XV: Miscellanea KAROL 1. MYSELS University of Southern California, Los Angeles
XTHE SOLUBILITY OF ACETATES Professor F. J. Allen of Purdue University pointed out that textbooks2 sometimes make the generalization that all acetates are soluble while in fact some are only slightly soluble or even quite insoluble. Silver acetate has a solubility quite comparable t o that of lead dhloride which is generally considered as only slightly soluble. At 25'C. the silver salt dissolves to the extent of 11.2 g./l. while solubility of the lead salt is 10.7 g./l. (1). On a mole basis it is less than twice as soluble so that in terms of equivalents silver acetate is actually less soluble than lead chloride (67 versus 76 meq./l.). Mercury(1) acetate is even less soluble: 1.024 g./l. a t 21°C. (1). For this compound there is another value in the literature: 7.5 g./l. a t 12°C.2 I have been able t o track this second value only to an original article (9)which does not mention this value or, in fact, any other aqueous solubility! Bismuth acetate, which is sometimes listed as insoluble, probably decomposes (5) in water. The basic acetate or bismuthyl acetate, BiOCIHaOn,which according t o Gmelins (4)is (or a t least was) an article of commerce, is insoluble in water. Beryllium forms an acetate Be(C,HZO2), (5) and a basic acetate Be4O(C2Ha0&(6) which are insoluble in cold water but dissolve with decomposition in hot water. However, soluble beryllium salts give no precipitate when mixed with solutions containing the acetate ion, even in the presence of an equivalent of OH-. Chromium(I1) acetate, a red compound, is said to be slightly soluble in water (7), but I could find no quantitative data. Finally, one might mention Paris green which is an insoluble copper acetatoarsenite. Thus i t seems that acetates cover a wide range of solubilities, but those generally encountered in elementary qualitative analysis after elimination of Group I are among the soluble ones.
' Suggestions of material suitable for this column are eagerly sought and will be acknowledged. They should be sent with as many details as possible to Karol J. Mysels, Chemistry Department, University of Southern California, Los Angeles 7, California. Contributors of dimmsions in a. form suitable for publication directly will he acknowledged as guest authors. Since the purpose of this column is to prevent the spread and continuation of errors and not the evaluation of individual texts, the source of the errors discussed will not be cited. The error must occur in at least two independent standard hooks to be presented.
THE EFFECT OF LIGHT ON REACTIONS
Professor Sementsov of Lafayette College directs attention to the fact that there are still books2 which state that the effect of light on certain reactions is catalytic. While t o some extent this is a matter of definition of the word used, it is likely that there is here an unintentional carry-over from times when the role of neither light nor catalysts was well understood and the latter term had a much broader meaning. A catalyst is a t present generally considered to alter the rate of a reaction and possess two characteristics: It is not consumed in the course of the reaction and it does not affect equilibrium. In contrast, light which affects a reaction must be absorbed and, in general, it does alter the equilibrium. That light can affect reactions only to the extent that i t is absorbed (and its energy used to activate some reacting species) is the old Grotthus (@-Draper (9) law, but the effect on equilibrium is perhaps less widely appreciated. The photo effect on equilibrium occurs when illumination is absorbed by one of the species and accelerates its reaction t o a greater extent than it affects the reverse reaction. Thus (10) a mixture of the dye thionine (0.01 g./l.) and iron(I1) sulfate (1.0 g./l.) is bleached by absorbed sunlight which accelerates the reduction of the dye to a leuco compound. I n the dark, however, equilibrium is re-established as the iron (111) ions thus produced slowly oxidize the leuco compound back to the dye. A particularly nice example of the shift of steady state produced by light has been given by Brode and his co-workers (11). There are two isomers of thioindigo, a ns and a trans, which have quite different absorption spectra and, therefore, colors. Illumination with light absorbed preferentially by one of these isomers causes a shift of composition towards the other, nonabsorbing, isomer. By illumination with light absorbed preferentially by this other isomer, the opposite shift can be produced a t will. Thus the dye solution adapts itself like a chameleon to the color of the light by which it is observed. The students are certainly less likely t o become confused if the action of light is described-just like the action of heat-as accelerating or causing a reaction and not as catalyzing it. DISCUSSED ELSEWHERE
A number of textbook errors have been discussed elsewhere in sufficient detail to make a full presentation JOURNAL OF CHEMICAL EDUCATION
in this column unnecessary. It may be well worthwhile, however, to mention their subject and give the reference for completeness of the record. Le Bell and Tetrahedral Carbon. Professor Sementsov has called attention (I$) to the fact that many textbooks credit, explicitly or implicitly, Le Bell (along with Van't Hoff) with originating the tetrahedron theory while in fact both introduced the concept of the unsymmetrical carbon atom, but Le Bell for a long time opposed the tetrahedral model and even tried to disprove it experimentally. The Production of Aluminum. Dr. B. Clifford Hendricks, Professor Emeritus of the University of Nebraska, now of Longview, Washington, has pointed out (13) that, contrary to common statements, cryolite is used up significantly in the Hall process of winning aluminum. This is of importance both in economics and in air polution. Fumaric Acid. Most biochemical texts erroneously give, according to Professor Patton of Colorado A. and M. (I,$), a cis formula for fumaric acid which in reality is the trans isomer of maleic acid.
'VOLUME35, NO. 1, JANUARY, 1958
LITERATURE CITED (1) SEIDELL,A,, "Solubilities of Inorganic and Metal Orgmlc Compounds, 3rd ed., I). Van Nostrand Co., Ine., New York, 1940, Vol. I. (2) WELSH,T. \V. B., A N D H. J. BRODERSON, J. Am. Chem. Soe., 37, 816 (1915). (3) SIDQWICK,Pi. V., "The Chemical Elements and Their Compounds," Oxford University Press, Ine., Xew York, 1950,p. 705. (4) "Gmelins Handbook of Inorganic Chemistry," System No. 19,Sth ed., Verlag Chemie, Berlin, 1927,p. 181. H.,2. anorg. Chert., 54, 21i (1907); alao ref. (5) STEINMETZ, (3) p. 205. Compl. rend., 133, 874 (6) URBAIN,G., AND H. LACOMBE, (1901);also ref. (31, p. 213. (7) Ref. (31, p. 1025. (8) GROTTHUS.C. J. D.. " O ~ t ~ ~ l Klassiker." d's No. 152. DRAPERJ: Ui., ~ h i l Mag. . (3), 19, 195'(1841); 23, 401 (1843);27, 327 (1845). BOWEN,E. J., "The Chemical Aspects of Light," Oxford University Press, Inc., New York, 1946, p. 284. BRODE,W. R., Am. Scientist, 43, 259 (1955); BRODE, W. R.,E. G. PERSON,AND G. M. WYJIAN,J . Am. Chem. Soc.. 76. 1034 (1954): WYMAN. G. M..' AND W. R. BRODE. J . Am. hem. h c . , 73, 1487 (i951). SEMENTSOV, A,, Am. Scientist, 43, 97 (1955). HENDRICKS, B. C., J. CHEM.EDUC.,32,56 (1955). PAT~ON A., R.,Chem. Eng. News,33,3147 (1955).
