.4 NEW METIJOD FOR DETERMINING VAPOR-
DENSITIES.’
BY PHILIP BLACKMAN
Part I The niethod here described is one of the simplest, least expensive, arid most accurate of all those hitherto discovered for determining vapor-densities. By means of it the actual volume of the vapor of a substance is directly measured.
RpFaratus A long tube A has one end, X, narrowed down leaving enough room for a Hofmann’s weighing-bottle to p a s s through; the other end is drawn out to a long, thin, capillary-tube, Y , left open. Method The bottle containirig the weighed substance is pushed through X, which is then connected by means of a piece of rubber pressure-tubing with another piece of long glass tubing B ; both il and B are placed vertically, and mercury p o u r d into B until the bottle in A rises to within about 8 cms from Y; the position of the X level of the mercury in A is carefully noted Fig. I with reference to some mark on the glass, and the capillary end Y is a t once sealed off by placing a flame to it. The tube A is now surrounded with a suitable heating jacket, and heated in the vapor of some liquid boiling above the temperature at which the substance experitiiented on vaporizes (to aid evaporation, the pressure in A is reduced by removing some mercury from B or by lowering the latter tube). When the substance has completely vaporized, the Compare Ber. chem. Ges., Berlin, 41,768 (1908).
680
Phil+ Blackman
n;ercury in B is brought on a level with that in A, the position of the level of the mercury in the latter being once more noted. ( I t is necessary to have sonie scale-any scale-etched on A to aid in ohserving the positions of the level of the mercury. A disused burette may with advantaqe be converted into the tube A, the tap being cut off and the glass fused to make an even smooth edge to serve for the end X , the other end being softened in the blowpipe-flame and drawn off). The tube A is next disconnected and placed vertically with the open end X to the top. Mercury is poured in from a good burette (reading accurately to 0.05 or 0.025 cc) until the position on the tube is reached where the mercury stood first; the volume thus found is the volumie of the air ericlosed; more mercury is added until the second position is attained, and the volume of the total mercury represents the volunie of the mixture of air and vapor. Theory Let t,' = the temperature of the air (when Y was scaled) ; t z o = the temperature of the heating vapor; v , = the volunie of the air (at tlO, as measured above); 'u, = volume of the air and vapor (at t,', as found above) ; w = weight of the substance; pi. p 2 = the initial and final atniospheric pressures, measured in mm (i, e., when sealing the tube and at the end of the experiment, respectively; generally, p1 = p,. The volume of the air at t,O =
therefore, the volume of vapor at t z o = v 2 , volunie oi air at t,
this volume, at o o and 760 mm,
=
Method for Determining Vapor-Densities
68 I
Hence, the density of the vapor is given by this last formula divided into w ;or, the vapor-density, compared with hydrogen at oo and 760 inm (assuming I gram of hydrogen occupies 11160 cc) is given by the formula (to be used in practice)
Precautions The amount of substance experimented on must be determined by the capacity of the apparatus. ( 2 ) To use the apparatus for another experiment, it is only necessary t o cut off the sealed end of the capillary-tube Y, and to thoroughly clean and dry the tube; when the capillary has becoine too short, the glass is softened in the blowpipe flame and drawn out t o a fresh capillary. (3) ,4ny number of determinations can be made, without reweighings etc., by simply passing through the heatingjacket the vapors of various suitable heating liquids in turn. (4) Dry, clean mercury only should be employed. (I)
Results If every precaution be taken t o measure accurately the various quantities, no difficulty should be found in obtaining results which differ very slightly from the required theoretical values. A successful experiment should occupy about 45 minutes. The author used water as the heating liquid (here t,
= IOO00).
Philip Blackman - _ _~_ _ _ Substance
Va
cc
$1
mni
Vapor-density
P,
--
mm
Found I'heory
C6H6
(C2H5)20 CH,. CO.CH, CH, .O H C,H,. OH CH,.CO.OC,H, CH,.CN
cs2
CZH2 CHJ C,H,Br CHC1, cc1,
40.15 762 30.4 765 32.2 751 39.7 759 41.55 765 29.8 767 37.15 755 42.3 754 35.0 749 41.75 750 48.25 753 33.3 76 7 27.05 767
762 764 752 759 765 766 754 755 750 750 754 767 768
16.0 ' 16.5 15.5 17.0 15.0
16.5 16.5 17.0 14.5
0.0870 0.0567 0.0495 0.0352 0.0529 0.0703 0.0409 0.0901 0.1508
38.97 39-00 35.24 37.00 28.92 29.00 15-79 16.00 2 2 . 2 1 23.00 43.25 44.00 19.62 20.52 37.01 38.06 76.82 77.92 71-44 70.93 55.41 54.48 55.84 59.67 7 4 3 3 76.90
In conclusion, I beg to offer my thanks to R. Blair, asq., Executive Officer of the London' County Council, and to the Authorities of the Hackney Technical Institute, for the facilities granted to me to carry out the research.
Part 11' The following modified form of the apparatus] though soniewhat more expensive and slightly less easy to manipulate, has nevertheless the advantage in that there will be no need to measure vl and v2, neither will there be any necessity t o seal the capillary Y. The capillary-tube V of A is replaced by a permanent thick-walled capillary-tube. To use the apparatus, the tube A must first be fixed in the heating-jacket, the capillary Y protruding through the other end; a small funnel is fastened t o the capillary by means of a piece of rubber tubing; the rubber-tube can be closed by means of a clip or pinch-cock. The weighing-bottle and contained substance is introduced through the open end X, the pressure-tubing connecting with B fixed on, the clip at Y opened, and mercury poured in until the weighing-bottle reaches t o nearly the capillary; Compare Ber. chem. Ges., Berlin, 41, 770 (1908).
Method for Determining Vapor-Densities
683
a little mercury is now poured into the funnel, and by lower-
ing -4 a thread of mercury is drawn down to close the capillary, after which the clip is closed, and the heating process proceeded with. The tube is permanently calibrated in cc (divided into smaller convenient divisions) from some definite mark on the capillary Y the sealing-thread of mercury being always brought to this mark. Thus v, and v 2 can be directly read. The heating-jacket and tube -4 may be made of jointedglass (similar to a condenser), and in this case, as A would never have to be disconnected from the heating-jacket, the capillary-end Y may v be terminated by a permanent funnel with a tap. With either of these two P= apparatus vl in any experiment may lie made very sniall; indeed by the followdevice, v, can be made equal to zero (and the formula €or the purpose of calculation 4 t,)/l&,vz). The reduces to 31068 w (273 weighing-bottle is put in so that it will stand in A stopper downwards; also it must be quite full of the substance experimented on to exclude all air. The mercury is allowed to rise above the clip or tap, and the latter is then closed. Thus x all air is removed from A. (If the weighing-bottle is placed stopper upwards, there Fig. 2 is risk of the stopper coming out and the specifically lighter substance being forced out through the capillary.) In very accurate work, in v, allowance rnust be made for the volumes of the weighing-bottle and the substance contained, and in v2 for the volume of the weighing-bottle. It may also be necessary to allow for the expansion of the glass of A and of the mercury in it. Only pressure-tubing (with a very narrow bore) must be used to connect A and B; ordinary rubber-tubing (with
+
Philip Blaclzman
684
a wide bore) is quite unsuitable, as it is difficult to fasten it quickly and securely to A and B, and also it is very liable to be distended by the weight of mercury thus introducing very appreciable errors.
Part I11 A #paratus The apparatus consists of a thick-walled, long glass tube A , sealed a t one end Y ; the other end X is narrowed down, being left wide enough to allow a Hofrnann’s weighing-bottle to pass through; A is E graduated in cms, beginning from Y . The end X can be connected by means of rubber pressuretubing ( IOO cm in length) with a long glass tube B. Method The pressure-tubing is fastened to B, which is fixed high up vertically; the other Y end of the pressure-tubing is closed by a clip or pinch-cock, and mercury poured into B until the pressure-tubing and part of B are filled. The tube A is placed upright, with X to the top, and filled nearly full with mercury; A the weighing-bottle (containing the weighed quantity of the substance to be experimented on) is dropped in through X, the free end of the pressure-tubing pushed on t o X, A inverted and fixed vertically with Y upwards -4 (this will cause the weighing-bottle to rise t o the top;. if it sticks, it will move up if the tube A is tapped), the clip or pinch-cock opened, the mercury in A and B allowed t o Fig. 3 come to rest, the volume of air inclosed in A noted, and the difference in height between the mercury levels in A and B measured.
-3
Compare Ber. chern. Ges., Berlin, 41, 881 (1908).
‘
Method for Determining Vafior-Densities
685
A is then surrounded b y a suitable heating-jacket and heated in the vapor of some liquid boiling at a temperature above that at which the substance vaporizes. When the latter has completely vaporized, the volume of the mixture of air and vapor is observed and the difference in height between the mercury levels in A and B measured. B may be raised or lowered to various positions to obtain different readings for v, and m, (see below) ; v, and m, might similarly be varied, but this is inadvisable, as on diminishing the pressure, the substance may he caused to evaporate and thus 'u, would not represent the true volume of the enclosed air. It is best t o hang a thermometer in the heating-jacket, whereby t,' may be correctly measured. To clean the tube A, it is washed out with suitable solvents, ending up with a little pure ether; a long glass tube is introduced through X and a current of air forced through by means of a pair of bellows, to dry the interior of A.
FormuEa. Let w
=
(For the proof see Part 11)
the weight of the substance;
PI, p 2 = the respective initial and final atmospheric pressures ; m,, m, = the differences in height between the mercury levels at the beginning and end of an experiment respectively ; v, = the volunie of the air enclosed (at tio); 'u, = the volume of the air and vapor (at t,') ; t,' = the initial temperature of the air; t,' = the temperature of the vapor; the vapor-density is given by the forniula
( + m, when the mercury in B is on a higher level than that in A, and - m , when lower than that in A ; when measuring m,, the mercury in B should be on a higher level than that in A).
Philifi B l a c h a n
686
Results The author used water as the heating-liquid (here
t,
=
IOOO).
~____
_____
v2
CH,.CO.CH,. W=0.0471; t l = = 15'..5; p z = 759; v1*=2.5; m112,= I 1 2 ; theoretical v a p o r density = 29.00
537 404 316 IO1
- 43 -144 -221
C,H, (benzene). w=0.0511; v,=3.0; m,= 40; p1 = p z = 760; t, = 16'; vapor-density (theoretical) = 39.00
(C,H,) 2 0 * w = 0.0522 ; v1 = 3.5; m, = 29; pt = fi2 = 760; tI0 = 16' ; theoretical v a p o r density = 37.00
472 213 164
- 21 -144 -232 -298 -350
573 240 40 - 93 -189 -260 -316 -360
16.6 18.5 20.0
25.0 30.0 35.0 40.0 15.0 19.0 20.0
25.0 30.0 35.0 40.0 45.0
15.0 20.0
25.0 30.0 35.0 40.0 45.0 50.0
Calculated vapordensity
29.17 29-15 29.18 29. I9 29.18 29-17 29-15 38 * 50 38.48 38.49 38.47 38.50 38.49 38.50 38.51 36.82 36.81 36.81 36.79 36.82 36.81 36.80 36.81
Part IV Iflien tubc A is being heated, it will he found that a few small bubbles of air, retained by the niercury, rise up and mingle wilh the air and vapor-; their volume is very small, but it is advisable to allow for them by redetermining 71, at the end of the experiment; to do so, tube A is allowed to cool down to t,, the temperature of the air and tube R is raised till the Compare Uer. chem. Ges., Berlin, 41,1588 (1go8),
Method for Determifiing Vapor-Densities
687
mercury in it is on a level with that in A, when vl is read off afresh. (The increase in v, thus determined varies between 0.I and 0.4 cc). Seeing that the method permits of vaporizing substances urider diminished pressure, i. e . , at $,-- m,, by using small quantities, substances may be completely vaporized at temperatures considerably below their ordinary (i.e . , at 760 nim pressure) boiling points. As illustrative instances, the following deterniinations were made at 100'. ~
~~~
~
Substance
w
t1°
Pl+
_
-
m1
Pz--m,
Vl
v2
Found Theory
_
_
CHBr, 10.0784 16 C,H,N (pyridine) 0.0349 14 C,H,,. OH 10.0205 I 5
_
_
_
160 250 250
760 754 756
~
~
0.2
50
0.3 45 0 . 5 24
-~-
127.99 i26.44 38.56 39.50 43-15 44-00
If every precaution be taken to measure accurately the various quantities, and also the following corrections be allowed for (a) the volumes of the weighing bottle and of the contained substance when measuring vl, ( b ) the volume of the weighing bottle when measurini v2, (c) the vapor tension of mercury above IOO', (d) the curvature of the mercury meniscus both for z', and v,, ( e ) the expansion of the glass tube A and(/) the expansion of the mercury, the method may be used for determining accurately the constituents by weight of a mixture of two substances or to determine the amount of an impurity in a substance. Thus, let w l ,w,, be the required weights of the two substances whose vapor-densities (supposed known) are d,, d,, respectively. Then v, (reduced to o o and 7ho mm) =
.
273v2(P* +>I. 764273 4) '
+
vl (reduced to o o and 760 mm)
=
Philip Blackman
688
the difference between these two quantities
=
the volume of the vapor at o o and 760 mm
by simplifying this equation, we obtain
+
and w, w, = w (by actual weighing), whence wl,w,, can be solved. In the following experiments, corrections (a) and ( b ) above mentioned were made. tzo = 100'. Substance Substance
Percentage error
I 2
-I/-I
CH,. CO. CH,
0.0238 0.0246 0.0656 I5
C,H,. O.C,H,
-I C,H, CCI,
3-2533.5
774
1--I-I ~
lo. 1286 13.5
I
764
1
3.3 35.9
SI.2
0.0418 0.0410 -1.2 10.0327 10.0326 0.0959
0.0960
4. I
+o. I
Part VI It will be found much more advantageous t o use the method here given than that in Part IV, as not only is it simpler and easier to calculate by its means on account of its consisting of only one equation, but also it gives directly the percentage composition, whereas by the former method, the percentage composition can only be obtained by the Compare Ber. chem. Ges., Berlin, 41, 1590, 2487 (1908).
Method for Determining Vapor-Densities
689
further calculation of the quantities, Ioow,,lw and roo I oow,/w. Let wl be the percentage weight required of one of the components (of vapor-density d,, supposed to be known), then ~ o o - - - w ,is that of the other constituent (of vapordensity d,, also supposed known), and w, I O O - - W , or I O O is the total weight of the mixture. Now a t o o and 760 mm pressure ( I ) the volume of the first constituent = 11160w,/d,; (2) the volume of the second component = 1 1 1 6 0 (100 -~ ~ l ) ~ d , ; (3) the volume of the mixture =
+
11160
x
IOO
vapor-density of the mixture
--_
11160
x
IOoi
(I% f '%)vz(%
31068,4273
+
ti) - (pi + % ) v 1 ( ~ 7 3 + +-ft,)(273 + 4)
tz)
1.>
but, ( I ) (2) = ( 3 ) ; hence, on simplifying the resulting - w,) expression, we get the equation d2w, d , ( ~ o o
+
whence, w,and IOO - w,, can be solved. It should not be found difficult to so arrange the practical work that m, = 0, m, = o and p1 = p , generally, that the calculations become much simplified. In the results here given, t , = 100'; also, the calculated percentage results are exactly identical with those as calculat,ed by the equations in Part IV reduced t o percentages.
Philip Blackman
‘1
44.2 17
2o
‘!
0
I 0.4 48.0 16
I
I Percent-
1
ages
26.14 0.0628 I 73.86 0.1250
58.78
-~~ 52.01
48.6 16
0.0882 47.99
CSZ
cc1,
-1-1-
I
!
82.33 46.0 17.5 0.1264 766
CH,.COH 767
CZH4C1Z
0 ~
I
17.67
---I 0 1
80.28 0.3 49.5 18 Io.06jo) 19.72
Part VI The apparatus here described has this great advantage over those previously described : the difficulties of measuring air in the tube and making the necessary allowances in the calculations are absent. The method may also be looked upon as a modification or variation of Hofrnann’s apparatus. A wide tube closed a t one end A is filled with dry, clean mercury, and any air-bubbles are carefully removed. A small stoppered weighing-bottle completely filled (to exclude air) with the weighed quantity of substance to be experimented on is placed upon the mercury, and a rubber cork fitted with a glass U-tube (with narrow bore), flush with the stopper as shown (Fig. 4),is forced into the neck B of the tube, thereby pushing the weighing-bottle inwards and causing the excess of mercury to fill the U-tube and some to perhaps escape; the tubes should thus be air-free. The stopper should be tied on to the lip round the neck with wire and the tube inverted; if the weighing-bottle does not rise upwards, slight tapping on
Method for Determirzing Vapor-nensities
691
the tube-wall will cause it to do so. The tube is now surrounded with a suitable heating jacket and heated in the vapor of some liquid boiling at a temperature above that at which the substances vaporizes (Fig. 5). When the volume of the vapor in the tube remains constant, the position of the merB cury-meniscus is noted and the difference in height (m)between the levels of the mercury in the U-tube and m e a s u r i n g - t u b e measured. The measuring-tube is removed, emptied of mercury, and (without removing the weighing bottle) water is poured in from a burette to reach the position the mercury occupied; the A volume of.the water gives the Fig. 4 Fig. 5 Fig. 6 volume of the vapor. When the substance is vaporizing the overflowing mercury from the U-tube should be caught in a beaker. If the U-tube be short and a wide tube attacked to it by means of a short piece of rubber pressure-tubing, mercury can be poured in so as to vary m (Fig. 6 ) . This will be found especidly useful if too great a quantity of substance has been introduced whose vapor would at comparatively low pressures occupy a larger volume than that afforded by the measuring-tube (compare first, second, ninth, and eleventh results). If the measuring-tube be graduated in cc (from closed end) the volume of the vapor (subtracting the volume of the weighing-bottle) is read off directly. Fig. 7 shows how the measuring-tube may Fig. 7 be made to consist of a thick-walled test-tube, and heated in a deep beaker. This simpler form must only be used with the following limitations : ( I ) the heating liquid
B
Philip Blackman.
692
must not act upon the rubber; ( 2 ) the temperature must not be raised too high to affect the rubber. The last two results were obtained with this apparatus. Let w = the weight of the substance; v = the volume of the vapor; p = the atmospheric pressure; m = the difference in height between the mercury levels; t = the temperature of the heating-vapor; the vapor-density is given by the formula 31068
(273
+
(P * m>v
+
t>
( m when the mercury-level in the measuring tube is on a lower level than that in the U-tube, and -m when on a higher level). The apparatus is very cheap, easily manipulated, and capable of giving quickly very accurate results. For very accurate work the following corrections must be made: ( a ) for the vapor pressure of mercury above 100'; ( b ) for the curvative of the mercury meniscus; (C) for the expansion of mercury; and (d) for the expansion of glass. Results The average time of anexperiment was about sixty minutes. __
t
Substance
OC
m mm
Vapor-density 2,
cc
CH,.OH CzH6.0H C,H, CS, C,H,CI, (CZH5)Z0, (CH,),CO CH,I CH,.CN CCl, CHCI, CH,.CO,C,H! 'ZH61
C,H5Br
0.0842 0.0869 0.0809 0.0865 0.0821 0.0847 0.0839 0.0877 0.0842 0.0812 0.0878 0.0870 0.0812 0.0825
761 761 768 760 759 758 760 760 763 763 764 ,760 764 766
'
IO0 IO0 IO0 IO0 IO0
80 80 80 IO0 IO0
IO0 IO0 IO0 IO0
$405 +278 32 33 - I3 - 43 - 48 - 94 +360 -163 - 47
++
- 20
+ 93 +I11
= steam. 8oo = benzene vapor.
IOOO
Hackney Technical Institute, Londolz.
51.6 42.9
30.0
33.7 26.3 35.1 44.7 20.3 40.2 20. I
24.1 32.3 14.3 I 9.6
Found
I'heory
16.23
16.00 23.00 39.00 38.06 49.45 37.00 29.00 70.93
22-59
39.06 37.48 48.49 37.01 28.91 71.14 21.62 78.02 58.88 42.18 76.78 55.62
20.52
76.90 59.67 44.00 77.92 54.48
