NOTES
reaction of porous glass than of pure silica can be attributed to the presence of surface boron, molecular HzO adsorbed on B-OH groups and boria islands reacting with siloxanes adjacent to the adsorption centers.
Acknowledgment. Support by the Office of Naval Research through 'Ontract 404(19) and NSF Grant GP1134 is gratefully acknowledged.
Anomalous pH Changes in Solutions of Inert
308 1
and a recorder; a tube connected on the outside by Tygon tubing to the test-gas tank and leading into a gas dispersion cylinder submerged in the solution in the flask; an outlet for the gas and for connection to the vacuum pump during degassing. The flask was clamped to a shaker for the degassing process, during which the electrodes were replaced by ground-glass stoppers. This was necessary because the electrodes .. were sensitive to the agitation and since KC1 appeared to be drawn out of the calomel electrode by the reduced pressure. After the degassing, the electrodes were reinserted, with a slight positive pressure of the test gas introduced to prevent any appreciable pickup of carbon dioxide from the air during the exchange It should be noted that it is essential to have the apparatus air-tight, and that several runs had to be discontinued because of leaks which allowed admission of carbon dioxide from the air and prevented the attainment of the maximum pH values. Another difficulty in earlier runs was the attainment of equilibrium. When bubbles having volumes of about 1 cc were used with a typical flow rate of 80 cc/min, the final pH readings were not reproducible. In order to obtain the results reported, it was necessary to use a dispersion cylinder which produced thousands of tiny bubbles per minute, thus allowing the process to reach equilibrium within 1 or 2 hr. -
Gases Possibly Indicating Their Basicity
by Eugene 11. Holleran, John T. Hennessy,l and Frank R. L,aPietra' Chemistry Department, St. John's University, Jamaica, .Yew York ll4SZ (Receiaed October 13, 1966)
The purpose of this note is to report observed increases in the pH of neutral water samples caused by dissolving relatively inactive gases such as argon, neon, helium, and nitrogen.
Experimental Section Half-liter samples of freshly distilled water were obtained from a Corning water distillation apparatus (Node1 BG 2). The pH of these samples was usually between 5.1 and 5.8. The first step was to degas the water sample. In some runs, this was accomplished by shaking under reduced pressure, thereby removing the carbon dioxide and other gases and bringing the pH after several hours to about 7.0. The test gas was then bubbled through the water, causing a further increase in pH which was followed continuously with a pH meter and recorder during the solution of the gas. The rising pH reached a steady value after from 1 to 2 hr depending upon the gas, the rate of flow, and the dispersion of the bubbles. In other runs, the first mechanical degassing was omitted, and the test gas was introduced immediately into the distilled water sample. In these runs, the bubbling of the test gas removed the other gases even more rapidly than the mechanical degassing, and the same final pH values resulted at saturation. On most runs, the saturated solution was degassed and then resaturated several times in succession, with the pH showing a corresponding rise and fall between 7.0 and the saturation value. The apparatus consisted of a 500-cc three-neck roundbottom flask fitted with the following: glass and calomel electrodes connected to a Corning Model 12 pH meter
^
Discussion of Results The experimental results are shown in Table I. Helium, argon, and nitrogen give nearly the same saturation value, about S.0, while neon gives about 7.5. There are several possible explanations for these results, aside from the indicated basicity of the test gases. First, there is the possibility of contamination by material dissolving from the glass flask or from the Tygon tubing. To investigate this possibility, we made several runs with the glass flask coated by a layer of the inert material Desikote. This did not change the results. I n addition, it was found that continued shaking of a water sample in the glass flask did not appreciably change the pH, whether it was at the degassed value of approximately 7, or at the lower values of 5 or 6 due to carbon dioxide, or at the higher values of about 8 due to the inert gas. Finally, cut-up pieces of the Tygon tubing were included in the water in several of these tests, and these also did not change the pH. The most convincing demonstration that the results could not possibly be due to any ionic or nonvolatile contamination is their reversibility, that is, the fact that the pH changes from 7 to S to 7 to S to 7, etc., upon (1) This report is based on the M.S. dissertations of J. T. H. and F. R. L.
Volume 7 1 , Number 9 August 1967
NOTES
3082
successive saturations and degassings of the test gas into and from the same water sample. The possibility of a gaseous impurity in the test gas as obtained from the tank also seems to be eliminated. Both regular and research grades of argon gave the same 02,Kr, and Ne) in the results. The impurities (N2, research grade (Matheson) were listed in parts per million, and presumably any unidentified impurity was present only in smaller amounts. It therefore seems most unlikely that enough of a contaminant such as ammonia could have accumulated during the 1 or 2 hr of bubbling at the typical rate of 80 cc/min to have raised the OH- concentration from to lo-' mole/l. Nevertheless, to remove any such possibility, ~
~
checked against standards and occasionally exchanged with an alternate set. The best check of the proper functioning of the electrodes was provided by several experiments in which indicators were used. For example, using the indicator neutral red, a nearly linear calibration was found for the ratio of the two absorbance peak heights at 530 and 460 mp plotted against the pH of standard buffered solutions. An argon run not listed in Table I was made with this indicator, and samples were taken when the pH meter readings were 6.91 and 7.59. The spectrophotometric values were 6.94 and 7.82, an agreement which confirms the reliability of the meter readings. On runs Ar-2 and Ar-3, the indicator phenol
~~
Table I : pH Changes Caused by Saturating Water with Inert Gases Run no.
Initial p H of sample
pH after degassing
p H after bubbling
Ar-1 Ar-2 Ar-3 Ar-4 Ar-5 Ar-6 Ar-7 He-1 He-2 He-3 He-4 N1-1 N2-2 Ne- 1
5.49 5.49 5.60 5.49 5.73 5.63 5.75 5.32 5.43 5 53 5.57 5.35 5.61 5.81
6.97 6.99 6.85 7.04 7.00
7.97" 8.00 8.00 7.97 8.03 7.98 7.96 7.95 7.97 8.00 7.94 8.13 7.86 7.50
7.00
7.01
pH after degassing
The Journal of Physical Chemistry
bubbling
pH after degassing
6.85 7.00 6.93
7.71b 7.98 8.02
6.94 7.01 7.01
7.09 7.05 7.06 7.04 6.99 7.00 6.94
8.02 7.88 8.07 7.98 7.90 8.12 7.50
7.19 7.01
a Bubbling was continued for 30 hr after reaching steady p H with no further change. the system, a frequent difficulty.
a trap of perchloric acid was inserted between the gas tank and the reaction flask in runs hr-5 and He-3. This introduced an acid contamination in the gas which had to be removed by a subsequent trap containing solid sodium hydroxide. The resulting pH rise was the same in these runs as in the others, and this would have been unlikely if contamination from the final sodium hydroxide trap had occurred. An even more conclusive result was obtained by using a liquid air cold trap on runs Ar-6 and Ar-7. The flow rate was such that most of the argon was condensed in the trap. Presumably, all ammonia would then also have been condensed. The possibility of electrode malfunction was also considered, since the electrodes were found to be very sensitive to pressure changes and to the bubbling. (Of course, bubbles had to be kept from the tip of the calomel electrode.) The electrodes were frequently
pH after
7.06 7.02 7.03 6.93
p H after bubbling
8.07 8.20 7.50
* Low value may be due to leakage of air into
red was included in the solution, and a visual comparison with standards of the resulting color changes showed roughly the same pH changes as were indicated by the electrodes and the pH meter. Thus in run Ar-3 the indicator method corroborates one of the important features of this phenomenon, namely its reversibility. The pH changes observed were thus shown to represent actual changes in the solution and not some change in the reliability of the electrodes caused by the test gas. It appears certain, then, that these inert gases do produce a definite decrease in the activity of hydrogen ions in water. Two possible theoretical explanations for this experimental result suggest themselves. One of these is that the physical process of dissolving the gas could account for the pH rise by changing the structure of the water and so changing the activity of H+. Although the observed magnitude of the effect
NOTES
3083
is not directly related to the molar solubility of the gases, it is possible that the size of the molecules as well as their concentration could have an effect on the structural changes. The second possible explanation is that the dissolved gas acts as a Lewis base, forming weak coordinate bonds with hydrogen ions. This would result in the formation of such species as ArH+, N,H+, NeHf, and HeH+, which might be called argonium, nitrogonium,2 neonium, and helonium ions, in analogy to hydronium and ammonium. In support of this hypothesis, it can be noted that the existence of NeHf and ArH+ was predicted by Funga on the basis of his calculations of the electronegativities of the noble gases. Also, the species HeH +, ArH + NeH +,and KrH + have been observed by mass s p e c t r ~ r n e t r y . ~The - ~ complexes ArHCl and XeHCl have also been reported in spectroscopic analyses.' Assuming the basicity mechanism, we can postulate the following procer-s, using argon as an example. First, the gaseous argon is in equilibrium with the dissolved argon, whose concentration is proportional to the partial pressure of the gas Ark)
Ar(aq)
++
[Ar(aq)l = KJ'
(1)
The dissolved argon then acts as a Lewis base Ar(aq)
+ HzO ++ ArH+ + OH-
[OH-][hrH+] = Kb[Ar(aq)] = K P
(2)
Kb is the base constant of the aqueous species, and K is KbK,,an equilibrium constant for the over-all process Ar(g) HzO ++ ArH+ OH-
+
+
and a base constant for the gaseous species. By using the expressions for K, and for charge conservat ion
K, [H+]
=
[H+][OH-]
+ [ArH+] = [OH-]
t 3)
we find the following relations for the saturated solution 2A
=
KP
log (1 =
+ KbK,P/K,)
10-14(102* - 1)
(4)
in which A is the pH rise. It may be noted that K P must be a t least of the order of if any appreciable p H change is to be found. For P of 1 atm and A of 1.0 pH unit, K is about 10-12. The experimental results indicate that gaseous nitrogen, argon, and helium are of about equal base strength, while neon appears to be weaker. The basic strength of the gas might be assumed from simple con-
siderations to increase with two factors, the electron density in the exterior shell and the polarizability. Since these properties vary in opposite directions as one proceeds through the noble gases from neon to radon, the relative basicities even in this series would be difficult to predict and could possibly exhibit a maximum. It should be of interest to measure the results for various other gases. A modified apparatus would allow the use of a limited sample of gas rather than bubbling it off into the atmosphere. Control of temperature and pressure could also be introduced, although the precision obtained here indicates that the results are not very sensitive to the ordinary variations in room temperature and pressure. Also of interest would be a test of the basicity hypothesis by the law of mass action. One way to do this would be to vary the pressure of the test gas. According to eq 4, doubiing the pressure of argon, for example, would further increase the pH by 0.3 unit. Another method would be to dissolve the gases in dilute strong acid solutions. The second part of eq 3 would then include on the right a term M , for molarity of the acid, and eq 4 would be modified accordingly. The acid concentration would have to be kept small to prevent appreciable changes in the activity coefficients. Significantly, the observed pH increases reported here, whether explained by a basicity of the gases or by an increased structure of the water, are consistent with experimental solubility measurements. These show the gases to be more soluble in acids than in most electrolyte solutions or even pure water in some cases. For example, in studying the radon content of 193 hot springs in Japan, SugiharaS found that the lower the natural pH of the water, the greater the concentration of radon. In a study of the solubility of helium and argon in electrolyte solutions, .ikerlofg found that these gases are more soluble in perchloric acid solutions than in water. This salting-in is in marked contrast to the salting-out effect observed with most electrolyte solutions. Ruetschi and Amlielo found gaseous hydrogen to be (2) The name nitronium is already used for NO2 +. (3) B. Fung, J . Phys. Chem., 69, 596 (1965). (4) K. D. Bainbridge, Phys. Res., 4 4 , 57 (1933). (5) F. J. Norton, National Bureau of Standards Circular No. 582, U. S. Government Printing Office, Washington. D. C . , 1953,p 201. ( 6 ) M. Pahl and U. Wymer, 2. .Vatzirfoi-sch., 13a, 745 (1958). (7) D. H. Rank, P. Sitaram, U. A. Glickman, and T. A. Wiggins, J. Chem. Phys., 39, 2673 (1963). (8) T. Sugihara, Il'ippon Kagaku Zasshi. 81, 1064 (1960). (9) G. Akerlof, J . Am. Chem. ~S'oc.,57, 1196 (1935). (10) P. Ruetschi and R. F. Amlie, J . Phus. Chem., 70, 718 (1966).
Volume 71, Number 9 August 1967
3084
more soluble in H2S04 than in KOH. Morrison and Johnstone" found greatly reduced salting-out effects for helium and krypton in hydrochloric acid. In nitric acid they found these two gases to be even more soluble than in pure water. They also found these and other gases to be salted-in by solutions of NMeJ and NE t4Br. Diamond12 suggested that such salting-in may be due to the association of the large positive ion and the nonelectrolyte caused by the enhanced water structure. Desnoyers, Pelletier, and J o l i c ~ e u r ' further ~ investigated salting-in by quaternary ammonium salts and agreed that salting-in generally seems to be due to an association of neutral molecules with structure-inducing ions. Following this lead, it appears that the salting-in or reduced salting-out of inert gases by acid solutions is due to an association of the neutral molecules with hydrogen ions. Whether this association is caused by an exceptional ability of hydrogen ions to induce structure in the water or to their ability to form weak bonds with nonelectrolyte molecules by accepting a share of the exterior electrons is not certain. The latter explanation perhaps more easily explains both this solubility phenomenon and the pH rises reported in this note. (11) T. J. Morrison and N. B. B. Johnstone, J . Chem. SOC.,3655 (1955). (12) R. M. Diamond, J . Phys. Chem., 67, 2513 (1963). (13) J. E. Desnoyers, G. E. Pelletier, and C. Jolicoeur, Can. J . Chem., 43, 3232 (1965).
The Effect of Additives on the Radiolysis of 1,2-Dichloroethane
by Hisashi 'Ueda
NOTES
enhanced cross-linking of polyvinyl chloride irradiated with NlIa gas.3 Such a phenomenon, however, will be well understood if some ionic processes are considered.2 In the present study, 1,2-dichloroethane has been chosen as a model compound for polyvinyl chloride to provide information about the ionic species which can be expected to exist in the primary processes. As l,&dichloroethane is a good solvent, there is little difficulty in dissolving a small quantity of the additives which will react with the ionic species to be formed. A radiolysis study of this compound has already been tried,4 but the G values in that study are not to be compared with those in the present paper since the experimental conditions are greatly different. The sample vessel containing 1.00 ml of 1,2-dichloroethane and the additive was irradiated with a 100-curie cobalt-60 source at 17 f 3". The total dose given was 1.70 X 1020 ev at the dose rate of 7.00 X lo** ev/hr. The total amount of the decomposition was less than 0.5% of the dichloroethane used. The amount of the additive was 3-5% of the dichloroethane used, and therefore it is assumed that there always was plenty of the additive in the irradiated system and the concentration effect of the additive was already saturated at these concentrations. The reagents used were special reagent grade and were used as received except 1,2-dichloroethane, which was distilled prior to use. The degassing of the sample was repeated three times prior to irradiation. After irradiation, the sample vessel was connected to the distillation system. Three heat baths, namely a t 77, 193, and 253"Ii, were employed to carry out the distillation. The specimen was separated into four fractions. The most volatile fraction was not collected. The fractions at 77-193°K and 193-235°K were analyzed with a reduced pressure type gas chromatograph using a column packed with dioctyl phthalate(9O0), and Table I : Classification of the Additives
Depaement of Chemistry, Tokyo Institute of Technology, Ohokayama, Meguro, Tokyo, Japan (Received January 6 , 1967)
CHCI=CHCl (cis- and CH*=CHz
The present author studied the effects of gases on irradiated polyvinyl chloride by esr.l In that study it was found t'hat free radicals produced in the polymer change their structures and reactivities soon after their formation, which process was termed as "radical transformation." The irregular effects of gases observed in that study were mostly attributable to the radical transformation processes. However, there were some phenomena which were impossible to explain by free radical transformations.2 One example is the The Journal of Physical Chemistry
-
Products
I I1 I11 IV
-
+ +-
HC1
-
++
CHz=CHCl
-
trans-)
-
-
-
(1) Z. Kuri, H. Ueda, and S. Shida, J . Chem. Phys., 32, 371 (1960). (2) H. Ueda, Kogyo Kuguku Zasshi, 69, 1527 (1966). (3) 2. Kuri, H. Ueda, S. Shida, and K. Shinohara, J . Polymer Sci., 43, 570 (1960). (4) R. H. Schuler and W. H. Hamill, J . Am. Chem. Soc., 74, 6171
(1952).
