T H E MICRODETERNI IXATION OF VAPOR DENSITY D. A. PEAK
AND
R. A. ROBINSOn’
Department os Chemistry, University College, Auckland, N e w Zealand Receiaed March 2, 1934
In a recent research we had need of an apparatus for determining the vapor densities of a series of compounds which could be vaporized only under reduced pressure. Such determinations have been made in the past, but usually with a constant volume apparatus; the apparatus now described possesses features such as the simplicity of manipulation and the rapidity with which determinations are made which should commend it to anyone who is determining the vapor densities of substances which decompose on boiling at atmospheric pressures or substances available to the extent of only a few milligrams. The method is also useful for substances whose high boiling point renders the determination of their vapor density a t ordinary pressures somewhat difficult. The apparatus, shown in figure 1, consists of a vaporization tube, A, 1 meter in length and of 300-cc. capacity, closed with a well-fitting rubber bung and surrounded with a copper jacket, B. The latter is lagged with asbestos, fitted with a side-arm condenser, C, and is attached to the flask, D, in which the bath liquid is heated. The vaporization tube connects to two side tubes, one, on the left, containing a phial with a few milligrams of the substance under examination and a nail, which, when pulled by a magnet, ejects the phial into the vaporization tube. The other side tube leads to a manometer, E, containing a light liquid of low vapor pressure (ethyl phthalate or nitrobenzene), to one arm of a Hempel buret, F, containing clean, dry mercury, and to a tap, G, through which the apparatus was exhausted by means of a Cenco pump. The other arm of the Hempel buret was connected through a reservoir of 2-liters capacity, H, to stabilize the pressure to the tap, K, and thence to the common lead to the pump. Another tap, not shown in the diagram, enabled the pump to be cut off from the rest of the apparatus. All joints were sealed with Picein wax. In practice it was found that, owing to minute traces of volatile material in the gauge liquid which could not be removed even by long heating i n vacuo, the manometer, E, did not record absolute pressures. It was used, therefore, merely as a pressure indicator and a McLeod gauge, 31, was incorporated in the system. In making a determination the bath was heated until the liquid was re941
942
D. A. PEAK AND R. A . ROBlNSON
fluxing well up the condenser arm, and the apparatus was exhausted with all the taps open until a pressure of 5 to 10 mm. of mercury was registered
.IL-
E
f
FIQ. 1. APPARATUSFOIL DETERMINATION OF VAPORDENSITY CNDER REDUCED PRESSDRE TABLE 1 BATH L I Q U I D
TEMPERATURE
BUBBTANCE
--
degrees C.
Nitrobenzene. . . . . . . . . Nitrobenzene. . . . . . . . . Nitrobenzene. . . . . . . . . Water. . . . . . . . . . . . . . . . Xylene. . . . . . . . . . . . . . a-Pinene . . . . . . . . . . . . . a-Pinene. . . . . . . . . . . . .
211 211 211 100 143 156 156
IOLECULAR WEIQIlT (CALCD.) __.-
mg.
Dibromobenzene Dibromobenzene Anthracene Acetoacetic ester Carvone Geranyl acetate Geranyl acetate
4
4 5 5 5
224 246 182 128 146
236 236 178 130 150 196 196
on the manometer, E, when the taps were closed. The phial was projected into the vaporization tube, its fall being broken by a small quantity of
MICRODETERMINATION OF VAPOR DENSITY
943
fusible metal a t the bottom of the tube (fusible metal is superior to glass wool, which is liable to evolve occluded gases under reduced pressure). Vaporization occurs in two minutes a t the most, the gauge, E, being maintained at a constant level during the vaporization by lowering the right hand arm of the Hempel buret. When the vaporization was complete the taps G and K were opened, the gauge, E, restored to its original level by further slight evacuation, and the pressure recorded with the McLeod gauge. The calculation follows that of the ordinary Victor Meyer determina tion. The method is superior to the constant volume method under reduced pressure since (1) it avoids the necessity for capillary tubing outside the heating jacket, and (2) it does not require a knowledge of the volume of the apparatus and the temperature of the jacket. One precaution must be mentioned. On admitting air at the conclusion of an experiment a film of moisture invariably condenses on the sides of the Hempel buret. This is revaporized during another experjment and causes a considerable error. It is necessary at the end of a determination to let the vacuum down with the mercury column fully raised and then to lower the column after reevacuation and to warm the sides of the buret. The buret arms may be enclosed in water jackets at constant temperature. Table 1 exemplifies some of the results obtained. The apparatus gives results correct to 3 to 5 per cent with ordinary precautions, an error which might easily be reduced to 1to 2 per cent if so desired.
