chemical vignettes ROBERT C. BRASTED Unwsrr~tyof Mmnelota M8nneopohs. 55455
Heavy, Heavy (~etals) Hang Over Our Heads Inorganic Preparative (Descriptive) Chemistry, General Chemistry, Biochemifry, Toxicology, and Environmental Problems
Many of us in the well over 30 (and therefore suspect) age group look back over our immediately past academic year of teaching wit,h more than a little frustration, knowing that u7e have had all too little time t,o devote to what we consider important parts of our teaching-the descriptive portions of our metal and nonmetal chemistry. This writer is not convinced that many of our environmental ills might have been avoided or a t least minimized if port,ions of our introductory and even advanced courses had had a different emphasis. Perhaps we have lacked the interest or just the time to produce interesting ways of relating to this kind of chemistry. Some may teach (with apology) such a process as the production of "chlorine with caustic." This as everyone knows is "dead" chemistry, yet the Federal Water Quality Administration, FWQA (among ot,her alphabetical organizations dedicated to digging our way out of ecological dilemmas), having been provided wit,h information and analytical tools of incredible sensitivity, announces that some 80 thousand tons of "lost" mercury should be accounted for. How many graduates of our first year courses in chemist,ry will see the relat,ion between "chlorine-caustic" and heavy metals? 3iuch of t,his "lost" mercury has been "found" locat,ed in the general vicinity of "chlorine-caustic" plants. Disturbing quantit,ies are in the North Woods regions, in the streams and lakes. Again, the reason is not hard to find if we look for paper and pulp processing. When t,he total number of Mat,hieson E-8, DeNora, and CastnerIiellner cells (using mercury cathodes) still in operat,ion is recognized, t,he loss of t,his amount of mercury is understandable if not forgivable. Now, where have we as t,eachers of chemistry been remiss or even derelict in our duties? This writer feels t,hat use should have taken time in our instruction to produce interesting lecture material in the overall treatment of (a) sodium chloride, other t,han F centers, Nadelung constants, etc; (b) elect,rolytic processes, ot,her t,han mathematical derivations; (c) toxicology of heavy metals, in addition t,o or even other than separation processes in qualitative flow she&; and ((1) the chemistry of the halogcns including their preparation and uses, in addition to t,he encrgies of dissociat,ionand RIO treatment of diat,omicmolecules. Still a conjecture hut not an unreasonable one, we might then have impressed the chemical enginecrs as 756
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Journal o f Chemical Education
well as technicians opcrat,ing t,hc mercury cells that good housekeeping is worth the effort. The following are a few places that this xriter t,ries t,o tie toget,her both the chemist,ry of t,he elements (nonmetals and metals), some indu~t~rial chemistry and especially "living" systems. For the writer, being basically an inorganic-type character, it has meant, more than a littk art,iculation xit,h the "lesser breeds" such as organic chemist,^, physiological chemists, toxicologists, and pharmacists. I t has been both instructive and refreshing to realize that even these people do not have answers that he t.hought would be common knowledge. 1) Chlorine-caustic production using mercury cells should be compared and cont,rasted with molten and noncaustic met,hods. A worthwhile lesson is t.aken from a recent talk by Professor Nelson Leonard of t,he University of Illinois (a master of synthetic processes) who makes an issue of examining the synthetic rout.es (as well as reverse synthetic routes), evaluating the unwanted, unsalable, and harmful by-products of a process. Maybe our final desired product can be produced by modifications of raw materials or changing individual synthesis steps. It is, of course, possible to produce chlorine without a mercury cathode, in fact without electrolysis. We never seem to get something for nothing in this day and age. The new route may well be more expensive than the old. 2) Why is a heavy metal so often poisonous? Here, as was the case of H,S, CO, HCN (see a previous Vignette) everyone knows that mercury is a posion, but too few texts tell us why. Data reported on mercury loss do not make it clear whether the mercury as an environmental pollnt,ant is the mercury (II), the vapor, the massive metal, or alkyl mercurials (the latter of which are the major environmental hazard). The general public may be forgiven when they become jaded by reading meaningless or questionable toxic limits. The full answers as to the toxicity of heavy metals are not clear cut but they would seem important enough for our elementary texts to give a few words on the subject. The chemistry resembles that of other enzyme inhibitors, CO, H2S, et al. It is a different enzyme system but there are areas of similarity. The sulfhydryl group (-SH), part of many proteins, is a very fine coordinator or complexing ligand for metal ions especially heavy metals (Ag(I), Pb(II), Hg(I1)). The complex, unfortunately, does not have the enzyme activity. Hour many present remember a chemical known as BAL, British Anti-Lewisite? This is nothing more than a simple molecule with t v o sulfhydryl groups (CH20HCHSHCH,SH). It has a great affinity
for both heavy metals and pseudometals (As) The arsenic ingredient of one of the war gases is neutralized by just the process that is giving our body fits when the mercury(I1) ion "sees" the -SH group of the enzyme. The toxicity of mercury is further increased by its effect in various transport processes and by causing chromosome breaks. 3) Why the different effects for the ( a ) soluble Hg(I1) species, the (b) massive elemental Hgo, (c) the alkyl mercurials, and the (d) vapor of mercury. The last mentioned is always a subject of deep concern when found on the lab floors or in the drainage traps. The mercury lost from the cathode of the electrolytic cells may be of the kinds: (a), (b) or (d). In the reverse (or denuding) process used to strip the cathode of its caustic and hydrogen forming capacity, some mercury may be oxidized and through poor housekeeping or ignorance may be released as the oxide or chloride as effluent to the streams, finally settling in beds or being assimilated by the plant and fish life. Where proper settling is not used the element itself is lost. This per se may not be poisonous until oxidized and thence assimilated by the water life. A Hgo cocktail is not recommended for even the social drinker, but there have been cases of ingestion of massive quantities of the metal which if eliminated soon by normal "channels" may have only the effect of discouraging (violently) further use as a beverage. The high toxic quality of the vapor is more because of its rate of adsorption through the skin. The mercury vapor is rapidly absorbed and changed into an ionic form in the lungs;
the liquid form meets with more formidable protective barriers. The toxicity of ingested mercury compounds is quite variable, and again depends on the ease of transport across the various protective barriers. It is hoped that here the general thesis of the Vignette continues: we can make essential descriptive and preparative chemistry not only palatable but justifiable with a few "life and death" stories. Those of us who for decades have enjoyed wetting a line in the lakes and streams of the north country, including Canada, are more cognizant of the fact that better descriptive chemistry taught over these years might have made it unnecessary to close certain areas for recreational and commercial fishing. The increased sensitivity of analytical devices has necessitated re-evaluation of toxicities, levels of body tolerance as well as interpretation of what we really mean by "zero ppm." It is doubtful, for instance, than any living organism of higher order will any longer show zero ppm of DDT and quite possibly no one will have zero ppm of the heavy metals. When diverse and conflicting levels of toxicity are published every other Thursday the public quite understandably generates a credibility gap. Unrealistic or unjustified levels whether on heavy metals or chlorinated hydrocarbons may adversely affector eliminate otherwise legitimate industries. Let us hope that our clientele in the major and the nonmajor courses in chemistry will be able to make value judgments when inevitable restrictive legistation is presented.
The Twelfth Annual Summer Conference, The California Association of Chemistry Teachers Asilomar Conference Grounds, Monterey Peninsula, California, August 16-22, 1970
Volume 47, Number I 7 , November 1970
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