Measurement of TMAE and TEA vapor pressures

416

Nuclear Instruments and Methods in Physics Research A270 (1988) 416-418 North-Holland, Amsterdam

MEASUREMENT

OF TMAE AND TEA VAPOR PRESSURES

D.F. A N D E R S O N Particle Detector Group, Fermi National Accelerator Laboratory, Batavia 1L 60510 USA

Received 4 February 1988

A new measurement of the vapor pressure of the photosensitive gas TMAE, a compilation of earlier measurements, and a measurement of the vapor pressure of TEA are presented.

1. Introduction The photosensitive gases terakis(dimethylamino)ethylene, TMAE, and triethylamine, TEA, have been used for many years in such applications as Cherenkov ring imaging, RICH, and to detect the scintillation light from gas scintillation proportional counters (see ref. [1] for a review of the subject). Because TEA has a substantial vapor pressure at room temperature, its concentration is usually adjusted to be considerably less than the saturated concentration. TMAE, on the other hand, has a very low vapor pressure and therefore is normally used at the highest pressure allowed by the operating temperature of the instrument and the surrounding environment. Thus it is important to know the functional form of the vapor pressure of T M A E so that the concentration can be accurately controlled. From the very introduction of T M A E to the field of instrumentation, it was realized that the published vapor pressures were incorrect. The published pressures [2] proved to be off by factors of about 60 and 20 at temperatures of 30 and 90 ° C, respectively. Because of the obvious discrepancy between the published and the observed pressures, this was one of the first quantities measured by this author in the evaluation of TMAE. The vapor pressure of TMAE was expected to obey the Clausius-Clapeyron equation: P = P0 exp

T0 -

'

(1)

where Po a n d To are reference pressure in Torr and temperature in K. A H is the molar heat of vaporization and R is the molar gas constant. Fitting the data to the Clausius-Clapeyron equation, the functional form of the vapor pressure was determined to be [3]: P°ld = 0"577 exp[6454( 1300

1)].

(2)

The value TO= 300 K was chosen for convenience. 0168-9002/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

This early measurement was made with a small sample of material, purified as well as was possible at the time. Because of the very low vapor pressure of TMAE, the pressure is very sensitive to any contamination of the material. With improvements in purification techniques, small disagreements to the value reported above began to be reported [4]. For example, there has been a measurement made for the Delphi barrel RICH detector [5] that yielded a vapor pressure of: PDdphi = 0.563 exp 6285 300

'

(3)

where again the pressure is in Torr. Because of these differences we have made a new evaluation of the T M A E vapor pressure. In a search of the literature, we have only been able to find the vapor pressure of TEA for three temperatures. These data cannot be fitted with the Clausius-Clapeyron equation. We have therefore made a measurement of the vapor pressure of TEA which is reported here.

2. Material preparation Over the last few years, a great deal of effort has been made in learning how to work with TMAE. Besides the fact that it reacts with oxygen, it also reacts with many other materials such as rubber. Therefore, even T M A E fresh from the supplier must be purified before use. There are three important steps in the purification of TMAE: washing, drying and final purification. The T M A E is washed in the following manner: Under nitrogen, the T M A E is shaken with a small portion of deoxygenated water in a separating funnel. (The oxygen has first been removed from the water by bubbling nitrogen or argon through it for 12 h.) The water is then removed and the procedure is repeated

417

D.F. Anderson / T M A E and TEA vapor pressures

until the water is d e a r . This step removes the oxidation p r o d u c t s of tetramethyl oxamide, tetromethyl urea, a n d dimethyl amine. This step noticeably lightens the color of the T M A E . T h e T M A E is then dried b y storing it in tightly closed bottles c o n t a i n i n g 4 or 5 A molecular sieve. T h e oxygen has b e e n removed from the molecular sieve b y evacuation a n d then back-filling with dry nitrogen. F o r m o s t applications, the T M A E is n o w ready for use. T h e final purification was p e r f o r m e d b y v a c u u m distilling the T M A E (at 50 ° C) o n t o a mixture of molecular sieve a n d activated a l u m i n a t h a t has b e e n outgassed u n d e r v a c u u m at a b o u t 400 ° C for several hours. T h e first a n d last fractions of the T M A E are discarded in the distillation step. It should b e n o t e d t h a t whenever working with T M A E with a v a c u u m system, a liquid n i t r o g e n cold trap should b e used. T h e T M A E will react with the seals in the p u m p , a n d cause t h e m to deteriorate in a very short time. T h e T E A used was reagent grade. T h e purification consisted of only the v a c u u m distillation step described above. A b o u t the first a n d the last 10% of the material were discarded. T h e last fraction h a d a noticeable amber tint to it, while T E A itself is a clear liquid.

T(°C) 60

50

I

i

4-0

I

i

30

I

t

I

20 I

10

I

i

0

I

n

TMAE

6285 ( I IaQ

O--i IT )

%" a

O b--

~T

n

P=O .500 P=0.500

0.1

s.o

6372 ( 11300- I / T ) e e6 a 7 2 t l / 3 ° ° - ~

o x~ •

"~.x

A

I

I

i

I

i

s.~

3.~

3.3

~.4

3.5

T(K)

-1

XIO

1. s

6

a

Fig. 1. TMAE vapor pressure as a function of the reciprocal of the absolute temperature and as a function of the temperature in o C. The fits Pold (dashed line), PDelphi (upper solid fine), and Pnew(lower solid line) are also shown. The four data points labeled A are taken from ref. [4].

3. Measurement procedure The sample of T M A E or T E A to b e m e a s u r e d was kept at a c o n s t a n t t e m p e r a t u r e b y a circulating water bath, while the t e m p e r a t u r e was read with a therm o m e t e r t h a t was m a r k e d in units of 0.2 ° C. T h e pressure was read with a calibrated Barocel pressure sensor (Datametrics) with a full r a n g e of 100 Torr. Since some of the m e a s u r e m e n t s were m a d e with the T M A E above r o o m temperature, all m e a s u r e m e n t s were m a d e in a n oven kept at least 10 ° C above the T M A E temperature. It was f o u n d in the m e a s u r e m e n t s of the v a p o r pressure of T M A E that even t h o u g h the material was very pure a n d kept in a clean e n v i r o n m e n t , reproducible results could not b e o b t a i n e d w i t h o u t periodically p u m p i n g o n the sample to remove some high-pressure c o n t a m i n a n t . Therefore, the sample was exposed to a v a c u u m for several minutes each day. T h e v a c u u m was supplied by a n o r d i n a r y rotary oil p u m p (ballast o p e n to remove any trace vapors) a n d a liquid-nitrogen trap.

shown. T h e best fit of the T M A E C l a u s i u s - C l a p e y r o n e q u a t i o n is P , ew = 0.500 exp 6372

300

data

to

"

the

(4)

Fig. 1 also shows the fits PoM (dashed line), PDdphi T(*C) 30 I

20 I

I

10 I

I

I

0

-~ 0

I

I

TEA

100

rc 0 F.-

10

[]B I B.3

4. Vapor pressures Figs. 1 a n d 2 show the v a p o r pressure as a f u n c t i o n of the reciprocal of the absolute t e m p e r a t u r e for T M A E a n d TEA, respectively. T h e t e m p e r a t u r e in ° C is also

I 3.¢

I 3.S

I 3.6

I 3.7

I 3.8

I 3.9

T(K) -1 XIO a Fig. 2. TEA vapor pressure as a function of the reciprocal of the absolute temperature and as a function of the temperature in o C. The two data points labeled A and B are taken from refs. [5] and [6], respectively.

418

D.F Anderson / TMAE and TEA vapor pressures

(upper solid line), and Pnew (lower solid line). The four data points labeled A are taken from ref. [4]. The small discrepancy between these four data points and eq. (4) are not considered significant since the least significant figure is 0.01 Torr. There has also been a measurement made recently [6] that confirms the old T M A E vapor pressure given by eq. (2). Eq. (4) yields a vapor pressure at room temperature (20 ° C) of 0.304 Torr, whereas eqs. (2) and (3) gave a values of 0.349 Torr and 0.345 Torr, respectively. Eq. (4) gives a vapor pressure that is about 12% lower than eqs. (2) and (3). The best fit to the T E A data in fig. 2 is P T ~ = 73.2 exp 4299 300

"

centration of only 36 p p m of dimethyl amine (a known contaminant in T M A E ) would account for the difference in the old and new fits to the vapor pressure. Henry's law states that for nonpolar solvents, such as T M A E , the mole fraction of solute is proportional to its pressure. Thus, such a contaminant as dimethyl amine, which has a vapor pressure of about 1260 T o r t at 20 o C, should have a very high solubility. Another consequence of Raoult's rule is that since T M A E is by far the majority material in any sample studied, any noticeable effect of a contaminant on the vapor pressure will be to increase the pressure. Therefore, an erroneous measurement, barring systematic problems, will tend to be too high.

(5)

Fig. 2 also includes two values of the vapor pressure taken from the literature [7,8]. It should be noted that the boiling point ( P = 760 Tort) given by eq. (5) is 85.4 ° C. The value given for the boiling point of T E A is given as 89.3 ° C in the literature [91.

5. D i s c u s s i o n

The fits to the T M A E vapor pressure all have essentially the same slope and seem to fall into two groups differing by a little more than 10% at room temperature. We believe that this may reflect a small amount of contamination in some of the sample studied. Since T M A E has such a small vapor pressure, it requires a very small amount of contaminant with a high vapor pressure to substantially increase the measured pressure. In a mixture, the pressure of the contaminant is equal to the vapor pressure of the material times its mole fraction (Raoult's rule). Thus, at 2 0 ° C a con-

References

[1] D.F. Anderson, IEEE Trans. Nucl. Sci. NS-32 (1985) 495. [2] V.N. Wiberg and J.W. Buchler, Z. Naturforsch. 19B (1964) 5. [3] D.F. Anderson, IEEE Trans. Nucl. Sci. NS-28 (1981) 842. [4] R.A. Holroyd, J.M. Presses, and C.L. Woody, Nucl. Instr. and Meth. A261 (1987) 440. [5] Y. Giomataris, H. Hofmann, S. Maltezoz, and E. Rosso, DELPHI 86-17 (1986). [6] R. Bouclier et al., Test of an Electromagnetic Calorimeter Using BaF2 Scintillators and Photosensitive Wire Chambers Between 1 and 9 GeV, submitted to Nucl. Instr. and Meth. (1987). [7] G. Charpak and F. Sauli, Nucl. Instr. and Meth. 225 (1984) 627. [8] Ph. Mangeot et al., Nucl. Instr. and Meth. 216 (1983) 79. [9] CRC Handbook of Chemistry and Physics, 67th ed., editor in chief R.C. Weast, (CRC Press Inc., Boca Raton, F1, 1986).