John Tanaka
University of Connecticut Storrs, 06268
Encapsulation in Borosilicate Glass Ampoules
With the introduction of many air sensitive compounds into undergraduate laboratories, the techniques of encapsulation have become increasingly important. The classical method used is to thicken and narrow down a large piece of tuhing a t the point of seal-off. Although this technique has served well through the years, it suffers from several disadvantages. In order t o prepare an even seal-off, rotation of the glass is important. The average student generally does not have the skill to rotate glass to make a good symmetric seal-off point. A more serious objection to this classical technique is that strains are set up in the thickened seal-off which are difficult even for accomplished glass blowers to anneal out. Researchers dealing with Carius tuhes empirically learn this annealing technique after many tries. When incompletely annealed, even ampoules which contain pressures less than one atmosphere will often crack a t the seal due to the strains imparted during the sealing operation. The experience in our laboratory has shown that students have no difficulty sealing small diameter standard wall tuhing. An explanatory sequence incorporated into the experimental writeup seems to provide adequate directions. Practice becomes necessary only when the glass diameter exceeds approximately 6 mm 0.d. An analysis of the sealing characteristics shows that the following differences can he noted in sealing glass of varying outside diameter with a gas-oxygen flame while pumping on the tube. A 3 mm a d . tube under vacuum cannot be blown in even with a very sharp flame. Using an O X - 3 tip on a National hand torch, a very sharp flame 4 cm long witb a blue cone 7 mm long would not pop a hale in the 3 mm tuhe. With a 4 mm tube, the same sharp flame above would not blow a hole when held on one side of the tube only. However, a hole could be created by "chasing" a thin spat as rapidly as possible. A 5 mm tube will suck in a hole if the same sharp flame is held on one side. However, there is no danger of a hole if a bushier flame is used. A 6 mm tube when heated on one side witb a larger flame 9 to 10 mm wide with a blue cone 9 to 10 mm long with the entire flame extendins about 25 cm will musk the wall to suck and touch the other side without crentmp, a hale. The same holds true wrh a 7 mm tuhe. The a mm tube. however. will implnd~when hented un one side only. \Vith proper direction, 10 mm ruhescan besealed by student-.
Elfect of Strains
When the seals made under vacuum in standard wall tuhing are examined in polarized light for strains, there are none visible in the 3 and 4 m m tuhes. A strain is harely discernable in the 5 mm tuhe and becomes quite pronounced as the tubing size increases. I n the larger sized tuhes, there is not only a strain in the undistorted part of the glass hut also in the heavy part created by the collapsed walls of the tube. In the 14 mm tubes which had been thickened, narrowed and annealed before sealing, the strains which were introduced were sharper and more serious than those in the 10 mm tuhe. Polarized light showed that these were serious enough so that there was a likelihood that a spontaneous crack might occur. In order to test the weakening effect of the strain to pressure within the ampoule, ampoules of three sizes of seal-off were prepared witb known quantities of carbon dioxide (see the figure). 224
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Weakening effect of the strain to pressure within
the ampoule.
Since deviations from ideality of carbon dioxide are well known (I), the pressure inside the ampoule could be estimated. In one series of ampoules, varying amounts of COz were sealed into 10 mm o.d. standard wall borosilicate tubes witb the seal being made directly in the 10 mm tube (tube B of figure). In another series, 5 mm 0.d. standard wall borosilicate glass tubes were filled with known amounts of C 0 2 and sealed (tube C of figure). In the third series, a 3 m m a d . standard wall borasilicate tube was sealed to 10 mm tubing and annealed before filling the ampoule with COa and sealing the 3 mm 0.d. tube (tube G of figure). The results are tabulated in Table 1. The data clearly indicates that strains at the seal govern the strength of the ampoule. In the 10 mm seal where the strain is most pronounced, the ampoule fails at 21 to 23 atm pressure. In the 5 mm seal where the strain is just perceptible, the ampoule fails at 40 to 42 atm pressure. When the seal is made in 3 mm tubing, the ampoule withstands at least 73 atm. Indeed, a 10 mm standard wall ampoule was prepared half filled witb liquid COZ at room temperature and then taken through the critical phenomena without bursting. Characteristics of Ampoules
The recovery of COz from these ampoules was initiated when i t was discovered during the course of these experiments that leaks sometimes developed. A quantitative recovery within the limits of experimental error in gas measurement indicated no leak. In cases of leakage the final pressure in the ampoule could be calculated from the amount of COz recovered. The magnitude of the final pressure indicated that the leak was almost always a very small one. Examination of the ampoule after recovery indicated that in every case the leak occurred a t the sealoff. These leaks were so small that indirect buzzing with a Tesla coil did not indicate a pin hole. Only when the area was directly buzzed was a pin hole enlarged sufficiently to be seen. All the leaks occurred a t the shoulder where the glass was pulled in during the sealing operation. None occurred as a channel through the wad of glass formed by the collapsed wall. At one time i t w a s theorized that the pin hole would always occur on the side where the wall was pulled in more than half the diameter of the tubing, but this did not always seem to he the case. It was significant that leaks never occurred a t pressures very far from the bursting pressures. On the basis of these observations,
it is almost certain that the directly sealed 10 mm tubes containing 23.9, 24.6, and 27.7 atm of COz had sprung leaks. A 5 mm tube seal-off was selected for comparing a number of different ampoule designs for relative strength. This selection was based on a number of factors. When an ampoule of any size is to he attached to a vacuum system, a 3 mm tube does not offer much mechanical strength. When solids are to be encapsulated, a 5 mm tube offers less problems in ampoule filling than a smaller diameter tube. Furthermore, a 5 mm tube can be nicely cleaned before sealing with a length of pipe cleaner. This is a definite advantage not shared by the classical narrowed sealoff. One of the ampoule designs selected was a 10 mm tube with a thin bent break off tip ring sealed into a tube. Besides this classical break ampoule, standard wall ampoules of 10 mm, 15 mm, and 20 mm were tested. Originally, it was intended that 25 mm ampoules also be tested. However, the 20 mm tubes ~ p t u r e dwith such a loud bang that cowardice dictated the cancellation of the 25 mm tube experiments. The surprising result of this series of experiments was that the bursting pressures of allthese ampoule designs were identical. It was concluded that the governing factor was the strength of the seal-off rather than the ampoule design. The results are summarized in Table 2. It had always been our feeling that an ampoule with a 3 mm break-off tip adapted to a vacuum tube opener (2) was far superior to the thin bent break-off tip for pressure work because of the Bourdon gauge principle operating on the latter. The strength of the thin break-off was therefore a surprise. It was as much a surprise to find
Table 1.
Table 2. Comparison of Different Ampoule Designs tor Relative Strength
co*
Pressure (Pht.
in atml
Results
Recoverva
10 mm ad., ClassicalBreak Ampoule Guant. (22 hr40min) Quant. (22 hr 25 mi") 35.7 0 Q u a n t (57 hr 30min) 39.9 0 Quant. (51 hr 50 mi") 40.0 leak 3.11f16.62 (14 hr45 mi") 9 atm 40.9 0 Quant. (14 hr 25 mi") 44.9 X 46.9 leak 10 mm o.d. + 3 mm tio
0.99f19.71 (39 hr) 3 atm
+ 5 mm seal-off
Weakening Effect of the Strain to Pressure Within the Ampoule
COaPressure (Pi,,. in atm)
40.2 41.0 41.9
Result'
10 mm 0.d.. 3 mm seal-off 38.6 0 41.7 0 42.2 0 42.3 0 44.5 0 45.6 0 46.9 0 48.9 0 49.6 0 52.8 0 54.8 0 58.8 0 60.5 leak?
Quant. (46 hr 30 mi") Quant. (24 hr 50 min) Ouant. (2.5hr) $"ant. (23 h r l 0 min) Quant. (21 hr40 min)
~ u a n t(23 . hr 30 mini Quant. (14 hr 15 min) Quant. (11 hr 50 min) Quant. (144 hr 25 mi") 27.2130.8 (120 hrl ~ u a h t(24 . hr) Quant. (20 hr 30 min)
..
5 mm a d . , seal an tube itself 5
that the failure of the 5 mm tube seal-off was dominant over the failure of a 20 mm tuhe. Shriver (3) calculates that a 20 mm standard wall tuhe should burst at about a thud the pressure required to burst a 5 mm 0.d. standard wall tuhe. It should be strongly emphasized that the high pressures observed in these experiments are not to be taken as the inherent strength of glass for just any material encapsulated in ampoules. Shriver (3) calculates bursting pressures of 12 at; for 5 mm, 7.1 atm for 10 mm, 5.6 aim for 15 mm, and 4.3 atm for 20 mm 0.d. tubing. The values given by Shriver are not as bad as they might seem in comparison with the results being reported on the experi-
Quant. (15 hr) Quant. (15 hr 25 mi")
15 mm 0.d. + 5 mm o.d. seal-off 32.1 0 Quant. (17 hr 25 min) 39.2 leak 1.05/11.85 (19 hr 10 min) 4.14atm 40.2 0 Quant. (20 hr 30 min) 41.1 0 Quant. (21 hr 30min) 42.6 leak 0.782/13.8 (22 hr 15 min) 2.98 atm X (28 min) 45.7 45.8 leak 0.514f15.02 (14 hr) 2.02atm 45.9 X ( 1 hr +) 45.9 leak 0.78/14.94 (19 hr30 mi") 3.06 atm 47.6 X (13 hr 10 min) 49.4 X (3 hr 45 min) 52.0 X ( 1 hr +) 53.1 X (3 hr) 54.4 X ( 1 hr 22 mi") 56.4 X (45 mi") 56.6 X ( 1 hr +) 59.1 X (55 min) 20 mm o.d. 5 mm 0.d. seal-off
+
n 0 0 0 0 0 X (11 hr +) 0 X ( 3 hr +) leak X (?) X (45 min +) 0 X (25 min) X (18 mi") X (50 min) X (11 rnin) X (22 min) X (18min) X (5 min)
0 0 X 115min +l
10 mm a d . , seal an tube itself 4 0 6 0 19.5 0 21.4 0 23 X (15 mi") 23.9 0 24.6 0 25.3 X (?I 26.3 X (18 min) 26.9 X (30 mi") 26.6 X (48 mi") 27.7 0 27.9 X (25 mi") 27.9 X (35 min) 28.2 X (20 min) X (4 mi") 34.7 X (3 mi") 43.4
" X indicates spontaneous rupture; 0 indicates no failure
39.6 42.4 42.6 43.7
0 leak leak leak X (35 min) X (13 min) X (2 hr 14 mi") X (25 min +l 35/41 (19 hr) 6.5 atrn
-
" X indicates spontaneous rupture; 0 indicates no failure. b M ~ l recovered/mmoles e~ put in (time in ampoule); pressure a t time of recovery. Volume 50, Number 3. March 1973
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225
ments with COz. Glass strength depends a great deal on the surface condition. Scratches and etches definitely weaken t h e glass. T h e improvement of the ease in cracking glass when scored and wetted is well known t o glass workers. A student in our laboratory some years ago prepared nitrosyl chloride and found t h a t the sealed ampoules of about 15 m m 0.d. could h e kept indefinitely in the refrigerator b u t would consistently pop on leaving on the laboratory desk overnight. Since the vapor pressure of nitrosyl chloride is 1.27 a t m a t O", 1.86 a t m a t 10", 2.67 a t m a t 20". and 3.7 a t m a t 30°C (4), it would seem on the basis of this latter experience t h a t Shiver's values are too high rather than too low. It is probably correct t o say t h a t dry COz has a pronounced strengthening effect on glass. If it is assumed t h a t t h e mechanical strength of glass is a function of lattice defects, flaws a n d surface cracks with the latter predominating (5), and t h a t the strength is further affected by corrosive action of polar liquids on the surface cracks (6-9), then it might he surmised t h a t the effect of the COz is t o scavenge water from the surface cracks. It is also not surprising t h a t NOCl would have a corrosive action on the sqrface cracks. Conclusion For general use ampoules are best prepared with 5 m m tubes a s seal-off points. This is a size which can h e easily sealed with little or no practice. T h e strains introduced in sealing are minimal. T h e entire seal-off area can b e readily cleaned of solids using a pipe cleaner. By using a section of 5 m m tube 3 t o 5 cm long a s a seal-off point, a great deal of latitude is allowed the beginner a s t o t h e exact m o t t o h e heated. For high nressure work 3 m m seal-off; can b e recommended whenever t h e equipment desien makes a small seal-off of this tvoe feasible. ~ l c a u s eglass ampoules on bunting scatter large numbers of small glass particles, care must be taken in how much pressure a n ampoule is forced t o contain. T h e guidelines given by S h i v e r are reasonable. However, it must h e recognized t h a t the strength of t h e ampoule will he a function of the material contained, the ampoule design, and t h e quality of the seal-off. In general, t h e quality of the seal-off is more critical t h a n ampoule design. It is very important t o recognize t h a t ampoules under a given set of conditions may hold a great deal more or a great deal less pressure than indicated hy the guidelines.
ampoules were then determined by measuring the length. For all ampoules, the volume of the seal-off stub was calculated in the same way. The 15 and 20 mm ampoules which were about 5.5 cm long were calibrated by determining the weight of the water used to fill the arhpaule. The volume of the main body of the ampoule was obtained by subtracting the weight of water in the 5 rnm stem estimated from the linear length of the water column in the stem. The total volume of the sealed ampoule was determined by adding the volume of the 5 mm stub as estimated from its linear length to the volume determined for the main body. The thin hent-tip break ampoules were calibrated by measuring the lengths of the appendage as well as the main body of the ampoule. Because of the uncertainty of these measurements, the ampoules which did not burst were broken and the volume of the bent section determined by weighing the water which it cantained. The determined volume and the estimated volume varied by 0.20 to0.53 ml for ampoulevolumes of about 8 ml. The ampoules were filled by measuring out a known number of millimoles of C02 by pressure-volume measurements. Pressures over 725 mm were not used. When necessary, several portions of C0z were measured and transferred to the ampoule by eondensing with liquid nitrogen. Deviations from ideality at pressures less than one atmosphere were ignored. To calculate the pressure in the ampoule, Michel's data (1) for 25.053-C were used to convert the nRT values to the real internal pressure. Since the room temperature varied from 23 to 25°C the accuracy of the P,,t was probably 2-3%. If an ampoule did not explode after a given period of time, it was gingerly lifted out of the 5 gal steel solvent can with a pair of long rubber tipped tongs. The ampoule was immediately placed in liquid nitrogen. When the C02 was frozen, the seal-off stub was broken and the ampoule quickly placed in a vessel which was cooled and attached to a vacuum line. When the vessel containing the ampoule was well cooled, the air in the vessel was evacuated slowly through two nitrogen cooled traps. After thoroughly evacuating, any carhon dioxide which might have escaped the vessel was returned by warming the two traps. The vessel was then warmed to -63°C. the first trap cooled to -111°C and the C01 caught in a liquid nitrogen cooled trap. This procedure separated the carhon dioxide from the moisture collected on the autside of the ampoule during the cooling process. Experiments with known amounts of carhon dioxide proved that the procedure was capable of quantitative recovery. The two portions of the ampoule from the recovery process were then sealed to joints so they could be attached to the vacuum line. Pin holes could be demonstrated only at the seal-off point and only the seal-off points for the ampoules where there was lass of carbon dioxide. Literature Cited Micheh. A,, andMicheh, C., Roc. RQY.Soe L o n d o ~Sor A, 1S3.201 (1936). (2) S t a k , A,. "Hydride~of Boron and Silicon." Cornell University Press, Ithaca, New Ymk, 1957. p. 180. (3) Shrivcr, D. F.. "The Manipulation of Air Sensitive Compounds,)i McCraw-Hill Bmk Company. New Yak. 1969. p. 215. (4) National Research Council, "Internatiansl Critical Tables," McCraw-Hill Bmk Company, New Yark. 1928, Vol. 3, p. 229. (51 Weyl. W.A.,Glorrlnd..27.17 (1946). (6)Bsks, T. C.,sndPrs%ton,F. W.,J AppliedPhys., 17.17+88(1946). 17) Elliott, H.A., J. Appi Phvr.. 23.22&5(1958). (8) Benedieks, C., and Rubens, G.,Jm7hantoirtsAnn..129.37-1% (1945). (91 Mmrtby, V. K.. and Tmley, F. V.. J. A m e r Comm. Soe.. 39.215-17 (1956). (1)
Experimental The COz used was purified in a standard vacuum line by trap to trap distillation through -63' and -11l0C traps to liquid nitrogen. The volume of the ampoules were determined in several ways. The 5 ml and 10 ml tube ampoules which were 14 to 15 cm long were calibrated far volume per linear length by weighing a known height of water in a portion of a tube. The volumes of the actual
Acknowledgment for Mt. Holyoke Conference The Journal of Chemical Education acknowledges with appreciation the assistance of the fallowing staff memhers of Mt. Holyoke College, South Hadley, Massachusetts for supplying the photographs for the Mt. Holyake Conference report which appeared in the January issue: Miss Irma Rabino. director of Public Information. Mr. Michael D. Feinstein. Associate Director of Publications. ~how&.ra~her, and Miss Mary C. ~'orbitl,Asr~stantto thr 1)irecl'or of I1~lblicInformation. We regret chat this acknowledgment was not included in the puhlirnrion of the r e p m .
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Journal of Chemical Education
