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A microthermogravimetric system for the measurement of vapour pressure by a transpiration method
Thermochimica Acta, 217 (1993) 175-186 Elsevier Science Publishers B.V., A m s t e r d a m
17 5
A microthermogravilnetric system for the m e a s u r e m e n t of vapour pressure by a transpiration m e t h o d S.R. Dharwadkar *, A.S. Kerkar and M.S. Samant Applied Chemistry Division, Bhabii~i Atomic. Research Ceoo, ~, Frombay, Bombay-400085 (India) (Rec e i ve d 22 July 1992)
Abstract
A novel automatic recording microthermogravimetric system for the m e a s u r e m e n t o f vapour pressures at high t e m p e r a t u r e s by transpiration m e t h o d was designed and fabricated. The p e r f o r m a n c e of the system was ch eck ed by measuring the vapour pressure of anhydrous c a d m i u m chloride in flowing dry argon in the t e m p e r a t u r e range 713-833 K. T h e vapour pressure of c a d m i u m chloride in this rmlge could be expressed as I n ( P / P a ) = - ( 2 0 109.10 ± 262.32)/T + (29.46 4- 0.344) The enthalpy of sublimation of CdCI2 derived from this equation at the m e a n t e m p e r a t u r e of investigation was found to be 167.2.4- 2.2 kJ mol- J and c o m p a r e d very well with the value of 166.5 4- 4.7 kJ mo1-3 r e p o r t e d by Skudlarski et el. (J. C~.aem. Therrnodyn., 19 (1987) 857) from their recent K n u d s e n effusion mass spectrometric measu remen ts . The present system can be used for m e a s u r e m e n t of vapour pressures up to 1425 K and is the first of its kind in which the mass loss due to vaporization is m o n i t o r e d continuously during the conventional transpiration experinaent. The system described in this p aper effects considerable saving of time in the experiment and minimizes the errors associated with mass m e a s u r e m e n t s in the conventional transpiralion m e t h o d currently in use. INTRODUCTION
A transpiration technique designed primarily to measure the vapour pressures of liquids and solutions at normal laboratory temperatures by Regnauit [1] in the first half of the nineteenth century has been adopted successfully in recent years in the measurement of vapour pressures of materials up to temperatures as high as 2500 K [ 2 ] . Vapour pressures b e t w e e n 10 -3Pa and 10kPa can be measured by this technique in the controlled reactive or inert gaseous environment [3]. The experimental method currently in use is, however, quite cumbersome and t i m e
* C o r r e s p o n d i n g author. 0040-6031/93/$06.00 ( ~ 1993 - Elsevier Science Publishers B.V. All rights reserved
S.R. Dharwadkar et al./Thermochiml Acta 217 (1993) 175-186
•176
consuming. W e describe in this p a p e r a simple system built in o u r l a b o r a t o r y , e m p l o y i n g an a u t o m a t i c r e c o r d i n g m i c r o t h e r m o g r a v i m e t r i e system. T h e i n s t r u m e n t is not only simple to o p e r a t e bvt also yields the most reliable data in the s h o r t e s t possible time. • T h e p e r f o r m a n c e of this a p p a r a t u s was tested by m e a s u r i n g the v a p o u r p r e s s u r e of p u r e a n h y d r o u s c a d m i u m chloride in the t e m p e r a t u r e r a n g e 713-833 K. C a d m i u m c h l o r i d e i s k n o w n to v a p o r i z e c o n g r u e n t l y in this range, •giving p r e d o m i n a n t l y CdCl2(g) as the c a d m i u m b e a r i n g species in the v a p o u r phase [4,5]. T h e v a p o u r p r e s s u r e and the e n t h a l p y of sublimation of CdClz at the m e a n t e m p e r a t u r e derived from the data of the p r e s e n t study are in excellent a g r e e m e n t with those r e p o r t e d recently from K n u d s e n effusion mass s p e c t r o m e t r i c studies [5]. PRINCIPLE
OF THE
TECHNIQUE
A detailed a c c o u n t of the t r a n s p i r a t i o n t e c h n i q u e , including the theory, can be f o u n d in several excellent reviews [3, 6]. In this t e c h n i q u e the mass loss of the s a m p l e m a i n t a i n e d at a c o n s t a n t t e m p e r a t u r e in the uniform t e m p e r a t u r e zone of the f u r n a c e is m e a s u r e d in the p r e s e n c e of a gas (generally t e r m e d the carrier gas) flowing o v e r it at a c o n s t a n t rate. T h e flow r a t e of the carrier gas is chosen so t h a t the t h e r m o d y n a m i c e q u i l i b r i u m b e t w e e n the v a p o u r and the vaporizing m a t e r i a l is virtually u n d i s t u r b e d . This :~. e s t a b l i s h e d e x p e r i m e n t a l l y (see m e a s u r e m e n t of v a p o u r p r e s s u r e of CdCI2). T h e v a p o u r p r e s s u r e p of the m a t e r i a l u n d e r investigation is t h e n calculated from the relation p = nvRT/V¢
= (Wv/Mv)RT/V~
(1)
w h e r e nv is the n u m b e r of moles of the v a p o u r t r a n s p o r t e d by V~ iitres of the carrier gas s w e p t •over the sample, T is the a m b i e n t t e m p e r a t u r e at which the v o l u m e of the carrier gas swept over the s a m p l e is collected, IV,. is the m a s s loss of the sample, My is the m o l e c u l a r weight of the v a p o u r species a n d R is the gas constant. E q u a t i o n (1) is b a s e d on the following a s s u m p t i o n s . (a) T h e v a p o u r b e h a v e s ideally. (b) T h e t h e r m o d y n a m i c e q u i l i b r i u m b e t w e e n the v a p o u r a n d the v a p o u r i z i n g material is Und;.z*.urbed by the flow of the carrier gas. (c) All the v a p o u r is t r a n s p o r t e d by the c a r r i e r gas alone. T r a n s p i r a t i o n e x p e r i m e n t s are generally p e r f o r m e d at a total p r e s s u r e of o n e a t m o s p h e r e , for which the ideal gas e q u a t i o n can be a s s u m e d to be valid. E x p e r i m e n t s are also d e s i g n e d to m e e t closely the criteria Jn a s s u m p t i o n s ( b ) a n d ( e ) b y c h o o s i n g a p p r o p r i a t e flow rates of the carrier gas s w e p t over the sample. T h e r a n g e of flow r a t e s is such t h a t the relative c o n t r i b u t i o n t o the mass loss of the s a m p l e due to o t h e r processes such as diffusion is insignificant c o m p a r e d to the mass loss c a u s e d by the v a p o u r
S.R. Dharwadkar et aLIThermochim. Acta 217 (1993) 175-186
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Fig. 1. Schematic of transpiration apparatus: A, ceramic tube; B, sample boat; C, ceramic block; D, capillary; E, detachable condenser tube; G, condensed vapour.
t r a n s p o r t e d by the carrier gas. M o r e o v e r , it is e n s u r e d that the flow rates c h o s e n are not too fast to disturb the t h e r m o d y n a m i c equilibrium b e t w e e n the sample and the vapour. T h a t these conditions are m e t in the experiment• is d e d u c e d from the p l a t e a u in the plot of the a p p a r e n t p r e s s u r e versus the flow r a t e of the carrier gas (see m e a s u r e m e n t of v a p o u r p r e s s u r e of CdCI2). TRANSPIRATION APPARATUS
T h e c o n v e n t i o n a l t r a n s p i r a t i o n a p p a r a t u s (Fig. 1) consists essentially of the c e r a m i c t u b e A h e a t e d by a resistance w o u n d furnace. T h e s a m p l e u n d e r investigation is k e p t in the uniform t e m p e r a t u r e zone. T h e sample b o a t B is located b e t w e e n the ceramic block C and the capillary D. T h e carrier gas p r e h e a t e d by contact with the ceramic block sweeps the v a p o u r t h r o u g h the capillary a n d the v a p o u r c o n d e n s e s in the t e m p e r a t u r e g r a d i e n t region d o w n s t r e a m . A d e t a c h a b l e c o n d e n s e r tube E is g e n e r a l l y placed i m m e d i a t e l y after the capillary so t h a t the v a p o u r leaving the capillary c o n d e n s e s in this t u b e at G. T h e ceramic block i m p r o v e s the t e m p e r a t u r e uniformity in the r e a c t i o n zone. EXPERIMENTAL PROCEDURE
T h e t r a n s p i r a t i o n e x p e r i m e n t consists of the following two parts. (1) D e t e r m i n a t i o n of the p l a t e a u in the plot of the a p p a r e n t p r e s s u r e versus the flow r a t e of the carrier gas at the suitable t e m p e r a t u r e ( p r e f e r a b l y the m e a n ) in the r a n g e of m e a s u r e m e n t s and; (2) D e t e r m i n a t i o n of equilibrium v a p o u r p r e s s u r e s at v a r i o u s t e m p e r a tures f r o m which the m e a n e n t h a l p y of v a p o r i z a t i o n is derived. T h e e x p e r i m e n t a l quantities m e a s u r e d to obtain the a b o v e i n f o r m a t i o n are as f o l l o w s . . . . . (a) T h e mass loss Wv of the s a m p l e d u e to the v a p o u r transpo1~ed away f r o m it by the carrier gas; (b) the r a t e of flow of the c a r r i e r gas o v e r the sample and the total time for which it is passed, from which the total v o l u m e of the gas swept o v e r the s a m p l e V= is calculated; (c) the t e m p e r a t u r e a t which the e x p e r i m e n t is p e r f o r m e d ;
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(d) t h e a m b i e n t t e m p e r a t u r e T at which the v o l u m e of t h e c a r r i e r gas is measured. • T h e accuracy of the results o b t a i n e d d e p e n d s on the accuracy with which these quantities a r e m e a s u r e d . M e a s u r e m e n t o f m a s s loss T h e mass loss o f the s a m p l e c o n t a i n e d in the b o a t r a n g e s f r o m a few milligrams to h u n d r e d s of m i c r o g r a m s a n d is m o n i t o r e d by any one of: (1) weighing the s a m p l e with the b o a t b e f o r e a n d after the e x p e r i m e n t [71; ( 2 ) in the e x p e r i m e n t s in which t h e s a t u r a t e d v a p o u r is carried into the c o n d e n s e r a n d d e p o s i t e d (see T r a n s p i r a t i o n a p p a r a t u s ) , weighing the c o n d e n s e r b e f o r e and after the e x p e r i m e n t to give the m a s s of the v a p o u r t r a n s p o r t e d into it d u r i n g the e x p e r i m e n t and h e n c e the m a s s loss of the sample; (3) analysing the v a p o u r d e p o s i t e d in the c o n d e n s e r chemically, e m ploying sensitive analytical t e c h n i q u e s [8]. O f the a b o v e t h r e e m e t h o d s the chemical analysis of the c o n d e n s a t e in the c o n d e n s e r p r o v i d e s the m o s t a c c u r a t e i n f o r m a t i o n , b u t is quite t i m e - c o n s u m i n g . In the first of t h e s e m e t h o d s t h e r e is an inevitable e r r o r d u e t o v a p o r i z a t i o n of the s a m p l e d u r i n g a t t a i n m e n t of the t e m p e r a t u r e of the e x p e r i m e n t , which can be m i n i m i z e d only by increasing the total time of the e x p e r i m e n t . M o r e o v e r , the total w e i g h t of the s a m p l e with the b o a t is too large c o m p a r e d to the m a s s loss of the s a m p l e i n v o l v e d in the e x p e r i m e n t . T h e d e t e r m i n a t i o n of t h e m a s s loss f r o m the v a p o u r d e p o s i t e d in the c o n d e n s e r by w e i g h i n g it b e f o r e a n d after the e x p e r i m e n t also gives results of p o o r accuracy b e c a u s e of the relatively small increase in the w e i g h t o f the collector c o m p a r e d to its total weight. F u r t h e r , t h e r e can also b e c o n s i d e r a b l e weighing e r r o r c a u s e d by the s a m p l e ' s characteristics (e.g. it m a y be hygroscopic) if a d e q u a t e care is not exercised d u r i n g cooling of the sample. T h e e x p e r i m e n t in t h e c o n v e n t i o n a l t r a n s p i r a t i o n m e t h o d suffers f r o m the i n h e r e n t mass e r r o r s d e s c r i b e d a b o v e . It is also very c u m b e r s o m e a n d m u s t be s t o p p e d to o b t a i n the m a s s loss d a t a at each t e m p e r a t u r e a n d flow rate. M e a s u r e m e n t o f the f l o w rate a n d the total v o l u m e o f the carrier gas s w e p t over the s a m p l e •T h e m e a s u r e m e n t relatively simple a n d total v o l u m e of the monitored using a
of the v o l u m e of the gas s w e p t o v e r t h e ' s a m p l e is involves m u c h less error. T h e flow r a t e and h e n c e the c a r r i e r gas s w e p t o v e r the s a m p l e is r e g u l a t e d a n d f l o w m e t e r c a l i b r a t e d by the s o a p film t e c h n i q u e
S.R. Dharwadkar et ai./Thermochim. Acta 217 (1993) 175-186
179
d e s c r i b e d e l s e w h e r e [9]. T h e flow rate at the exit of t h e system w a s m e a s u r e d by a r o t a m e t e r calibrated against the capillary flow m e t e r .
Measurement o f temperature T h e accurate m e a s u r e m e n t of t e m p e r a t u r e is d o n e by using t h e r m o couples c a l i b r a t e d at the m e l t i n g points of p u r e m e t a l s as r e c o m m e n d e d in ref. 10. T h e details of calibration are d e s c r i b e d in ref. 11. T h e Pt-Pt13% R h t h e r m o c o u p l e was used in the p r e s e n t system. SPECIAL FEATURES oF THE PRESENT APPARATUS
T h e novelty of the p r e s e n t a p p a r a t u s lies in the a u t o m a t i c r e c o r d i n g of the t e m p e r a t u r e a n d the mass c h a n g e in the s a m p l e u n d e r investigation. T h e o t h e r features, h o w e v e r , are identical to t h o s e f o u n d in m o s t of t h e t r a n s p i r a t i o n systems used by o t h e r investigators [12]. T h e a d v a n t a g e s of a u t o m a t i c r e c o r d i n g of the mass loss are discussed at the e n d of this paper.
Details o f design and construction In the p r e s e n t system (Fig. 2) the s a m p l e A u n d e r investigation ( s p r e a d u n i f o r m l y ) is c o n t a i n e d in a t i e r e d s a m p l e h o l d e r B c o m p r i s i n g t h r e e pans 10 m m in d i a m e t e r a n d 2 m m deep. T h e pans are m a d e of p l a t i n u m foil a n d s t a c k e d one a b o v e the o t h e r , leaving a gap of a b o u t 1 m m b e t w e e n each pan. T h e t h r e e t i e r e d s a m p l e h o l d e r is s u s p e n d e d on one a r m of an electronic r e c o r d i n g type m i c r o b a l a n c e C ( S a r t o r i u s m o d e l A.n.ll) with the help of a 0 . 2 m m d i a m e t e r h a n g d o w n wire D m a d e of p l a t i n u m p l a t i n u m / 1 3 % r h o d i u m alloys. T h e o t h e r a r m of the b a l a n c e carries a silica p a n E for t a r i n g the w e i g h t of t h e s a m p l e and t h e c o n t a i n e r located in the furnace. T h e 0.2 m m d i a m e t e r h a n g d o w n wire carrying the t i e r e d s a m p l e p a n passes t h r o u g h a 4 m m internal d i a m e t e r ( I D ) silica tube F. F serves two p u r p o s e s : s u p p o r t i n g the t h e r m o c o u p l e G with the tip located n e a r the s a m p l e pan; carrying the gas e n t e r i n g f r o m the r e a r p o r t H in the b a l a n c e h o u s i n g to the s a m p l e site after getting p r e h e a t e d to the b~.mperature of t h e s a m p l e on passing t h r o u g h the t e m p e r a t u r e g r a d i e n t along its p a t h , T h e c e n t r e t u b e F also facilitates location of the baffles I in a position which helps to i m p r o v e the t e m p e r a t u r e u n i f o r m i t y in the s a m p l e r e g i o n a n d also to m i n i m i z e the diffusion c o n t r i b u t i o n to the ~otal m a s s loss of the sample. O f the t h r e e baffles located i n t h e u n i f o r m t e m p e r a t u r e region of the reaction tube, t h e first is m a d e of Pt-20% R h alloy a n d the o t h e r two of
S,R. Dharwadkar et al./Thermochim. Acta 217 (1993) 175-186 /~-D
/~-C
CAPILLARY FLOW METER POTENTIOMETER OR RECORDER
H COLD JUNCTION
O -RINGS
TO ROTAME TE|
CONSTRICTION
~
A K
lACE
Fig. 2. Diagram of the transpiration apparatus incorporating the microthermogravimetric system: A, sample; B, tiered sample holder; C, electronic recording-type microbalance; D, hangdown wire (0.2 mm in diameter); E, silica pan for weight taring; F, 4 mm internal diameter (ID) silica tube; G, thermocouple; H, rear port in the balance housing; I, baffles made of (i) P t - 2 0 % R h plate and (ii) alumina discs; J, 14 mm ID silica tube; K, capillary (1.5 mm diameter hole); L, 28 mm ID silica tube; M, 8 mm diameter hole; N, layers of platinum coating.
recrystallized alumina. T h e s e baffles have a 3 m m d i a m e t e r side h o l e , w h i c h guides• the t h e r m o c o u p l e s l e e v e and facilitates the location o f the t h e r m o e o u p l e near the s a m p l e . T h e s a m p l e pan is s u r r o u n d e d by the 14 m m I D silica tube J. T h i s tube is constricted a b o u t 2.5 cm from its closed end. T h e constriction acts as a support to the baffles, w h i c h can slide• freely over the central 4 m m ID silica
S.R. Dharwadkar
ct
al./Thermochim. Acta 217 (1993) 175-i86
181
tube and the t h e r m o c o u p l e sleeve fixed on its side. The baffles sitting on the constricted part i m p r o v e the uniform t e m p e r a t u r e in t h e s a m p l e region and also minimize the diffusion of the v a p o u r in the u p w a r d direction because these virtually isolate the sample region f r o m the rest of the reaction tube, T h e closed end of the 14 m m d i a m e t e r silica tube is provided with a 1.5 m m d i a m e t e r hole K. The carrier gas entering the reaction c h a m b e r through the 4 m m I D central tube sweeps the v a p o u r out t h r o u g h this hole, which then condenses on the inner and o u t e r surfaces respectively of the 2 8 and 14 m m I D silica tubes in the annular space, in the t e m p e r a t u r e g r a d i e n t . T h e 28 m m ID tube L surrounding the 14 m m I D tube is closed at one end and provided with an 8 m m d i a m e t e r hole M a b o u t 15 m m f r o m i t s open end. T h e two tubes are fixed to the balance h o u s i n g t h r o u g h a p p r o p r i a t e m e t a l collars and O-ring fittings. T h e carrier gas transporting the v a p o u r t h r o u g h the 1.5 m m d i a m e t e r hole located at the d o s e d end of the 14 m m d i a m e t e r silica tube comes out through the 8 m m d i a m e t e r side hole M in the 28 m m I D tube and passes through the r o t a m e t e r connected at the outlet. T h e o u t e r surfaces of both the 14 and 28 m m I D tubes are coated with a p l a t i n u m layer near their closed ends. T h e metallic layer on the 28 m m I D tube is e a r t h e d to avoid accumulation of static charge caused by the gases r u b b i n g on the silica surfaces. M E A S U R E M E N T O F V A P O U R P R E S S U R E O F CdCI~
To check the p e r f o r m a n c e of the present system the v a p o u r pressure of a n h y d r o u s c a d m i u m chloride (CdCi2> was m e a s u r e d in the t e m p e r a t u r e range 713-833 K in flowing pure argon gas dried by passing over a n h y d r o u s m a g n e s i u m perchlorate. CdCl2 is k n o w n to vaporize congruently, g i v i n g p r e d o m i n a n t l y a single molecular species CdCl2(g) in the v a p o u r phase [4,5]. in the present e x p e r i m e n t s a n h y d r o u s CdCI2 ( A n a l a R grade) (obtained by d e h y d r a t i o n of the hydrate and stored in a m a g n e s i u m perchlorate dess~.c~,tor before use) was heated to the desired t e m p e r a t u r e in flowing d r y argon. T h e t e m p e r a t u r e was maintained constant to 4-1 K with the help of a p r o p o r t i o n a l - t y p e t e m p e r a t u r e p r o g r a m m e r ( I n d o t h e r m Model MPC-500). A t zero time after the t h e r m a l equilibrium was established the total mass loss indicated on the L E D m o n i t o r of the control unit of the electronic balance was tared and the continuous m o n i t o r i n g of the mass loss was started as a function of time. A typical mass loss record for CdCI2 at 762 K is shown in Fig, 3, The isothermal rate o f m a s s l o s s w a s found t o b e i n d e p e n d e n t of time at each t e m p e r a t u r e ; this is typical of the vaporization process. Figure 4 shows a plot of the a p p a r e n t pressure of CdCI_, as a •function o f flow rate at 773 K. The v a p o u r pressures were calculated from eqn. (1). The various quantities m e a s u r e d and the final values of the c o m p u t e d pressures are listed in Tables 1 and 2. I t c a n be seen f r o m Fig. 4 that the a p p a r e n t
S.R. D h a r w a d k a r et aL/Thermochim. Acta 217 (1993) 175-186
182 START'
¼ !
=
03 O ..J O't
TI ME,~=,.
Fig. 3. A typical m a s s loss vs. time trace recorded at 762 K ( f l o w rate 2490 ml h r - i ) .
vapour pressure of CdCI2 is virtually independent of the flow rate in the region 1 7 0 0 - 2 6 0 0 m l hr -t, implying that the equilibrium b e t w e e n the vapour and the solid is maintained in this flow rate region [3], The experiments at other temperatures were performed at the flow rates of 2100 m l / h r - ' . Figure 5 shows a plot of l n ( P / P a ) vs. 1 / T for the vaporization of CdCI2. The vap:.~ur pressure of CdCI2 as a function of temperature could be expressed in this range by the equation l n ( P / P a ) = - ( 2 0 109.10 4- 262.32) + (29.46 4- 0.344) T 46.0
T=773K
4 2.0 .
u~
38.0
f.n
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• 34.0 30.0"
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26.0
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22.0 18.0 5O0
I
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1500
i
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FLOW RATE OF ARGON{m; Fig. 4. A plot o f apparent pressure v~,, ~0
183
S.R. Dltar~vadkar et aL/Thermochim. Acta 217 (1993) 175-186 TABLE
1
A p p a r e n t p r e s s u r e o f CdCl2(g) as a f u n c t i o n o f flow rate o f a r g o n at 773 K F l o w rate o f argon (ml hr -~)
Volume of argon gas s w e p t (ml)
M a s s loss o f CdCl2 p e r litre o f a r g o n ( m g l -I)
Apparent p r e s s u r e (Pa)
1040 1310 1720 2010 2280 2520 2890 3430 3890 4080
1080 1420 2220 2010 2590 3180 4180 4890 6320 6950
2.87 2.66 2.31 2.30 2.22 2.25 2.15 1.98 1.83 1.78
38.5 35.7 31~0 30.9 29.8 30.2 28.8 26.6 24.6 23.8
The enthaipy of sublimation of CdCI2 derived from this plot at the m e a n t e m p e r a t u r e of m e a s u r e m e n t s is 167.2 ± 2 . 2 k J m o l - ' . This is in excellent a g r e e m e n t with the value of 166.5 ± 4.7 kJ mol-~ o b t a i n e d by Skudlarski et al. [5] in their recent K n u d s e n effusion mass spectrometric m e a s u r e m e n t s in the t e m p e r a t u r e interval about 100°C below that in the present study. The o b t a i n e d in the third law value of the e n t h a l p y of sublimation AHz~t.,,,b~ ° present w o r k from the v a p o u r pressures m e a s u r e d at different t e m p e r a t u r e s TABLE
2
T h e e n t h a l p y o f s u b l i m a t i o n o f CdCi., f r o m v a p o u r p r e s s u r e d a t a Temperature {1< ~
Mass t r a n s f e r per litre o f c a r r i e r gas ( m g i -I)
Pressure (Pa)
AH~.~c,..h~ ( k J tool - ')
713 729 744 745 760 768 773 788 801 816 833
0.27 0.49 0.86 0.86 1.40 1.88 2.31 3.76 5.91 9.65 17.96
3.6 6.6 11.5 I 1.5 18.8 25.2 31.0 50.4 79.3 129.5 241.0
176.8 176.8 176.6 176.9 177.0 176.8 176.5 175.4 176.0 175.5 I74.6
( t h i r d law)
A v e r a g e 176.4 ± 0.7
S.R. Dharwadkar et al./Thermochim. Acta 217 (1993) 175-186
184
6.o • THtS WORK h B L O O M A N D W E L C H ( R e f . 15~ E) S K U D L A R S K | et. at (Ref. 5 ) x NIWA (Ref.14.. p.427
o
4.0
3.0 eL
z.o
4.) ft.
1,0
--
"-,j,:
mm
_1
0.0 - 1.0 %
- 2.0
-5.0
~"~X -4.0
I
i
1,20
;
I
,
~
,
1.30
I
~
,
l
1.40
1.50
1.60
1.70
103 K . / T Fig. 5. A plot of ln(P/Pa) vs. 1 / T for CdCI2. TABLE
3
T h e enthalpy of sublimation of CdCI2 from various investigators
Method of measurement K n u d s c n ulTusit~n Tran~l~irttil,~n K n u d s u , ctfJ,, h~, Transi;i!:~li::,~
Transpiration Transpiration
Ref.
AH,.*,~(.~,b, (kJ m o l -~)
Second law
Third law
187.9 219.1 191.0 193.1 180.0 188.6
177.8 1~7.5 182.3 177.1 189.6 176.4
4- 14.5 4- 1.6 4- 1.7 4- 16.5 4- 6.2 4- 2.2
4- 0.4 4- 8.2 4- 0.8 4- 1.3 + 1.9 4- 0.7
5 13 14 15 16
This work
a n d e m p l o y i n g the free energy functions used in ref, 5 was f o u n d to be 176.4 ± 0.7 kJ m o l - ' . Similarly, the s e c o n d law value for the enthalpy of sublimation o f CdCI2 derived from the enthalpy i n c r e m e n t data used in ref. 5 was 188.6 ± 2.2 kJ m o l -t. T h e s e values are in g o o d a g r e e m e n t with those c o m p i l e d by S k u d l a r s k i et al. [5] from the w o r k o f various investigators (Table 3).
S,R. Dharwadkar et al./Thermochim. Acta 217 (1993) 175-186
185
DISCUSSION
T h e r e have b e e n several m e a s u r e m e n t s of the v a p o u r p r e s s u r e s of CdCI2 e m p l o y i n g both t r a n s p i r a t i o n and K n u d s e n effusion techniques. T h e results o b t a i n e d by various investigators have recently b e e n s u m m a r i z e d in ref. 5. T h e exeellent a g r e e m e n t b e t w e e n o u r value of the e n t h a l p y of sublimation of CdC12 (167.2-+ 2.2 kJ mol -i) and the value of Skudlarski et al. [5] at the m e a n t e m p e r a t u r e of the two studies by i n d e p e n d e n t t e c h n i q u e s establishes t h e accuracy of o u r m e a s u r e l n e n t s . T h e results o b t a i n e d by o t h e r investigators e m p l o y i n g t r a n s p i r a t i o n [15] and K n u d s e n effusion t e c h n i q u e s [5, 14] are i n c o r p o r a t e d in Fig. 5 along with o u r results for c o m p a r i s o n . It can be seen that the e n t h a i p y o f sublimation o b t a i n e d in the t r a n s p i r a t i o n e x p e r i m e n t s is h i g h e r in some eases t h a n t h a t derived from K n u d s e n effusion m e a s u r e m e n t s . This could p e r h a p s be due to the different t e m p e r a t u r e intervals over which these m e a z u r e m e n t s w e r e m a d e . M o s t of the t r a n s p i r a t i o n m e a s u r e m e n t s w e r e m a d e up to the trielting point of CdCI,.. T h e v a p o u r p r e s s u r e s of c a d m i u m - b e a r i n g species n e a r the melting point of CdCl~ are well b e y o n d the K n u d s e n effusion range and, f u r t h e r m o r e , the c o n t r i b u t i o n of the d i m e r to tbe total p r e s s u r e is also significant. Both these factors would result in an incrt.'ased slope of the plot of l n ( P / P a ) vs. 1/T and h e n c e the derived e n t h a l p y of vaporization. A l t h o u g h Skudlarski et al. [5] in their m e a s u r e m e n t s have r e p o r t e d t h a t the v a p o u r of CdCI2 consists p r e d o m i n a n t l y of the m o n o m e r in their range of t e m p e r a t u r e m e a s u r e m e n t , Moss [16] has o b s e r v e d a b o u t 15% d i m e r c o n t r i b u t i o n in the v a p o u r a b o v e solid CdCI~ n e a r its melting point. In o u r p r e s e n t t r a n s p i r a t i o n d a t a t h e r e is significant difference in the m e a n e n t h a l p y of v a p o r i z a t i o n derived from the l n ( P / P a ) vs. 1 / T plot if the value of the p r e s s u r e derived at the highest t e m p e r a t u r e of the p r e s e n t m e a s u r e m e n t (833 K) is included in the linear least s q u a r e s analysis of the data. T h e m e a n value for the e n t h a l p y of sublimation u n d e r this condition w o r k s out to be 170.9-+ 3.1 kJ moi -1. T h e d i m e r c o n t r i b u t i o n to the total p r e s s u r e at this t e m p e r a t u r e is k n o w n to be m o r e t h a n 10% [13, 16], so the point at 833 K was ignored in the analysis of the p r e s e n t data. CONCLUSION
It is seen f r o m the p r e s e n t work t h a t the a u t o m a t i c r e c o r d i n g t r a n s p i r a t i o n system d e s i g n e d a n d fabricated in om l a b o r a t o r y yields most reliable d a t a of a high d e g r e e o f accuracy i , the s h o r t e s t possible time. This is the first time that the m i c r o t h e r m o b a l a n c e has b e e n a d o p t e d in on-line m o n i t o r i n g of the mass loss in a c o n v e n t i o n a l t r a n s p i r a t i o n e x p e r i m e n t . T h e design of the system is simple and with little alteration the unit can be used for n o r m a l t h e r m o g r a v i m e t r i e investigations. T h e r e l i a b i l i t y of the results
J86
S.R. Dharwadkar et al./Tiiermochim. Acta 217 (1993) i75-i86
obtained with this instrument has recently been demonstrated in our studies on the vaporization of molybdenum trioxide [17]. Several investigations on the thermodynamics of tratisiti0n metal molybdates are currently in
progress using this sy'~l~!m, The t et!iperttttll't~ i~taiige of measurement in the present apparatus can be increased substantially by replacing the silica tubes by those of recrystald alumina. This is ctiri'ently being attempted. REFERENCES I R.V. Regnault, Ann. Chim., 15 (1845) 129. 2 M. "~ -,r,baum an d P.D. H u n t , J. Nucl. Mater., 34 (1970) 86. 3 U. ~ .~ten ilild ~V,E. Bell, in J.L. Margrave (Ed.), The Characterization of High T. q" a~ure Vap0urs, Wiley, New York, 1967 p. 91. 4 K. Hilp;rt, in Struciure and Bonding, Voi. 73, Springer-Verlag, Berlin, 1990, p. 163. 5 K. Sku~larski, J: Dudek and J. Kapala, J. Chem. Thermodyn., 19 (1987) 857. 6 P.G. Wahlbeck, High Temp. Sci., 21 (1986) 189. 7 S.N. Tripathi, A.S. Kerkar, G. Chattopadhyay and M.S. Chandrasekharaiah, J. Am. Ceram. Soe., 68 (1985) 232. 8 S.R. Dharwadkar, S.N. Tripathi, M.D. Karkhanavala antl M,S. Chandrasekharaiah, in Thermodynamics of Nuclear Materials, Int. Atomic Energy Agency Syrup. No. 190/32, Vienna, 1975, 455. 9 0 . M . Sreedharan, S.R. Dharv~adkar and M.S. Chandrasekharaiah, Bhabha Atomic Research Centre, Internal Rep. No. 1-239, 1973. 10 Internntional practical temperature scale, Metrologia., 5 (1969) 35. 11 O.M. Sreedharan, A.S. Kerkar and M.S. Chandrasekharaiah, Bhabha Atomic Research Centre, Internal Rep. No. 1-261, I973. 12 K.A. Sense, M.J. Snyder and J,W. Clegg, J. Phys. Chem., 58 (1954) 223. 13 F.J. Keneshea and D.D. Cubicciotti, J. Chem. Phys., 40 (1964) 1778. 14 O. Kubaschewski and C.B. Alcock, Metallurgical Thermochemistry, 5th e d n . , Pergamon, Oxt'ord, 1979, p. 427. 15 H. Bloom and B.I. Welch, J. Phys. Chem., 62 (I958) 1594. 16 H.I. Moss, Thesis, Indiana University, 1960; quo:,.,d by L. Topor, Rev, Roum, Chim., i7 (1972) 1503. 17 M.S. Samant, A.S. Kerkar, S.R. Bharadwaj and S.R. Dharwadkar, J. Alloys Compounds, 187 (1992) 373.
17 5
A microthermogravilnetric system for the m e a s u r e m e n t of vapour pressure by a transpiration m e t h o d S.R. Dharwadkar *, A.S. Kerkar and M.S. Samant Applied Chemistry Division, Bhabii~i Atomic. Research Ceoo, ~, Frombay, Bombay-400085 (India) (Rec e i ve d 22 July 1992)
Abstract
A novel automatic recording microthermogravimetric system for the m e a s u r e m e n t o f vapour pressures at high t e m p e r a t u r e s by transpiration m e t h o d was designed and fabricated. The p e r f o r m a n c e of the system was ch eck ed by measuring the vapour pressure of anhydrous c a d m i u m chloride in flowing dry argon in the t e m p e r a t u r e range 713-833 K. T h e vapour pressure of c a d m i u m chloride in this rmlge could be expressed as I n ( P / P a ) = - ( 2 0 109.10 ± 262.32)/T + (29.46 4- 0.344) The enthalpy of sublimation of CdCI2 derived from this equation at the m e a n t e m p e r a t u r e of investigation was found to be 167.2.4- 2.2 kJ mol- J and c o m p a r e d very well with the value of 166.5 4- 4.7 kJ mo1-3 r e p o r t e d by Skudlarski et el. (J. C~.aem. Therrnodyn., 19 (1987) 857) from their recent K n u d s e n effusion mass spectrometric measu remen ts . The present system can be used for m e a s u r e m e n t of vapour pressures up to 1425 K and is the first of its kind in which the mass loss due to vaporization is m o n i t o r e d continuously during the conventional transpiration experinaent. The system described in this p aper effects considerable saving of time in the experiment and minimizes the errors associated with mass m e a s u r e m e n t s in the conventional transpiralion m e t h o d currently in use. INTRODUCTION
A transpiration technique designed primarily to measure the vapour pressures of liquids and solutions at normal laboratory temperatures by Regnauit [1] in the first half of the nineteenth century has been adopted successfully in recent years in the measurement of vapour pressures of materials up to temperatures as high as 2500 K [ 2 ] . Vapour pressures b e t w e e n 10 -3Pa and 10kPa can be measured by this technique in the controlled reactive or inert gaseous environment [3]. The experimental method currently in use is, however, quite cumbersome and t i m e
* C o r r e s p o n d i n g author. 0040-6031/93/$06.00 ( ~ 1993 - Elsevier Science Publishers B.V. All rights reserved
S.R. Dharwadkar et al./Thermochiml Acta 217 (1993) 175-186
•176
consuming. W e describe in this p a p e r a simple system built in o u r l a b o r a t o r y , e m p l o y i n g an a u t o m a t i c r e c o r d i n g m i c r o t h e r m o g r a v i m e t r i e system. T h e i n s t r u m e n t is not only simple to o p e r a t e bvt also yields the most reliable data in the s h o r t e s t possible time. • T h e p e r f o r m a n c e of this a p p a r a t u s was tested by m e a s u r i n g the v a p o u r p r e s s u r e of p u r e a n h y d r o u s c a d m i u m chloride in the t e m p e r a t u r e r a n g e 713-833 K. C a d m i u m c h l o r i d e i s k n o w n to v a p o r i z e c o n g r u e n t l y in this range, •giving p r e d o m i n a n t l y CdCl2(g) as the c a d m i u m b e a r i n g species in the v a p o u r phase [4,5]. T h e v a p o u r p r e s s u r e and the e n t h a l p y of sublimation of CdClz at the m e a n t e m p e r a t u r e derived from the data of the p r e s e n t study are in excellent a g r e e m e n t with those r e p o r t e d recently from K n u d s e n effusion mass s p e c t r o m e t r i c studies [5]. PRINCIPLE
OF THE
TECHNIQUE
A detailed a c c o u n t of the t r a n s p i r a t i o n t e c h n i q u e , including the theory, can be f o u n d in several excellent reviews [3, 6]. In this t e c h n i q u e the mass loss of the s a m p l e m a i n t a i n e d at a c o n s t a n t t e m p e r a t u r e in the uniform t e m p e r a t u r e zone of the f u r n a c e is m e a s u r e d in the p r e s e n c e of a gas (generally t e r m e d the carrier gas) flowing o v e r it at a c o n s t a n t rate. T h e flow r a t e of the carrier gas is chosen so t h a t the t h e r m o d y n a m i c e q u i l i b r i u m b e t w e e n the v a p o u r and the vaporizing m a t e r i a l is virtually u n d i s t u r b e d . This :~. e s t a b l i s h e d e x p e r i m e n t a l l y (see m e a s u r e m e n t of v a p o u r p r e s s u r e of CdCI2). T h e v a p o u r p r e s s u r e p of the m a t e r i a l u n d e r investigation is t h e n calculated from the relation p = nvRT/V¢
= (Wv/Mv)RT/V~
(1)
w h e r e nv is the n u m b e r of moles of the v a p o u r t r a n s p o r t e d by V~ iitres of the carrier gas s w e p t •over the sample, T is the a m b i e n t t e m p e r a t u r e at which the v o l u m e of the carrier gas swept over the s a m p l e is collected, IV,. is the m a s s loss of the sample, My is the m o l e c u l a r weight of the v a p o u r species a n d R is the gas constant. E q u a t i o n (1) is b a s e d on the following a s s u m p t i o n s . (a) T h e v a p o u r b e h a v e s ideally. (b) T h e t h e r m o d y n a m i c e q u i l i b r i u m b e t w e e n the v a p o u r a n d the v a p o u r i z i n g material is Und;.z*.urbed by the flow of the carrier gas. (c) All the v a p o u r is t r a n s p o r t e d by the c a r r i e r gas alone. T r a n s p i r a t i o n e x p e r i m e n t s are generally p e r f o r m e d at a total p r e s s u r e of o n e a t m o s p h e r e , for which the ideal gas e q u a t i o n can be a s s u m e d to be valid. E x p e r i m e n t s are also d e s i g n e d to m e e t closely the criteria Jn a s s u m p t i o n s ( b ) a n d ( e ) b y c h o o s i n g a p p r o p r i a t e flow rates of the carrier gas s w e p t over the sample. T h e r a n g e of flow r a t e s is such t h a t the relative c o n t r i b u t i o n t o the mass loss of the s a m p l e due to o t h e r processes such as diffusion is insignificant c o m p a r e d to the mass loss c a u s e d by the v a p o u r
S.R. Dharwadkar et aLIThermochim. Acta 217 (1993) 175-186
I77
__'-f,4/4444x'# / GAS
INLET.--~
-J- N
,r
c
a
,B
\
°
r
Fig. 1. Schematic of transpiration apparatus: A, ceramic tube; B, sample boat; C, ceramic block; D, capillary; E, detachable condenser tube; G, condensed vapour.
t r a n s p o r t e d by the carrier gas. M o r e o v e r , it is e n s u r e d that the flow rates c h o s e n are not too fast to disturb the t h e r m o d y n a m i c equilibrium b e t w e e n the sample and the vapour. T h a t these conditions are m e t in the experiment• is d e d u c e d from the p l a t e a u in the plot of the a p p a r e n t p r e s s u r e versus the flow r a t e of the carrier gas (see m e a s u r e m e n t of v a p o u r p r e s s u r e of CdCI2). TRANSPIRATION APPARATUS
T h e c o n v e n t i o n a l t r a n s p i r a t i o n a p p a r a t u s (Fig. 1) consists essentially of the c e r a m i c t u b e A h e a t e d by a resistance w o u n d furnace. T h e s a m p l e u n d e r investigation is k e p t in the uniform t e m p e r a t u r e zone. T h e sample b o a t B is located b e t w e e n the ceramic block C and the capillary D. T h e carrier gas p r e h e a t e d by contact with the ceramic block sweeps the v a p o u r t h r o u g h the capillary a n d the v a p o u r c o n d e n s e s in the t e m p e r a t u r e g r a d i e n t region d o w n s t r e a m . A d e t a c h a b l e c o n d e n s e r tube E is g e n e r a l l y placed i m m e d i a t e l y after the capillary so t h a t the v a p o u r leaving the capillary c o n d e n s e s in this t u b e at G. T h e ceramic block i m p r o v e s the t e m p e r a t u r e uniformity in the r e a c t i o n zone. EXPERIMENTAL PROCEDURE
T h e t r a n s p i r a t i o n e x p e r i m e n t consists of the following two parts. (1) D e t e r m i n a t i o n of the p l a t e a u in the plot of the a p p a r e n t p r e s s u r e versus the flow r a t e of the carrier gas at the suitable t e m p e r a t u r e ( p r e f e r a b l y the m e a n ) in the r a n g e of m e a s u r e m e n t s and; (2) D e t e r m i n a t i o n of equilibrium v a p o u r p r e s s u r e s at v a r i o u s t e m p e r a tures f r o m which the m e a n e n t h a l p y of v a p o r i z a t i o n is derived. T h e e x p e r i m e n t a l quantities m e a s u r e d to obtain the a b o v e i n f o r m a t i o n are as f o l l o w s . . . . . (a) T h e mass loss Wv of the s a m p l e d u e to the v a p o u r transpo1~ed away f r o m it by the carrier gas; (b) the r a t e of flow of the c a r r i e r gas o v e r the sample and the total time for which it is passed, from which the total v o l u m e of the gas swept o v e r the s a m p l e V= is calculated; (c) the t e m p e r a t u r e a t which the e x p e r i m e n t is p e r f o r m e d ;
178
S.R. Dharwadkar et aL/Thermochim. Acta 217 (1993) 175-186
(d) t h e a m b i e n t t e m p e r a t u r e T at which the v o l u m e of t h e c a r r i e r gas is measured. • T h e accuracy of the results o b t a i n e d d e p e n d s on the accuracy with which these quantities a r e m e a s u r e d . M e a s u r e m e n t o f m a s s loss T h e mass loss o f the s a m p l e c o n t a i n e d in the b o a t r a n g e s f r o m a few milligrams to h u n d r e d s of m i c r o g r a m s a n d is m o n i t o r e d by any one of: (1) weighing the s a m p l e with the b o a t b e f o r e a n d after the e x p e r i m e n t [71; ( 2 ) in the e x p e r i m e n t s in which t h e s a t u r a t e d v a p o u r is carried into the c o n d e n s e r a n d d e p o s i t e d (see T r a n s p i r a t i o n a p p a r a t u s ) , weighing the c o n d e n s e r b e f o r e and after the e x p e r i m e n t to give the m a s s of the v a p o u r t r a n s p o r t e d into it d u r i n g the e x p e r i m e n t and h e n c e the m a s s loss of the sample; (3) analysing the v a p o u r d e p o s i t e d in the c o n d e n s e r chemically, e m ploying sensitive analytical t e c h n i q u e s [8]. O f the a b o v e t h r e e m e t h o d s the chemical analysis of the c o n d e n s a t e in the c o n d e n s e r p r o v i d e s the m o s t a c c u r a t e i n f o r m a t i o n , b u t is quite t i m e - c o n s u m i n g . In the first of t h e s e m e t h o d s t h e r e is an inevitable e r r o r d u e t o v a p o r i z a t i o n of the s a m p l e d u r i n g a t t a i n m e n t of the t e m p e r a t u r e of the e x p e r i m e n t , which can be m i n i m i z e d only by increasing the total time of the e x p e r i m e n t . M o r e o v e r , the total w e i g h t of the s a m p l e with the b o a t is too large c o m p a r e d to the m a s s loss of the s a m p l e i n v o l v e d in the e x p e r i m e n t . T h e d e t e r m i n a t i o n of t h e m a s s loss f r o m the v a p o u r d e p o s i t e d in the c o n d e n s e r by w e i g h i n g it b e f o r e a n d after the e x p e r i m e n t also gives results of p o o r accuracy b e c a u s e of the relatively small increase in the w e i g h t o f the collector c o m p a r e d to its total weight. F u r t h e r , t h e r e can also b e c o n s i d e r a b l e weighing e r r o r c a u s e d by the s a m p l e ' s characteristics (e.g. it m a y be hygroscopic) if a d e q u a t e care is not exercised d u r i n g cooling of the sample. T h e e x p e r i m e n t in t h e c o n v e n t i o n a l t r a n s p i r a t i o n m e t h o d suffers f r o m the i n h e r e n t mass e r r o r s d e s c r i b e d a b o v e . It is also very c u m b e r s o m e a n d m u s t be s t o p p e d to o b t a i n the m a s s loss d a t a at each t e m p e r a t u r e a n d flow rate. M e a s u r e m e n t o f the f l o w rate a n d the total v o l u m e o f the carrier gas s w e p t over the s a m p l e •T h e m e a s u r e m e n t relatively simple a n d total v o l u m e of the monitored using a
of the v o l u m e of the gas s w e p t o v e r t h e ' s a m p l e is involves m u c h less error. T h e flow r a t e and h e n c e the c a r r i e r gas s w e p t o v e r the s a m p l e is r e g u l a t e d a n d f l o w m e t e r c a l i b r a t e d by the s o a p film t e c h n i q u e
S.R. Dharwadkar et ai./Thermochim. Acta 217 (1993) 175-186
179
d e s c r i b e d e l s e w h e r e [9]. T h e flow rate at the exit of t h e system w a s m e a s u r e d by a r o t a m e t e r calibrated against the capillary flow m e t e r .
Measurement o f temperature T h e accurate m e a s u r e m e n t of t e m p e r a t u r e is d o n e by using t h e r m o couples c a l i b r a t e d at the m e l t i n g points of p u r e m e t a l s as r e c o m m e n d e d in ref. 10. T h e details of calibration are d e s c r i b e d in ref. 11. T h e Pt-Pt13% R h t h e r m o c o u p l e was used in the p r e s e n t system. SPECIAL FEATURES oF THE PRESENT APPARATUS
T h e novelty of the p r e s e n t a p p a r a t u s lies in the a u t o m a t i c r e c o r d i n g of the t e m p e r a t u r e a n d the mass c h a n g e in the s a m p l e u n d e r investigation. T h e o t h e r features, h o w e v e r , are identical to t h o s e f o u n d in m o s t of t h e t r a n s p i r a t i o n systems used by o t h e r investigators [12]. T h e a d v a n t a g e s of a u t o m a t i c r e c o r d i n g of the mass loss are discussed at the e n d of this paper.
Details o f design and construction In the p r e s e n t system (Fig. 2) the s a m p l e A u n d e r investigation ( s p r e a d u n i f o r m l y ) is c o n t a i n e d in a t i e r e d s a m p l e h o l d e r B c o m p r i s i n g t h r e e pans 10 m m in d i a m e t e r a n d 2 m m deep. T h e pans are m a d e of p l a t i n u m foil a n d s t a c k e d one a b o v e the o t h e r , leaving a gap of a b o u t 1 m m b e t w e e n each pan. T h e t h r e e t i e r e d s a m p l e h o l d e r is s u s p e n d e d on one a r m of an electronic r e c o r d i n g type m i c r o b a l a n c e C ( S a r t o r i u s m o d e l A.n.ll) with the help of a 0 . 2 m m d i a m e t e r h a n g d o w n wire D m a d e of p l a t i n u m p l a t i n u m / 1 3 % r h o d i u m alloys. T h e o t h e r a r m of the b a l a n c e carries a silica p a n E for t a r i n g the w e i g h t of t h e s a m p l e and t h e c o n t a i n e r located in the furnace. T h e 0.2 m m d i a m e t e r h a n g d o w n wire carrying the t i e r e d s a m p l e p a n passes t h r o u g h a 4 m m internal d i a m e t e r ( I D ) silica tube F. F serves two p u r p o s e s : s u p p o r t i n g the t h e r m o c o u p l e G with the tip located n e a r the s a m p l e pan; carrying the gas e n t e r i n g f r o m the r e a r p o r t H in the b a l a n c e h o u s i n g to the s a m p l e site after getting p r e h e a t e d to the b~.mperature of t h e s a m p l e on passing t h r o u g h the t e m p e r a t u r e g r a d i e n t along its p a t h , T h e c e n t r e t u b e F also facilitates location of the baffles I in a position which helps to i m p r o v e the t e m p e r a t u r e u n i f o r m i t y in the s a m p l e r e g i o n a n d also to m i n i m i z e the diffusion c o n t r i b u t i o n to the ~otal m a s s loss of the sample. O f the t h r e e baffles located i n t h e u n i f o r m t e m p e r a t u r e region of the reaction tube, t h e first is m a d e of Pt-20% R h alloy a n d the o t h e r two of
S,R. Dharwadkar et al./Thermochim. Acta 217 (1993) 175-186 /~-D
/~-C
CAPILLARY FLOW METER POTENTIOMETER OR RECORDER
H COLD JUNCTION
O -RINGS
TO ROTAME TE|
CONSTRICTION
~
A K
lACE
Fig. 2. Diagram of the transpiration apparatus incorporating the microthermogravimetric system: A, sample; B, tiered sample holder; C, electronic recording-type microbalance; D, hangdown wire (0.2 mm in diameter); E, silica pan for weight taring; F, 4 mm internal diameter (ID) silica tube; G, thermocouple; H, rear port in the balance housing; I, baffles made of (i) P t - 2 0 % R h plate and (ii) alumina discs; J, 14 mm ID silica tube; K, capillary (1.5 mm diameter hole); L, 28 mm ID silica tube; M, 8 mm diameter hole; N, layers of platinum coating.
recrystallized alumina. T h e s e baffles have a 3 m m d i a m e t e r side h o l e , w h i c h guides• the t h e r m o c o u p l e s l e e v e and facilitates the location o f the t h e r m o e o u p l e near the s a m p l e . T h e s a m p l e pan is s u r r o u n d e d by the 14 m m I D silica tube J. T h i s tube is constricted a b o u t 2.5 cm from its closed end. T h e constriction acts as a support to the baffles, w h i c h can slide• freely over the central 4 m m ID silica
S.R. Dharwadkar
ct
al./Thermochim. Acta 217 (1993) 175-i86
181
tube and the t h e r m o c o u p l e sleeve fixed on its side. The baffles sitting on the constricted part i m p r o v e the uniform t e m p e r a t u r e in t h e s a m p l e region and also minimize the diffusion of the v a p o u r in the u p w a r d direction because these virtually isolate the sample region f r o m the rest of the reaction tube, T h e closed end of the 14 m m d i a m e t e r silica tube is provided with a 1.5 m m d i a m e t e r hole K. The carrier gas entering the reaction c h a m b e r through the 4 m m I D central tube sweeps the v a p o u r out t h r o u g h this hole, which then condenses on the inner and o u t e r surfaces respectively of the 2 8 and 14 m m I D silica tubes in the annular space, in the t e m p e r a t u r e g r a d i e n t . T h e 28 m m ID tube L surrounding the 14 m m I D tube is closed at one end and provided with an 8 m m d i a m e t e r hole M a b o u t 15 m m f r o m i t s open end. T h e two tubes are fixed to the balance h o u s i n g t h r o u g h a p p r o p r i a t e m e t a l collars and O-ring fittings. T h e carrier gas transporting the v a p o u r t h r o u g h the 1.5 m m d i a m e t e r hole located at the d o s e d end of the 14 m m d i a m e t e r silica tube comes out through the 8 m m d i a m e t e r side hole M in the 28 m m I D tube and passes through the r o t a m e t e r connected at the outlet. T h e o u t e r surfaces of both the 14 and 28 m m I D tubes are coated with a p l a t i n u m layer near their closed ends. T h e metallic layer on the 28 m m I D tube is e a r t h e d to avoid accumulation of static charge caused by the gases r u b b i n g on the silica surfaces. M E A S U R E M E N T O F V A P O U R P R E S S U R E O F CdCI~
To check the p e r f o r m a n c e of the present system the v a p o u r pressure of a n h y d r o u s c a d m i u m chloride (CdCi2> was m e a s u r e d in the t e m p e r a t u r e range 713-833 K in flowing pure argon gas dried by passing over a n h y d r o u s m a g n e s i u m perchlorate. CdCl2 is k n o w n to vaporize congruently, g i v i n g p r e d o m i n a n t l y a single molecular species CdCl2(g) in the v a p o u r phase [4,5]. in the present e x p e r i m e n t s a n h y d r o u s CdCI2 ( A n a l a R grade) (obtained by d e h y d r a t i o n of the hydrate and stored in a m a g n e s i u m perchlorate dess~.c~,tor before use) was heated to the desired t e m p e r a t u r e in flowing d r y argon. T h e t e m p e r a t u r e was maintained constant to 4-1 K with the help of a p r o p o r t i o n a l - t y p e t e m p e r a t u r e p r o g r a m m e r ( I n d o t h e r m Model MPC-500). A t zero time after the t h e r m a l equilibrium was established the total mass loss indicated on the L E D m o n i t o r of the control unit of the electronic balance was tared and the continuous m o n i t o r i n g of the mass loss was started as a function of time. A typical mass loss record for CdCI2 at 762 K is shown in Fig, 3, The isothermal rate o f m a s s l o s s w a s found t o b e i n d e p e n d e n t of time at each t e m p e r a t u r e ; this is typical of the vaporization process. Figure 4 shows a plot of the a p p a r e n t pressure of CdCI_, as a •function o f flow rate at 773 K. The v a p o u r pressures were calculated from eqn. (1). The various quantities m e a s u r e d and the final values of the c o m p u t e d pressures are listed in Tables 1 and 2. I t c a n be seen f r o m Fig. 4 that the a p p a r e n t
S.R. D h a r w a d k a r et aL/Thermochim. Acta 217 (1993) 175-186
182 START'
¼ !
=
03 O ..J O't
TI ME,~=,.
Fig. 3. A typical m a s s loss vs. time trace recorded at 762 K ( f l o w rate 2490 ml h r - i ) .
vapour pressure of CdCI2 is virtually independent of the flow rate in the region 1 7 0 0 - 2 6 0 0 m l hr -t, implying that the equilibrium b e t w e e n the vapour and the solid is maintained in this flow rate region [3], The experiments at other temperatures were performed at the flow rates of 2100 m l / h r - ' . Figure 5 shows a plot of l n ( P / P a ) vs. 1 / T for the vaporization of CdCI2. The vap:.~ur pressure of CdCI2 as a function of temperature could be expressed in this range by the equation l n ( P / P a ) = - ( 2 0 109.10 4- 262.32) + (29.46 4- 0.344) T 46.0
T=773K
4 2.0 .
u~
38.0
f.n
~
~.
• 34.0 30.0"
~Z
26.0
~D.
22.0 18.0 5O0
I
I
1500
i
I
I
ZSO0
FLOW RATE OF ARGON{m; Fig. 4. A plot o f apparent pressure v~,, ~0
183
S.R. Dltar~vadkar et aL/Thermochim. Acta 217 (1993) 175-186 TABLE
1
A p p a r e n t p r e s s u r e o f CdCl2(g) as a f u n c t i o n o f flow rate o f a r g o n at 773 K F l o w rate o f argon (ml hr -~)
Volume of argon gas s w e p t (ml)
M a s s loss o f CdCl2 p e r litre o f a r g o n ( m g l -I)
Apparent p r e s s u r e (Pa)
1040 1310 1720 2010 2280 2520 2890 3430 3890 4080
1080 1420 2220 2010 2590 3180 4180 4890 6320 6950
2.87 2.66 2.31 2.30 2.22 2.25 2.15 1.98 1.83 1.78
38.5 35.7 31~0 30.9 29.8 30.2 28.8 26.6 24.6 23.8
The enthaipy of sublimation of CdCI2 derived from this plot at the m e a n t e m p e r a t u r e of m e a s u r e m e n t s is 167.2 ± 2 . 2 k J m o l - ' . This is in excellent a g r e e m e n t with the value of 166.5 ± 4.7 kJ mol-~ o b t a i n e d by Skudlarski et al. [5] in their recent K n u d s e n effusion mass spectrometric m e a s u r e m e n t s in the t e m p e r a t u r e interval about 100°C below that in the present study. The o b t a i n e d in the third law value of the e n t h a l p y of sublimation AHz~t.,,,b~ ° present w o r k from the v a p o u r pressures m e a s u r e d at different t e m p e r a t u r e s TABLE
2
T h e e n t h a l p y o f s u b l i m a t i o n o f CdCi., f r o m v a p o u r p r e s s u r e d a t a Temperature {1< ~
Mass t r a n s f e r per litre o f c a r r i e r gas ( m g i -I)
Pressure (Pa)
AH~.~c,..h~ ( k J tool - ')
713 729 744 745 760 768 773 788 801 816 833
0.27 0.49 0.86 0.86 1.40 1.88 2.31 3.76 5.91 9.65 17.96
3.6 6.6 11.5 I 1.5 18.8 25.2 31.0 50.4 79.3 129.5 241.0
176.8 176.8 176.6 176.9 177.0 176.8 176.5 175.4 176.0 175.5 I74.6
( t h i r d law)
A v e r a g e 176.4 ± 0.7
S.R. Dharwadkar et al./Thermochim. Acta 217 (1993) 175-186
184
6.o • THtS WORK h B L O O M A N D W E L C H ( R e f . 15~ E) S K U D L A R S K | et. at (Ref. 5 ) x NIWA (Ref.14.. p.427
o
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-5.0
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i
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;
I
,
~
,
1.30
I
~
,
l
1.40
1.50
1.60
1.70
103 K . / T Fig. 5. A plot of ln(P/Pa) vs. 1 / T for CdCI2. TABLE
3
T h e enthalpy of sublimation of CdCI2 from various investigators
Method of measurement K n u d s c n ulTusit~n Tran~l~irttil,~n K n u d s u , ctfJ,, h~, Transi;i!:~li::,~
Transpiration Transpiration
Ref.
AH,.*,~(.~,b, (kJ m o l -~)
Second law
Third law
187.9 219.1 191.0 193.1 180.0 188.6
177.8 1~7.5 182.3 177.1 189.6 176.4
4- 14.5 4- 1.6 4- 1.7 4- 16.5 4- 6.2 4- 2.2
4- 0.4 4- 8.2 4- 0.8 4- 1.3 + 1.9 4- 0.7
5 13 14 15 16
This work
a n d e m p l o y i n g the free energy functions used in ref, 5 was f o u n d to be 176.4 ± 0.7 kJ m o l - ' . Similarly, the s e c o n d law value for the enthalpy of sublimation o f CdCI2 derived from the enthalpy i n c r e m e n t data used in ref. 5 was 188.6 ± 2.2 kJ m o l -t. T h e s e values are in g o o d a g r e e m e n t with those c o m p i l e d by S k u d l a r s k i et al. [5] from the w o r k o f various investigators (Table 3).
S,R. Dharwadkar et al./Thermochim. Acta 217 (1993) 175-186
185
DISCUSSION
T h e r e have b e e n several m e a s u r e m e n t s of the v a p o u r p r e s s u r e s of CdCI2 e m p l o y i n g both t r a n s p i r a t i o n and K n u d s e n effusion techniques. T h e results o b t a i n e d by various investigators have recently b e e n s u m m a r i z e d in ref. 5. T h e exeellent a g r e e m e n t b e t w e e n o u r value of the e n t h a l p y of sublimation of CdC12 (167.2-+ 2.2 kJ mol -i) and the value of Skudlarski et al. [5] at the m e a n t e m p e r a t u r e of the two studies by i n d e p e n d e n t t e c h n i q u e s establishes t h e accuracy of o u r m e a s u r e l n e n t s . T h e results o b t a i n e d by o t h e r investigators e m p l o y i n g t r a n s p i r a t i o n [15] and K n u d s e n effusion t e c h n i q u e s [5, 14] are i n c o r p o r a t e d in Fig. 5 along with o u r results for c o m p a r i s o n . It can be seen that the e n t h a i p y o f sublimation o b t a i n e d in the t r a n s p i r a t i o n e x p e r i m e n t s is h i g h e r in some eases t h a n t h a t derived from K n u d s e n effusion m e a s u r e m e n t s . This could p e r h a p s be due to the different t e m p e r a t u r e intervals over which these m e a z u r e m e n t s w e r e m a d e . M o s t of the t r a n s p i r a t i o n m e a s u r e m e n t s w e r e m a d e up to the trielting point of CdCI,.. T h e v a p o u r p r e s s u r e s of c a d m i u m - b e a r i n g species n e a r the melting point of CdCl~ are well b e y o n d the K n u d s e n effusion range and, f u r t h e r m o r e , the c o n t r i b u t i o n of the d i m e r to tbe total p r e s s u r e is also significant. Both these factors would result in an incrt.'ased slope of the plot of l n ( P / P a ) vs. 1/T and h e n c e the derived e n t h a l p y of vaporization. A l t h o u g h Skudlarski et al. [5] in their m e a s u r e m e n t s have r e p o r t e d t h a t the v a p o u r of CdCI2 consists p r e d o m i n a n t l y of the m o n o m e r in their range of t e m p e r a t u r e m e a s u r e m e n t , Moss [16] has o b s e r v e d a b o u t 15% d i m e r c o n t r i b u t i o n in the v a p o u r a b o v e solid CdCI~ n e a r its melting point. In o u r p r e s e n t t r a n s p i r a t i o n d a t a t h e r e is significant difference in the m e a n e n t h a l p y of v a p o r i z a t i o n derived from the l n ( P / P a ) vs. 1 / T plot if the value of the p r e s s u r e derived at the highest t e m p e r a t u r e of the p r e s e n t m e a s u r e m e n t (833 K) is included in the linear least s q u a r e s analysis of the data. T h e m e a n value for the e n t h a l p y of sublimation u n d e r this condition w o r k s out to be 170.9-+ 3.1 kJ moi -1. T h e d i m e r c o n t r i b u t i o n to the total p r e s s u r e at this t e m p e r a t u r e is k n o w n to be m o r e t h a n 10% [13, 16], so the point at 833 K was ignored in the analysis of the p r e s e n t data. CONCLUSION
It is seen f r o m the p r e s e n t work t h a t the a u t o m a t i c r e c o r d i n g t r a n s p i r a t i o n system d e s i g n e d a n d fabricated in om l a b o r a t o r y yields most reliable d a t a of a high d e g r e e o f accuracy i , the s h o r t e s t possible time. This is the first time that the m i c r o t h e r m o b a l a n c e has b e e n a d o p t e d in on-line m o n i t o r i n g of the mass loss in a c o n v e n t i o n a l t r a n s p i r a t i o n e x p e r i m e n t . T h e design of the system is simple and with little alteration the unit can be used for n o r m a l t h e r m o g r a v i m e t r i e investigations. T h e r e l i a b i l i t y of the results
J86
S.R. Dharwadkar et al./Tiiermochim. Acta 217 (1993) i75-i86
obtained with this instrument has recently been demonstrated in our studies on the vaporization of molybdenum trioxide [17]. Several investigations on the thermodynamics of tratisiti0n metal molybdates are currently in
progress using this sy'~l~!m, The t et!iperttttll't~ i~taiige of measurement in the present apparatus can be increased substantially by replacing the silica tubes by those of recrystald alumina. This is ctiri'ently being attempted. REFERENCES I R.V. Regnault, Ann. Chim., 15 (1845) 129. 2 M. "~ -,r,baum an d P.D. H u n t , J. Nucl. Mater., 34 (1970) 86. 3 U. ~ .~ten ilild ~V,E. Bell, in J.L. Margrave (Ed.), The Characterization of High T. q" a~ure Vap0urs, Wiley, New York, 1967 p. 91. 4 K. Hilp;rt, in Struciure and Bonding, Voi. 73, Springer-Verlag, Berlin, 1990, p. 163. 5 K. Sku~larski, J: Dudek and J. Kapala, J. Chem. Thermodyn., 19 (1987) 857. 6 P.G. Wahlbeck, High Temp. Sci., 21 (1986) 189. 7 S.N. Tripathi, A.S. Kerkar, G. Chattopadhyay and M.S. Chandrasekharaiah, J. Am. Ceram. Soe., 68 (1985) 232. 8 S.R. Dharwadkar, S.N. Tripathi, M.D. Karkhanavala antl M,S. Chandrasekharaiah, in Thermodynamics of Nuclear Materials, Int. Atomic Energy Agency Syrup. No. 190/32, Vienna, 1975, 455. 9 0 . M . Sreedharan, S.R. Dharv~adkar and M.S. Chandrasekharaiah, Bhabha Atomic Research Centre, Internal Rep. No. 1-239, 1973. 10 Internntional practical temperature scale, Metrologia., 5 (1969) 35. 11 O.M. Sreedharan, A.S. Kerkar and M.S. Chandrasekharaiah, Bhabha Atomic Research Centre, Internal Rep. No. 1-261, I973. 12 K.A. Sense, M.J. Snyder and J,W. Clegg, J. Phys. Chem., 58 (1954) 223. 13 F.J. Keneshea and D.D. Cubicciotti, J. Chem. Phys., 40 (1964) 1778. 14 O. Kubaschewski and C.B. Alcock, Metallurgical Thermochemistry, 5th e d n . , Pergamon, Oxt'ord, 1979, p. 427. 15 H. Bloom and B.I. Welch, J. Phys. Chem., 62 (I958) 1594. 16 H.I. Moss, Thesis, Indiana University, 1960; quo:,.,d by L. Topor, Rev, Roum, Chim., i7 (1972) 1503. 17 M.S. Samant, A.S. Kerkar, S.R. Bharadwaj and S.R. Dharwadkar, J. Alloys Compounds, 187 (1992) 373.