J. A. CAMPBELL Harvey Mudd College Clorernont, California 9171 1
Questions 458. Radon is oxidized between - 195' and 25'C by chlorine fluorides, bromine fluorides, iodine heptafluoride and fiTiF6- ion in hydrogen fluoride with the formation of stable solutions of radon fluoride (nonvolatile below 200°C). Electromigration studies show that the radon is present as a cation, possibly Rn2+ or RnF+, in several conducting solutions. [STEIN, L., Science, 168, 363 (1970)l. Which ion is more likely? Why might it be stable in liquid HF, but not in water? 459. Pressures as low as torr are attained in high vacuum systems. How many gaseous molecules per cm3in such avacuum? 460. A human thyroid requires about 50 r g of iodine per day [RIGGS,D. S., Pharmacol. Rev., 4 , 284 (1952)l and humans excrete, on average, 150 ~g of iodine per day. At what level would you "iodize" salt (add KI to NaC1) to minimize general iodine deficiency? If KI is added to moist salt, the iodine is rapidly lost. How? Suggest an alternative method of iodizing the salt cheaply [SCRIMSHAW, el al., Lancet, 265 166 (1953)l. [Commercial level is 0.01-0.0005%.] 461. When sponges of two different species are ground up, the pieces dyed red and blue, respectively, then put back into a single dish, the red and blue pieces wander around combining with their olrn color to reconstitute larger sponges. How would you decide
whether or not it was the added dye which led to the final sorting out? 462. Suggest mechanisms by which a dye molecule might adhere to a fabric fiber. Would a single dye be equally effectiveon cellulose and on nylon, for example? 463. Grinding and polishing surfaces of two pieces made of the same metal first decreases then increases the friction between them. Suggest a mechanism a t the molecular level. Dissimilar surfaces also decrease in friction, then increase upon polishing but the increase is much less than a i t h identical substances. Why? Be specific. 464. Acridine dyes are powerful mutants and bind to DNA by sliding between adjacent base pairs forcing them to move from 3.4 to 6.8 A apart. Interpret this binding and suggest a molecular mechanism for the mutagenic action. [BRENNER, S., el al., J . Mol. Biol., 3,121 (1961)l.
This column is composed of questions (plus possible, but certainly not uniquely satisfactory, answers) requiring no more than a concurrent first-year, college level course, e data handbook and a willingness t o apply fundamental chemical ideas t o the systems which surround us (or even are inside us). Contributions far possible inclusion are solicited. Initiated in the January 1972 issue of THE JOURNAL.
Answers ASS. Liquid H F contains H F (plus its polymers) and H' and F - solvated by H F molecnles. A Rnl+ ion waidd be expected to solvate with the small F- ions consistent with the high charge densities on bot,h ions. Thus Rn+' should be less common bhan R n F + ions. The electron affinitty of F-, or H F , is considerably greater than t,hat of OH-, or HIO, so electron transfer t o Rn2+is less likely in H F than in HIO. I n other words H F is a poorer reducing agent t,han is HIO so Rn2+shouldbe less readily reduced in H F than in water. A59. 1lorr = (1/760) atm PV = (N/No)RT (N/V)
=
NoP/RT(molecules/cma)
=
fi X l W 5 (moleoules/mole) X (10P/R00) atm to 1 ~ ifig.~ . 80 (ema atm/mole 'K) X 300'K
(NIL')
=
3 X 10' (malecules/cm5)
A60. Since d l iodine is eventually excreted, only 150 r g per day need be supplied, not 200 rg. I t seems reasonable that, to the closest power of 10, a person needs abont 1 g of
salt per day (not 10 g or 0.1 g). 1 5 0 r g i s 150 X 10- X 100/1 = 1.50 X 10-27, iodide shanld be in salt, which is just. above the range given. Presumably same iodine is provided in other ioods thus allowing the amoont in salt to be reduced and still provide the required amount,. Oxygen oxidizes I- to I>which can volat,ilize and be loqt to the potential consumer. Oxidized iodine in t,he form of KIOj can be used in amounts about (130 90)/130 times that of K I since human physiology can convert IOJ- to hormones as effectively as i t does I-. This should emphasize t o you the wide adaptability of living systems a t themolecular level. A61. Grind the senmate SDonnes. " , seDarate the nieces into two equal sebs, and dye one of each blue, the other red. Mix the fragments of eaeh sponge with its own kind and see if color sorting does or doesn't occur. Also repeat the original experiment but (1) reverse the color of dying between the two sponges, (2) try one dyed and the other undyed (using both combinations), (3) try ot,her dyes, and (4) try t,he separately ground sponges with one half-dyed and the other undyed. A62. It would appear dye molecules and/or particles conld either be chemically bound t o fibers or mechenically trapped
+
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among them. The binding could he d i r e d y to groups in the fiber or through an intermediate bridge molecule. All these methods are used. Bonds can he covdent, ionic, or dipole. The most common chemical linkages are through polar g r o u p (e.g., OH, NH, 0) incorporated in the fibers. Color fast dyes must have at least two properties: (1) the dye does not readily wash out due to hydrolysis of its bonds or mechanical attrition, (2) the dye is not readily oxidized, reduced, or photochemically decolorized. Since dyes, perforce, absorb light, photochemical color fading is one of the more difficult changes to minimize. Light absorption leads to local high energy centers which are apt to ho chemically reactive. Cellulose contains mainly OH as passihle honding groups, andnylonmainly NH. Strongly eciddyesshould bind to both hut the best dyes for each may well be designed for that particular flher. A63. Rough surfaces exhibit friction because of surface nnevenness leading to hills and valleys which "catch" on one another. ~olishing smooths these- out reducing friction. Further polishing leads to the production of large areas which can come into good contact leading to chemical bond form* tion and local welding. The friction increases. If the two
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surfaces are of identical substances it is conceivable that very careful polishing will lead to such strong honding that the joint will almost disappear. For example, highly polished metals do adhere tightly if the film of .gas adsorbed on the surfaces is removed. Dissimilar surfaces cannot match atoms so well and do not form such tight adhesion. I t is good practioe in hearings, for example, to use two different metals to minimize the occurrence (undesirable) of "seizure."
and dyes made from it can form strong 7-7interactions, perpendicular to the plane of the acridine moleeole. The base constituents of DNA (adenosine, thymine, cytosine, guanine) can also form such 7- bonds. Interjection of acridine hetween the base residues of DNA would certainly change the properties of the DNA spiral and thus result in genetic effects. The scridine w-w honds may well he stronger than those found in normal DNA end so greatly hinder the DNA spiral from the unwinding which is necessary ior gene replication and protein synthesis.
