to ignition from static electricity, heat, friction, even a light switch being turned on, DO NOT discount this warning i have had a best friend blown through a wall of a house when someone there flipped the light switch, luckily he lived but i don't think you'd want to want to test your luck. Then finally for use with acetone use in the same fashion as with the toluene. Good Luck and Great Dreams. Pyrex out
