NOTES
+
3165
+
3N2 Clz 20, and this would account for the experimentally determined stoichiometry with respect to N consumption and 0 production. It also suggests that the appropriate value for [Clz]is [ClN0lt/2. This mechanism excludes the possibility of determining the rate of reaction i in this system, but suggests that the sequence of reactions i-iv is fast and stoichiometric and suitable for titrating N atoms. The data do, however, show a high degree of internal consistency with respect to such a mechanism. A complete discussion would be unacceptably lengthy here, but an outline of the analysis is as follows. The following equations may be derived from the detailed steady-state treatment of reactions 1-6.
+
C1z,6~10 but considering the approximation involved in the calculation, they tend to support the proposed mechanism. The weak air afterglow emission observed when ClNO is in excess of that required a t the end point indicates that 0 and NO are both present. At this point all of the N atoms will have been consumed, and the reaction 0 ClNO + C10 NO becomes important. The nitric oxide concentration will probably be low due to the reaction NO C10 + NO2 C1," and the emission, the intensity of which is proportional to [0][NO], is weak. The appearance of the air afterglow does, however, allow one to visually bracket the end point in much the same way as with NO. When the end point has been attained, the flow of N atoms orginally present will be twice the flow of added nitrosyl chloride.
+
+
+
+
Acknowledgment. This research was supported in part by the U. S. Atomic Energy Commission. (7) F. Kaufman, Progr. Reaction Kinetics, 1, 31 (1961). (8) C. G. Freeman and L. F, Phillips, J. Phys. Chem., 72, 3028 (1968).
[C1NOII
+ kWN+ 2krN[?S]o[M] (11)
where IC, refers to the appropriate third-order recombination rate constant; a = ka/k2; k , is the appropriate wall recombination rate constant; and [nil, is the concentration of nitrogen atoms in the absence of ClNO. Examination of absolute values of N and 0 down the flow tube showed that, after the onset of 0 atom decay the ratio [N]/[O] remains constant for about threequarters of the observed decay. At the highest pressures and highest [O] the value of the recombination term in eq 1 is at least 20 times smaller than the firstorder terms. On this basis the observed first-order decay for 0 atoms and their dependence on [CINO], can is calculated for be understood. A value of 9.5 X the recombination coefficient of 0 atoms and is in reasonable agreement with literature value^.^^^ On the other hand, the recombination term in eq I1 is about one-tenth to one-fourth the observed initial N atom decay rates which could account for the curvature of N atom decay plots. Equation I1 also reflects the pressure effects observed for the dependence of initial N atom decay rate on [CINO]t. The data allow calculation of 3 X for the wall recombination coefficient for N atoms. Finally, it is possible to extract from eq I and I1 estimates for kl and k4 if a value of a! is assumed. Calculations were made with the aid of a PDP-8 computer to determine the best value of a! consistent with all the experimental data and then to calculate kl and kq. The values so obtained were for a! = 0.6; kl = 2.3 A 0.4 X and k4 = 5.8 k 1.4 X cm* molecule-' sec-'. These values differ by about a factor of 3 from those reported from direct measurement of N Clz9 and 0
+
+
(9) K. S. Raxworthy and L. F. Phillips, Can. J. Chem., 42, 2928 (1964). (10) M. A. A. Clyne and J. A. Coxon, Trans. Faraday Soc., 62, 2175 (1966). (11) J. A. Coxon, ibid., 64, 2118 (1968).
A n Isopiestic Vapor Pressure Study of the System Potassium Chloride-Sodium Chloride in Deuterium Oxide Solution at 25" by R. A. Robinson State University of New York at Binohamton, Binghamton, New York 16901, and the National Bureau of Standards, Washington, D. C . ~ O S S J J (Received March 8,1969)
The activity coefficient of zinc sulfate has been found' to be somewhat lower in heavy water than in light water; this is also true for cesium chloride,2 but the contrary behavior is found for lithium chloridej2 its activity coefficient being higher in deuterium oxide. While it might be anticipated that the osmotic coefficient of a mixture of two salts in deuterium oxide would be close to that found with ordinary water as solvent, it is, nevertheless, worthwhile studying one system for confirmation. Since the salt pair potassium chloridesodium chloride in ordinary water has already been studied in detailJ3a few vapor pressure measurements of this salt pair in deuterium oxide have now been made. (1) J. C. Rasiah, Thesis, University of Pittsburgh, 1965. (2) R. E. Kerman, Thesis, University of Pittsburgh, 1964. (3) R. A. Robinson, J. Phys. Chem., 65, 662 (1961).
Volume 76, Number 0 September 1969
3166
NOTES
Table I : Osmotic Coefficients in Deuterium Oxide Solution PNaC I
PN&l
mKC1
mNaCl
PKCI
(in Dz0)
(in Hz0)
1.0650 1.7439 2.4487 3.2969 4.3427
1.0174 1.6338 2.2475 2,9657 3.8024
0.8978 0.9070 0.9226 0.9448 0.9749
0.9398 0.9681 1.0052 1.0504 1.1134
0.9361 0.9634 0.9977 1,0430 1.1015
osmotic coefficients derived therefrom on the as sumption that the osmotic coefficients of the two potassium chloride solutions were the same as those in ordinary water at the same aquamolality. Following the treatment of Scatchard,6 we define a function A = 2V
- 2yApAo - 2 y B ( P B o
+
where A = KCI, B = NaC1, Y A = mA/(mA mB), Y B = mB/(mA mB), and the superscripts designate the osmotic coefficients of a salt in its own solution mB. c p ~ Ois obtained at an aquamolality m = mA from table^;^ BO is also obtained from tables4 together with the correction, discussed above, for the difference between ordinary water and heavy water as solvent. Values of A obtained in this way are given in the fifth column of Table 11. It has already been shown7 that for this salt pair in ordinary water as solvent
+
Table 11: Osmotic Coefficients of Deuterium Oxide Solutions of Potassium Chloride Sodium Chloride
+
1.7439 1.3179 1.1192 0.8312 0.7075 0.5853 0 3857 3.2969 2.9394 2.4783 2.0285 1.5258 1.1690 0.5923 I
0 0.4073 0.5948 0.8644 0.9792 1.0961 1.2790 0 0.3345 0.7605 1.1700 1.6217 1.9397 2,4441
...
...
...
0.9168 0.9228 0.9329 0.9377 0.9407 0 9501
0.9175 0.9234 0,9334 0.9374 0.9423 0.9504
- 0.0110 - 0.0130 - 0,0130 - 0.0118 -0.0148 - 0.0096
0,9516 0,9619 0.9739 0.9897 1.0021 1.0259
0.9519 0.9622 0.9744 0.9899 1.0021 1,0239
-0.0112 -0.0216 - 0.0270 - 0,0278 - 0.0252 -0.0124
I
... - 0.0095 -0,0118 -0.0129 -0.0124 -0.0116 - 0.0090
...
-0.0105 -0.0210 - 0,0259 - 0.0273 - 0.0252 -0.0164
The deuterium oxide contained 99.8 mol % ’ D20. Each salt was recrystallized from water. All results are given in terms of aquamolality, moles of salt in 55.51 mol of solvent. A few measurements were made with solutions of single salts, the aquamolalities of solutions of potassium chloride and of sodium chloride of the same vapor pressure being given in Table I. If it be assumed that potassium chloride in deuterium oxide has the same osmotic coefficient (at the same aquamolality) as it has in ordinary water, the data in the third column can be interpolated from tabulated values4 and values of the osmotic coefficient of sodium chloride in deuterium oxide calculated. These can be compared with the corresponding values in ordinary water.4 The differC I DzO) ence can be represented by the equation ~ N ~ (in - (PNaC1 (in H20) = 0.0030m~,cl. The difference is somewhat larger than that found previously5 and is considered more reliable. Table I1 gives the molalities of some mixtures in deuterium oxide in vapor phase equilibrium with solutions of potassium chloride in deuterium oxide and the
The Journal of Physical Chemistru
(1)
+
A
=
YAYBm(boi
+bod
(2)
with bol = -0.0253 mol-2 kg-’ and bo?: = -0.00299 mol-’ kg-’. Values of A calculated using these values of bo’ and bozare given in the last column of Table 11; a comparison of the last two columns shows that the interaction coefficients for the system in ordinary water are applicable to the system in deuterium oxide. Another way in which to make a comparison is to use these values of bol and boz to calculate osmotic coefficients by means of eq 1 and eq 2. Values so calculated are given in the fourth column of the table; it can be seen that the equations set up for solutions in ordinary mater predict very well the observed values in heavy water. Objection may be made to the assumption that the osmotic coefficient of potassium chloride is the same in ordinary and in heavy water; however, if the calculation is repeated with the converse assumption of the equality of the osmotic coefficients of sodium chloride in the two solvents, one arrives at exactly the same conclusion. There is no detectable difference (within the limit of experimental error) in the behavior of potassium chloride-sodium chloride mixtures in ordinary water and in deuterium oxide, provided that the results are expressed on the aquamolality scale. (4) R. A. Robinson and R. H. Stokes, “Electrolyte Solutions,” revised ed, Butterworth and Co. Ltd., London, 1968, Appendix 8.3. (5) R. A. Robinson, Trans. Faraday SOC.,35, 1220 (1939). (6) G. Soatchard, J . Amer. Chem. SOC.,83, 2636 (1961); 90, 3124 (1968). (7) R. M. Rush and R. A. Robinson, J . Tenn. Acad. Sci., 43, 22 (1968).
