Experiment planning in the new culture of laboratory safety should include minimization of the material used at each step of an experiment. Consider two simple examples: (1) Transferring a liquid reaction mixture or other solution from one flask to another container usually requires the use of a solvent to rinse out the flask. During this procedure, the worker should use the smallest amount of solvent possible that will enable a complete transfer. (2) Celite® is often used during filtrations to keep the pores of filter papers or filter frits from becoming clogged. When putting the Celite® in place, the worker should carefully determine the minimum amount needed to be effective.
To enhance safety and minimize the environmental consequences of an experiment, careful thought should be given to the materials to be used and the scale of an experiment. Traditionally, chemists have chosen reagents and materials for experiments to meet scientific criteria without always giving careful consideration to waste minimization or environmental objectives. In synthetic procedures, overall yield and purity of the desired product were usually regarded as the most important factors. Material substitution emerged as an important consideration in manufacturing process design because of the large quantities of chemicals involved. The following questions should now be considered when choosing a material to be used as a reagent or solvent in an experimental procedure:
Can this material be replaced by one that will expose the experimenter, and others who handle it, to a lower order of potential hazard?
Can this material be replaced by one that will reduce or eliminate the generation of hazardous waste and the consequent cost of waste disposal?
The following examples illustrate applications of these principles to common laboratory procedures:
A standard general chemistry experiment designed to study Beer's law involves the use of a considerable volume of a copper-ammonia complex. When this volume is multiplied by the number of students in a general chemistry class, a waste disposal problem is created, because a large quantity of copper should not be released directly into a sewage treatment system. The experiment has been modified to use an iron-salicylic acid complex instead, resulting in a waste product that can be disposed of via the sanitary sewer without causing environmental harm (although specific regulations must be consulted).
Liquid scintillation counting of low-level radioactive samples using flammable solvent-based cocktails (e.g., based on xylene, toluene, or dioxane) requires precautions because of the flammability of the solvent and generates large volumes of waste, which must be disposed of by incineration. Substitution of nonflammable, water-miscible cocktails eliminates the fire hazard and generates aqueous waste, which can be disposed of via the sanitary sewer rather than by incineration in many localities. Implementation of this strategy at one major university resulted in a substantial reduction in the volume of flammable organic waste sent out for incineration. Acceptance of the new practice was achieved following demonstrations by key research groups that the new cocktails gave results comparable to those from the flammable solvent-based cocktails.
Phosgene is a highly toxic gas used as a reagent in many organic transformations. Its use requires proper precautions to deal with the containment of the gas and the handling and disposal of cylinders. Commercially available substitutes such as diphosgene (trichloromethyl) chloroformate, a liquid, or triphosgene bis(trichloromethyl)carbonate, a low-melting solid, can often be substituted for phosgene by appropriate adjustment of experimental conditions or can be used to generate phosgene only on demand. Both chemicals are highly toxic themselves, but they offer a means to avoid the problems associated with handling a toxic gas.
Many widely used reagents contain toxic heavy metals, such as chromium and mercury. Waste containing these materials cannot be incinerated and must be handled separately for disposal. Thus, substitution of other reagents for heavy metal reagents will almost always be beneficial with respect to hazard and waste minimization. Chromic acid cleaning solutions for glassware can be replaced by proprietary detergents used, if necessary, along with ultrasonic baths. Various chromium(VI) oxidants have been important in synthetic organic chemistry, but their use can often be avoided by the substitution of organic oxidants. The Swern oxidation of alcohols (oxalyl chloride/dimethyl sulfoxide) produces relatively innocuous by-products, which can be handled with other organic waste. Other oxidation reagents tailored to the specific needs of a given transformation are available.
Fluorine and fluorinating reagents such as perchloryl fluoride are among the most demanding reagents to handle because of their high reactivity and toxicity. Accordingly, there has been considerable incentive to develop substitutes for these materials. One example is F-TEDA-BF4, or 1-chloromethyl-4fluoro-1,4-diazonia [2.2.2] bicycloctane bis(tetrafluoroborate). This reagent can be substituted for more hazardous reagents in many fluorination procedures.