A I N EASY METHOD FOR DETERMINING \',\PORDENSITIES BY PHILIP RLACKMAh
PART I
The apparatus here described has to be especially made, but is then always ready for use. It is more suitable for experimenting upon liquid substances than upon solids. It consists of a long capillary tube, from 300-400 cm in length ithe longer the better), bent into the form of a closed U' (or Jvith still more folds) so t h a t it occupies as little length as possible and a t the same time presents a compact form. It is open a t both ends with a tap a t one end. The total length of tube must be carefully graduated in cubic centimeters divided into millimeters (in an accurate piece of apparatus the divisions should be in the ratio of the relative corresponding internal volumes of the bore, especially in the neighborhoods of the bends). Some mercury is placed in a small vessel, and covered with a layer of the substance t o be experimented on. The tap-end of the capillary is placed into the mercury, the t a p being open, and a short thread ( I or 2 cm) of the metal is sucked u p ; the capillary is raised till the end dips into the la>-er of substance above and some of it (4-7 mm) drawn in; the capillary is non- a t once lowered into the mercury and a second length of it sucked up, sufficient t o close the t a p completely, which is then shut off. The length of the thread of substance is carefully measured, and the capillary (except the tap) completely surrounded by a suitable heating jacket, and heated in a horizontal position in the vapor of some substance boiling at a temperature above t h a t a t which the substance experimented upon boils a t ordinary pressures. TT'hen the substance in the bore has completely vaporized and the movable mercury thread has come t o rest, the length of the capillary from one
Easy Method lor Deterrnining Vapor-densities
533
mercurj--thread to the other is determined (see Fig. I ) . The atmospheric pressure and vaporizing temperature are required known.
Fig
I
Let
I L
the length of the thread of substance introduced, = the length of the thread of 1,aporized substance; t , = the temperature of the heating niedium; s = the specific gravity (as measured on the metric system) of the substance when introduced into the capillarj-; fi = the atmospheric pressure; d = the vapor density required; n = the area of the cross-section of the bore of the capillary tylbe ithis is not required t o be known). The weight of the Tubstance introduced = sln . . . . , . ( I ) ; its volume a t t 2 0 and pressure p = Ln, n-hich a t o o and 7 6 0 =
mm. pressure
=
@oc
?73 760(273 -- t,)
__
,
.,. . ,
(2);
hence its densitj7
=
( I ) + ( 2 ) , or, if conipared 113th hydrogen a t o o and 760 nini pressure (assuming that I gram of hydrogen occupies I I 160 cc.)
d = 11160
(I)
+ (2)
PART I1 If the t a p end of the apparatus be bent at right angles to the plaiae of the rest of the apparatus (otherwise bending it will render it impossible for the movable mercury-thread t o be seen or the readings t o be observed), the apparatus will admit of its being heated in a thermostat t o any desired temperature, a method of procedure which will in many cases or for special purposes be found much more convenient and
advantageous than a lieatiii~-jacket-method. \T-ith this apparatus the niercur? -thread closing the t a p must lit, long cnough to fill the \\hole of the right-angled bend -1lio the formula to bc ernplo\ c d in the calculation of resulti obtaintd n itl: thi- apparatub i i more correctl! n ritten ti
;io68
\ / ( 2j 3
-
( P* 1-) 6 : 1
2
tL) '
in \\ hich 6 = the \pecific qra\it\- of the heatinq liquid, 1 = the depth of the m o l able niercurJ--tliread below the surface of the heatiiig ixcdium, and 13.6 = t h e specific qral-itl- of niercury. It i i a ~ncll . honel-er t o point out t h a t the quantitJ8.l 13.6 nil1 rarely amount to a i much as I mrn. and may for all practical purpo\es he neglected and the leis coinplicatecl formula used The tap-cnd must be outside the heatinq niediuni. The other, open end of the apparatus may be immersed in the liquid, thouqh of courw t h e apparatus may be so made t h a t this end too 15 at right angle5 to the plane of the rest of the apparatus anti thus projects outside the heating liquid (in n hich case the quantity 6 1 13.6 becomes unnecessary), but x-erj littlt advantage nil1 be gained by thi3 Seithcr apparatus can be used n ith substances T\ hich act upon mercury. -Ithigh temperatures p must be nritten p -- q , where q i5 the vapor-tension of mercurj-. The mercury employed must be quite clean. Every time before an e s perinient is performed the apparatus must be thoroughly cleaned and dried IYhen the movable mercury-thread applies comes t o rest a t an upright (not horizontal-this onlj- to the apparatus in Part I if placed so that the plane of the apparatus is vertical and not horizontal) bend, p must be written p + 711, m being the x-ertical distance of one end of t h a t thread above the other end; to avoid the possibilitj- of this occurring and the consequent correction, care should be taken that before heating the apparatus is in such a position t h a t its plane lies horizontally.
Easy n/f ethod )or Determining l.'apo.i-densities
53 j
Results.-It is not at all a difficult matter to measure I correctly to fourths or even fifths of a rnm by the use of an ordinar>- magnifying glass held over the scale and the thread to be measured. In connection with this it must be borne in mind that the chief source of error lies in the accurate determination of I . I n ver!- exact work an allowance in 1 (which, however, is very small) must be made for the convexivity of the mercury ends contiguous to the thread of substance. In all cases here given t? = rooo. d ~
Found
CHJ C,H,I C,H,Br C,H,Cl, CH 3 . C 0,C H CHCl, C,H,, NO1 C,,H, ~
Theory
70.9 77.9 54.5
49.4 44.0
59.7 j8.j 39.0
PART I11 The apparatus here described, though more xpensive to make than those already detailed, nil1 hon-el-er be found to possess these important ad\-antages, it-hich will more than repay the initial cost of manufacture. These advantages are that. (11 Greater accuracj- can be obtained in the measurement of I , because the thread of substance introduced may he made several cm long. ( 2 ) The apparatus need not necessarily be of great lengtli ( 1 2 0 cni in lcngth is quite sufficient, if not more than I cm length of the thread of substance be introduced), though the longer the length of the capillarj- tube is the greater I may be made and thus increased accuracy is obtained. ;-l;b;bauztus.--The bore of the capillary tube, 8-10 cm on either side of the tap, should be of a v e i j ' siiinll dianzeter, the
Philip Blackman
536
rest of the capillary tube having a comparatively large diameter (of course the bore of the latter must not be too wide t h a t a thread of mercury cannot remain unbroken in it). The radius, Y, of the cross-section of the narrower bore, and the radius, R, t h a t of the larger bore, must be accurately known, these being permanent constants for any one piece of apparatus. -2s t o the method for graduating the total length of the capillary tube, the general direction already given in Part I apply here. Method.--r\lercury is drawn in until the whole of the narrow bore and I or 2 cm of the beginning of the wider bore are filled. This is inimediately followed by a thread, several cm long, of the substance to be experimented on, and behind this another thread of mercury t o close the t a p is drawn in, after which 1 is measured. The apparatus is heated, and the lengths I , of the narrower bore and I, of the wider bore occupied by the vapor are determined. Formulae.-The veight of substance = d l s , and the volume occupied by the vapor = rrr2Z, aR2L. Substituting these values for the quantities sln and Lcr respectively in the theoretical portion given in Part I,
+
d=
31o68rZls(273 t?) .. P(?,’lC It%)
+
. .
-1s Y is small in comparison with R , r2lCis very small compared with R2L, and is consequently a negligible quantity for all ordinary purposes. This consideration admits of the simplification of the formula t o
In practice it may be of great advantage to calculate beforehand for any one apparatus the quantities r z and R2, and further work out the quantity 31068r2/R2, say equal t o the known factor K, which is used as a constant for t h a t
Easy
lethod j o y Determining Vapor-densities
537
particular apparatus, and the simpler, and almost scarcely less accurate, formula can now be written
In conclusion, I must once more offer niy sincere thanks to R. Blair, Esq., Education Officer t o the London County Council, and t o the authorities of the Hackney Technical Institute for the continued facilities granted t o me t o carry out research work. Loizdoiz,
A\-.
E. ( E x g l a n d )
