chemical vignettes ROBERT C. BRASTED Urvemty of Monnesota Monneapolm, 55455
Some "Real Life" Applications of Solubility Iron, Iron Everywhere buf Not a Drop to Drink Agriculture, Soils, General, and Inorganic Chemistry
The visitor to our 50th state, who having enjoyed the bikini clad beaches of Waikiki, travels from the ocean up the gentle slopes of the Schofield plain of Oahu where the pineapple grows in barn-red soil may not have his mind troubled by chemical problems. However, on the assumption that chemistry, like gold, is where you find it, there is an interesting lesson in solubilitv related to the growth of this delightful fruit.. The red soil mentioned is well in excess of 20% iron, yet the pineapple plant starves in this soil for the want of iron. One only has to look at the solubility of 10e2O3(or Fe(OH)a) to understand why this should be RO. Even the unbelievable adaptiveness of nature has not provided the plant with the mechanism for converting iron(lI1) to more soluble iron(II), a form the plant can use. Give nature credit, though, if the iron were water soluble, just for the pineapple's benefit, our oceans, seas, rivers, and lakes would have a color more like the Hawaiian pineapple soil. Although Hawaii has never been noted for its heavy chemical industry, sulfuric acid was produced there in substantial quantities for a number of purposes, one of which was to convert scrap iron to iron(I1) sulfate. This product is sprayed on the pineapple leaf to provide this transition metal ion needed for a healthy plant-in a soil that is substantially composed of iron (see Chemistry in Hawaii, Chem. and Eng. News,25,496 (1947)). Still another segment of transition metal chemistry may find its way into this discussion. The soil on this island is also very high in insoluble manganese, the oxidation-state of which is favorable to keeping the iron in the oxidized, insoluble state. Geologically the soil is new with a content of humus and reducing materials so low that little or no iron reduction is possible. An experiment devised some years ago for the general chemistry classes a t the University of Hawaii used as its theme the deterrninatiou of the iron/manganese ratio. The writer has found that such experiments designed for a local situation can make the difference between a vital laboratory experience and one that is ~ r e t t vmuch "ho hum". The purpose of "Chemical Vignettes" is to illustrate spplicsr tions of ehemist,ry,especially in engineering and other scientific areas. Readers are encouraged to send items appropriate to this column to the author. 634
/
journal o f Chemical Education
The writer recognizes that the above illustration of solubility (or lack of it) has a thousand and one extehsions. Where the pineapple was frustrated because of inability to get a t an insoluble species, the radiologist and the medicine-man-type of doctor often depend upons this same unavailability of ions. Whether a chemical is healthful or hurtful to the body is often pretty much a matter of solubility. Your internal plumbing becomes opaque to X-rays after quaffing a BaSOl cocktail. The medical profession knows full well that the Ba2+is not healthful, but he also knows that the sulfate ties this ion up very efficiently. Similarly the medication (for another phase of internal plumbing), calomel, Hg2Cl2,stimulates (rather than kills) the liver and other secretive organs because only an infinitesimally small amount of mercury is liberated by the insoluble mercury(1) chloride. You win some and you lose some in this game of solubility.
Solubility and the Chemistry of the Covalent Bond More on DDT-A
SubsfifutedAlkyl Halide
Organic and General Chemistry, Agriculture, Entomology, and Soils
With a more sophisticated approach to the nature of the chemical bond than was used a decade or more ago, most of us do not limit our discussion of the nature and prediction of solubility only to the "like solute likes the solvent molecules" principle. However, it is not a bad "ball park" approach. Demonstrations involving the solubility of such covalent systems as iodine in carbon tetrachloride, or the insolubility of CClp in water have stood the tests of time. A variation on the theme permits the student to widen his horizons and better understand the current legislative bickerings involving DDT and other halogenated hydrocarbon insecticides. The former
I
CI-C-CI
I
is not difficult to relat,e structurally to carbon tetrachloride. As such it should have similar solubility tendencies. Chloradane
would be similar a t least insofar as its C-C1 structure is concerned. Having established these covalent molecule generalities, it is reasonable that they might well be expected to dissolve in covalent fatty materials. The student should not be surprised to find that they concentrate because of mutual solubility in the fatty tissues of fish, birds, and game. Since nerve tissue is fat-like i t is further expected that the insecticide might also localize in this part of the body. Perhaps of equal importance from the standpoint of our general solubility (polar versus nonpolar species) discussion is the fact that DDT would not be leeched from the soil by rain water. The bond strengths of C-C and C-CI (about 80 kcal) suggest low reactivity and hence persistence in the soil-which property both have--an estimated half life of 10-15 yr for DDT. Fortunately for man (and even more fortunate for women) the propagation of our kind does not depend upon egg shell development and structure or we might have become more concerned about DDT and its ecological
effect longer ago. It is distressing to find so little on these compounds in so-called recent introductory texts that would help the first-year student understand a subject so widely discussed in popular news media. The implications of some simple blackboard type chemistry (actually reactions that are possible only because of some sophisticated biochemistry) involving DDT are frightening. A typical reaction of alkyl halides is dehydrohalogenation. DDT undergoes this detoxifying reaction in the presence of the enzyme DDT-dehydrohalogenase to form DDE ensyme
(p-CICsH&2HCI, DDT
+(p-CICs&)C=CCb DDE
+ HC1
The low slow curve is delivered by a mutant fly recently developed which produces this detoxifying enzyme. There now exist fly populations immune to DDT. The fact that this phenomenon is not new and that immume populations have developed before DDT was used is but small comfort. John W. Hill
Wisconsin State University River Falls, Wisconsin 54022
Volume 47, Number 9, September 1970 / 635
