IVOTES
Dec., 1963 that of Lewis and Herndon, only one reaction possessing established kinetics has been studied in such a system. DeGraaf and Iiwartj describe a stirred flow reactor (capacity flow reactor) in which they studied the pyrolysis of ethyl acetate. Our technique and apparatus are much simpler than theirs and yield results which are consistent with those of previous workers. In a stirred flow reactor for the first-order reaction A -+ B -4-C, the first-order rate constant is given by the equation k = (U'V)[(B)/(A)].In this equation, U is the flow rate of gas, V is the volume of the reactor, and (€3) and (A) are the concentrations of one of the products and of the reactant, respectively. Average rate constants determined in this manner, the standard deviations of the rate constants, and the number of runs at each temperature are given in Table I. TABLE I DECOMPOSITION OF CHLOROCYCLOHEXASE~ Temp., O C .
N o . of runs
Average k, set.-'
Standard deviation, set.-'
17.6 X l o w 6 0.60 X 10-5 350 17 18.0 x 10-4 1.38 x 10-4 385 15 80.6 x 10-4 2 . 5 2 x 10-4 414 11 3.73 x 10-3 439 13 32.2 x 1 0 - 8 1 84 X 452 18 59.5 x 10-3 0.91 x 10-2 476 18 15.5 X lod2 a Swinbourne2 found k = 16.70 X 10-5 see.-' at 350" and sec.-l a t 385'. IC = 15.59 X
Our results give k = 1013.s8f 0.25 exp(-50,200 f 800/RT) which is the same within experimental error as found by Swinbourne. The constants in this Arrhenius equation were calculated in the usual manner from the rate data iin Table I by the method of least squares, and the limits of error are standard deviations. The stirred flow reactor was assumed to be well stirred by diffusion. Experiments have been completed to prove this point and will be reported elsewhere. However, the fact that a large change in flow rate has little effect upon the calculated rate constant as shown in Table I1 supports this view.
I ~ E C O ? d P O S I T I O NO F
Blow rate, nil./sec.
2.230 1,634 1.189 0.700 0,326
TABLE I1 CHLOROCYCLOHEXAKE AT 414" Rate constant, sec.-l
8.264 8.224 8.032 7.717 8 011
X X X IOW3
x
2843
Experimental Apparatus and Procedure.-The flow system was constructed from Pyrex glass. In this system a stream of prepurified nitrogen controlled by a needle valve was directed into a vaporizer containing chlorocyclohexane. The gas stream which now contained chlorocyclohexane vapor then passed into the reactor and was finally led t o a soap-bubble flow meter. For nearly all of the experiments the vaporizer was maintained a t 0". The reactor had a volume of 52.8 ml. and was immersed in a molten lead constant-temperature bath. The volume of the tubing leading to and from the reactor which was in the temperature bath was less than 0.2 ml. The bath was heated electrically and controlled by a proportional controller to f 0 . l " . The temperature of the reacting gas was measured by a ChromelAlumel thermocouple inserted in the reactor, and the thermocouple was calibrated against a platinum resistance thermometer. The flow rate of the exit gas was calculated by measuring the time required for a soap bubble to traverse 50 ml. The flow meter was a 50-ml. buret and the time was measured with a 0.1sec. electric timer. The gas stream was sampled just after it left the reactor and was analyzed by gas chromatography using a Wilkens Instrument Company "Hi-Fi" chromatograph. Known mixtures of chlorocyclohexane and cyclohexene were used to calibrate the hydrogen ionization detector. With five different mixtures ranging from a cyclohexene to chlorocyclohexane ratio of '/a to 3 , the per cent standard deviation from the average sensitivity factor was 0.3. Integrated areas were used in all of the experiments reported in this paper. Analysis of Error.-The equation which is used to calculate the rate constants is k = ( U / V ) [(B)/(A)]and the symbols are defined in the discussion. The flow rate, U , in this equation is not the measured flow rate but must be corrected to the temperature of the reactor. The equation then becomes k = ( T P / T ~ ( V) / V ) . (B)/(A) where T , and T I are the temperatures of the reactor and flow meter, respectively. The temperatures were meaeured with calibrated thermometers and thermocouples and are known to better than l t 0 . l o a t room temperature and f 0 . 6 " a t temperatures above 300'. Random errors exist in the measurements of flow rates, volume of reactor, and integrated areas. From the precision with which each of theEe quantities could be measured it is estimated that each is known to better than 0.4%. Two possible systematic errors remain to be discussed. The products of the reaction, hydrogen chloride and cyclohexene, might be soluble in the soap solution used in the flow meter, and this would create a negative error in the flow rate. The partial pressure of the soap sclution would generate an error in the opposite direction. The partial pressure of chlorocyclohexane is less than 2 mm. a t 0 ° , and the partial pressure of the soap solution is 18 mm. a t 30", and, since these experiments were all carried out at atmospheric pressure (about 750 mm.), the maximum possible systematic error is +2.4%. I t is possible that the maximum systematic error is much less. The resultant error in the calculated rate constant is 3=2.5% as estimated by the method described by Beers.6 (6) Y. Beers, "Introduction t o t h e Theory of Error," Addison-Wesley Publishing Co., Reading, Mass., 1957, p. 34.
10-3
X
We also observed fast initial rates for the decomposition as previously reported by Sminbourne.2 However, two or three da,ys of continued pyrolysis in the reactor was sufficient to reduce the value for the rate constant to a reproducible figure. The stirred flow reactor method is subject to certain systematic and random errors which are not present in the more conventional kinetic experiments, and these are discussed in the Experimental section of this paper. However, it is evident that our method gives results which agree with the previous results and, moreover, is useful over a much larger range of temperature. (4) M. F. R. AIulcahy a n d D. J. Williams, Australian J . Chem., 14, 534 (1961). (5) J. de Graaf and H. K u a r t , J. Phys. Chem., 67, 1458 (1963).
XONINDEPENDEKCE OF MOBILITIES OF COUNTERIONS I N A CATION EXCHAXGE MEMBRANE BY W. G. S. STEPHEKS AKD F. S. STEVEN Department of Physiology and Biochemistry, St. Salvator's Colleee, St. Andrews Universzty, St. Andreus, F i f e , Scotland Received M a y 33,1063
Variations in the equivalent conductivity of one species of counterions produced by the presence of varying equivalent fractions of a second species have been studied, using Permaplex C-20 sulfonated polystyrene membrane. Experimental Strips 15 cm. long and 1 em. wide were cut from a 1-mm. thick sheet of membrane. The strips were converted into the
2844
Vol. 67
NOTEES
30
calcium ions t o the total conductivity was allowed for on the basis of unchanged equivalent conductivity : errors arising from the possible inaccuracy of this assumption are unlikely to be very significant except a t the extreme bottom end of the curve, where the contributions of the two ionic species to the total conductivity are of the same order of magnitude. The broken lines in Fig. 1 indicate the results of preliminary trials using silyer (valency 1, Xo/X, = 45.5) and aluminum (valency 3, Xo/X, = 199). Due to the difficulty of estimating aluminum uptake by titration, the aluminum curve has been based on the amount of aluminum offered to the membrane, assuming complete uptake. The actual uptake must be less than this, so that the true curve for aluminum must lie to the right of that plotted.
26
d
:-20 m u
.-cR
.* +
f: 15
'D
x 13
E
3
g 10
.*
6
T H E INFLUENCE OF UREA ON T H E FLUORESCENCE OF AQUEOUS DYE SOLUTIONS
c
BY K. K. ROHATGI~ AND G. S. SINGHAL
0 0
0.2 0.4 0.6 0.8 Fraction of exchange capacity in H + form.
1.0
Fig. I.-Influence of Ag+, Ca+2, and Al+3 on equivalent conductivity of H + in Permaplex C-20 as a function of equivalent fraction of H+. required form and washed in distilled water. The resistance was determined by passing a constant current from an earth-free source between two platinum electrodes applied to the ends of a strip, and measuring the potential drop between a second pair of electrodes spaced 10 cm. apart with a valve-voltmeter of 100megohm input resistance.
Results For a strip of uniform cross section, it is readily shown that if the total exchange capacity of a strip of over-all length L (em.) is C (mequiv.), and if the resistance of a 10-cm. length is R (kilohms), the equivalent conductivity of the ion species under observation in the wet membrane is given by the expression
X
10L
= __ pmho/cni.
CR
Mean results for six strips of Perinaplex C-20 are given in Table I. The maximum "spread" for any ion was less than 5%. TABLEI F~QUIVALEST COKDUCTIVITIES OF IKDITIDUAL COUNTERTONS AT
Ion
H+
Am
27.9
xo/xm
11.3
Ag
18" +
1.17 45.5
c a +2
0.79 64
~ 1 + 3
0 2 199
Figure 1 shows the influence of the presence of varying amoupts of ions of higher affinity on the equivalent conductivity of the hydrogen ion in the membrane. The full line shows the effect of substitution of calcium for hydrogen. The equivalent conductivity of the hydrogen ion is 27.9 with the resin fully in the hydrogen form. As the calcium content is increased, the conductivity of the remaining hydrogen ions falls, reaching a level of about 6 d= 2 when there is only 2.5 % hydrogen remaining. This is the limit of the technique used, as the amount of hydrogen is becoming too small for reliable estimation by titration. The contribution of the
Physzcal Chemzstry Department, Jadavpur Unauersaty, Calcutta, Indaa Recezved June 1, 1968
Vrea is well known for its property of bringing about denaturation of protein molecules. Easy dissolving of denatured protein in strong urea solutions might be taken as an indication that urea is able to weaken hydrophobic as well as hydrogen bonds.2 The work of Alexander and Staceys on the colloidal behavior of dyes, and very recently of Mukerjee and co-t~orkers,~ on the effect of urea on self-association of methylene blue show that the presence of urea in the solution helps to break up the dimers or higher associates present in the aqueous solution of the dye with little effect on the polarity of the water molecules. With this property oi urea in view, it was considered proper to investigate the effect of urea on the fluorescence of dye solutions, which might be helpful in elucidating the mechanism of concentration quenching and in separating quenching due to association from other types of quenching involved in Concentrated solutions. Experimental The effect of added urea was studied on absorption spectrum, emission spectrum, and fluorescence yield of fluorescein and rhodamin B solutions. The effect was studied at 1 X 1 x 10-2, and 5 X 10-1 M concentrations of fluorescein in aqueand 1 X ous sodium hydroxide solution (pH 12) and 1 X 10-3 M concentration of rhodamin B hydrochloride in water. Fluorescence intensity of the solution was measured in a Brice Phoenix universal light scattering apparatus at an angle of 135" to the direction of radiation. The 436 and 365 mp lines of mercury were used for excitation of fluorescein and 546 mp for rhodamin B solution. A suitable glass filter was placed in front of the photomultiplier to cut off stray incident radiation. The absorption by the solutions a t the corresponding wave lengths was also measured in the same instrument with the photomultiplier positioned a t 0". The respective concentration of urea was taken as blank in all absorption measurements. The fluorescence spectra were measured on a Beckman DU 4700' spectrophotometer with spectrofluorimeter attachment using fluorescent glass a5 the reference standard. The absorption spectra of concentrated solutions were taken by sandwiching two to three drops (1) Indian Institute of Technologv, Kanpur; India. (2) W . Kauzmann, Advan. Protein Chem., 14, 1 (1959). (3) P. -4lexander and K. A. Stacey, Proc. Roy. Yoc. (London), A212, 274 (1952). (4)(a) P. Mukerjee a n d A. K. Ghosh, J . Phys. Chem., 67, 193 (1963); (bl P. 3lukerjee and A. Ray, ibid.. 67, 190 (1963).
