A SEW APPLICATIOS OF .iZEOTROPISlI ni- MAL-RICE
LECAT
More than twelve years ago,’ the author, applying the binary azeotropism, devised a new method available for the determination of the “chemical functions” in organic compounds boiling without decomposition. At that time, however, the knowledge about the relations between the azeotropic behaviour of binary liquid systems and the chemical constitution of its components was still rudimental and possibility of applying the method remained rather questionable. Severtheless a first step was made towards the “azeotropic analysis.” Since then, careful and exceedingly tedious systematic experimental investigations* by the author have given a much better insight in this question. Our knowledge a t present is even sufficient to allow us to state the possibility of t,he detection of chemical functions by azeotropic method. The present writing will constitute a second step. Two or three more are still needed to complete the solution of the question, as will be discussed later. Review of Some Notions on Azeotropism Before describing the method, it may be well to review some available definitions and properties in connection with azeotropism, since this phenomenon is generally still imperfectly understood. Under constant pressure, a single chemical compound, if it boils without decomposition, distills isothermally. The converse however is not true : a liquid distilling isothermally under constant pressure is not necessarily a single substance. I n other words a liquid bulk is not always zeotropic.3 According to the chemical nature of its components, to their relative proportions, and also t o the constant’ value of the pressure, it may happen that during the whole distillation the boiling point ta remains constant (as for a single substance). The temperature t.t mill be necessarily either above or below the boiling point of any of the components. In this case there is said to be azeotropism and the distillate is at every moment an azeotropea. Only when we are dealing with a single compound, will every isobaric distillation be wholly isothermal. Indeed the azetropic composition has been found always to vary with the pressure (and also with the temperature, the latter being correlated to the first).l Thus it is most probable that a binary A I . Lecat: “Az&otropisrne”, 29 (1918). The author has devoted t o it a number of writings in various scientific journals. See especially “ L’AzBotropisme dans les s y s t h e s binaires contenant une substance hydroxyl6e” Rendiconti R . Accad. Lincei, (6) 9, I I Z I (1929) 1se.y R. A. L.]. One will find there references to the principal other papers published on the subject. 3 I t is stupefying and deplorable that even recent university textbooks of physics make so heavy an error. Roscoe (1861) was the first to show it, for the aqueous mineral acids. 1
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A N E W APPLICATION OF AZEOTROPISM
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liquid bulk can only be azeotropic for one single pressure PA,perhaps for two, this case being however highly improbable (experiment will only decide the possibility). Frequently it has been observed that under a certain pressure the azeotropic concentration X A of one of the constituents disappears, thus separating zeo- and azeotropi~m.~ This last term, as above defined, has not exactly the same meaning as that given to it previously by the author. I t is rather the etymological meaning, that is more extensive. To put the two again in concordance, it will be best to call it azeotropism Zato sensu.6 It includes indeed the two following cases: a) If boiling under a pressure pa, the liquid bulk is homogeneous, we are dealing with azeotropisin stricto sensu, also called, more frequently and indeed more adequately, merely azeotropism.’ This property was still in 191; regarded as a rather uncommon one. I n fact, it is very frequent. The researches which the author carried out during the past twenty-two years have detected more than three thousand cases of binary azeotropism, six thousand pairs of liquids being investigated.* Moreover, a number of empirical formulae, established in recent years’ (by the author), answer, in a majority of cases, and with very much confidence, the question whether any two liquids are likely to form an azeotrope. Eventually, these formulae1° give with fnir accuracy, the two azeotropic constants of the system, i . e . the azeotropic temperature and concentration (for the pressure considered). The azeotropes (as understood to be stricto sensu) are either more or less volatile than their components, and named respectively positive or negative, as azeotropism itself.” The azeotropic shift, say 6 , respectively a lowering or a raising, is the difference between the boiling points of the azeotrope and that of the most or the least volatile component. One agrees t o use the sign (which besides remains unexpressed) for a lowering and the - sign for a raise. The maximum boiling point (negative azeotropism) is not met with so frequently as the minimum boiling point (positive azeotropism). The occurrence of the first, which appears to be possible only when dealing with two components,’? is about ten times less frequent than positive binary azeotropism. The cases referring to more than two components are out of consideration here.
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Axeotropism is thus not always absolute, as van der Waals taught. e In its etymological meaning, azeotropism would also include the case of a single substance, but evidently it is better to exclude this case. At the azeotropic point the ehuZlitionisobars are tangential to the dewisohars. * T h e y will be collected in a volume “Tables of Sormal Binary Azeotropes,” to be gubljshed in 1930. I n the meantime, the author’s fifteen lists of “Nouveaux az6otropes inaires” may be consulted. References to them will be found in the Ann. SOC.Scient. Brux., 49 B, 10g (19.~9). The first ones however, already in 1917. See especially R.A.L., and also the writings to which reference is made there. I n 1918, the author devised the expreesiona: “az6otropisme de premiere e8 h e , adotropisme de seconde esp&ce,” that are less convenient than the terminology o r the present text, used since 1926. l2 No ternary mixture of maximum boiling point has yet been discovered.
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MAURICE LECAT
b) As long as during its isobaric distillation, a liquid bulk 11 is heterogeneous (Le. composed of more than one phase), its boiling point, say t h , remains constant. The converse is also true; indeed, if during at least a part of the distillation, the liquid is homogeneous, its boiling point will vary continuously. Rut whether constant or varying, it can never be higher than that of the least volatile component. Let us suppose the system t o be binary. Two possibilities may occur, one according to th being lower than the boiling temperatures, t l and t2, of the constituents I and 2 ; another to th being intermediate. In the first eventuality, there exists an initial composition of 11,say xj, that remains constant, as well as t h r during the whole distillation. The liquid bulk is then said to be ezctectic.I3 Its shift is always a lowering, since the eutectisni is only a variety of positive azeotropism. i.1eutectic isa heterogeneous azeotrope). I n the second case no coniposition whatevpr will maintain the heterogeneity of the boiling liquid (neither of th) during the whole of the distillation; but the boiling point however remains constnnt during R part of the operation. There is a trrciisz‘ticii~pioi‘tit.14 Its vnpor, contrarily to tho eutectic one, gives, at its deiving point, a homogeneous’: condensntc, n-hosc composition differs from that of the vapor. Any fraction colltcteil duririg the isothermal (and isopiestical or isobnricj distillation is homogeneous :it its boiling point and even lowx-.’6 Trcir~sitioii is only pwsible ~ l i r nthe two components a r c partly immiscil-)le i t he converse hoircver not being t r u q . Occurring very seldom, it will not bc usrd here, and vie only mrntioncd it in order to be complete. Summing up, we m:iy deduce the esistence of four different kinds of constant boiling points, corresponding t o positivi. and negative azeotropism, t o the eutectism, :inti to trnnsition.” Obviouslyj this latter case is to be (axcludrd from the nzclotropism itven i i i t o scrie.iO ; these include then only the (strict) azeotropism and the eutectism; the azeotropca itrto serzsu including the t u ( c ozcotropcs, merely cnllcd ( i x o t c o p c s ) and the eutectics. They are all c:illeti r ~ o ~ ~ / /if~ tthc i / , :twotropic coniposition is that relative to the normal pressure. For practical laboratory purposes they are undoubtedly the most interesting, since the atmospheric presjure is often near t o 7 6 0 mni (of nicrcury) and 1n:ry tlicn easily he rctluccd to the normal. lJ The (,omliositioii of the eritecti
