NOTES ON ANALYTICAL PROCEDURES Analysis of n-Butane-lsobutane Mixtures by the Density Method ERNEST S O L O M O N , The M. W. Kellogg Company, Jersey City, N. 1.
A
RECENT paper by Leighton and Heldman ( 1 ) prompts a brief description of a method that has been employed succeesfully for several y e m in this laboratory for’the analysis of mixtures of n-butane and isobutane. The method is simiiar to that employed by Leighton and Heldman; however, a descrip tion of the apparatus employed may assist other laboratories in assembling a simple and compact analytical unit. The butane sample, which has been freed of olefins and of lighter and heavier hydrocarbons, is condensed into the inner chamber of a triple-walled Dewar flask. The intermediate chamber, containing li uid propane, is surrounded by an outer evacuated chamber an%is further insulated with aluminum foil in which appropriately placed windows have been cut. The temperature of the butane sample is adjFted, by regulating the pressure over the boihng propane, until a small glasa float of a propriate density neither rises nor sinks. The pressure over t i e propane is rapidly adjusted by either applying a reemre of nitrogen or evacuating with a water as irator througt a ballast volume. A small Nichrome heating coifkmersed in the propane assists in the rapid attainment of the desired equilibrium tem-
perature; a reflux condenser is provided to return vaporized propane to the intermediate chamber.
It has been found necessary to calibrate for equilibrium flotation temperature with a few known mixtures of n- and isobutane since the relationships between isobutane concentration and either propane pressure or sample temperature (expressed as millivolts measured on a multijunction thermocouple immersed in the liquid) are not quite linear. Using calibration charts the method can readily yield results accurate to * 1% in about 15 to 20 minutes from the time the sample is introduced into the sample chamber until the apparatus is ready for the next sample. About 10 ml. of liquid provide a convenient sample for this technique, although there is no reason why this cannot be readily reduced, LITERATURE CITED
(1) Leighton and Heldman, J. Am. Chem. Soc., 65,2276 (1943) ; cf.
Randall and Longtin, IND. ENG.CHDM.,ANAL. ED.,11, 44 (1939), for generalized discussion of flotation analysis.
Fixing Rubber Connections HERBERT Z. L I T T M A N , Research Department, Palestine Potrsh, Ltd., Jerusalem, Palestine
IT IS
general practice to fasten the joint between a glass tube and a rubber tube by means of a small cord, waxed thread, or copper wire. With tubes of small diameter, this method is troublesome and frequently leads to breakage. It is also a d i e advantage that the connection cannot be removed without c u t 4
ting the wire or cord. In certain cases the following method is very convenient, especially if only a small internal pressure preveils and it is desirable to remove and replace the connection frequently: Cut a small ring, A , from a rubber tube of a diameter e q d to or slightly larger than the tube, B, which is to be fixed on the lsss tube, Lubricate the ring wrth glycerol and ush it on a cork kxer sharpener C (Fi ure 1, movement 1). Tien transfer the the inner diameter of which ia larger ring to a cork borer, than the outer diameter of the rubber.tube to be fixed (movement 21. Then d i the ~ end of the rubber tube into the cork-borer and transfer the rubber ring onto the rubber tube (movement :T a). With. the aid of glycerol it is now esey to put the rubber tube with ite rubber sling I! upon a glass tube, where it will hold tightly.
8,
;I
\Af Figure 1
Figur 4 348
The method is especially ueeful if a thermometer is to be introduced into an apparatus through a glass tube (Figure 2). Here a wide rubber tube which goes over the glass tube haa to be used, and consequently it will be too wide for the thermometer. The connection between the thermometer and rubber tube can be made by a rubber ring cut from a heavy-walled rubber tube. It is easy to slip the rubber ring from the cork borer onto the rubber tube. This should be done after the thermometer is in position, so that no form haa to be applied to the thermometer. Anyone trying the method will quickly learn to make the right choice of diameter, wall thickness, and length of the rings, according to the necessary tightness or ease of connecting and removing.
