K. Keith lnnes Vanderbilt Universitv Nashville. Tennessee
Viscosities of Butanol-Heptane Mixtures
1
A physical chemistry experiment
The Ostwald viscometer used in most physical chemistry courses is perhaps too simple a device. The usual attitudes of manual, student, and instructor toward an introductory relative viscosity experiment are perfunctory. We have attempted to make the experiment more memorable by using i t to introduce the students to problems of temperature control below O°C and to simple research in an old field. Since only equipment available to most physical chemistry classes has been used, the student experiment, as well as t,he new results, may be of general interest. The work is begun by discussing with the students the qualitative relationship of liquid viscosity to liquid structure. It is emphasized that the most useful general relation, q = AeBIT, for the viscosity, 7, of simple liquids holdx within the accuracy of most experiments from temperatures slightly above the melting points to those slightly below the respective boiling points (1). The absolute rate theory interpretation of this result is discussed briefly (2). The students are then asked to make their own estimates of what common liquid
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I 30
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I 3.8
3.4
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4.2
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lo' T The viscoritieg of mixtures of n-heptane and n-butmol ar functions of lhe absolute temperature. The mole fraclion of butand is indicated a t the upper end of each line.
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Journal o f Chemical Education
in each of the classes, polar and non-polar, would offer the most critical test of the above equation; that is, what material is liquid over the largest range of temperature. n-Propyl alcohol (range, 220') and n-heptane (190') are accepted as reasonable answers. The students find in the literature that viscosities of propanol and heptane seem to be readily available over temperature ranges of only 70' and 163", respectively (5, 4). n-Butyl alcohol is therefore substituted for propanol and data from -51 to 100°C (5) are found to be well represented by the equation q (cp) = 0.0011 X exp (2306/T). Data for heptane taken a t temperatures from -73 to 90°C are almost as well represented by q(cp) = 0.178 exp (989/T). It is noted that, unlike most constants of rate processes, the constants of these equations have been determined over large temperature ranges, of order 1.0 T to 1.8 T(M& 360°K), so that inaccuracies should be relatively small. It is stressed that the larger coefficient in the exponent for butanol is typical of hydrogen-bonding substances. The problem of representing the viscosities of liquid mixtures is then introduced (2, 6-7). From the foregoing it appears that the most concise and funda mental presentation of data would be the relation of the mole fraction, X, of the mixture to the two constants, A and B, of the above equation. The probable reason that this course seldom has been followed is low accuracy of the values of A and B which can be derived from existing data. The constants usually are determined over temperature ranges of order 1.0 T to 1.1 T and the largest range presently available seems to be 1.0 T to 1.5 T (8). .It is proposed that i t would be worth while to study mixtures of two liquids for which these ranges could be increased appreciably. One could then hope to interpret A and B as functions of X. The students are reminded that there will not be many liquid pairs which will remain completely miscible while the absolute temperature is reduced by almost one-half. Butanol and heptane may be such a pair and would be most attractive because of the data already discussed for the pure substances and because reagent grade material is inexpensive in each case. Hop-ever, a literature study shows only that the two liquids form an azeotropic mixture (bp minimum of 93'C) a t 18 mole % butanol (9) and that some heats of mixing have been measured a t temperatures as low as 8'C (10). It is therefore necessary for the students to test &xtures. These tests indicate complete miscibility between -80 and 90°C. Consultation of a reference work for research (11) shows that the Ostwald viscometer is suitable 'for viscosities between 0.22 cp (heptane a t +90°C) and 36.1 cp (butanol a t -51°C) and makes clear the simple
precautions necessary for its use a t room temperatures and above. Problems of adaptation of the Ostwald viscometer to low temperatures are considered. An unsilvered Dewar flask is taped (except for an observation port) for use as a thermostat. A toluol thermometer graduated in tenths of degrees and a solid COracetone mixture complete the equipment for temperature control. To avoid condensation of atmospheric moisture in the viscometer, a CaClz tube is attached to each arm. To check the effect of a large temperature change on the standardization of the viscometer, pure heptane is used a t -70% and a t other temperatures if necessary. The density of each mixture a t each temperature is measured with a Westphal balance. We find that the student measurements proceed smoothly a t both low and high temperatures. Commercial reagent grade materials obtained over a period of three years reproduce the literature values of the viscosities of the pure liquids a t 20°C. Each pair of students prepares one solution, studies it over the temperature range -65 to +70°C (1.0 T to 1.7 T), and determines values of A and B of the above equation such that the data obtained are represented with'm experimental error, that is within 2 6 % . As the students prepare to summarize their results, their attention is directed to the many proposed methods of analysis. Viscosities of liquid mixtures have been discussed from the purely empirical view (7), from the semi-empirical view (Z, 5, 6 ) , and from the simple theoretical view (6). The second view is useful for interpreting the A and B values determined above. A more sophisticated approach cannot be justified for a hydrogen-bonding liquid. Student values of A and B determined from the accompanying figure are presented as functions of the mole fraction of butan01 in the table. A decreases sharply as X increases. B increases with X, slowly a t first and then more Constants of the Equation 7 = AeBIT for Mixtures of n-C4HsOHand n-C,Hla
rapidly, and in a third stage from about X = 0.6 to X = 1.0, follows the linear interpolation between the values of the pure liquids. In the absolute rate theory B is approximately proportional to the heat of activation for flow. Much of the difference of the B's of pure heptane and pure butanol is expected to be accounted for by the necessity of breaking the hydrogen bonds of associated butanol to form the activated state. From the present results, one would conclude that, when butanol is diluted with heptane to X below 0.4, the heat of activation drops sharply because no hydrogen bonds need to be broken before a molecule can move relative to its surroundings. This interpretation is consistent with the values of A in the table and with the formation of an azeotropic mixture a t X = 0.18. It may be noted that heats of mixing of the two liquids led independently to the conclusiou that the double molecules of pure butanol are entirely dissociated in dilute solution (10). The success of the simple approach outlined here shows that the undergraduate laboratory student may reasonably expect some of his afternoons to lead to significant new results. This extension of the effective temperature range for viscosities of mixtures easily could be doubled if one wished to introduce the student to the use of liquid nitrogen and organic solvents for temnnrature control between 143 and 210°K. Literature Cited
(1) INNES,K. K., J. P h y ~C h . , 60; 817 (1956). (2) GLASSTONE, S., LAIDLER,K. J., AND EYEING,H., "The Theory of Rate Processes," McGraw-Hill Book Co., Inc., New York, 1941, p. 484. (3) "Handbook of Chemistry and Physics," 34th ed., Chemical Rubber Publishing Co., Cleveland, 1952, p. 1886. (4) JOENSON, J. F., AND LETOUENEAU, R, L., J. Am. Chem. Soc., 75,1743 (1953). (5) P ~ T I N G T OJ. N ,R., "An Advanced Treatise on Physical Chemistrv." Vol. 2. Lonemans. Green & Co.. Inc., New ...-., -.- - ,=. (6) MOELWYN-HUGKES, E. A,, "Physical Chemistry,'' PergamonPress, New York, 1957, p. 771. T. K., "The Properties of (7) REID, R. C., AND SEERWOOD, Gases and Liquids," McGraw-Hill Book Co., Inc., New York, 1958, p. 216. (8) BEAMLEY, A,, J . C h .Soc., 109, 451 (1916). ( 9) HORSLEY. L. H.. Editor. "Aeeotro~icData," American ~~, he mi mi society; ~sshidgton,D. c., 1952. (10) TON ELBE,G., J. Chon. Phya., 2 , 73 (1934). (11) WEISSBERGER, A. (Editor), "Techniques in Organic Chemistry," Vol. 1, Interscience Publishers, Ino., New York, 1949, p. 331.
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