5 84
INDUSTRIAL AND ENGINEERING CHEMISTRY
literature available t o the author shows, his results are the most accurate and complete data available for temperatures between 20" and 100" C. The average accuracy of the results in Table I1 is about *0.25 centipoise, and the uncertainty of any one result is only as great as is caused by an uncertainty of *O.l " C. in the temperature measurement, which is a t least *O0.20per cent. The accuracy a t 20" C. is obvious from the results in Table I. There remains the question as to how the results would be affected if aniline was not absolutely pure. The viscosity of a sample of aniline that was darker in color and had a density greater by 0.003 than the product used in the viscosity measurements was observed to be 0 6 per cent higher than t h a t of the purified material. Another sample with the same density as used in the T iecosity measurements had the same viscosity. Aniline with a density of 1.014 showed a viscosity of 4.32 centipoises; accordingly, a density change of * 0.0003 corresponds to a +0.07 per cent change in the viscosity. Since an uncertainty of this order is acceptable, the question was not investigated further. Aniline must be distilled for viscosity measurements, but this is also true of water. m h e n it is protected from light and the atmosphere, aniline can be kept 2 months without change. Aniline vapor attacks paper. The vapor is poisonous only in concentrations higher than those likely to occur incident to viscosity measurements-at least, the author noticed no ill effects. Furthermore, the viscosity of aniline is not especially high. However, in stepwise standardization beginning directly with water, the first step is most uncertain as a consequence of turbulence and surface tension. A viscosity of 4.4 centipoises is sufficiently large to permit convenient standardization of an oil with a viscosity of about 25 centipoises. The latter can then be used for the calibration and control of instruments.
Advantages of Aniline One great advantage of aniline lies in the fact that its surface tension is only slightly larger than that of lubricating oil. Barr (I) has pointed out the relatively large effect of surface
A Liquid Bath Melting Point Apparatus
T
HE melting point apparatus represented in Figure 1 combines a high degree of generalconvenience, moderate rapidity, and a very sat,isfactory level Of accuracy. The design of the outfit and the dimensions indicated are such as to avoid the disadvantages of a large liquid bath, and also difficulties in uniform heat distribution and in heat regulation as the point is approached, which nlay be encountered in the use of very small baths. If strongly made Of Pyrex or similar glass, the apparabus is substantial and may be regarded as no more fragile than other glass apparatus. Severa1 outfits of this tYPe have heen in use for a number of years, one in the hands of students, without breakage. The capacity of bulb A is about 100 cc. The cap, H,seated in a ground joint, has three tubulures, one for the melting point thermometer, F , one for the auxiliary thermometer, G, and the third for the wiper, E. The profile of the apparatus is an unbroken vertical line, with only a convex curvature along the vertical axis. This produces a noticeable horizontal magnification of
VOL. 10, NO. 10
tension-for example, with capillaries of range 1 of the British Standard BESA-188, an error of 0.8 per cent arises from this cause on changing from water to oil. It is, however, possible to eliminate the influence of surface tension in capillary apparatus. Surface tension had no effect on the author's measurements, because active surfaces are not present in the falling-body apparatus. Aniline is far more suitable than water for measurements a t high temperatures. It can be used in a closed apparatus, such as is provided in the Steiner viscometer, a t 100" C. and above without danger of evaporation. Furthermore, according t o Bingham ( 2 ) , the uncertainty in the present accepted values for the viscosity of water amounts to 0.5 per cent-that is, 1 0 . 2 5 per cent. Thus, aniline passes the test in this respect. Whereas water can be employed for calibrations of the required accuracy only at 20" C., distilled aniline having a density of 1.0208 may, with the aid of the results given in this paper, be used for calibration a t temperatures from 20" to 100"C. I n view of the fact that the constants of many viscometers 1-ary with the temperature, and for other reasons, the ability to calibrate instruments a t the higher temperatures appears to be extremely important.
Literature Cited Barr, Guy, T o d d Petroleum Congress, London, 1.933, Proc. 2, Vol. F, p. 50s. Bingham, E. C., Ibid., p. 512. Bingham, E. C., Klooster, H. S. van, and Kleinspehn, W. G., J. Phys. Chem., 24, 1 (1920); International Critical Tables, Vol. VII, p. 217, New Tork, hIcGraw-Hill Book Go., 1930. Erk, S., "Ziihigkeitsmessungen an Flussigkeiten und Cntersuchungen von Viskosimetern," Berlin, V. D. I. Verlag, 1927. Ibid., page 24. Ibid., page 27. International Critical Tables, Vol. 111, p 29, New York, M c Graw-Hill Book Co., 1928. Landolt-Bdrnstein Tabellen, 5th ed.. Vol. I, p . 367, Berlin, Julius Springer, 1922, Vol. 11,D. 1232. (9) Ibid., 1st Suppl., p. 77, 1927 RECEIVED September 9,1937.
E. C. WAGNER AND J. F. MEYER University of Pennsylvania, Philadelphia, Pa.
objects nithin the bath, but no distortion. The "bridge," B, is attached at two points near the base of the neck and supports the small horizontal platform, D. In depressions a, b, b are seated, respectively, the melting point thermometer and the melting point tubes, The support, D, and the tubulures mentioned are so placed with respect t o each other that the thermometer is parallel rvith the front wall and about 5 mm. from it, and that the melting point tubes, inserted through the oblique tubulures shoX1-n in Figure 1, are seated beside and slightly forward of the bulb of the thermometer, in such position that the rear illumination of the charge in the melting point tube is unimpaired. Klien the bath is at room temperature the immersion of the thermometer is about 20 mm. The melting point tubes are about 7 cm. long, The sclew-type stirrer, C, extends obliquely through opening 0 and between the two supporting arms of B t o a point near the bottom of the bath and directly beneath platform D, and is rotated by a small disk motor with rheostat control. The \Tiper, E , is made from two pipe cleaners joined by twisting two ends together, a free end being then bent into a loop of such diameter that manipulation of the wiper will clear the glass lvall of drops or liquid film which decreases the visibilitv of the thermometer. The apparatus is seated in a 4-cm. circular hole in an
OCTOBER 15, 1938
ANALYTICAL EDITION
asbestos board (not shown), and ip held in place by means of a clamp attached at 0. The gas supply is regulated by use of a glms stopcock with an elongated handle. The melting point thermometer is of the enclosed-scale type, and has the general dimensions of the familiar Anschutz thermometers. It is calibrated for total immemion, with a range from 30" to 300" C . in 1" intervals. The over-all length is 180 mm., the length t o the 300" mark is 120 mm., and the diameter is 5 mm. The bulb is 5 mm. high and 5 mm. in diameter at the top, tapering to 3 mm. at the end. The entire scale of the thermometer is within the apparatus. The divisions of the scale are too small to be read satisfactorily lyith the unaided eye, but by use of the buret reader, M,readings t o 0.1" can be made with ease. A collar of rubber tubing (not shoa-n), which makes contact with the tubulure of cap H , assists in keeping the thermometer in position. A set of short-scale thermometers may be used instead of the single thermometer described.
TABLEI. MELTINGPOINTDATA Substance Benzoic acid Benzanilide
@
92 mm.
APPARATUS
Melting Point Observed t'a 122.2 76 122.5 86 163.0 95 163.2 100 242.2 132 242.4 136 134 242.4 242.7 138 242.0 135
No 72 72 113 113 182 182 182 183 182
Formula used: Correction = N ( t
r FIGURE 1. MELTINGPOIXT
585
Stem Correction +0.5 +0.4 $1.2 4-1.1 $3.3 3.2 3.3 3.1 3.2
Calibration Melting Accepted Correc- Point Melting tion Corrected Point -1.1 121.6 -1.1 121.8 l21:7 -1.5 162.7 . ... -1.5 1 6 2 . 8 163 0.0 245.5 ,,.
... ,,, ... . ..
245.6
245.7 245.8 245.2 Av. 245.6
. ..,
,,
,,,
.. ,
244.8
- t')0.000154.
The auxiliary thermometer, G, includes the range 30" to 180' C. on a scale which occupies the lower 55 mm. of the stem-i. e., which is wholly within the apparatus when the thermometer is in the topmost position to which it need be raised in use. This thermometer has a small bulb and is employed in the usual manner for determination of the emergent stem corrections to be applied to the readings of the melting point thermometer. T h e auxiliary thermometer is held at any desired level by means of a collar of rubber tubing which rests on the tubulure of H. The biconvex lens. N , is dispensable, but is desirable as it improves the viqibility of the melting point phenomena. The telescope, M , and the lens, N , are supported by clamps on a small ring stand. A racking device would be an added convenience. For the trials whose results appear in Table I, thermometer F was calibrated by direct comparison with Anschiia thermometers tested b y the Bureau of Standards. The melting point tube was in each case introduced when the temperature of the bath was 10" below the melting point of the substance, with the thermometer showing a rise in temperature of 2' per minute. T h e melting point was taken as the temperature at which a liquid meniscus first appeared. The data recorded in Table I indicate the magnitudes of the stem corrections a t several temperatures, the precision of the results, and their accuracy, as determined with several compounds purified by repeated crystallizations. RECEIVED
.4pril 27, 1938.
