Apparatus for determining boiling point


pressure bulb. By operating the bulb and stopcock, the mercury is caused to rise in the shorter limb until it reaches within about 2 mm. of the ground...
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MAY. 1947

APPARATUS FOR DETERMINING BOILING POINTS 1. M. SIMMONS The Scots College, Sydn'ey,

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APPARATUS to be described facilitates the determination of the normal boilimg point of a sample of about one-third ml. of any liquid which does not attack mercury or glass, within the range 30 to 200°C. No calculated corrections for variation in atmospheric pressure or for emergent thermometric column are required, and, because the method is static, superheating does not occur. The procedure is very rapid, the apparatus is simple, easily cleaned and charged. The boiling points of mixtures which undergo change in composition on distillation are determined by this method as easily as are those of pure liquids or azeotropes. ,The accuracy of the apparatus as a t present used is fO.l°C. The apparatus is shown in the figure. It consists of a glass U-tube of bore about 7-mm. diameter, having its longer limb about 150 mm. in length; the shorter limb is about 90 mm. long and can be closed by a ground glass plug P. The plug is surrounded by the mercury cup M. Both limbs are graduated in millimeters from a datum line near the bend. The plug P is removed and mercury is poured into the clean, dry tube. The tube is tilted from time to time dnrine the uourine to urevent the adherence of air

of a glak rok Mercury is added until it reacges halfway up the scale on the shorter limb when the limbs are vertical. A rubber tube is then attached to the longer limb, the other end of the tube being attached to a stopcock and pressure bulb. By operating the bulb and stopcock, the mercury is caused to rise in the shorter limb until it reaches within about 2 mm. of the ground socket for the

N. S. W.,Australia

JOURNAL OF CHEMICAL EDUCATION

is such as will compensate for the variation of H from 760 mm., and the vapor pressure of the sample is then 760 nun. The flame is then moved slightly so that it does not strike directly through the hole in the asbestos mat, and the hath temperature therefore slowly falls. The rate of cooling can easily be adjusted to less than 0.5' per minute. Condensation slowly occurs in the shorter limb and the mercury rises therein. When the mercury in the longer limb again stands at (H a b - 760) mm. the thermometer is again read. The bath temperature is caused to rise and fall until consecutive boiling points differ by less than 0.1'. It is found that the depth of the liquid layer of sample a t the time of reading the boiling point is about 3 mm. and that the height of the column of vapor above it is about 40 mm. Hence the volume of vapor is about 13 times that of the liquid. The mass of this vapor is a small fraction of the mass of the liquid; for water it is of the order of one per cent. Therefore, if the sample undergoes change in composition on distillation, the change in composition produced by the formation of the relatively small mass of vapor is generally negligible. This change can be reduced still further by enclosing larger samples of liquid between the mercury and the plug. It must be pointed out that the apparatus determines the temperature a t which the total vapor pressure of the actual liquid sample reaches 760 mm., i. e., the true boiling point of the sample. Hence if the sample contains dissolved air, moisture, or other impurities, the hoiling point found will differ from the condensation temperature determined by the usual distillation method, for thk latter is really the boiling point of the boiled-out and partly fractionated sample. Thus the present method is particularly suited to determining the boiling points of solutions and also to investigating the purity of a sample as received, The present arrangement is found to be much easier to manipulate than the somewhat similar apparatus described by Jones,= Schleierma~her,~ and Pregl.' In these latter methods the sample is subjected to partial evaporation during the process of charging the tube, hut the indefinite change in composition thereby produced in liquid mixtures is avoided in the present apB - paratus. 980.63

plug. About one-third ml. of the liquid whose hoiling point is to be determined is poured onto the mercury in the shorter limb. Any air bubbles adhering to the mercury or glass are removed with the aid of a pipet. The glass plug is placed in its socket, care being taken that no air is entrapped. The plug is secured by springs or a rubber hand attached to horns on the plug and mercury cup. Mercury is then poured into the mercury cup t,o seal the plug, and the rubber tube is removed from the longer limh. The U-tube thus charged is clamped with its limbs vertical in a large beaker containing a transparent liquid of high boiling point, e. g., medicinal p a r a h . Verticality of the limbs is ensured by lowering a small plumbline down the longer limb. A short-stem thermometer, calibrated for total immersion, is clamped so that its bulb is near the middle of the scale on the shorter limb and so that the graduation representing the expected hoiling point of the sample is immersed in the bath liquid: The beaker is heated by standmg it on an asbestos mat having a central hole through which a flame plays directly on the bottom of the beaker. The bath liquid is continually stirred by mechanically or manually raising and lowering a large annular metal stirrer ( S in the figure). The barometric height is read and corrected to an equivalent column of ice-cold mercury a t sea-lei.el a t latitude 45" in the usual way.' Let this correctedpressure be H mm. of Hg. The positions of the mercury menisci in the two limbs are read; it does not matter whether vaporization has yet commenced. Let these readings he a and b mm. respectively, the lowest graduation of each scale being taken as 0. When the temperature of the hath approaches the expected hoiling point of the sample, the flame is adjusted so that the temperiture of the bath rises not more than 0.5" per minute. When the boiling point is approached, the sample begins t o vaporize and the mercury below i t is depressed, thus caufing the mercury to rise in the longer limb. The thermometer is read a t the instant that the mercury in the longer limh reaches (H a the graduation given by the expression b - 760) mm.. for then the difference in mercury levels

+ +

+ +

+

(1 --at) B -X (1 Bt) where B is the barometric reading in mm., or an$ B are the coefficients of linem expansion of the scde and ouh~calexpansion of mercury respectively, and g is the acceleration due to gravity at the place where the barometer is located, snd t is the air temperature.

' e. g., by applying the formula H

=

+

JONES,C., J. Chem. Sac., 33, 175 (1878). a S c n ~ ~ m ~A,.~, Ber. a c dlseh. ~ ~ ehem. ~ . Ges.. 24. 944 (18911. PREGL,F., "Quantitative Orgmie Microanalysis," ' J. A. Churchill, Ltd., London, 1937, p. 225.

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