"Melting Points Are Uncorrected"-True or False? George V. D. Tiers 3M Corporate Research Laboratory, St. Paul, MN 55133
The classical capillary melting-point d e t e r m i n a t i ~ n , ~ , ~ immersion depth, often "76 mm immersion". (The location of an integral ground joint defines the immersion depth for providing a simple measurement of an unpredictable charsuch thermometers.) The temperature scale is graduated on acteristic property of solid organic compounds, together the assumption that the (hot) fluid does not contact the stem with a good indication of purity, has been of central imporabove that mark and that the exposed stem is a t room temtance to the development of organic chemistry for well over a nerature. -25 Temoeratures read on such thermomcentury. Even today, amid the proliferation of amazing (and kters, when used as intended, are inherently "corrected" amazingly expensive) instrumental methods, the melting (and should be true temoeratures). I t is therefore point retains its value to the laboratory worker; and it also . .false and misleading to designate such readings as "uncorrected': furnishes an essential link to the massive preinstrumental literature. It was thus more than a little surprising to discov"Uncorrected" Meltlng Points: Analysis of Information er that. of those articles and notes in a major international Does i t happen? Consider this: of the 88 papers reporting organic chemistry publication3 in which melting points are "uncorrected" melting points, 18 used the Thomas-Hoover reported, a huge 65% stated "melting points are uncorrecapparatus, nine had Biichi equipment, three listed Electroted", only 3% claimed "corrected" melting points, and the thermal, and four used Fisher-Johns (a hot-stage device). As remaining 32% made no statement a t all.' From a more purchased, all 34 had partial-immersion thermometers redetailed inspection of the papers there emerges a very strong auirine no "correction". Another 309 are nresumed to have presumption that, in the great majority of cases, the asserbeen designed similarly, hut pertinent information is not a t tion "uncorrected" is false to fact! hand. The remainine 24 did not snecifv an aooaratus. It is How can this regrettable (yet, as we will see, fortunate) surely unlikely thaivery many 01these diverse pieces of state of affairs have come about? T o answer this, we must understand what the terms "uncorrected" and "corrected equipment had had their thermometers removed and replaced by total-immersion types.'O The conclusion is inessignify. They apply only to thermometers. The overwhelmcapahle-it does happen that "corrected", true temperaing majority of capillary (and hot-stage microscopic) melting points have been measured by means of one of two types of tures are routinely described as "uncorrected". Why? ~erc~r~-in-~ thermometers. lass Of these, when used in typical fashion, the "total-immersion" type gives increasingly 'Morton. A. A. Laboratory Technique in Organic Chemisfry: erroneous readings, the higher the temperature, and thus McGraw-Hill; New York, 1938; pp 21-39. requires "correction". Kamm. 0. Qualitative Organic Analysis, 2nd sd.; Wiley: New York, 1932: pp 123-126; Cheronis. N. D. Semimicro and Macro Correction of Totai-Immersion Thermometers OrganicChemistiy Crowell: New York, 1942: pp 47-51: McElvain, S. M. The Characterization of Organic Compounds; Macmillan: New The older, and inherently more accurate, "total-immerYo*: 1945; pp 12-17: Shrlner, R. L.; Fuson. R. C. The Systematic sion" type only gives a true reading when immersed in the ldentification of Organic Compounds. 3rd ed.; Wiley: New York, 1948: working fluid to a point a t least above the mercury level (as nn 19-23. r r ~- - the name implies); the degree scale isso engraved. If now one Speciflcally. 136 papers in J. Org. Chem. 1988, 53.4164-4627. were to withdraw such a thermometer from the (hot) fluid 4905-5177. and 5599-5784. A cursory examinat on of other major until the room-temperature (say 25 OC) mark on the scale is journals suggests little difference in this regard. just exposed, and the emergent stem were to cool to that A similar survey of J. Chem. Educ. 1986-1988 found papers only temperature, the mercury column would shrink more than in this latter category. the glass, and an erroneously low reading must result. The For this case E = 0.00017(Tn- 25)2. The exact general formula is glven by Sturtevant,J. M. In Physical error, E (in "C), which must be added to the thermometer Methods of Organic Chemistry, 3rd ed.; Weissberger. A., Ed.: Intersreading, TR, to find the true temperature, may he found hy cience: New York. 1959; Vol. 1. Part 1. Chapter 6. p 264. An an approximate formula'.5 to he only 1.0 " C a t TR= 100 "C, approximate general formula presented there was also glven by but goes to 5.2 OC a t 200 "C and up to 12.9 OC at TR= 300 Morton1along wlth constants (differential expansion coefficients)for OC.6 The usual presentation of the e q u a t i ~ n ' .tends ~ ~ ~ to "norma1" and for borosilicate thermometers. conceal its quadratic character. Unfortunately, all ordinary 'From personal discussions wlth many highly skilled chemists modern laboratory thermometers hearing no contrary desighaving academic andlor Industrial backgrounds, I find that these facts nation are of this type.7 (Note, however, that those bearing are not widely appreciated. integral ground joints are normally exceptions to this rule.) Failure precisely to satisfy these conditions results in less ultimate accuracy: for example, the use of such a thermometer in Me Most of the older melting-point data were of necessity taken "total immersion" mode would result in overestimatingthe true temwith total-immersion thermometers, often corrected for the perature by about the same amounts as calculated above. It is emergent stem. Some workers, however, found such correcapparent that small immersion discrepancies will cause only small tion impractical, dubious, or inconvenient, and resorted to errors, perhaps within the precision of the thermometer reading. admitting their inaccuracy by appending "uncorr." (unkorr. Yanagimoto hot-state microscopic, five: Kofler hot-stage microin German) to their data. scopic, five; Reichert hot-stage, five; Mel-Temp, 10; Gallenkamp. Mettler, Yamato, Sybron-Thermolyne MP-12615, and Boetius Partial-lmmerslon Thermometers PHMKOS, one each. Some of these may not utilize thermometers. The other relatively common type of thermometer, dating j0 In one of our laboratories someone did exchange the fine. back at least 50 years,' is the "partial-immersion" type; i t accurate thermometer in a new Biichi 510 apparatus for a common bears (in the United States) an etched ring a t the intended total Immersion one-because the laner was "easier to read"!
-
258
Journal of Chemical Education
Posslble Causes and Effects
Perhaps the senior scientist, knowing the above, queries the . younger . team members, who replv "uncorrected" hecause they have not personally d ~ n e & ~ t b iabout n ~ i t and do not realize they are giving the wrong answer.7 Others, . aware that corrections are very small (at row temperatures!) consider the matter to be unworthy of their attention. Some authors may be confusing "uncalihrated" with "uncorrected". There may also he cases of "conditioned reflex". What harm results? Fortunately not very much, for the lower melting points suffer little uncertainty, and (if the above analysis is valid) for the higher ones the statement "uncorrected" can he ignored in most cases. The damage, aside from possible embarrassment, lies in creating lingering uncertainty about data that should be definite and above reproach; i t is unnecessary.
formed statement be made; for example, rather than naming the apparatus used, the Experimental section of a paper should name the thermometer used.lL Thirdly, since the objective is to determine with maximal accuracy the true melting temperature, journal editors, authors, and researchers should begin to use the term "certified" to indicate claims for accuracy by the manufacturer of an apparatus or thermometer (when known), or preferably the term "calibrated" to signify that a series of high-purity materials of well-established melting points have been run and that any ohsewed discrepancies have been used to prepare a calibration graph or table (rechecked occasionally) from which true temperatures can be determined. Notably, none of the 136 papers surveyed above mentioned calibration of their equipment.
Recommendations
What can be done? Firstlv. informed researchers and authors should determine which statement applies and use it properly. Secondly, journal editors can require that an in-
"For example, the generic term "ASTM 2 C imprinted on the back of a thermometer specifies 76 mm immersion and reading to 300 OC wiihin a prescribed accuracy, regardless of manufacturer.
Volume 67
Number 3
March 1990
259
