T o the Editor: The conclusiou reached by Racusen' in his recent article on paper chromatography is one which has no foundation a t the present time. There is no indication that we can "expect that some day we may have a system of organic analysis based entirely on paper chromatography . . . containing very exact relationships regardiug structure and migration rate." Chromatography 1s a t present very popular and has achieved many important results, but it has several limitations which should be realized by all chemists, especially teachers of chemistry. The principal use of all differential migration methods (including paper chromatography) is for the separation of mixtures. Besides this, chromatographic sequences (R values) partially describe or characterize substances; they provide clues to the kind, location, and number of functional groups present in organic substances. Many reports can be found in the literature of attempts to relate molecular structure and chromatographic bel~avior.~Yo simple relationship has ever been found. In fact, there is such great variation in sorhability with solvent, sorbent, solute (concentration and molecular structure), and temperature, that a widely applicable generalization will likely never be Chromatographic data is useful in addition to decisive, conventional, chemical methods for determining compositiou and structure. Migration data alone cannot reveal con~position,structure, or functional groups.4 Only if pure reference substances are available for comparison of migration behavior can an unknown he identified chromatographically. The following quotation taken from a recent review emphasizes this alternative view:5 There is a limited systematic basis for deriving the molecular structure from differential migration behwiovior. Highly selective with respert to separations, the methods are non-specific with respert to strurture.
It seems reasonable to expect that some day we may have a system of organic mslysis based entirely on paper chromatography. The scheme might well contain very exact relationships regarding structure and migration rate
While I did not claim that such "relationships" are a prerequisite to identification, it does seem likely that they would prove useful in identifying the members of a homologous or other closely related series. Naturally, such an investigation would require close control of temperature, solvent composition, paper quality, etc. These requirements are within reach of today's technology. DAVIDRACUSEN UNIVERSITY OF VERMONT B~LINGTON
T o the Editor: In the May 1962 issue an article described the use of hypodermic syringes for an experimental verification of Boyle's law (THISJOURNAL 39, 252 (1962)). I should like to call attention to a similar article published by George Towe and Georgeanne R. Caughlan in the American Journal of Physics, 29, 706 (1961). The use of a hypodermic syringe for Boyle's law demonstration was suggested by the Physical Science Study Committee in Experiment 220 of their revision for the second portion of preliminary edition of Laboratory Guide No. 2, September 25, 1958. The Physics Laboratory a t Montana State College has been using the apparatus described in the article for a number of years and it has proved to be a very satisfactory experiment.
The enormous variation in chromatographic systems is further demonstrated by the fact that no useful theory has ever beeu developed for the prediction of the conditions required to perform a given separation. The chromatographer must rely on intuition, analogy, empiricism, and trial-aud-error; I see no indication that any of this is about to change.
' RACUSEN,D., J. CAEM.EDUC., 39,484 (1962).
STRAIN,H. H., "Chlompl&~tPigments and Chromatographic Analysis," (32nd Annual Priestly Lectures), Pennsylvania. State University, University Park, 1958. S. R., J . Ch~omalog, 1, 537 (1958). Roux, D. G., EVELYN, ' STRAIN,H. H., '4nal. Ckem., 30,228 (1958). 'STRAIN,H. H.,.4nel. Ckem., 33,1733 (1961). a
T o the Editor: It was not my intention to exclude chemical characterization as a means of identification. In the context of my article color reactions on paper were taken as a necegsary condition to identification. My statement, quoted by Sherma, reads in its entirety:
T o the Editor: Dr. Strong's letter (January, 1963) brings up a very important and much neglected fact about the teaching of thermodynamics. It is certainly not apparent to the student that thermodynamics is actually good for anything! One way to increase student awareness is t'o have them locate and prepare a brief written report on three thermodynamic articles which have appeared in major journals within the last year. They report what was measured and how, the purpose of the research, and its importance. Typically, a class of 20 students will come up with more than 50 different articles. JESSEH. DAY
Volume 40, Number 4, April 1963
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