Patrick G. McCormick Marquette University Milwaukee, Wisconsin 53233
Reaction Rate of Ethyl Acetate Hydrolysis by Osdlometry
High frequency conductance, or 0scillometry, is usually introduced to the student in a physical chemistry course, or one in instrumental analysis. Especially in the latter case, not only is the theory presented, but also the practical situations in which the technique is applied to advantage. Among the most important advantages of the technique is complete avoidance of solution-electrode contact. Another is the sensitivity to small changes in the sample conductance and the speed with which those changes can be detected. Unfortunately, aU too often, upon entering the laboratory and finally confronting the instrument, the student finds that the experiments he is to perform do not take advantage of these features. Indeed, one of the most common experiments performed with the oscillometer is a titration, not even illustrating these features. To be sure, the determination of loading curves does illustrate pertinent theory, and measurement of dielectric constants and small concentrations are typical applications of oscillometry. To the student, however, they offer little challenge, and provide little data. A more pertinent experiment is the determination of the second-order rate constant in the alkaline hydrolysis of ethyl acetate. Such an experiment not only takes full advantage of the unique features mentioned previously, but also provides values which lend themselves to analysis by statistical measures. The experiment demonstrates vividly an important instrumental method, offers a good introduction to automatic recording of results, and provides data of real significance from which much additional information can he extracted. The a ~s~~ l i c a t i oofn hieh freauencv oscillators to the measurement of rate constants has been described previously (1-S), and their suitability for such determinations has been fully established. These workers made use of rather elaborate temperature-regulating equipment which pumped the sample solution (I) or the thermoregulating liquid (2) through a cell placed within the coil of a tuned oscillator circuit. The presence of hydroxyl ions in the solution caused a change in the capacitance of the solution and a corresponding imbalance was indicated in the oscillator circuit. As hydroxyl ions were consumed in the reaction, the circuit indicated a reduction in the imbalance condition. In effect, then, the oscillator monitored instantaneously the concentration of hydroxyl ions in solution. This indication was of a nature which could easily be displayed on a strip-chart recorder providing a record of time versus a function which could he related to concentration. Mathe~~L
558
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matical analysis of the cuwe provided the desired rate constant. The entire experiment took 20 minutes or less. Such an experiment seems ideally suited to the student laboratory. The primary drawback is the elaborate nature of the temperature-regulating and circulating system. Nonetheless, an attempt was made to adapt the method for use with the Sargent Model V Chemical oscillometer, an instrument found in many laboratories. During initial studies two important discoveries were made. First, if all solutions, the instrument, and the cell are a t room tempera-. ture to begin with, and one uses a large volume of sample (ethyl acetate) solution, the addition of a small volume of sufficiently dilute NaOH will not cause any significant temperature change during the relatively short time required for the reaction to take place. It would appear that the cell plus sample solution provide sufficient capacity for temperature compensation under these conditions and that no more elaborate regulation is necessary. I t should be noted, of course, that one does not have the ability to choose a particular temperature, being forced to work a t ambient. However, one need not be concerned about significant deviations from that. temperature during the course of a single experiment. Second, if the NaOH can be added and efficiently stirred into the solution in the cellwithin the risetime of the recorder (1-2 sec in this case), extremely wellshaped decay curves can be obtained which provide data amenable to direct calculation of the rate constant. With careful technique, the experiment gave theoretical results at two temperatures for which literature values were available. In the hands of students, excellent results were achieved, and in cases where results deviated from theoretical values by more than could he accounted for by experimental error, the evaluation of probable sources of error provided further insight into the experiment and the technique. Literature values for the rate constant were available for temperatures of 25' and 30°C. An Arrhenius plot of log k versus 1 / T was prepared from these two values and values a t other temperatures near this range were obtained for comparison with experimental results. The agreement is quite good. (See Table 2 below.) Ethyl acetate was chosen for study in developing this experiment, but other esters can undoubtedly be used as demonstrated by Flom and Elving (2). Conceivably, an unknown ester could be given and its identity deduced from a calculation of the rate of its alkaline hydrolysis.
Experimental
Procedure
Apparatus
A stock solution of 0.10 F NaOH was prepared and standardized against potassium acid phthalate in the nsttsl manner. A stock solution of ethyl acetate approximately 0.05 F was prepared by diluting 10 ml of reagent grade material to 2 liters with distilled water. The exact concentration of this solution was determined by completely hydrolyzing a known quantity in a measured excess of NaOH, hack-titrating the excess with standard acid. This determination, due to volatility of ethyl acetate, should be carried out every one or two days, since the exact value is needed in the cdculrttions. These concentrations, though not those re* ommended by Flom and Elving, were found to give the h a t curves, and resulted in the best rate constant values Prior to performing the experiment, a11 solutions and equipment were allowed to reach temperature equilibrium for several hours in the laboratory. The oseillometer was turned on a t least one hour before use to reduce the likelihood of drift during a run. Just before starting the run, the oscillometer meter was set to mid-scale (zero current) using the center adjust and zero set controls. When exactly balanced, the recorder pen was set a t zero using the recorder zero control. The center adjust control on the oscillometer was then used to move the meter needle to ahout 16 to 17 pA to the left of zero. If the recorder did not deflect upscale, the recorder-input leads were reversed. (Deflections on the meter during a run were always to the left of zero.) The range attenuator on the recorder was used to bring the pen to the top of the scale. This would allow the values ohtained during a run to use as much a9 possible of the chart. The recorder chart meed was set to Medium I1 . in./min). . . E:xartly 100.0 ml of the ethyl acetate solution wa-5 pipetted into the cell, ~ n rhe d temperature was read with a thermometer. The center ndjust and Zen) set mntrola on the oscillometer were used to return the meter needle to zero. (Note that throughout the entire experiment none of the variable capacitors are used, nor is the internal standard.) If the recorder pen did not return exactly to zero, it was adjusted accordingly. The svrinee was filled with exsctlv 1 ml of base. no air bubbles heing t;ap@d. The recorder was"started seveisl subdivisions before a major grid line on the chart paper, and the syringe and stirrer were poised over the cell, one in each hand. Just as the pen reached the bare of a major uharr division, rhc contents of the yringe were rapidly ejected direvtlg into the solution in the cell, and the stirrer was nlunmd into the cell. to ahout ' 1 , of its denth. One ouick un-anddown motion was d e . and the stirrer \as removed from the cell. If done quirkly, the pen had nor renrhcd the peak of its travel irp the chart lwfure the stirrer was removed from the cell. A looj~filtingrap (an aluminum foil moisrure cup serves very well) was carefully placed on top of the cell to reduce volatility losses of ethyl acetate during the experiment. The system was allowed to remain undisturbed for 18 to 20 ruin, durine which time the recorder nroduced a. trace similar to that bhown ill Figure 2. Thp rene;ion aaq considered rmnplete when no change was observed in the recorded vnlue for 2 to 3 min.
The instrument used was the Sargent Oscillameter, Model V, equipped with a large cell and holder. The instrument was connected to a Sargent SR recorder through the recorder jack on the back of the ascillometer. A shielded cable was constructed which connected to pins 1 and 3 of the female plug on the instrument, the ground wire connecting to the instrument chassis via, the plug casing. The recorder was used without a range plug, in which configuration it had a 125-mV range. The oscillometer output was still too large for accommodation by this range, so a 1:200 voltaee divider was constructed from orecision l-Mee and 5-
scale on the recorder by reducing the range by means of the range ~ ran be arr~mmodaredhy attcnunrion vontrol. L R I ~ CrGrentq using larger voltage dividers, I:&O0 heiug sl~ltir.ent to display thr rntlre ourput of the o:(.illom~ter cimtir. It .h011111 tm mted thnt rhic nrmngemntt COIIIIPCIS the reronlrr to thp o~rillomerrr i r i a d n j u t t I n t . Thus, the revorder will follow accurately rurrrnt vdurs whirh are olT+.nle on the mrter. As noted above, two requirements were necessary to ensure well-formed curves: rapid, accurate base addition, and rapid stirring. Flom and Elving added base with s. pipet, starting the recorder a t half-addition. In the present work, a more rapid method was sought which offered at least the accuracy of pipets. It was discovered that disposable plastic Tuberculin syringes' without needles, worked extremely well. The syringe has a, one ml capacity, and is not affected by the NaOH solution. Without a needle attached, it om he emptied very rapidly. The graduations allow the plunger to he set quite reproducibly. To test the accuracy of such a method, a randomly selected syringe was calibrated for delivery using water and weighing the ejected amount. Ordinary care was exercised, hut no special precautions or techniques were employed. As shown in Table 1, the syringe showed an accuracy and precision superior to what might be expected of a Class A l-ml pipet. Table 1. Trial
Calibration of Plastic I-ml Syringe Weight dispensed (g)
Temperature = 2:3.0eC; density of water - 0.00757 e, ml. averare weight drlivered = 1.00107 g; average volnme delivered = l .(kl34 ml; s t d drv. = 0.0001; ml. The design of the cell, heing of annular crass-section, makes stirring difficult. Glass stirring rods could damage the cell, and magnetic stirring would not he efficient, or rapid. A plastic stirring rod was used with some success, hut, especially in the hands of students, often proved inefficient as indicsted by poorly formed curves. A plastic stirring apparatus was fabricated from Teflon and Nylon as shown in Figure 1. The Teflon disk was a/* in. thick, though this dimension is not critical. The Nylon handle was made from '/&in. diameter stock, cut down to '/ain. at one end to fit into one of the small holes as shown. The handle was 7 in. long. This stirrer fits into the annular cell, and two rapid updown motions are sufficient to affect complete stirring of the cell contents. This motion a n he produced, with very little practice, in far less than the time required for the recorder pen to resch its peak value.
'Available from American Hospital Supply, Evttnston, Ill., or from Pharmrtsesl Laboratories, Glendde, Calif.
Figure 1.
Dotoilr d plarticstirring apparatus. Volume 48, Number 8, August 1971
Table 2.
2
TIME (MINI
Figure 2. Typieol recorder tracing during a wn 0.000986 F, tamp = 25'C).
10
= 0.0482 F, b =
Calculation
The data treatment is fully developed by Flom and Elving (%), and will only be outlined here. The curve is extrapolated to give values at time zero (6) and infinite time (i,). The initial concentration of base is proportional to lie - i-1, and the decrease after time t is proportional to (io - it)/(& - i-), which expression is written as a for convenience. It is shown by Flom and Elving (%) that substitution of this expression into the second-order equation results in the following expression
where a and b are the concentrations of ethyl acetate and sodium hydroxide, respectively. This expression originally derives from MacImes (4) and was utilized by Jensen, Watson, and Beckham (I) for similar data. A plot of t versus the log term of the equation should yield a straight line, the slope of which is 2.303/k(a - b) for a second-order reaction. Time and current (arbitrary recorder units) data are read from the chart, and the log term calculated for each current value. These data are plotted and the slope of the straight lme is used in determining the rate constant. Since this is a very time-consuming task, a short FORTRAN IV computer program was written to take time and current data directly, and calculate a least squares slope of the plot mentioned above. From this value, the computer evaluates the rate constant. (A copy of the program will be sent on request.) It should be mentioned that lmearity in the plot only extends for 13 to 15 min of data, so points beyond that value should he checked for fit, lest the slope be unduly influenced by them. Results
As mentioned previously, values for the rate constant for ethyl acetate hydrolysis a t 25°C and 30°C are given in the literature. Theoretical values a t other temperatures can be deduced from a plot of log k versus 1/T constructed from these two known values. In Table 2 are shown some typical results obtained both by the author and by senior undergraduate and first-year graduate students. Comparison is made with theoretical values determined as described above. It can be seen that values correspond quite well with theory. It is expected that other esters would give similarly satisfactory results. Flom and Elving 560
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lournal of Chemical Education
Rate Constant Values for Alkaline Hydrolysis of Ethyl Acetate
Ternperature ('C)
This Work (l/rnole-sec)
Theoretical (l/mole-sec)
24 25 28 30 32
0.104 0.110 0.134 0.145 0.158
0.104 0.110 (8, 8 ) 0.132 0.145 (1, 9) 0.1.55
(9) give values for the rate constant of hydrolysis of methyl and ethyl chloroacetate a t both 20°C and 30°C, so these compounds might also be used as described in this paper. The rates for these hydrolyses are much faster than for ethyl acetate, and the experiment would he expected to he proportionally more challenging. Conclusion
As described here, the high frequency conductance method can he used to determine a t ypical secondorder rate constant with very satisfactory results by any careful student. The experiment offers many significant improvements over other typical oscillometry student experiments (5-7) including (1) a clear demonstration of some unique advantages offered by the technique itself, (2) the opportunity for the student to obtain very close to theoretical results in a reasonable length of time, (3) the opportunity to compare classes of similar compounds such as acetates and chloroacetates, or ethyl acetate and ethyl proprionate, and (4) the possibility of determining an unknown from a study of its kinetic behavior. Additional features of the experiment include an opportunity to use a computer as a computational tool to greatly reduce the time required for calculation. The program used in this calculation is not a t all complicated and anyone with any FORTRAN skills could easily write his own. Finally, by making measurements of the rate constant a t two temperatures, additional thermodynamic information can be deduced such as the energy of activation for the reaction. The experiment is quite versatile, and can be presented in several ways to serve the best interests of a variety of course objectives. Acknowledgment
The author wishes to express his appreciation of the time and helpful suggestions offered by the students who ran this experiment. Special thanks are due P. J. Lamothe. Literature Cited F. W.. WATSON,G. M..
(1) Jassew.
-1770 ..- (10S1> - -- .,.
AND
BEOIEAM,d.
B., ~ n d Chcm., . 23,
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(2) Fmu, D. G., AND ELVINO, P. d., A d . Cham., 25, 541 (1953). (31 EWINQ, P. J.. Awn LArsrrz, I., J . Amcr. Chcm. Soc., 77, 3217 (1955). (4) MAOINNEB,D. A.. "The Principles of Electroohemistry;' Reinhdd Publishiw Corp., N e w York, 1939, pp. 378--80. (5) MELOAN, C. E.. AND KIBER,R. "Probleme and Experiments in Inatrumental Analysis." Merril Books. Inc.. Columbus, Ohio, 1963, pp.
W..
-.
,A""G G S
H. H.,MERRIPT,L. L.. AND DEAN,J. A,, "Instrumental Method. of Analysis," 4th ed.. D. Van Nastrand Co.. Prinoeton. N. J . . 1965, pp. 741-2. (7) D m * a ~ r ,P., "Instrumental Analysis," The Maomillan Co., New York, 1957, p. 355. , G., Tmna. Famday Soe., 40, 352 (1944). (81 D n e r a ~ K. (9) Rmcnss, L. T., Ann.. 238, 276 (1887). (6) W r m m o .
