9
T H E ADSORPTION O F NITROGEN PEROXIDE BY SILICA GEL BY R A M E S C . RAY
1. Introduction
X considerable amount of work has been done on the adsorption of gases by porous substances and several theories have been proposed to explain the phenomena, but not one of them is applicable to all cases, or quite adequate from a practical point of view. This is mainly due to the fact that very little is known about the na,ture of the solid adsorbent, and any theory of adsorption which disregards the nature of both the adsorbent and the adsorbed substance must be incomplete from a theoretical standpoint. The specific and selective influence of the adsorbent on gaseous as well as liquid adsorption has been shown by several workers. In discussing a paper by Chaney,l Sheldon stated that he was successful in obtaining a charcoal which a t liquid air temperatures took up relatively more hydrogen than nitrogen. The preferential adsorption from the gas.phase, and the influence of the chemical composition of the adsorbent were confirmed quantitatively by Briggs,2who also studied the adsorption of nitrogen and hydrogen by charcoal and silica at liquid air temperatures. He found that the striking partiality of charcoal and carbonaceous adsorbents for hydrogen stood out in high relief when these substances were compared with silica gel. That the preference was not due to the state of porosity of the adsorbent was indicated by the fact that the non-porous graphite had, at -19o0C, a, H/N ratio almost the same as that of the very porous blood charcoal. One must assume the existence of high specific attraction between the two elements carbon and hydrogen, The selectivity of adsorption froin liquids has been mentioned by Chaney, Ray, and St. John.3 They state that the preferential adsorption can be readily demonstrated by shaking up a mixture of water and benzene with activated carbon and silica gel respectively. The carbon will adsorb the benzene, and if enough benzene be present to saturate the carbon, the water will be completely rejected. The iilica gel, on the other hand, will take up the water and reject the benzene. These facts prove beyond doubt that every theory of adsorption must take into account the specific chemical nature of the adsorbent. The theory of capillary adsorption supported by Patrick and McGavack4 must recognize the predominating influence of specific chemical factors, inasmuch as capillary phenomena exhibit certain sharply contrasting aspects depending upon whether the capillaries are wetted by the liquid or not. Such differences in the wetting action reveal Trans. Am. Electrochem. Soc. 36,91 (1919).
* Proc. Roy. Soc. 100 A, 88 (1921). 3
Ind. Eng. Chem. 15, 1244 (1923). J. Am. Chem. Soc. 42, 946 (1920).
ADSORPTION O F NITROGEN P E R O X I D E BY SILICA GEL
75
the operation of specific chemical or polar forces, which are not explicable on any simple mathematical concept of relative capillary diameters. Langniuir’s theory1 of one layer adsorption appears to be well established for small amounts of gases which are held with extreme tenacity. The view that the maximum adsorption from the gas phase cannot exceed a monomolecular layer has, however, been much criticised. On the basis of their own measurements on the adsorption of gases on a known surface of glass wool, as well as of the data obtained by ?c/Iiilfarth,2Evans and George3 have shown that the adsorption layer may be many molecules thick. Further, Langmuir’s equation, in its present form, deals with plane or smooth surfaces only, and is not applicable to contiguous surfaces and hence t o the measurements of adsorption by porous bodies. Rilson4 has demonstrated that although Langmuir’s view explains the adsorption of small quantities of gases under very low pressures, and although capillary condensation may represent fairly adequately the slightly depressed vapour pressures in capillaries of moderate size, there is a wide intermediate range of adsorption in which neither theory Seems t o be satisfactory. Of the several adsorption formulas, the one proposed by Freundlichj is the simplest and the most widely applicable. It is purely an empirical relation and is too elastic and pliable. But the greatest drawback of this equation is that, it is not possible to predict what the adsorption would be a,t a particular temperature, knowing the adsorption a t another temperature. The isotherm has to be determined separately for each temperature in order to obtain the proper value of the constants to be used in the equation. I n this respect the formulas proposed by Polanyi6 and developed further by Berhyi’ as well as that by Williamss are more satisfactory, although they are still far from complete, and there is no means, at present, of calculating even roughly the amount of adsorption, a t a given temperature and pressure, of a new substance for which the constants to be used in the equations are not known. Patrick and NcGavackg determined the adsorption of sulphur dioxide by silica gel at various temperatures and pressures. X small amount of mater is always associated with silica gel. The water content of the gel can be lowered by heating it t o increasingly higher temperatures. Only a definite quantity of mater is removed a t each temperature and when the percentages of water are plotted as a function of the temperature a straight line is obtained. The whole of the water is removed froin the gel at about 700°, but a waterfree gel obtained in this way loses almost entirely its adsorptive power. Patrick and McGavack found that a gel containing five to eight per cent. of J. rlm. Chem. Soc. 39, 1848 (1917); 40, 1361 (1918). Ann. Physik. 3, 328 (1900) Proc. Roy. Soc. 103 A, 190 (1923). Phys. Rev. ( 2 ) 16, 15 (1920). “Kapillarcheinie,” p. 127. Second edition. Ber. 16, 1012 (1914); 18, 55 (1916); Z. Elektrochem. 26, 307 (1920). Z. physik. Chem. 94, 628 (1920); 105, 5 5 (1923). Proc. Roy. Soc Edin. 38,23 (1918);39,48 (1919); Proc. Roy. Soc. 96A,287, 298 (1919). 9 Lor. cit.
76
RAMES C . RAY
water seemed to be the most active. The adsorption isotherms for sulphur dixoide with gels containing 8.01 and 4.85 per cent. of water lay practically on the same line, indicating that the maximum value of the adsorption woulc be possessed by a gel containing an amount of water lying between these twc values. The fact that the gas was soluble in water appeared to make nc difference. I n a later paper, however, Patrick and Davidheiserl, in order t c bring the results of their work on the adsorption of ammonia by silica gel intc conformity with the theory of capillary adsorption, assumed that the amount of adsorption of ammonia was considerably affected by the presence of ever; a very small quantity of water in the gel. The present investigation wat undertaken in order to find out if the small amount of water in the gel, whick possessed practically no vapour pressure almost up to the temperature t c which it had been previously heated and did not suffer any appreciable diminution under a very high vacuum, really exerted any influence on the amount of adsorption. Far this reason, nitrogen peroxide, which would react with the water, was selected. Moreover, a suitable medium for absorbing nitrogen peroxide is also important from a technical point of view. 2.
Apparatus and Materials
As nitrogen peroxide attacks rubber, the different parts of the apparatut had either to be sealed together or connected by ground glass joints. Preliminary experiments showed that the amount of adsorption was fairly large and as the gas attacks mercury and dissolves in or reacts with almost all liquids, it was found convenient to weigh the bulb containing the gel after each experiment; the increase in the weight of the bulb gave the amounts adsorbed. The arrangement of the apparatus is shown diagrammatically in Fig. I . It may be divided into two parts, the first! from A to J for the preparation and purification of nitrogen peroxide, and the second from K to N for determining the adsorption of the gas a t different pressures and temperatures. Large quantities of nitrogen peroxide can best be prepared by heating a mixture of finely powdered arsenious oxide, concentrated sulphuric acid and fuming nitric acid (sp.gr. I . 5 ) as described by Cundall2. The crude product, a t first, contains nitrogen trioxide, but Cohen and Calvert3 have shown that although liquid trioxide is practically unacted upon by oxygen, rapid combination takes place between oxygen and gaseous nitrogen trioxide which is thus oxidised to the peroxide. The tetroxide is not further attacked by oxygen; although it is oxidised by ozone. On the contrary, the pentoxide undergoes spontaneous decomposition into tetroxide and oxygen. The decompo. sition of the peroxide is negligible under the conditions of experiment. Thit J. Am. Chem. SOC.44, I (1922). J. Chem. SOC.59, I 0 7 7 (1891). J. Chem. SOC.71, 1052 (1897).
ADSORPTION O F XITROGEK PEROXlDE BY SILICA GEL
77
nethod was used later by Frankland and Evans who1 found it to be most 3atisfactory; all the lower oxides could be oxidised by prolonged treatment gith oxygen. The mixture recommended by Cundall2 was heated in the distilling flask A. The evolved gases were first passed through the U-tube €3 filled with glass wool, then after passing through a series of tubes containing anhydrous calcium nitrate and phosphorus pentoxide, condensed in the glass spiral C packed in ice, and collected in the vessel D which was also kept cold with ice and water. The liquid was redistilled, at as low a temperature as possible, into
FIG.I
the succeeding vessels F, H and J through phosphorus pentoxide tubes and the bulbs E, G and I where the gaseous oxide was mixed with a current of dry oxygen. The final product, when cooled with a freezing mixture, formed a perfectly white solid which melted at - 10.2' ( - 10.8' according to Scheffer according to Egerton, and -9.6' according to Guye and Treub, -10.5' and Drouginine). The boiling point was found to be 22.4'. Finally, the pure nitrogen peroxide was distilled into the bulb L through a phosphorus pentoxide tube. The stem of the bulb L passed through a rubber cork which fitted closely the mouth of an unsilvered cylindrical Dewar vessel M of about three litres capacity. Besides the bulb the cork was bored to contain a normal pentane thermometer reading from + I 5' to - I soo, a glass tube for making connection J. Chem. SOC.79, 1359 (1901).
* LOC.cit.
.
78
HAMES C. RAY
with a water pump, and a glass tube drawn to a capillary a t its lower end which reached to the bottom of the vacuum vessel, the top being closed by a piece of rubber pressure tubing and a screw clip. The purpose of these will be explained later. The bulb L was provided with a three-way stopcock a, one arm of which was connected with the reservoir J containing pure nitrogen peroxide, the other arm to the adsorption bulb ?J, and then through another three-way stopcock b, to a McLeod gauge and a two unit Langmuir mercury vapour pump. A short piece of capillary tube carrying a stopcock x was ground to fit the neck of the adsorption bulb N. The other end of the capillary tubing served for connection with the rest of the apparatus through a ground glass joint, so that the adsorption bulb with the capillary stem could be detached when required arid weighed. Adsorption isotherms were determined at I j ' , 5 7 O , 80" and 100". The vapours of briskly boiling acetone, benzene and water in the double-walled vessel P gave the last three temperatures. 15' was the air temperature which was kept constant by circulating, through the annular space of the vessel I? a current of water, the rate of flow of which could be varied. The mouth of P was closed with cotton wool, and a thermometer was placed inside, the thermometer bulb being in contact with the adsorption bulb. Different pressures of nitrogen peroxide were obtained by maintaining the bulb L a t different temperatures, the vapour pressure being regulated by the coldest part of the apparatus. A large number of investigations have been published on the vapour pressure of nitrogen peroxide, notably by Ramsay and Young,' Guye and Drouginine,2 Scheffer and Treub3 and Egerton.4 The results of Scheffer and Treub are in good agreement with those of Ramsay and Young. Those of Guye and Drouginine also agree well for the undercooled liquid a t low temperatures but are somewhat higher than those of Scheffer and Treub and Ramsay and Young for temperatures above - 20'. This fact is ascribed by Scheffer and Treub to the method employed by Guye and Drouginine for the measurement of statical vapour pressures. The experimental results of Egerton nearly agree with those of Scheffer and Treub for temperatures above -30' but the rate of decrease of vapour pressure is much greater. Guye and Drouginine's results decrease still less rapidly. However, by taking the mean of the closely agreeing results, it is possible to construct a smooth vapour pressure-temperature curve which probably represents the true values of the vapour pressure of nitrogen peroxide a t different temperatures. From this curve, the vapour pressure a t any temperature between +43' and - jo" can be obtained. The vapour pressures of nitrogen peroxide can also be calculated approximately correctly from van der Waals' T
i
-
equation log - =a("P T 147 atmospheres and
I),
taking the mean value for a as 4.17 and for T and
171.2'
respectively.
'Phil. Trans. 177, 109 (1886). J. Chim. phys. 8, 473 (1910). Proc. Akad. Wet. Amsterdam, 14, 536 (1911); Z.phyeik. Chem. 81, 308 (1919). J. Chem. SOC. 105, 647 (1914).
2
i-,
ADSORPTION O F NITROGEN PEROXIDE BY SILICA GEL
79
The silica gel was obtained from the Kestner Evaporator and Engineering Co., Ltd. The gel was activated by heating it for about three hours at 200' in a U-tube contained in an oil bath at zooo through which was drawn a current of air, freed from carbon dioxide and moisture and, previously heated t o the same temperature by passing through a glass spiral placed in the same bath, The water content of the gel was determined by blasting a weighed quantity of the activated substahce in a platinum crucible to a constant weight. All taps were lubricated with viscous metaphosphoric acid except a short length of the ends exposed to the air; these were lubricated with a small amount of rubber greese. The pentane thermometer was calibrated by comparison with a tension-thermometer of the type described by Stock, Henning and KUSS.~ 3.
Method of Experiment
The bulb L, which contained nitrogen peroxide, was first cooled with liquid air and thoroughly evacuated. Solid nitrogen peroxide does not possess any appreciable vapour pressure at liquid air temperatures. The volume of the adsorption bulb N together with the capillary stem up to the stopcock x was accurately determined. A weighed quantity of the activated silica gel was then introduced into the adsorption bulb. Generally 12-1 j grams of the gel were used for each experiment. The bulb was finally attached to the apparatus and connection was made with the pump. When the pressure, as indicated by the McLeod gauge, was of the order of 10-5, the tap x was closed, air was introduced through the three way stopcock b, and the loss of weight of the adsorption bulb was determined. It was again connected with the apparatus and maintained at the desired temperature. The part of the apparatus between the taps a, b and x was then completely pumped out. I n the meantime the bulb L was cooled to the required temperature. Temperatures below -10' were obtained by the method employed by Steele and Bagster.2 Liquid sulphur dioxide or liquid ammonia was introduced into the unsilveredDewar vessel 34 which was also connected with a water pump by a piece of pressure rubber tubing which could be more or less closed by means of a screw clip. To obtain any desired temperature below the boiling point of sulphur dioxide or ammonia, the pump was set in action and a regulated stream of air was admitted through the capillary reaching to the bottom of the Dewar vessel. The sulphur dioxide or ammonia boiled under the reduced pressure and its temperature fell. The stream of air served to prevent superheating and subsequent bumping, and also to stir the liquid and obtain uniform temperature throughout the bath. When the temperature was near the desired point, the pressure tubing was partly closed by means of the screw clip, thus checking the rate of withdrawal of the sulphur dioxide or the ammonia vapour7 and consequently the rate of evaporation, so lessening the heat absorption and fall of temperature. I n a short time a state of equili1Ber. 54, 1119 (1921). J. Chem. SOC. 97,2613 (1910).
80
RAMES C. HAY
brium was established, and by this means the temperature could be kept constant to 0.1'. If it varied occasionally, a slight turn of the screw clip sufficed to adjust it again. If the Dewar vessel was nearly filled a t the beginning, it was possible to run from 2 to 3 hours without adding a fresh quantity of liquid. Filling the Dewar vessel with powdered ice gave oo, and temperatures above '0 were obtained by placing the bulb L in a well-stirred mixture of ice and water in contact with. a thermometer. When both the bulbs L and N were a t the desired temperatures, the stopcock leading to the pump was closed and connection was made between the two bulbs. Adsorption took place very rapidly and was almost complete within a few minutes, but as the adsorption took place with the evolution of a large amount of heat, the bulbs were left in connection from 40 to 45 minutes in order to make sure that the gel had regained the temperature of the bath. The stopcocks a and x were then closed, and air was introduced through the three-way tap b. The adsorption bulb with the capillary stem was taken out and weighed. After allowing for the weight of nitrogen peroxide in the space not occupied by the gel, the increase in weight of the bulb gave the amounts of gas adsorbed. 4.
Experimental Results
The experiments could be repeated, and the difference between the results of t,wo determinations a t the same temperature and pressure never exceeded 2 . 5 per cent. The results given below representt he mean of two or more experiments :-
TABLEI Water content of the gel = 6.02 per cent. Amount of adsorption in grams of nitrogen peroxide per IOO grams of the gel Temperature of the bath
Pressure of nitrogen peroxide in mm. of murcury
490 450 351 2 70 150 88 30
+12' +IO0
+
5 O
O0
- IO0 -20'
- 34'
- 40'
I1
I
X m
X m
at 15"
a&"
at 80'
at 100"
56.80 5548 50.43 45.64 37.27 30.16 20.85 14.49
20.46 19.65 17.80 15.78
14.82 14.37
9.92 9.38 8.07 6.85 4.89 3.56 1.84
The isotherms are plotted in Fig. X
X-
x
Gi
2.
12.17
9.59 6.42 4. I4
12.40
10.69 8.05 5.92 3.38 1.99
I .02
They agree well with Freundlich's
equation, log - =log a + - log p, except a t pressures approaching saturation. m n The values of Freundlich's constants a and ~ / for n different temperatures are tabulated below :-
81
ADSORPTION O F NITROGEN PEROXIDE BY SILICA BEL
TABLE I1 Temperatures. . . . . . . . . . . . . . . . 1 So a . . . . . . . . . . . . . . . 14.00 I /n
5.
...............
0.358
I ooo
SO0
5 7 O
3.98
I .89
0.96
0.418
0.529
0.602
Influence of the Water Content of the Gel on the Amount of Adsorption
In order to find out if there was really any connection between the amount of water present in the gel and the amount of adsorption, samples of gel were heated to increasingly higher temperatures in an electric heater. At each
uO
12
24
36
48
60
GRAMS N/TROGEN PERUX~OL? ADSORBED FER /OD GNS. GEZ FIG.2
temperature, only a definite quantity of water appeared to be given up, no further loss of water seeming t o take place even on prolonged heating at a particular temperature. I n this way, gels contitining different amounts of water were obtained. The water content was determined as before by heating, in the blow-pipe flame, a weighed quantity of the gel in a platinum crucible t o constant weight. Adsorption isotherms were obtained a t 15' in the manner previously described, using gels with 6.80, 5.16, 4.68 and 3 - 2 7 per cent. of water. The following results were obtained :-
82
HAMES C . RAY
TABLE
111
Temperature of Adsorption:-I so Amount of adsorption in grams of nitrogen peroxide per ( = x/m) Pressure of nitrogen
Temperature of the bath
perioxide in mm. of murcury
+ so
490 450 351
O0
270
- IO0
I50
+I2O
+IO0
-I 2 O -zoo
-34O -35O
- 40' a. . . . . . . I/n.. . . .
x/m with gel containing 6.80% of water 57.22
55.64 51.29 47.=2 38.81
---
x/m with gel containing j.167, of
water
56.12
__-
IOO
grams of the gel
x/m with gel-containing 4.687c of mater
__54.30
---
44.26
--_
50.48 44.54 35.42
45.30
---
37.70 33.39 25.57
---
34.72 28.93
___
-_-
138 88 30 26
32 ' 58 22.86
---
__-
I1
16.47
---
___
15.96 0.328
12.32 0.392
12.12
.......... ..........
d m with gel containing 3.2770 of water
18.91
---
17.78
0.401
---
13.22
--_
8.6j 8.34 0.425
These results are also represented graphically in Fig. 3. It will be noticed that the isotherms with gels containing 6.80, 6.02, 5.16 and 4.68 per cent. of water lie practically on two lines quite close to each other, showing that the presence in the gel of an amount of water lying between these two values does not affect the amount of adsorption of nitrogen peroxide to any large extent. When the percentage of water in the gel falls to 3.27, however, the adsorptive capacity of the gel is considerably diminished. The loss of the adsorptive power is, no doubt, due to the reduction of the adsorptive space. In order to obtain a gel with very low water content, it has to be subjected to prolonged heating a t a fairly high temperature, and in this process the gel undergoes incipient fusion which is revealed by the formation of a skin. Briggs' has suggested that the adsorptive power of any given material depends upon two factors, namely, (a) the degree of its porosity on the microscopic and ultramicroscopic scale, and (b) the degree of porosity on the molar scale. He believes that the heating, without destroying the coarser passages, vitrefies the silica and blots out the finest openings upon which adsorption so largely depends. 6. The Effect of Repeated Adsorption on the Same Sample of Gel
In all the experiments described so far, fresh quantities of gel were used for each experiment. The following experiments were undertaken in order to find out whether the adsorbed nitrogen peroxide could be completely 1
LOC.cit.
ADSORPTION O F SITROGEK PEROXIDE BY SILICA GEL
83
recovered, and whether the adsorptive capacity of the gel deteriorated by repeated adsorption. The experimental arrangement was the same as described previously with the exception that an adsorption bulb of a different type was used. The form of adsorption bulb employed in these experiments is shown in Fig. 4. At the bottom of the bulb was sealed a capillary tube which was bent in the form of a U and carried a stopcock. A weighed quantity of the gel was introduced into the bulb, and the amount of adsorp-
FIG.3
tion at a particular temperature and pressure was determined as before. In order to remove the adsorbed gases the bulb was detached from the rest of the apparatus, and a current of hot air, dried by passing through a series of tubes containing sodalime and phosphorus pentoxide, was drawn through the bulb until no more nitrogen peroxide came off. The bulb was again attached to the pump and thoroughly evacuated until the McLeod gauge indicated no pressure. After closing the taps, the bulb was taken out and weighed. It was found that a slight permanent increase in the weight of the gel took place. If the amount of adsorption was small at first, then a further increase in weight took place after the removal of the gases of the second adsorption, slightly inore after that of the third, and so on until a stage was reached after which no further increase in weight occurred. On the other
84
RAMES C. R.4Y
hand, if the amount of adsorption was large a t the first adsorption, after the preliminary increase in the weight of the gel, no further increase took place after subsequent adsorptions, and when the permanent increase in the weight of the gel was complete, the whole of the adsorbed nitrogen peroxide could be removed by passing a current of hot air for a sufficiently long time. It should be mentioned in this connection that the gel turned bluish-green when it adsorbed nitrogen peroxide for the first time. When hot air was drawn through it for sometime the green colour disappeared and the gel assumed an orange-yellow colour and finally became quite colourless. The change of colour also occurred at subsequent adsorptions until the permanent increase in the weight of the gel was complete, after which the gel, although still capable of adsorbing fairly large amounts of nitrogen peroxide, did not turn green but acquired only an orange-yellow colour after adsorption. The production of the green colour was, no doubt, due to the formation of nitrogen trioxide. With small quantities of water nitrogen peroxide reacts according to the following equa tion : 2Nz04+H20 = ? r T 2 0 3 + 2HN03. When subsequently air was drawn through the gel the nitrogen trioxide was oxidised to peroxide nd finally removed1. Thus the water originally present in the gel was gradually completely replaced by nitric acid, when the gel no longer turned green on adsorption of nitrogen peroxide and there was no further increase in weight. McBain2 has shown that the phenomenon of adsorption sometimes consists of two processes-namely, a surface condensation which takes place rapidly, followed by a slow diffusion into the interior of the solid with the formation of a true solid solution. I n view of this fact, FIG.4 namely that ordinary adsorption of a gas by a porous solid may be a combination of both processes. McBain has proposed to employ the term sorption when referring to absorption as a whole, to call diffusion of the gas into the interior absorption, and t o restrict the word adsorption to the first stage of sorption, namely surface condensation. The small amount of water, which cannot be removed by evacuation and only a definite amount of which comes off at each temperature when the gel is progressively heated, is possibly present either as a solid solution or in the interior of the gel. The water, however, does not exert any influence on the amount of adsorption. After the adsorption is complete, the adsorbed nitrogen peroxide slowly reacts with the water forming nitrogen trioxide and nitric acid, and producing the bluish-green colour. During the 1Eng. Pat. 319 (1911); Eng. Pat. 1180061 (1916). ZPhil.Mag. (6) 18, 916 (1909).
85
ADSORPTIOIT O F S I T R O G E S PEROXIDE BY SILICA GEL
reverse process of removing the adsorbed gases, the nitrogen trioxide becomes oxidised to tetroxide which, with the excess of the adsorbed nitrogen peroxide, is removed by the current of air. The water is thus gradually replaced by nitric acid. Finally, a gel is obtained in which the water is completely substituted by the acid. This gel no longer assumes a green colour when it adsorbs nitrogen peroxide, and it is possible to remove the whole of the adsorbed gas from it. There is also no increase in weight of the gel after the removal of the adsorbed gases as in the case when water is present. An adsorption isotherm mas determined a t 15' with a gel in which the water had been wholly replaced by nitric acid. The results obtained are recorded in the following table:
TABLEIV Temperature of the bath Pressure of nitrogen peroxide in mm. mercury Amount of adsorption in grams (f) per IOO grams of the gel
+IZO
+IO0
490
450
+so
oo
351
270
-Ioo
150
-20°
88
-34O
40°
,30
I1
5 4 . 2 6 52.31 48.70 43.38 34.43 27.08 16.69 10.61
a = I o . I 8 and 1/n=o.446 The above results are plotted in Fig. 5 . An adsorption isotherm at the same temperature with a gel containing 6.02 per cent. of water is also given for comparison. It is evident from the curves that the adsorptive capacity of the gel, when the water is completely replaced by nitric acid, is only slightly diminished. It is interesting to note that there appears to be a definite relation between the amount of water originally present in the gel and the amount of nitric acid which takes the place of the water. Samples of gel which originally contained 6.02 and 4.68 per cent. of water underwent a permanent increase of weight of 3.23 and 2 . 5 2 per cent. respectively, i.e., 6.02 and 4.68 per cent. of water were replaced by 9.25 and 7 . 2 0 per cent. of nitric acid. The ratios of the percentage of nitric acid which takes the place of water to the percentage of water originally present in the gel are 1-54 and As the ratio of the densities of the two substances is I . 5 2 in the two cases. also approximately the same, it appears that the amount of water in the gel is replaced by nearly the same volume of nitric acid.
7. Summary ( I ) The adsorption of nitrogen peroxide by silica gel has been determined a t different temperatures and pressures. The results are given in Table I and adsorption isotherms at IS', 5 7 O , 80' and 100' are plotted in Fig. 2 .
86
RAMES C. RAY
( 2 ) The influence of the water content of the gel on the adsorption of nitrogen peroxide has been studied. The results are recorded in Table 111. It was found that within certain limits the water content of the gel exerted no influence on the amount of adsorption. Isotherms with gels containing j . 1 6 and 4.68 per cent. of water lie practically on the same line. (Fig. 3).
FIG.5
(3) The effect cf repeated adsorption of nitrogen peroxide on the same sample of gel has been investigated. It was found that by repeated adsorption the water in the gel was replaced by approximately the same volume of nitric acid. The adsorptive capacity of the gel is only slightly diminished when the water is replaced by nitric acid. (Table I V and Fig. 5). The adsorbed nitrogen peroxide could be removed from a gel containing nitric acid by drawing a current of hot and dry air through it. (4) Freundlich's empirical equation was found to hold for all cases investigated, except when the saturation pressures were approached. This investigation was undertaken a t the suggestion of Prof. F. G. Donnan. My best thanks are due to him for procuring the silica gel, and for his kind interest and helpful criticisms. I must also express my thanks t o Dr. D. C. Jones for his assistance in fixing up the apparatus. T h e W i l l i a m Rainsay Labortories of Inorganic and Physical Chemistry, University College, London.
