edited by MIRIAMC. NAGEL Avo" High Schwl Avan. CT 06001
Begone Odious Benzene, Carbon Tet, and Chloroform Terry S. Carlton Oberlin College. Oberlin, OH 44074 Benzene, carbon tetrachloride, and chloroform are now known to be too toxic for routine laboratory use. Therefore, some teachers have eliminated all three solvents from their instructional laboratories, in spite of their widespread use in crvstallizations, extractions, -physical measurements, and cleaning. The resulting reduction in exposure to these toxic substep forward, &mething which can be acstances is a complished immediately. For decades to come, however, scientists will continue to encounter in the literature countless procedures that use these solvents. Rarely do these procedures mention toxicity. Chemistry teachers have a responsibility to warn students of hazards from these solvents, to call attention to safe substitutes, and to describe precautions for working with highly toxic solvents in those rare instances when no acceptable substitute is available. ~
Acute and Chronic Toxicity I t is important that students appreciate the distinction between acute and chronic toxicities of chemicals. Harmful effects of acute poisoning become evident either immediately or in a matter ofdays. Chronic poisoning results from repeated exposure to much smaller doses, and its effects may be delayed for vears or even decades. Chronic ~oisonineis insidious. I t may arise from exposures too low to give any warning of daneer. even if the eventual result is disahilitv or death. A;U& effects of benzene and chloroform incfude depression of the central nervous svstem. Acute effects of carbon tetrathe kidneys and liver. A(tUv~fleclc chloride in~ludedama~e'to of all three solvents can he fatal, set chronic rffrcts represent the greater hazard in most laboratory settings. Continued exposure to benzene eventually destroys the ability of bone marrow to make circulating blood cells. I t also may cause leukemia. Liver damage is a chronic effect of carbon tetrachloride and chloroform. Carbon tetrachloride also can damage the kidneys and cause visual disturhances. Chloroform has been found to cause cancer in mice and rats. The Ease of Substituting Safer Solvents Unfortunately, these toxic solvents continue to be used much more widely than is necessary. Inadequate awareness of their severe toxicity is only one reason for this situation. Editor's Note: At the secondary schwl level, purchase, use, storage. and ultimately disposal of toxic organic solvents should be kept to an absolute minimum. Safety has to be a major concern for Me secondery school teacher who is working with inexperienced students and with limited facilities-as much or more than it Is atthe college level. Where it is still appropriate to use toxic organic solvents, the suggestions provided here should be seriously considered.
Also to blame are concerns over the presumed difficulty of selecting substitutes and the possibiliiy that such substitutes would perform unsatisfactorily. To allay such worries, teachers should promote a few specific substitutes that will prove satisfactory in most applications. Some possible alternative solvents appear in the table. Listed as a basis for selecting substitutes are dielectric con. vaoor nressures. as well as nerstants. hoiline ~ o i n t s and missible levek bf exposure. ?;he [deal substitute has dielectric constant and boilina ~ o i nnearlv t the same as those of the solvent it replaces. 1nsome situations one must choose between these pro~erties.One matches dielectric constants in applications to liquid chromatography and in replacing one component of a mixed solvent. Boiling point is important when the solvent is to be removed by evaporation. Cyclohexane and toluene are good substitutes for benzene. Toluene is more similar in its solvent action, hut cyclohexane is more volatile. Avoid grades of toluene that contain appreciable amounts of benzene. Cyclohexane is the. primp candidate as a suhstitute for carbon tetrachloride unless the suhstitute needs UI he denser than water. In that case, tetrachloroethylene provides a good match in solvent abilitv, whereas dichloromethane mav be better if volatility is important. The best all-around substitute for chloroform is l,l,l-tricbloroethane. If high volatility is particularly important, dichloromethane can be used instead. Ethyl acetate and ethyl ether would be acceptable when density~isirrelevant, hut it might be better not to mention these possibilities to beginning students, lest they be intimidated by the task of making a choice from among many possibilities. What matters most is not that they necessarily make the optimum choice, hut that they choose any solvent that will do the job in place of highly toxic chloroform.
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Using Toxic Solvents Safely When benzene, carbon tetrachloride, o r chloroform cannot be avoided, a hood must be used. Gloves a r e mandatory protection when one is pouring benzene o r ProDertles of Solvents Solvent
Benzene Carbon tetrachlaide Chlorofnm Cyclahexane Dichloromethane Ethyl acetate Ethyl ether Tetrachlaroethylene Toluene 1.1.1-Trichloroethane
€25
Bp. 'C
2.3 2.2 4.7 2.0
80 77 61 81 40 77 35 121 111 74
8.9 6.0 4.2 2.2 2.4 7.5
p,./torr 95 110 195 98 436 91 537 18 28 121
Limit8 10 10 50 300 500 400 400 100 200 350
"Limit" is the average permissibls exposure (in pa* per millionby volume in thestmoaphere) for any eight-howwaCiw day. as establishedunder the Occupational Safety and nealm na.
530
Journal of Chemical Education
carbon tetrachloride and a r e advisable for chloroform. Natural rubber. however, is too Dermeahle to all three solvents to offer much protection. ~ e o ~ r e nrubber e and normal ~olvvinvlchloride are acce~table,but nitrile rubber and hi&-grade polvyinyl chloridr are better. Students should he warned tu treat even the suhstitute solvents with care. They too are toxic, though considerably less so than the solvents they replace. Some are flammahle as well. Gloves are advisable for toluene and tetrachlorwrhvlene. Like benzene and carbon tetrachloride, they dissolve'fatty romnonents of the skin. This not onlv ~ e r m i t entrv s to the bid; through the skin, hut may lead todermatitis aH well. Hoods may also he needed for the substitute solvents, depending on how well the lahoratory is ventilated. Even a relatively high permissible concentration in the air, such as 500 ppm, corresponds to a partial pressure of only 0.4 torr. For every solvent in the table, the vapor pressure at 25'C is a t least two orders of magnitude larger than the permissible partial pressure. Putting Our Own House in Order
A recent article in THIS JOURNAL' de~loredthe re valence of lahoratory manuals that call for henlene unnecessarily and with virtually no warning of its toxicity. The situation w ~ t h respect to carbon tetrachloride and chlorotorm also leaves much w he desired. Students (and instructors) continue to he
exposed needlessly to toxic vapors. Furthermore, students who are required to use toxic solvents in their courses are less likely to heed exhortations to avoid those same solvents in their future work. Fortunately, instructors and authors will experience little difficulty in modifying experiments to incorporate safer solvents. Furthermore, not one of the recommended substitutes is appreciably more expensive than the toxic solvent it replaces; most are actually cheaper. In those rare instances in which no suitable substitute can he found, it is worth asking whether students could learn just as much from a different experiment. Influencing Students' Attitudes
In safety education the attitudes that students take away are as important as the factual knowledge. What attitudes may result from the foregoing treatment of toxic solvents? One is that the individual is responsible for his or her own safety in the laboratory. With unsafe procedures widespread in the scientific literature, one cannot rely on safe practices always being imposed by external authorities. A second lesson is that by applying chemical knowledge one frequently can replace a hazardous chemical by a safer one without sacrificing desirable properties. Active, questioning thinking is as important for safety as for other aspects of experimental success.
' Smith, R. M., J. CHEM.EDUC.,57, A85 (1980).
Volume 59 Number 6 June 1982
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