the students came to the conclusion that a double-beam instrument was needed to accomplish the task of handling the specified number of samples per day. Once this decision was reached, the specifications and capabilities of double-beam instrument systems were compared. F r a n k A. Settle, Jr. Project Select To the Editor: An article entitled "Project Select" appeared in the June 1979 issue of this Journal (pp. 411-12) whose objectives, as stated bv its author Frank Settle, Jr., were ". . . (1) to stress important to the selection of a particular type the of instrumentation, (2) to emphasize the relationship between instrumental parameters and the methods required for a specific analysis, and (3) to give students an introduction to evaluating state-of-the-art instrumentation through the use of basic concepts acquired in the course." We feel we must take exception to some of the points in this article, particularly the ultraviolet-visible assignment. 1) Of
the 5 instruments used in this section, 3 were double beam spectrophotometersand 2 were single beam spectrophotometers. There is no way that purchasers of these instruments would attempt to compare single beam instruments versus double beam instruments. The two are entirely different as far as usage, electronics, ete. The applications that may be performed necessitate which instruments will he considered for evaluation. 2) The purpose of the project could he served better by including tests which show each instrument to advantage, thus providing the student with a closer to real life situation. 3). Definitive criteria should be established. based on test reauiremmts, wavelength range required, the nerd wan, etc., whereby initrunwx~areevsluared. Ir w.essmlca in the aniclr that ptudenu erroneously picked highrr priced initrumrnrc yer dnul unr pulllished using this poor criteria. ~~
We feel that the bias in "Project Select" towards comparison of instrument specifications rather than comparison of specifications to the need and requirement of the analysis does a great disservice to the students. We are encouraged to see that subsequent experiments will include maintenance costs and instrument lifetimes as factors and suggest -- an additional factor, that of cost per test including labor and amortizing instrument coat over five vears. This more correctly represents a "real life" situation. We would hope that "Proiect Select" is continued and l w k forward to future reports on its success. ~
Ellen P. Aquilina Product Administrator James P. Malone Manager, Technical Seruices Bausch 8r Lomh Analvtical Svstems Division 820 Linden Avenue Rochester, NY 14625 To the Editor: I am writing in response to the comments of Ms. Aquilina and Mr. Malone concernine the "Proiect Select" article. The concern of the instrument vendor is understandable in view of the results of the experiment. The particular situation involving the selection of an ultraviolet-visible spectrophotometer was indeed biased toward the more rapid analyses afforded by double-beam instruments. I plan to expand the number of situations to include those which would favor single-beam instruments, and I feel confident that the Spectronic 21 will be most competitive. Since the initial use of Project Select, I have attempted to emphasize tostudents the newssitv of estahlishing definitive Eriteria for evaluation of instruments for a pakicular application. In the ultraviolet-visible application of the article,
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Virginia Military Institute Lexington, VA 24450 A Phase Rule Footnote
To the Editor: The note by Lenox and Hadley, [J.CHEM. EDUC., 56,460 (1979)], "On the Melting Point of Acetanilide," inspires the following remarks about the need for care in describing phase equilibria. The above authors speak of a "quadruple point" of the acetanilidelwater system, consisting of two liquid phases, one solid phase, and "the vapor." It is to such descriptions that we are reacting. The concern involved is much the same as discussed in some detail by Parker and Kristol [J. CHEM. EDUC., 51, 658 (1974)l. As a two-component system, the acetanilide-water one will.,bv.the ~ e be invariant when four ~ h a s e are s . h a s rule. in equilibrium. Such a condition is indeed a quadruple point. That is, while more than one quadruple point may exist (if, for example, various solid phases are possible), each is a sin~ularitv, iust a "ha~~enstantial" location on a continuous .. not . Tine or surface of phase kquilibrium. The ambiguity and hence source of possible confusion is in the use of the word "point."To illustrate, in a one-component system, the melting "point" varies with pressure; there is no unique value. It is a merely a loose convention to read "melting point" as the temperature of solid-liquid equilihrium under one atmosnhere nressure heelectine. in addition. the effect or air). By contrast, tKe triple point is defined of the uniauelv on stating the phases present. The ambimritv is thus in whether "point';meakj (a) ;condition of invar&&e, or (b) merely a temperature of phase equilihrium at some conventional choice of pressure, that is, a point on a continuous line of phase equilibrium. To return to the acetanilide-water case, air is present, and the system is actually a three-component one (if the constituents of air are not differentiated). A condition of four phase equilibrium is therefore not a quadruple point in sense (a) above; the system is not invariant. The so-called quadruple point is merely a temperature of four phase equilibrium under an arbitrarilv chosen oressure of air. In this sense anv eutectic mixture in a vessel containing air is at a "quadruple point." We brine up this matter not in a auibbling sense. hut rather out of resp&tfor the intrinsic clarit; of thermodynamics, and to add a voice urging equal clarity in language. We urge that the terms "triple point," "quadruple point," etc., be used to designate conditions of invariance, that is, in sense (a) above. Points on a continuous line or surface of phase equilihrium should be qualified, as in "normal boiling point," "melting point under one atmosphere," etc? After all, carelessness in language can he the beginning of carelessness in substance. Reerettablv. - . for examnle. . , there are " ~ h a s ediaerams" mesented in current literature that are no more than maps of aualitative observations. the boundarv lines erosslv.violatinr! fundamental phase rule principles.
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J a m e s B. Ellern A r t h u r W. Adamson University of Southern California Los Angeles, CA 90001 No extremism is intended. Thus in a lenethv textual discussion.
of the material. In concise technical papers, however, the qualification should always be present. Volume 57, Number 5, May 1980 I A171
