Ronald E. Marcotte Texas A&l University Kingsviile, TX 78363
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A Functional Fast Flow Kinetics Apparatus
I t is always a welcome event when advanced concepts or techniques can be included in the physical chemistry laboratory without involving or fabrication of exotic .the purchase equipment. In our own case, it was desired to introduce the more advanced techniques used in the study of the kinetics of fast reactions without eettine into the cost and comnlexities of an accelerated flow or stopped flow apparatus, oscilloscope cameras, etc. The continuous flow system described by Haggett, Jones, and Oldham1 seemed to be the ideal answer to our teaching needs since it utilized a "clock reaction" whose end point could be visually observed in a glass flow tube. Student response to the experiment as presented in Shoemaker, Garland, and Steinfeld2was initially very enthusiastic, but this enthusiasm soon turned to frustration when attempts were made to obtain kinetic data. The experiment had to be withdrawn while changes were made to both the apparatus and mocedure. Althoueb the resultine annaratus. which is .. shown in thr fiqre, still I w r s il s~~prrilcial resemblance 111 the original runficuratiml. numerous rcfinemcnti ha\.e been incorporated that make i t significantly more stable and controllable in its operation.
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Apparatus Briefly, the original a p p a r a t u ~ lutilized .~ a water aspirator (A) to siphon the reagents RI and Rz into a T-bore stopcock mixing chamber (MI and down a Pyrex* reaction tube (G). Stopcock (M) was also intended originally to adjust the ratio of RI to Rz. The linear distance to the clock reaction's end point was measured and converted into a reaction time using the measured flow data. Stopcock (N) was originally a straight bore type and was used to regulate the overall flow rate. Modifications Thr prinripnl prohlems encountered with the original apparatus concrrned its insuffirient rontrol orer the rclntivc f l w rates from the two reagent cylinders, the inability to maintain the end point a t one location or even within the bounds of the reactiontube, and the overly sensitive control of the total flow rate and hence the end point position. As the figure shows, the 2 mm T-bore sto~cockwas retained. but not for the ournose of adjusting the reagent ratio. An ordinary three-way k o p k c k will not, of course, change the relative flow rates as it is rotated since both arms are closed equally. The first modification was to file notches, as~mmetricallv,on the sto~cockbore. This proved unsatisfactory since it was not possible to get the notches sufficiently deep and properly contoured to give enough differential throttling without simultaneously reducing the total flow rate below acceptable limits. The operation was also somewhat unpredictable due to small and varying amounts of grease which tended to accumulate in the notches. The solution to this problem was to add a separate stopcock (D) for each reagent line. This was accomplished readily by using the replaceable, plug-in, Teflon* stopcocks from our student burets. The particular ones used here were Kimax* brand. Bv carefully slicing off the O-ring seal at the reagent end of the stopcock, one H i left with a 5 mm O.D. by 7 mm tubulation that readily accepts the rubber tube from the reagent reservoir. The replaceable buret tip was removed and the 7 mm O.D. arm of stopcock (M) was fitted snugly in its place. The stopcock was drilled out to a 3 mm bore to avoid unnecessary restrictions in the reagent lines. The two reagent 388 1 Journal of Chemical Education
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flow rates could now he adjusted independently to any desired ratio without undue diminuation of the flow rate. Several simply constructed mixing chambers were tried in an effort to eliminate stopcock (M), but none matched the rapid turbulent mixing generated by the sharp, right angle passages in the T-bore stopcock. Since turbulent mixing shamens the end ~ o i n t the . sto~cockwas retained as the mixing chamber rW. Thisobsr-rvation isconsistent with that of Hartridre and Ihirhtun:' il 'l'-iunrtiun mixer is adequate for reactions with halilives than 0.01 sec. The other problem area concerned the difficulty in positioning the end point within the bounds of the reaction tube a t a sufficiently stationary position to permit the collection of meaningful flow data. For the formaldehyde/bisulfite clock reacti~n,'.~ the procedure had to be done during the 40-160 sec available for each run. The ordinarv 2 mm elass sto~cock which was usrd originall" at point ront;ol the o;wall flow rarey was found to he very critical in its adiustrnent. The end point wo~lldfrquently trnvrrse the entire t l , tubeaher ~ a seemingly minute adjustment had been made. This behavior coupled kith the factthat the system takes several seconds to come to equilibrium after each minute adjustment, meant that one or two runs might be wasted just trying to find a suitable setting. The setting would of course be different for the next set of reagent concentrations used. The stopcock was thus discarded in favor of a 10-turn Teflon" needle valve (N). The end point could now be systematically maneuvered to the desired position, although about one third of the available reagents were still generally consumed in the process. It was now ohserved that although the end point could be positioned easily, it simply would not stay in one place more than a few seconds. The trouble was traced in part to the accumulation of liquid in the vacuum line with intermittent siphoning action
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Hamett. M. L.. Jones.. P... and Oldham. K. B.. J. CHEM. EDUC.. 40,367-ii963). Shoemaker. D. P.. Garland. C. W.. and Steinfeld. J. 1.. "Exoeri.. ..., ... - .
Hartridge, H. and Roughton, F. J. W., Proc. Roy. Soc. London, A104,376 (1923).
and to variations in water Dressure a t the aspirator. The vacuum lines were arranged so as to avoid low spots where spent reagents would accumulate and subsequently siphon over. Additional suction flasks were added to increase the ballast volume and thus diminish the effects of rapid fluctuations in aspirator action. The dramatic improvement came, however, when the presently used 12 1 round-bottomed flask (B) was added. A mercury manometer had been attached temporarily to the vacuum line for diagnostic purposes. It showed that with this flask in place the aspirator now required about 5 min to hring the system to its ultimate vacuum of around 33 mm Hg, and it showed that the vacuum never varied by so much as 1 mm Hg during the course of a run. This stability was reflected in the behavior of the end point which could now be set rapidly and whirb would remainstationary for 40 to 50sec if nwessarv. This large flask sbuuki, of course, he wrapped with tape and placed insome sort of container for safety purposes. The overall flow r a t e in the 1m long by 2 mm id. flow tube (.G,) were imnroved sliehtlv - .hv-these modifications. Some representative flow rates for the formaldehyde clock solutions' are shown in the table. These were taken a t 24.7"C with the needle valve (N) approximately one half open and a differential drivine Dressure along the flow tube of 7.9 mm Hg. Rough estimates of the Reynolds number for these flow velocities ranged from 1800-6300, indicating that the system may be in a state of transition to laminar flow at the lowest velocities. Turbulent flow is the desired mode of operation since it produces a sharper end point and a less sensitive dependence of flow rate on net driving pressure and solution v i ~ c o s i t y In . ~ addition, errors due to deviation from ideal "mass" flow5 are significantly smaller under turbulent flow conditions. Chance, B., J. Franklin Inst., 229,445,613,737, (1940). Roughton, F. J. W., in "Techniqueof Organic Chemistry," (Editors: Friess, S. L., Lewis, E. S., and Weissberger, A,), Vol. 8,2nd Ed., John Wiley and Sons, New York, 1963, p. 178.
Some Representative Flow Data
Solution Formaldehyde Number 1 concentration. M
Volume Flow Linear Flow Rate. mllsec Velocitv. cmlsec
Another subtle advantage of needle valve (N) can now be seen. Since the end noint can now he ~ositionedreadilv near the end of the flow t h e for any givenmixture, the appkatus can be adiusted to merate under conditions of maximum flow rate on any given run. This adjustment puts the operation of the apparatus as far into the turbulent flow region as possible. The reactions tested include the iodine clock, done with hydrogen peroxide and with iodate ions, and the formaldehyde clock. We have found the formaldehydeihisulfite reaction with phenolphthalein indicat~r'.~ to he more satisfactory because of its sharper end point, ease of reagent preparation, less hazardous reagents, and economy of operation. Summary
The changes described above stabilize the flow system and permit the flow parameters to be set quickly and efficiently. The exneriment is still a little more challeneine" than most of the other physical chemistry lab experiments. Not only must the usual eood lab techniaue be used. hut also the adiustment and data collection must he done quickly and efficiently during the brief time available. The challenge is now well within the average student's ability, and the student response has been enthusiastic.
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Acknowledgment
The author wishes to express his gratitude to Robert Wheeler, Clayton Hammock, and James Remlinger for spending many laboratory periods testing the apparatus in its various configurations.
Volume 57, Number 5, May 1980 1 389
