Capillary Tube Experiments with Gases LEON J. KAY London County Council, School of Building, Brixton, England
E School of Building in London has in all some 1500 students and their varied requirements form difficult problems for the science department. In chemistry the provision of a general course of fundamental character followed by specialized courses is found to be the most satisfactory method. For the general course, only a short time is available, and consequently it has been necessary to develop rapid and extremely simple technics to give students clear ideas of chemical principles. Many of these, of course, are well known, but some appear to be less familiar, in particular the use of long capillary tubes in dealing with gases and vapors. This is a most effective method of demonstration which has also the advantage of giving accurate results without the necessity of elaborate precautions or corrections which might tend to obscure the principle involved. As will be seen in the two examples described, fhe apparatus required does not involve any more than average laboratory skill in its construction or use.
'I"
DISSOCIATION
The phenomena of dissociation, although very important, are only rarely demonstrated since expensive time-consuming apparatus is usually thought necessary. However, the more significant facts may be shown using the apparatus illustrated in Figure 1.
GLASS
although not essential, is useful since it hinders the loss of ammonia. During sealing the wider tube should be kept as cool as possible. Next a one-cm. thread of mercury is introduced a t C, using a length of fine-drawn soft tubing as a funnel. After withdrawal of the funnel, the end D of the capillary is sealed. The apparatus is supported in an almost horizontal position, A being a t the lower end, and the position of the mercury thread markedband cut from a piece of rubber tubing is useful. The space between the mark and the sealed end of the capillary is divided into the fractions '/z, '/a, '/4, '/s, etc., and a mark placed a t each. These will record the position of the mercury when the dissociation pressure is 2,3,4,5, . . . atmospheres. Finally, the tube containing the dissociating substance is supported in an oil bath for heating and the capillary tube passed t h r o u ~ ha hole in an asbestos shjeld. The apparatus is now ready for use in showing: (1) The increase of pressure with temperature, and the determination of the relation between these quantities. (2) The reversal of dissociation on cooling. (3) With two or more, pieces of apparatus, the variation of p"ressures using diierent substances. (4) With two or more pieces of apparatus, the independence of dissociation pressure of the amount of substance used. The compound of calcium chloride with ammonia was first prepared by Berzelius and its dissociation was discovered by Faraday. It is easily prepared by placing a few grams of fused calcium chloride in a 350-cc. conical flask, cooled in a water bath, and passing in dry ammonia with the usual precautions. As the gas is absorbed, the calcium chloride swells, cracks, and falls to powder. When absorption is completed, the product has approximately the composition represented by the formula CaClr4NH3. The relation between dissociation pressure and temperature for this substance was expressed by C. Antoine' by the formula log fi = 2.1361
+ 0.0231
The substauce ZnCla.4NH3 is similarly prepared First, a 20-cm. length of one-em. bore tubing sealed a t one end, or a hard glass test tube of similar dimen- from anhydrous zinc chloride and for this C. Antoine sions, is about one-quarter filled with some dry dis- gave sociating substance, such as CaC1v4NH3 or ZnClzlog p = 4.9445 (2.4391 4NH3. A loose-fitting glass wool stopper is inserted t + 449 near the open end and then a meter capillary tube with The glass wool, 2-mm. bore is bent and fused on. I Comfit. rend., 107, 836 (1888). 580
-
x)
THE DETERMINATION OF VAPOR DENSITY
The method described does not give an absolute measure. Actually, two substances are used and their vapor densities compared, then if the value for one is known, that of the other can be calculated. The specific gravities of the two liquids at ordinary laboratory temperature must be known. A meter capillary tube as before described is carefully cleaned and dried and then prepared according to the following directions: (1) Seal one end. (2) Using the funnel, insert a thread of mercury (one cm. long, approximately) about halfway along the tube. (3) Support the tube vertically, closed end at the bottom, and on the thread of mercury insert a thread (0.5 to 1 cm.) of liquid of known vapor density. (4) Taking care to keep the tube cool, seal the top. (5) When the sealing has cooled, open the end sealed a t first, support the tube vertically, closed end down, and introduce a thread (0.5 to. 1cm.) of the liquid of unknown vapor density. (6) Seal the top of the tube. The apparatus is now ready for use. Diagrammatically, the preparation of the tube may be represented as in Figure 2. When the tube has cooled to atmospheric temperature, the lengths of the liquid threads are measured to 0.01 cm.
columns on either side of the mercury thread are measured to 0.1 cm.
CALCULATION OF RESULTS
Let A ccs. be the volume occupied by one gram mol of vapor in the conditions existing in the heated tube; d crns. be the diameter of the capillary bore, L and I cms. be the lengths of the threads of liquid, of known and unknown vapor density, respectively, as measured before heating; V and w crns. be the lengths of the respective vapor columns as measured during heating; S.G. and s.g. be the respective specific gravities of the liquids, and V.D. and v.d. be the respective vapor densities. Then the volume of the vapor of the liquid of known vapor density will be given by Volume =
* 4
x v
=
*d' L X S.G. 4 V.D.
X A
whence A
=
V X V.D. L X S.G.
Since the temperature and pressure conditions of the two vapors are alike, for the liquid of unknown vapor densitv we have :,
Hence
vX
V.D. = u X u.d.
L X S.G. - 1
The tube, resting in a few glass rings, is then placed horizontally in a wider tube and heated by vapor passing along the wider tube (Figure 3). Any convenient liquid may be used to provide the heating v a p o r a l l that is necessary is that both liquids should be completely vaporized. The temperature need not be fouud. Obviously, the comparatively high pressure arising in the tube will mean that a temperature considerably above the boiling point of either liquid is necessary. When both have vaporized, the lengths of the vapor
In this equation, all the quantities are known with the exception of the unknown vapor density, which can therefore be easily determined. This calculation is an approximate one, but when a more exact formula is applied it is fouud that the results agree within one per cent with those obtained using .the formula given. This exceedingly simple method gives results quite as good as those obtained by elaborate technics and with great saving of time. The liquids used as knowu or unknown can be any of the simple stable organic componndshydrocarbous, alcohols, esters, halogen derivatives, etc. The following have been selected as typical satisfactory compounds:
Benzene Methyl alcohol Ethyl alcohol Ethyl acetate Ethyl iodide Chloroform
S.G. at 20' (Landoll-Borndein) 0.8787 0.7915 0.7894 0.9005 1.934 1.4827
