Example
Determination of pH for c = 0.01 moWL (NH&HP04. From comparison of charge and concentration balances one obtains (7) [HI + 2[H3P0,1 + IH2PO41= IOHI + [PO,] + [NHsI
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ular formula. The better students in the class easily come up with a dozen for a molecule such as C6H6.I n fact, a recent enumeration (3)lists 217 possible topological (structural) isomers. Although many of these are unlikely to be stable entities, some are a t least a s stable as the ones cited in this article.
(see eq 5 a t q = n = 3). The preliminary evaluation (see figure) gives pH 8 for the point P of intersection of the corresponding resultant functions. At the point P, eq 7 can be simplified into the form = [NHd (8) [HzPO~I
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pH 8 lies close to the pk2 =log (KdKJ = 7.2 value and the expression for Z (eq 1) ought be reduced into the form
z = K,[HI + K ~ [ H I ~ Moreover we see also that IogK,' = pk,' = 9.35 for TQL' lies close to the evaluated pH value. I t is then advisable to write
Then we get, in turn, C.K~[HI~ 2~ KJHI + K ~ [ H=IK< ~ [HI + 1
Tadeusz Michalowski Faculty of Chemistry Jagiellonian University 30-060 Krakow, Poland To the Editor:
Obviously, there is more than one way to solve acid-base problems. The purpose of my paper was to point out that in this kind of chemical problem we have chemistry involved, rather than a cross point in a logarithmic plot or a n exact number coming from the use of purely mathematical techniques. Even though a method may include some extra algebraic manipulation, it may also include more chemistry. As a chemist, one may prefer not to do simply what can be done chemically. Aleiandro Olivieri Depanamento oe OJ mlca Anal llca F a c ~tad de C enc as Bloq~rnlcas y Farmace~tcas Suipacha 531 Rosario (2000),Argentina
Benzene Isomers? To the Editor:
After mentioning benzene ( I ) , Dewar benzene (2), benzvalene (3),and prismane (41, the recent essay on "The Diverse Nature of the CsH6 Molecule" (I)states that "the only remaining isomers of C6H6 t h a t are possible are the ...bicyclopropenyls" of which "there are three isomers, namely 1,l-, 1,2-, 2,2-." Again in referring to the synthesis of the latter two ofthese compounds (2)they are characterized a s "the last of the remaining benzene isomers". What about fulvene ( 5 ) and trimethylidenecyclopropane (6)to name just a couple? As a n introduction to the structural theory of organic chemistry I routinely ask students to draw a s many isomers as possible for a particular molec-
The author may have misread the statement of Billups and Haley (2)that 2,2-bicyclopropenyl(7) is the last of the remaining (CHI6 isomers. In fact, six (1-4,7,8) of the 217 C6H6isomers are composed solely of CH groups (4),but the 1,2-(9) and 1,l-bicyclopropenyls (10) mentioned in reference 1do not fit into this category Literature Cited 1. Patgieter,J.H. J. Chem. Ed., 1981,68,280. 2. Billuos. W. E.: Halev. M. M . A m u Cham Int. Ed. Enel.. - . 1963. 28.. 1711 3. ~ m k H.A"& : ~ h e n Int. z ~ d y ~ n g 1988,Z8,1627. l., 4. Bslaban.A.TRm Rournn. Chim l9BB.11, 1097:Warren,D. S.;Oimare,B.M. J.Am. Ckam Soc 1992,114,5378.
Manfred G. Reinecke Texas Christian University Fort Worth, TX 76129 To the Editor:
I want to thank Professor Reinecke for pointing out that the CfHf molecule is indeed much more diverse than the contentiof my article suggests. The phrase "last of the remaining benzene isomers" was perhaps a poor choice of words when one considers the contents of the article, but a t the same time it also indicates the main thrust of the article, and that is to discuss some of the less well-known benzene isomers. I t was never my intention to discuss the whole spectrum or even the well-known C6H6isomers, because, a s was pointed out by Professor Reinecke, it would be a n immense task, but to concentrate on the ones most closely related to benzene. A better choice of title and clearer explanation in the introductory part of the article Volume 69 Number 10 October 1992
859
would no doubt have caused less confusion about the purpose of the article. J . H. Potgieter Physical Metallurgy Division Mintek Private Bag X3015 Randburg 2125, South Africa
New RecrystallizationSolvent for Synthesis of a Bicyclo[2.2.l]heptene To the Editor:
Recent work in my laboratory has shown toluenelhexanes to be superior to EtOAc/MeOH as the recrystallization solvent for purifymg the hicyclo[2.2.llheptene DielsAlder adduct described in my recent article [1991,68,4261. Ernest A. Harrison, Jr. Penn State-York Campus York, PA 17403
A Teacher's Thank You to ACS To the Editoc
This is u letter ofgratitude to the mrrnherii of the American Chemical Soc~etv(br constant concern u h ~ u thc t high school chemistry curriculum and for their concrete contributions to the enrichment of chemistry a t the secondary level. After teaching chemistry for 40 years a t the high school level, I retired but could not resist the temptation to look back to the vears when the ACS was alwavs a t mv side. oKcring updated mater~alin lntrrcstlng ways I saw [he beemnines of the Chem~cr~l Bond A ~ u r o a c hand CHEM st;dy anld utilized the best of these piojects i n my classroom. The motivating lectures by chemists such as George Pimental and Art ~ a k ~ b eencouraged ll me to try harder& meet their standards. Locally the San Diego ACS chapter seemed always present to encourage. They awarded the achievement of my top students and made i t possible for me to receive the Conant Award in 1979, an award that awakened my district to the possibility that the quality of education oi their doorstep may he of a high level. Thank you ACS for the years of encouragement. Sometimes I feel that salarv increases do not mean a s much a s the feeling that otherscare and are by your side. You have h e l ~ e dto make mv career a memorable one and have helped many students to think of chemistry a s vital to their lives and as a possible career. Shirley E. Richardson Torrey Pines High Del Mar. California
A Potpourri of Comments To the Editor:
This letter comments on several items from the Mav and June, 1991 issues and thereby kills several birds with one stone-a choice of language which I hope i s not unfortunate. 1. The Aufbau scheme afferred by Singh and Dikshit (May 1991, p 396) is equivalent to those discussed by Darcey ( I ) , Parsons (21, and Freeman (3). If the upper blocks in Figures 3 and 4 of Singh and Dikshit are "shoved to the right" so that the ns blocks are alignedvertically, then the schemes are identical. 2. Scerri's pqint (May 1991, p 445) about Aufbaumnemonics is well made. I would add, however, that if one uses the Janet form (3)ofthe periodic table, then the scheme advocated by the authors in the previous item is more than a mnemonic; it is a compact uersion of theperiodic table itself 3. Thompson's mriqur M y 1991, p 100, of the usual rule* for slkmlficant dlprs ~3isoreIy needed: one hopes his recommrndntion is herded by textbook authors. 4. hlalynrux J u n e 1991, p 467 ha.; presented nn inrcresting solutron t u the 'logarithm-oi-unit." wohlpm. i..nfn,rtw
nately, I fmd prohle&s with the proposkd solution. For example, in finding the "absolute" entropy of a gas, suppose that I replace Molyneux's 4th stage (d, vaporization) by direct, calorimetric measurement of the enthalpy of vaporization W at some fixed T,and then calculate A s = A&/T; how do the "nidden" logarithmicunits get into the entropy?? Literature Cited
New Source for Starch Solutions: Biodegradable Pellets To the Editor:
Robert D. Freeman Oklahoma State University Stillwater, OK 74078
Your readers may be interested in the following easy and convenient way of preparing starch indicator for use in iodine titrations or in "clock" reactions. Shake one Eco-Foam peanut in 0.5-1 L of distilled or deionized water until i t disintegrates. Let i t stand for a few minutes, then pour off the clear liquid for use as starch indicator, which forms the characteristic blue starch-iodine complex. As you know, starch indicator must usually be freshly prepared by adding "soluble" starch powder to boiling water. Eco-Foam is a cornstarch-based, loose-fill packagingalternative to expanded polystyrene pellets. I t was featured i n Research & Development 11991, (March) 321. Starch indicator made from Eco-Foam is not only easy and convenient, but a n unlimited, free supply i s available from your chemistry storemom.
When we saw our paper "Experimental Demonstration of Corrosion Phenomena. Part 11. Corrosion Phenomena of Steel in Aqueous Media" [ J. Chem. Educ. 1991,68, 3513521 i n print, we realized that we had failed to cite the main reference that inspired our experiments: Sato, N.; Okamoto, G. In Comprehensiue n e a t of Electrochemistry; Vol. 4: Electrochemical Materials Science; Bockris, J. O'M., et al, Eds.; Plenum: New York, 1981; Chapter 4. We regret this oversight
John M. De Moura Laboratorv Coordinator ~nivdrsity01 Idaho Moscow. ID 83843
Jorge G. lbaiiez Universidad lberoamericana Prol. Paseo de la Reforma 880 01210, Mexico D.F.
860
Journal of Chemical Education
Adding an lmprotant Reference To the Editor:
