A Stabilized Direct-Reading John T. Stock
University of Connecticut Storrs
Conductance Apparatus
I
The hydrolysis of benzoyl chloride
M a n y of the applications of electrolytic conductometry involve measurements of the relative change in conductauce, rather than of the conductance itself. This change can be followed directly by applying a small lixed ac voltage to the electrodes and observing changes in the current that flows through the solution. Devices that employ low-frequency ac drawn from a lineoperated step-down transformer are useful for non-critical applications (1-3). A source of difficulty is inconstancy of readings, caused by line voltage drift or fluctuation. Controlled partial "clipping" of the low-voltage ac can be used as an alternative or adjunct to stabilization by feeding the device through a voltage-regulating transformer. Such clipping can be achieved by matched zener diodes arranged backto-back (4),or by a single zener diode that is fed through an inexpensive full-wave rectifier (5). Figure 1 is the circuit of a conductance device that uses single-zener regulation. The dependence of oub put voltage (and hence of apparent conductance) upon line voltage is about one-third of that encountered when regulation is omitted. Serving the dual purposes of
Figure 1. Circuit of conductance device. Ports lirt: Dl-bridge rectiRsr (Mallory F.W.501, D1-instrument rectifier [Conant type BJ, E-electrode 6.3-volt, 0.15-amp (General binding port.. F--fuse, '/camp, 1-lamp, 0 to 1-rnilliomp. RL-15-ohm [see text), R2Electric type 47), M-eter, 50-ohm (see text), Rr-25-ohm, S-line witch, T l r a n ~ f o r m ~ r12.6-volt , diode, 6-volt ISorkes Torzion output (Chicago Stoncor P8130J. 2-rener VR6AJ.
pilot and resistor that increases in value with voltage, the type 47 lamp slightly augments the regulating action of the zener diode. The device works well with an electrode system that can be constructed from two pieces of platinum wire in about 15 min (2). The well-ventilated Gin. cubical box used for assembly has a Lucite back to permit the components to be seen. Potentiometer Ra carries an inked scale and allows any ac voltage up to about 2.5 to be applied to the electrode binding posts. A wire link joining points X and Y is temporarily replaced by a milliammeter while the zener current is being set. This is done by successively adjusting R1and Rzuntil the voltage drop across R1 is 2.5 and the zener current is approximately 50 mamp. The variable resistors R1 and Rzcan then be replaced by lixed resistors of suitable values (typically 10 and 38 ohms respectively). Useful applications of the device are in conductometric titration (I), a study of the properties of strong and weak electrolytes (d), and experiments on the rate of hydrolysis of 2-iodo-2-methylpropane (6). A further example of the latter type is the determination of the energy of activation of the hydrolysis of benzoyl chloride in aqueous acetone. Archer and Hudson (7) used precision null-point conductometry and potentiometry in their careful study of this system. The hydrolysis cell (Fig. 2) is a screw-capped bottle approximately 25 mm in diameter and 60 mm high. Acetone-water mixture is introduced through hole A in the plastic cap. A short sleeve B of 6-mm outside diameter glass tubing is cemented into a second hole and serves as a guide for electrode system C. Rubber band D, cut from in. tubing, keeps the electrode system clear of the bottom of the cell. A horizontal spring clip mounted on the lower end of a vertical metal rod permits the cell to be supported so that the cap is well above the surface of the constant temperature bath. This is a 4-liter beaker filled with water and
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continuously stirred. The required temperature is obtained and maintained by the addition of crushed ice. Experimental Procedure
Adjust the bath temperature to 20.0°C and maintain this as closely as possible. Dilute 15 ml of distilled water to 100 ml with acetone and support the stoppered flask containing the mixture in the bath. Mount the hydrolysis cell in the bath and leave for about 30 minutes. Pipet 10 ml of the mixture into the cell. Connect the electrode leads to the binding posts of the conductance device and set the voltage control at 1.2.' Switch on. The meter reading- should remain at zero. Carefully remove the cell can and electrodes and hane Figure 2. Hydrolysis ud by the connecting leads. cell. use Microcap" ( ~ r & o n d Scientific Co., 500 Parkway, Broomall, Pa. 19008) to transfer 40 pl. of benzoyl chloride into the Remove the cell from the bath, temporarily close with a rubber stopper, shakevigorously for 15 sec, at once retun the cell to the bath, and replace the cap and electrode~.~ The meter reading should start to rise. When it reaches about 0.2 mamps, note the reading and simultaneously start a timer. At exactly 2, 4, 6, 8, etc., min after this, note the readings, stopping when about three readings above 0.7 mamps have been taken. Reject the cell contents, thoroughly rinse the cell and
a
1 This is the optimum voltage setting for an electrode system of average geometry. 'Beneoyl chloride should be taken from s freshly opened bottle. This compound is toxic and lachrymatory. Unless the Miorocqm is held almost horizontally, it will not fill with benzoyl chloride. Try out the filling and discharge before attempting a. run. A principal source of error is non-homogeneity of the reaction mixture. The drop of benzoyl chloride tends to sink before complete dissolution has occurred. 'The approximate ohsenmtion times for runs at 20°C and 15°C me 20 and 35 minutes respectively. A run at 10DCcan be attempted, hut at least 50 minutes must be allowed.
electrodes with water and then with acetone, and allow to drain (do not attempt to dry the electrodes). Make two repeat runs. Make a t least two runs at 15.0°C.' The reaction involved is first-order with respect to bensoyl chloride, and the velocity constant k is therefore given by: k=-
2.303 log (a -XI) (I- td (a - 21)
where a is the initial concentration of benzoyl chloride, while x, and x2 are the concentrations destroyed at times tl and & respectively. The values of x are proportional to the meter readings. If (& - tl) is fixed, then (a - xl)/(a - zl) is a constant e, and XI = c~ a ( l - c) (8). Suppose that the following results have been obtained: t (min): 0 2 4 6 8 10 ... z (mamp): 0.199 0.286 0.365 0.435 0.491 0.544 . . .
+
Take (& - tl) as 4 minutes. Then if xl is taken as 0.199, xs will be 0.365; if XI is taken as 0.286, xz will be 0.435. Hence construct a second table from the values of x above, e.g.,
Plot xl against zl and find c as the slope of the graph. Suppose this is 1.230. Then: k = (2.303/4) log 1.230 = 0.052 min-'. From the average values of the velocity constants a t the two temperatures, calculate the activation energy of the reaction. For 15% water medium over the temperature range 15" to 25"C, Archer and Hudson (1) found a value of 13,580 call. This apparatus was developed with the partial support of the National Science Foundation's program for the design of science teaching equipment. Delano W. Peckham performed many hydrolyses to establish the technique. Literature Cited (1) STOCK, J. T.,Metallurgia, 4 2 , 48 (1950) and references cited. (2) STOCK, J. T., J. CHEM.EDUC., 31,410 (19.54). E. E., AND NORDMAN, J., J. CAEM.EDUC., 43, (3) STEINBERG, 309 (1966). (4) Intern~tionslRectifier Corporation, "Zener Diode Handbook," El Segundo, 1961, p. 44. (5) IVES,R. L., Radio-Electronics, 38 (2), 55, (1967). (6) STOCK, J. T., School Sd.Rev., 31,336, (1950). (7) ARCHER, B. L., AND HUDSON, R. F., J. Chem. Sac., 1950. 3259. (8) SHORTER, J., AND HINSHELWOOD, C., J . Chem. Soe., 1949, 2412.
