THE ADSORPTION O F IIERCL-RY VAPOUR BY ACTIVATED CHARCOAL BY D. 0 . SHIELS
Introduction Apparently little work has been done on the adsorption of mercury vapour by activated charcoal, although it is of the first importance to know whether charcoal does adsorb mercury vapour or not, as it is usual in experiments on the adsorption of gases or vapours to measure the pressure by means of mercury manometers, and to use McLeod gauges containing mercury for the measurement of the pressure during evacuation of the charcoals previous to adsorption. Apparently it has usually been assumed that a t ordinary room temperature the adsorption would be very slight owing to the very low (0.001m.m.) vapour pressure of mercury a t .this temperature. Recently Zelinsky and Rakusin' have carried out experiments which appear to show definite adsorption of mercury vapour by activated birchwood charcoal. An increase of weight equivalent to 5.63% of the weight of the charcoal was obtained by exposing it to mercury vapour a t ordinary temperature. An increase equivalent to 4.6 j7c of the weight of the charcoal still remained after exposure to an atmosphere free from mercury vapour. Their method was to expase the charcoal in a dish to the vapour of mercury a t room temperature, the dish containing the charcoal and a dish containing mercury being kept in a desiccator without any drying agent. As considerable experimental work in adsorption of gases and vapours had shown no evidence of the adsorption of mercury by activated charcoal except in the slightest traces, and as the method adopted by the above authors is open to the objection that blank experiments were not done and the carbon may not have been in equilibrium with the water vapour in the atmosphere, it was decided to carry out experiments in a vacuum. While this work was in progress (August 1 9 2 7 ) a paper by Coolidgez appeared, in which he showed that the adsorption of mercury vapour by activated charcoal was very slight at room temperature (0.3 mgr. per gram of charcoal a t 20OC)*
Experimental Details The charcoal was a German gas-mask charcoal which had been extracted with concentrated hydrochloric acid, and then washed acid free. It had an ash content of less than 0.17~. Ber., 59, 2072 (1926). SOC., 49, 1949 (1927).
* J. Am. Chem.
ADSORPTIOS O F MERCURY VAPOR BY ACTIVATED CHARCOAL
I399
It was contained in a glass vessel C having a good vacuum tap, and could be attached by means of a ground joint to a bulb containing mercury, phosphorus pentoxide drying tubes, 1IcLeod gauge and mercury vapour pumps. The charcoal was evacuated to 6.7 X IO-5 m.m. while cold and then the temperature was raised to 400’C. The evacuation was continued for I + hours a t this temperature and then the charcoal was allowed to cool off with the pumps running. The final pressure at 2oocC was 6.7 x IO-j m.m. The container was then shut off, detached and weighed. The lubricant was carefully cleaned from the side arm of the container which had been inserted into the ground joint, and the external surface of the whole vessel cleaned with alcohol, ether, and then wiped with a damp clean cloth.
FIG.I
Weighing was done against a glass counter-poise of same external volume and nearly same surface area. I t received exactly the same treatment in cleaning with the exception of washing off the grease. Weighings were correct to 0.00003 grams. C was then re-attached to the apparatus, and the part between T, and TI evacuated to 3 X IO-. m.m. This part of the apparatus was then shut off from the pumps and the tap T, opened, and the charcoal left in contact with the mercury vapour for 13 hours. It was then detached and weighed. A similar process was carried out with a further interval of 43 hours. Table I shows the results obtained:-
TABLE I Weight of charcoal, evacuated = 0.9 j08 grms. Time
o hours 13 ” 56 ”
Increase in Weight
o ooooo grms. 0 OOOII
’I
0 00020
”
The above results confirm those of Coolidge so far as the order of magnitude is concerned, but are only about I 2 j o of those obtained by Zelinsky and Rakusin whose experiments were carried out in such a way that air had access to the charcoal. I t would therefore have contained about 1 7of~adsorbed
D. 0.SHIELS
I400
air, partly nitrogen and partly oxygen. It was thought that possibly in their case there had occurred an oxidation of the adsorbed mercury vapour by the previously adsorbed oxygen. The extremely low vapour pressure of the resulting mercuric oxide might account for the retention of an increase equal to 4Yc of the weight of the charcoal when it was exposed to an atmosphere flee from mercury. I n order to test this hypothesis in such a way as to eliminate any uncertainty due to water vapour the following method was adopted. After attaching the container C containing the same lot of charcoal to the ground joint the tap T, was opened through to the P206 tube and the air which had been a t atmospheric pressure between T, and TI allowed to expand
r
/
0
/
I
/
X'me 70
40
in 60
Hours. 80
100
eo
FIG.2
into the larger volume. The resulting pressure was about 8 m.m. This air was allowed to stand for 2 1 hours in contact with P20j. T, was then opened and the charcoal exposed to this low pressure of dry air for 13 hours. It was then evacuated in the cold to 3 X 1 0 5 m.m. and then removed and weighed. It might have been more suitable for the purpose proposed t o have the charcoal exposed to a constant pressure of dry air near atmospheric throughout, but the apparatus was not adapted for such a method. This would have given a greater quantity of adsorbed oxygen on the charcoal. The container C was then re-attached to the apparatus, the part between T, and TI evacuated to 1.6 X I O + m.m., and then T, was opened; after standing 183hours a t room temperature it was removed and weighed. The same process was gone through again with a 463 hours interval. Time
o hours 18: " 65 "
Increase in Weight 0.0000
0.0000j 0.00020
ADSORPTIOS O F JIERCURY V h P O R BY .4CTIVATED CHARCOAL
1401
The above results are plotted in Fig. 2 . Admittedly the results are hardly numerous enough t o establish any oxidative influence, but they do show that any adsorption of mercury vapour by evacuated charcoal is very small. There appears to be a probability that there is some slight effect of oxidation. It was decided to repeat the experiments of Zelinsky and Rakusin with the addition of blank experiments. Samples of the same kind of charcoal were contained in weighing bottles having same volume in order t o minimiae the effect on their relative weights of any changes in temperature or pressure of the air contained therein. The charcoal had stood for about 9 months in a glass stoppered bottle aftrr being dried a t IIOT. The weighing bottles containing the charcoal were weighed against a counterpoise of same volume to within I cc. and nearly same weight. One vias placed with the counterpoise in a clean empty desiccator, the lid of the weighing bottle being placed beside it, that of the counterpoke being greased and placed in position thus ensuring that the air contained in it was constant in weight. The other was placed in a similar desiccator in which was a shallow dish containing pure niercury which had been dried by running through filter paper. The lids of both desiccators were lubricated with Ramsay grease. Previous to weighing the lids of the desiccators were removed, the lids placed in the weighing bottles which were then removed, and the desiccator lids replaced, the whole operation being carried out as quickly as possible. Table I1 shows the increase in weight of the two Tyeighing bottles and contents over a period of some days.
TABLEI1 so
K t . Charcoal
2
=
1.0459 grms.
Exposed to mercury vapour Date
so. 3 1.0092 grms. S o t exposed t o mercury vapour
Increase per gram of charcoal
Increase per gram of charcoal
0 0000
0 0000
26th Augt. 27th ” 29th ” 30th ’ 32st -’ 1st Sept. 2nd ”
0 OOjOI
o 00308
Total :
0.02387
0 02269
o 00488 0 00523 o oo64y o 00209 o 00217
0
00342
o 00560 o 00633 0 OOZII
0 00215
This gives an increaqe per gram of o O O I 18 gm. ( = o I ? ( ( ) iiiore in the case of the charcoal exposed to nicxcury vapour than in the case of that not so exposed
D.
I402
0.
SHIELS
The above experiment is open to the objection that the charcoals might be adsorbing vapours from the lubricant. A similar experiment was therefore carried out using desiccators Those lids were not lubricated. Two fresh samples of the same kind of charcoal were used. TABLE
111
JTt. charcoal = 1.00345 Date
Sept. 3rd I' 14th
Increase in TYeight
1.00210
Increase in JVeight
0 0000
0.0000
o 20650
0.213j6
Mercury then placed in desiccator with S o . z Sept 14th to Octr. 13th o 0092j
0.01513
The excess of increase of weight of the charcoal KO.3 over that of the charcoal No. z was o.ooj06 gms. for 11 day interval. After exposing KO. 2 to mercury vapour the excess of the increase of KO.3 over that of Xo. z was 0.00588 gms. The difference may have been due to the fact that S o . 2 was adsorbing mercury vapour or to the fact that both charcoals were coming more nearly to equilibrium with the water vapour in the air. Both causes might operate simultaneously. It is not possible to distinguish between them. It has been shown, however, that the method of Zelinsky and Rakusin is not reliable.
summary The adsorption of mercury vapour by activated charcoal has been determined at room temperature (12-14~C)in a vacuum. The weight adsorbed is of the order of 0.2 mgr. per gram of charcoal. Experiments were done to test the adsorption of mercury vapour by activated charcoal in presence of air. No definite indication of such adsorption was found. Chemzsfry Department, 1'mLerszty o j Melbourne. Feh. 12, 1919.
