Dennis D. Davls New Mexico State University Las Cruces, 88001
Kenneth L. Stevenson Indiana-Purdue University at Fort Wayne Fort Wayne, 46805
T h e study of t h e kinetics of chemical reactions i n which gases a r e evolved or absorbed a t constant pressure is usually accomolished throueh t h e use of a eas buret. A constant pressure is maintained with a leveling bulb a n d t h e volume of t h e gas is read visually. Perhaps because of t h e tedium involved in using the gas buret many kinetic problems involving gas volumes a r e avoided, a n d laboratory manuals of physical chemistry are usually lacking in such experiments. One exception is a n experiment o n t h e kinetics of t h e iodide-catalyzed decomposition of H ~ 0 2It. ~ is obvious t h a t a n automatic recording.. pas buret would not only save time and effort in such measurements, but would also permit more precise measurements of rates, particularly for fast reactions. Several systems for a u t o m a ~ i crecord& of gas volume have heen described' "n rerent years, hut s u r h systems are rather complicated, bulky, a n d hence n o t amenable to quick s e t u p a n d use. W h a t h a s been needed is a simple, y e t precise piece of anoaratus which is a . ~. a l i c a b l to e a wide varietv of chemical .. reactions. In order to c a n y out sensitive studies of t h e kinetics of nhotodecomoosition of water beine oerformed in this lab-. oratory, such a recording gas microvolumeter was constructed a n d p u t into use.
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The heart of the system consists of the glass apparatus shown in Figure 1.The volumeter tube, which is made from aealibrated 1-ml oioet. . contains mercurv which is oushed into the mercurv reservoir brhin'd the tuhe by r k r t w n gas entering through therhree.way stoprock ar the nght. T h ~ s ofrourse, , r a w e r a slight rise in the level of mercury in the reservoir, but I he diameter of the reswwir ( 5 cm) is large enough that the increase in pressure in the reaction system is only 0.5 torr when 1 ml of gas is delivered to the volumeter tube. Such an increase is negligible when the reaction is occurring at or near normal atmospheric pressure. A 30-gauge resistance wire (Nickelchromium Alloy A) runs concentrically down the length of the volumeter tube. The wire oenetrates rubber stoooers .. which seal the ends of the tube and tension is maintained by the tightener, which is a modified rubber tubing screw clamp, shown s t the left end of the tube. A constant current is supplied to the wire by the simple circuit shown in Figure 2. Although a more elaborate circuit would allow for zeroing the recorder and scale adjustment, this very simple circuit works well and consists of a 1.5 V battery in series with a 7.8 K resistor. As the mercury moves through the volumeter tube the length of unsborted wire changes proportional to the change in volume of gas; hence the voltage drop across the wire varies linearly with the volume of gas in the tube. By using either a gassyringe, or the graduated scaleon the side of the tube, acalibration of volume versus voltage can be made. Figure 3 shows a calibration plot using the graduated scale. Such a calibration can be made very quickly by using the bulb ta pasition the mercury s t the tenth-milliliter marks on the tube. However, one needs to apply a correction for the volume occupied by the wire. Another method of calibrating the instrument is to inject measured amounts of gas into the reaction vessel using a gas syringe.
~igure1. me microvalumeter apparatus.
volumeter
millivolt recorder
Figure 2. Cinuit diagram. R, resistance: E, banery
.
'Author to whom correspondence should he addressed. ZSalzberg, H. W., Morrow, J. I., Cohen, S.R., and Green, M. E., "Physical Chemistry, A M d e m Laboratory Course," Academic Press, New Ynrk. 405. - ~ 1969. -...,r ~ o.~ - , ~ 3Rohwedder, W. K., Reu. Sci. Instr., 31,1734 (1966). 'Tinker, H. B., Craddock, J. H., and Paulik, F. E., Reu. Sci. Imtr., 39,590 (1968). &St.John, M. R., Constabarid, G., and Johnson J. F., Rev. Sci. Imtr., 39,1582 (1968). Thompson, C. D.,and Hackermsn,N.,Reu. Sei. Instru., 44,1029 (1973). ~
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Figre 3. Measwed EW(mv) amsr k-3 volumeter tubewrrsusWlumeter scale reading. The gas delivery tube ends in a syringe needle so that the reaction vessels can be sealed off with serum caps as shown in Figure 1.This allows for the use of a wide variety of reaction vessels such as test tubes, flasks, and spectrophotometer cells. In order to maintain the response of the volumeter at a maximum, the dead volume in the vessel and delivery tube should be as small as possible. A flexible delivery tube of Xe in. i.d. Tygon bas proved quite convenient. The rubber bulb on the mercury reservoir serves a s a mercury pump. By operating the stopcocks in the proper sequence, thegas in the tube can be vented and the mercurv returned to the startine.. .DOd u n during the course of a reaction without admitting air to the reaction system. Thid isa desirable feature when mnre than 1 ml uf gas is evolved in a reaction. The 3-way stopcocks at the ends of the volumeter tube allow for the tuhe to be flushed with cleaning solutions and gases without disturbing the mercuryor the reaction system. This also permits the tube to he filled with reaction gas far reactions in which gas is absorbed. In fact, the design of the device allows for the measurement of gas evolution or absorption rates with equal facility.
This instrument should be useful not only t o chemists, but t o biologists, as well, for studyingsuch processes as biological oxygen demand. I t can be operated a t pressures other than the ambient atmospheric pressure by adjusting the pressures in the reaction vessel and in the mercury reservoir by a common pressurizing system. Another method of carrying out reactions a t pressures other than atmospheric pressure would be t o position the mercury reservoirs above or below the volumeter tube so that a difference in mercury levels, between the reservoir and the tube, could be maintained during the reaction. It should be emphasized that the device is still quite useful even without a chart recorder, since an inexpensive millivoltmeter can be used in its dace. The mierovolumeter has heen used very successfully in this laboratory to measure hydrogen evolution rates for reactions of the
M"+
+ H + l:M"+' + -21 Hdg)
Figwe 4. Voiumetet output in ml versrs inadiati~tima of 2.0 mi of 0.010 MCuCl in 1 MHCl at 20% 665 torr. The light source is a 200-W short-arc mercury lamp.
where M"+are such cations asTiat, V2+,Cr2+,Fez+,EuZ+,and Cut. Figures 4 and 5 show actual recorder traces from the photolmis of Cuf solutions in HCI and HBr solutions, respectively. The vertical axes have been calibrated to read out directly in ml gas. The stoichiometry of the reaction is
Since both runs were carried out using 2.0 ml of 0.010 M Cut solutions, a t a temperature of 20°Cand a pressureof 665 tom, one would have expeded 0.276 ml of gas to he evolved. The traces show 0.282 ml for the HCI run, and 0.280 ml for the HBr run, or, yields of 102 and 101%, respectively. A square, silica cuvet of l-cm path length fitted with a rubber stopper was used. The solution was stirred with amicro stirring bar a t a reasonably rapid rate. There is ashort period following initiation of the reaction when no gas evolves, presumably because the solution has not become saturated with Hn(g). Following this starting period, a fairly constant rate is maintained, from which quite re~roduceableinitial rates can be obtained. Several hundred runs such as these have heen made, and although t h deta~ls ~ oirhe copper p h m ~ h t m ~ s twill r ? be published elsenhere,
Figure 5. Volumeter output versus irradiation time of 2.0 ml of 0.010 M CuBr in 1 MHBr, same conditions as in Figure 4. the data have been good enough to arrive a t a fairly detailed picture of the mechanism. The device's greatest asset is that it so greatly improves the speed and facility of taking this kind of data that many experiments are attempted which would athenvise not be because of their extreme tedium or complexity.
Volume 54, Number 6,June 1977 1 395
