723
ELECTRICAL CONDUCTAWES OF AQUEOUS SODIUM IODIDE of the reasons accounting for the variation in the amount of hydrogen observed in the products via mass spectral analysis of products desorbed from sodium hydroxidedoped and chromium oxide-doped catalysts. Chromium ions themselves may also present a strong adsorption site for hydrogen. Thus it appears that the main reaction of propanol on y-alumina is that of dehydration, involving primarily one type, the least acidic, of surface hydroxyl groups. Once these high-frequency surface sites have been removed by doping with sodium hydroxide, the hydrogenation reaction involving the Ala+ becomes dominant. The addition of chromium oxide to the catalyst creates more sites for dehydrogenation.
Adsorption of propanol on chromia-alumina catalyst, without sodium hydroxide treatment, gave similar results to those obtained on pure y-alumina. This suggested that for dehydrogenation to occur, the activity of the dehydrating sites must be first suppressed. Further studies using the intermediates observed in propanol dehydrogenation3 and the effect of preadsorbed hydrogen, water, and carbon monoxide will be reported later.
Acknowledgment. This work was supported by funds provided by the National Research Council of Canada and by the University of Alberta.
Electrical Conductances of Aqueous Sodium Iodide and the Comparative Thermodynamic Behavior of Aqueous Sodium Halide Solutions to 800" and 4000 Bars1 by Lawrence A. Dunn2 and William L. Marshall Reactor Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee
67850
(Received September 9, 1968)
From electrical conductance measurements on dilute (0.001-0.10 m) aqueous solutions, the ionization behavior of NaI was studied in the temperature range 0-800" and a t pressures to 4000 bars. Both the conventional ( K ) and complete (KO) ionization constants were calculated for comparison with published values for NaC1 and NaRr. As expected, XaI ionized to a greater extent than either NaBr or NaCl, the order being directly proportional to the anion size. The net change ( k ) in waters of solvation on ionization decreased from 10.2 for NaCl to 9.7 for NaI. For the temperature range 400-800", the vaii't Hoff isochore yielded standard thermodynamic functions for the complete equilibrium, NaX(so1vated) kHzO e Na+(solvated) X-(solvated). The AH" obtained, approximately constant with temperature, mas essentially the same as found for NaBr and NaCl. With the calculated values of AGO and an average value for AH" of -7.0 kcal mol-l, standard entropy changes of -88.3, -86.2, and -82.9 cal mol-I deg-I for the complete reactions were obtained for NaC1, KaBr, and NaI, respectively. The negative values of AS" show that order is increased by additional solvation on ionization of the electrolyte. Thus, a t high temperatures, inclusion of the solvent as a reactant provides a simple description whereby each equilibrium can be described by only three values ( k , AH", and AS") which are essentially independent both of temperature and pressure. The closeness in values of k and AS" for the three halides suggests that averaged values from these salts may be used to estimate the equilibrium properties of most 1-1 salts between 400 and 800".
+
Introduction The properties of aqueous electrolyte solutions at high temperatures and pressures, particularly in the supercritical region, are amenable to investigation by conductance techniques. This method has been applied in several recent publications from this laboratory to the study of the behavior of dilute aqueous solutions of KHS04,3NaCl,4NaBr,5and HBra to 800" and to pressures of 4000 bars. The results from these studies have indicated that, at high temperatures and pressures, aqueous electrolyte solutions exhibit a simplified be-
+
havior not evident a t lower temperatures. It was found that the isothermal limiting equivalent conductances of these salts in the temperature range 100-800" (1) Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corp. Presented before the Division of Physical Chemistry at the 156th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 8-13, 1968. (2) Mellon Institute, Carnegie-Mellon University, Pittsburgh, Pa. 15213. (3) A. S. Quist and W. L. Marshall, J . Phys. Chem., 70, 3714 (1966). (4) A. S. Quist and W.L. Marshall, ibid.,72,684 (1968). (5) A. 9.Quist and W. L. Marshall, ibid., 72, 2100 (1968). (6) A. S. Quist and W.L. Marshall, ibid., 72, 1545 (1968). Volume 73, Number 9 March 1969
LAWRENCE A. DUNNAND WILLIAML. MARSHALL
724 are linear functions of the solvent density.3-6 Moreover, at temperatures from 400 to 800", the limiting equivalent conductances for a particular electrolyte were found to be independent of the temperature at constant solvent den~ity.~-B This paper presents conductance measurements of dilute (0.001-0.10 m) aqueous NaI solutions at temperatures from 0 to 800" and at pressures to 4000 bars. From these measurements limiting equivalent conductances of NaI were obtained a t integral temperatures and densities. Ionization constants ( K ) for I-, were the conventional equilibrium, NaI $ Na+ also obtained where NaI behaves as a weak electrolyte. From these values the complete ionization constants (KO)for NaI were calculated at each temperature.'#* These results were used for calculation of thermodynamic quantities for NaI for comparison with the previously published results for NaCl and NaBr. For NaCl, NaBr, and NaI a unified behavior has been observed between 400 and 800" in the attainment of an essentially constant value of AH", independent both of pressure and temperature, that describes the behavior of all three electrolytes. Similarly constant values of AS" and of the net change in waters of solvation ( k ) for each electrolyte have also been calculated.
+
Experimental Section
A detailed description of the high-pressure conductance equipment and procedures used has been given p r e v i ~ u s l y . ~The conductance cell with no pressure seals in the high-temperature region was used exclusively in this work. All solutions were prepared by weight from reagent grade NaI (Mallinckrodt Chemical Works) and conductivity water. The salt was dried for 24 hr at 120" before use. The conductivity water was obtained by passing distilled water through an ionexchange column and then redistilling it twice from a fused quartz distillation unit. Conductivity measurements were made to 800" and 4000 bars on four solutions of NaI: 0.0009996, 0.005001, 0.05000, and 0.1000 m. Previous data of Quist and Marshallg for 0,01000 ?n NaI solutions under the same conditions have been included in the calculations. Four different inner electrodes were used for the conductance measurements. Their cell constants ranged from 0.495 to 2.104 cm-*, as determined fron 0.01 and 0.10 demal KC1 solutions a t 25.00 * 0.01". Results and Discussion Specific conductances and equivalent conductances were calculated from the experimental data in the manner described previ~usly.~An example of isothermal specific conductances of NaI solutions as a function of pressure at the temperatures of the measurements is shown in Figure 1. From comparable figures for all five molalities of NaI, specific conductances at integral pressures were obtained by interpolation. Isobaric The Journal of Physical Chsmi8tTy
3000
*
.-
-
e
-'E 2000 0
I
c I
Y
1.000
0 0
1000
2000
3000
4000
PRESSURE (barf)
Figure 1. Specific conductances of 0.05000 m NaI solutions as a function of pressure at several temperatures.
specific conductances for 0.05000 m NaI solutions 11s. temperature are shown in Figure 2, and isothermal equivalent conductances vs. solvent density in Figure 3. The other solutions of NaI exhibit similar behavior except that the density at which the equivalent conductance maximum occurs increases with increasing electrolyte molality. For 0.0009996 m NaI the maximum occurs near 0.50 g crnF3,while it occurs at about 0.70 g ~ m for - ~0.1000 m NaI solutions. Similar behavior has been observed for NaCI4 and NaBP solutions. From plots like Figure 3 equivalent conductances at integral densities were obtained for each molality of NaI at the experimental temperatures. These values were then plotted against temperature at constant solvent density, and from smoothed curves through the data, equivalent conductances at various temperatures and densities were obtained. These are presented in Table I for temperatures from 100 to 800", and include equivalent conductances for 0.01000 ?n NaI calculated from the specific conductances given el~ewhere.~Data obtained at 0 and 25" have been used only for comparative purposes as shown in Figures 1-3, and have not been included in Table I. The values in parentheses represent equivalent conductances at saturation vapor pressure at that temperature. Calculation of Limiting Equivalent Conductances. From the conductance values in Table I limiting equivalent conductances were calculated by the several methods described previou~ly,~ and with the computer programs developed At low temperatures and high densities the Robinson-Stokes equation,10 the (7) W. L. Marshall and A. S. Quist, Proc. Nat2. Acad. Sci., U.S., 5 8 , 901 (1967). (8) A. S. Quist and W. L. Marshall, J. Phys. Chem., 72, 1536 (1968). (9)A. 5. Quist and W. L.Marshall, i b i d . , in press. (10) R. A. Robinson and R. H. Stokes, J. Amer. Chem. Soc., 76, 1991 (1954).
ELECTRICAL CONDUCTANCES OF AQUEOGSSODIUM IODIDE
725
3000 I500
-
2500
5
1000
2 500
2000
Lo
sx
--
0
0.30
Is
'E t500
c
0.60 0.70 0.80 DENSITY ( g ~ r n - ~ )
0.90
1.00
equation, which includes an ionization constant, fitted the data satisfactorily. Limiting equivalent conductances calculated for NaI at the various temperatures and densities are included in Table I, where the (limiting) molality of NaI is set equal to zero. At densities of 0.65 g and above, the standard errors associated with the Ao(NaI) values of Table I are less than 1%. Below this density the uncertainty in the limiting equivalent conductances increases with increasing temperature and decreasing density. Figure 4 shows the linear relationship observed when isothermal values of Ao(Na1) from Table I are plotted against the density of the solvent. As found previously with NaCI4 and NaBr,6 the limiting equivalent conductance of NaI at constant density increases steadily with temperature, reaching a maximum, constant value at about 400" and above. The deviations from linearity (Figure 4) at high temperatures and at densities below 0.45 g ern+ may be due to difficulty in making accurate experimental measurements at sufficiently low electrolyte concentrations for reliable extrapolation to infinite dilution. The limitation under these conditions is the relatively high solvent conductance.a A linear equation describing the variation of the limiting equivalent conductance of NaI with solvent density (d) in the temperature range 400-800" is given as
IO00
500 Y
I 0 200
0
400 TEMPERATURE
600
800
C'( 1
Figure 2. Isobaric variation of the specific conductances of 0.05000 m NaI solutions a3 a funct,ion of temperatme at pressures from 500 to 4000 bars. 900
rT-[---
-,-
--T---T-l---T-,---
800 700
600
-' I
I
500
'E
s
-
0.50
Figure 4. Limiting equivalent conductances of NaI as a function of density at temperatures to 800".
Y
NE
0.40
400
q
300 200
Ao(NaI) = 1897 - 1210d
400 n
0.2
0.4
0.6
0.8
1.0
4.2
DENSITY ig
Figure 3. Equivalent conductances of 0.05000 m NaI solutions as a function of density at several temperatures.
Fuoss-Onsager-Skinner equation, l 1 and the Shedlovsky equation (including an ionization constant) l 2 gave essentially identical limiting equivalent conductances. As found previously for NaCI4 and NaBr,5 a t densities below 0.70 g cm-a the Robinson-Stokes equation did not represent the data as well as the other equations, while below about 0.60 g cm-a only the Shedlovsky
(1) Similar relationships were found previously for KHSO, (considered as a 1-1 electrolyte),a NaCl,4 NaBr,6 and HBrS6 Ao(KHS04) = 1740 - llOOd (2) Ao(NaC1) = 1876 - 1160d (3) Ao(NaBr) = 1880 - 1180d (4) ho(HBr) = 1840 - 560d (5) (11) R. M. Fuoes, L. Onsager, and J. F. Skinner, J. Phys. Chem., 69, 2681 (1965). (12) T. Shedlovsky, J. Franklin Institute, 225, 739 (1938); R.M. Fuoss and T. Shedlovsky, J . Amer. Chem. Soc., 71, 1496 (1949). Volume Y9, Number 9 ivarch 1969
726
LAWRENCE A. DUNNAND WILLIAML. MARSHALL
Table I: Equivalent Conductances (em2ohm-' equiv-1) of Dilute Aqueous NaI Solutions a t Various Temperatures and Densities Dmuity, 'I(%)
Iloh11tY
0.35
0.40
0.45
0.50
0.51
0.60
0.65
* 03-3
0.70
0.75
0.80
0.85
0.90
0.wwo
1w
(356) (345)
0.0009996 0.005001
0.05000 0.10w
0.00000 0.0009996 0.005001 0.01000
0.05000 0.1000
200
0.00000 0.0009996 0.005001 0.01000
0.05000 0,1000
250
0.00000 0.0009996 0.005001 0.01000 0.05000 0.1000
o.ww0 300
(986) (970) (920) (860)
0.0009996 0.005001 0.01000
0.05000
350
(810)
500
550
600
815 805 747 730 678 665
750 738 694 677 641 631
687 666 645 638 593 585
840
830
7w 767 721 702 661 650
118 701 666 659 609 599 739 724 679 669 613 604
802 733 717
0.0009996 0.005001 0,01000 0.05000 0.1000
1320 1076 825 695 495 475
1430 1175 1000 820 655 6W
1460 1190 1045 920 645 590
1310 1200 1030 925 705 655
1220 1180 1025 940 760 720
llB0 lis5 1020 965 192 755
1115 1102 990 945 798 765
10% 1015 940 900 795 767
9 70 968 890 858 770 749
9 34 907 846 814 738 724
867 847 790 766 710 697
799 785 735 716 668 658
0 .00000 0.0009996 0.001w1 0.01000 0.05000 0.1000
1440 935 695 575 400
1420 1100 875 715 550 500
1440 1145 980 860 590 545
1300 1185
1220 1175 1015 930
1170 1152
1005 900 680 630
745
705
1010 955 780 741
1110 1097 988 940 790 758
1040 1020 945 9bS 788 758
975 970 890 860 768 746
936 908 846 817 140 726
869 848 789 770 712 698
806 790 738 7 19 666 658
0.00000 0.0009996 0.005001 0.01000 0.05000 0.1000
1430 825 590 480
320 305
1380 1010 780 630 465 425
1400 1100 925 Bw 550 500
1290 1165 980 800 655 610
1220 1170 1005 920 725 685
1170 1150 1000 942 762 722
1110 1092 980 930 778 743
1040 1025 945 900 775 742
975 971 890 856 762 738
934 906 844 811 738 724
863 838 784 764 706 694
0.00000 0.0009996 0.005001 0.01m
1390 730 5 10 410 265 250
1370 945 710 565 4w 365
1380 1060 870 755 505 460
1290 1150 964 055 630
1220 1160 990 905 705 665
1179 1145 987 920 745 701
1105 1088 968 918 760 722
1040 1023 940 888 760 728
975 968 886 851 751 726
927 899
1340 660 450 350 220 205
1370 095 655 120 355
1360 1025 825 705 465 425
1280 1130 930
1160 1140 972 908 720 675
1095 1080 955 90 3 735
1040 1020 938 878 743 710
970 964
I55
1210 1150 970 M5 680 640
917 888 824 790 721 707
1330 610 405 310 190 175
1390 855 610 480 315 280
1340 1000 780 665
1270 1105 900 795 570 520
1210 1135 950 865 650 605
1170 1130 955 890 692 640
1090 1071 935 885 710 672
1020
960 956 870
0.0009996 0.005001 0.01000 0.05WO 0*10000
1390 5 70 370 285 165 150
1420 825 575 445 280 250
1360 980 740 630 400
1250 1075 860 760 535 485
1200 1115 920 835 610 570
1160 1120 935 870 665 612
1080 1061 915 865 688 638
1010 992 892 843 700 663
0.00000 O.WO9996 0.005001 0.01000 0.05000 0.1000
1200 535 340 245 140 125
1460 775 545 415 250 220
1360 950 705 595 370
1190 1090 885 800 570 525
1160 1110 915
330
1240 1045 820 720 490 445
632 577
1070 1048 897 045 653 612
0.00000 0.0009996
1310 505 310 220 122 110
1550 750 520 390 225 195
1370 935 670 560 345 305
1240 1010 775 680 445 400
1190 1060 850 765 525 480
1160 1095 892 825 595 540
1060 1032 875 820 620 578
0.00m 0.0009996 0.005001 0.01000
0.0009996 0.005001
0.m o o
0.005001 0.01000 0.05000 0 * 1000
380
315
435 395
360
185
830 605
848
The significance of this similarity in behavior of 1-1 electrolytes has already been discus~ed.4-~ Calculationof the Complete Ionization Constant of N u l . The ionization of NaI in aqueous solutions can be represented by the equation^^,^ KO
NaI(H20)j
632 611 600 586 550 538
868 822 180 722 707
771 748
0.01wo 0.05000 0.1000
800
586 569 559 544 516 505
8%
0.00000
750
644 621 610 602 562 556
772 754 700 687
0.1ow
7w
70 8 698 656 640 605 596
910 888
0.05000
650
540
964 960 888
0.1000
+ kHzOe Na(HzO),+ + I(H20)m- (6)
The Journal of Phusical Chemistry
526 511 502 488 469 458
780 7 70 720 694 640 629
1013 1002 930 890 798 773
0.05000
444 434 424 414 402 39 1
583 563 554 545 512 507
0.00000 0.0009996 0,005001 0.01000 0.1000
450
496 481 ,472 457 438 432 653 645 602 589 548
(760)
o.ww0
022) 015)
955 950 884 840 768 742
838
7w
880 844 735 710
1008 912 862 722 688
336
(670) (655) (630) (595) (570) (540)
0.1000
0.05000
400
888
1.00
329 320 315 M9 301
(340) (336)
0.01wo
150
0.95
836 804 732 718
834 717 693 955 94) 858 824 698 674
KO
= U N ~ ~ H ~ O ) , + ~ I ~ H , O ~ , - / ~ N ~ I ~ H(7a) ~O~~~H~O~
K/UH~O~ (7b) log K = log KO k log ~ H ~ O (8) where KO is the complete ionization constant including the hydration reaction and K is the conventional constant. The integers j , m, and n represent hydration =
+
ELECTRICAL CONDUCTANCES OF AQUEOUB SODIUM IODIDE
727
Table I1 : Negative Loghrithrn of the Convent,ional Ionization Constant, K , for the Dissociation of NaI into Na+ and I-. Standard State Is the Hypothetical 1 iM Solution Density, g c m - 8 -
r
T,OC
0.35
0.40
0.45
0.50
0.55
0.60
0.66
0.70
0.76
400 450 500 550 600 650 700 750
3.24 3.46 3.63 3.76 3.88 3.97 4.06 4.16 4.25
2.68 2.93 3.10 3.23 3.33 3.42 3.50 3.58 3.65
2.39 2.54 2.66 2.77 2.86 2.93 3.01 3.08 3.14
2.01 2.11 2.19 2.27 2.34 2.44 2.52 2.62 2.73
1.54 1.63 1.73 1.81 1.91 2.02 2.13 2.24 2.36
1.23 1.32 1.33 1.45 1.54 1.68 1.79 1.89 2.00
0.85 0.92 1.04 1.19 1.22 1.34 1.48 1.60 1.72
0.67 0,63 0.79 0.85 0.99 1.02
0.16 0.43 0.53 0.64 0.70 0.83
1.18
0.83
800
numbers of NaI, Ya+, and I-, respectively, while IC denotes the net change in waters of solvation resulting from the ionization of one mole of NaI. Over the complete density range from 400 t o BOO", values of the conventional ionization constant ( K ) were obtained from the experimental data using the Shedlovsky equation,12 with Ao(SaI) values calculated from eq 1. With this procedure the Shedlovsky equation contains only one parameter, the conventional ionization constant. Table I1 gives the calculated negative logarithms of the conventional constants at temperatures from 400 to 800" and densities from 0.35 t o 0.75 g cm+. These ionization constants are based on the standard state of unit molarity at each temperature and density. The average uncertainty associated with the values in Table I1 is approximately 0.03 pK unit, with the greatest uncertainties occurring at the highest densities at all temperatures.
Table 111: Valuew of the Logarithm of the Complete Ionization Constants (KO)and the Complete Free Energy Changes (AGO, kcal mol-') for the NaC1, NaBr, and Ea1 Equilibria
DENSITY ( g ern-))
0.35
0.40 0.45 0.50 0.55
0.65
With the concept of the complete ionization constant (including hydration) ,7,8 where aH20 is replaced by the molar concentration of water ( C H ~ Owith ) a hypothetical standard state of unit molarity at the particular density (or pressure) , isothermal plots of log K against provided linear relationships at the several log C H ~ O temperatures as shown in Figure 5 . The slope IC for NaI was independent of temperature in the range 400 to BOO", having an average least-squared value of 9.67 f 0.08. Corresponding least-squared values for NaCl and SaBr are included in Table IV. These values have been proposed to represent the net changes in waters of solvation on ionization of one mole of each electrolyte. From the value of IC = 9.67 for NaI (Figure 5 ) , together with the data of Table 11,values of log KO( NaI) have been calculated and are given in Table 111. Cor-
0.75
NaCl -7 log KO AP
7 -
T,o c -I
400 450 500 550 600 650 700 750 800
.+t;-2 'L
z
11
L
3
E
-3
-4
1.2
I
I
I
I
I
4.3
1.4
1.5
4,6
1.7
log CH
(moles/liter)
Figure 5. Log K(mo1ar units) for the equilibrium NaI e Na+ I- as a function of the logarithm of the molar concentration of water at temperatures from 400 to 800".
+
-17.10 -17.23 -17.31 -17.42 -17.52 -17.62 -17.70 -17.77 -17.83
52.7 57.0 61.2 65.6 70.0 74.4 78.8 83.2 87.6
- -- -NaBr log KO Aao
-16.49 -16.58 -16.73 -16.90 -17.04 -17.16 -17.27 -17.35 -17.38
50.8 54.9 59.2 63.7 68.1 72.5 76.9 81.2 85.3
log
NaI
KO
-15.87 -16.01 -16.12 -16.23 -16.32 -16.43 -16.52 -16.61 -16.71
AGO
48.9 53.0 57.0 61.1 65.2 69.4 73.6 77.8 82.1
responding values for NaC14 and NaBr5 have also been included for comparison. Comparative Thermodynamic Behavior of the Alkali Halides. Over the entire range of temperature ( T ) and density ( d ) studied, the order of association at constant T and d, is NaCl > NaBr > NaI. This behavior can be observed from a comparison of both the conventional ionization constants, presented in Table I1 for NaI and previously for the other two halide^,^^^ and the complete constants, presented in Table I11 for the Volume 78, Number 8 March 1969
LAWRENCE A. DUNNAND WILLIAML. MARBHALL
728 TEMPERATURE VC)
800
-15.0
600
700
500
400
Table IV: Thermodynamic Values ( A H " , AS') and k for the NaCI, NaBr, and NaI Equilibria, 400-800" Electrolyte
NaCl NaBr NaI
-16.0
k
1O.20=tO0.15 9.94 4C 0.48
9.67 & 0.08
AH', kcal mol-1
-7.04C0.7 -7.0 i:0.7 -7.0 i:0.7
AS",oal mol-1 deg-1
-88.34CO.1 & 0.3 -82.9 4C 0 . 1
-86.2
O$:
-m -17.0
-18.0
1.0
0.8
Figure 6. Log Ko (molar units) for the equilibrium NaX(H20)j kH,O e Na(HtO),+ 4- X(HtO),- as a function of T-I( OK) for NaC1, NaBr, and NaI.
+
three alkali halides. Thus, KaI is the most highly ionized electrolyte, while NaCl is the most highly associated electrolyte, with XaBr occupying an intermediate position. This pattern observed over an extreme range of temperature and density implies strongly that the halogen ion sizes, independent of the degree of hydration, remain in the order, I- > Br- > C1-, where the extent of association is expected to be inversely proportional t o the ion sizes. The comparative values of A G O for the complete equilibria, tabulated in Table I11 at 400-800°, show the relatively small differences in standard free energy changes among the three halides. The approximate constancy of k (the net change in waters of solvation) between 400 and 800" allows a simple comparison of the thermodynamic functions over this range of temperature, since for each electrolyte the respective equilibrium does not change in stoichiometry. Plots of the log KO data in Table I11 against T-1 (OK) are shown in Figure 6 for the three sodium halides. Least-squares analysis of the data indicated an average slope of 1540 =t160 for each of the electrolytes, within the precision of the measurements. This value of the slope yielded a single value of -7.0 kcal mol-1 for AH" for the complete equilibrium for each of the three electrolytes. With this value of AH" and the AGO values in Table 111,values for AS" for the complete reactions were calculated, and were found, within the precision of the measurements, to be independent of both density and temperature (400-800"). Comparative values for k , A H " , and AS" are given in Table I V for the three electrolytes. For the complete equilibria ( b l n K O / b P ) ~= - A V " / R T
0
(9) Thus AH" = AE" P A V " = AE". The negative values of A S " , approximately proportional to k , show
+
The Journal of Physical Chemistry
=
that increase in solvation upon ionization of the ion pairs provides greater order in the respective systems. Xinzple Description of Alkali Halide Behavior. By obtaining complete ionization constants, it has been found that at high temperatures not only does IC become constant with temperature and density, but also AH" and AX". Since AH"is a constant, independent of density and temperature (400-800") within the precision of the measurements for all three 1-1 salts, only AS" needs to be known for the particular salt in order to specify KO or A@'. To specify the fraction ionized at a particular temperature and pressure, we must know the additional single value for k. The closeness in values of k and AS" and the essentially identical value of A H o for these three alkali halides suggest that the degrees of ionization for most 1-1 salts are similar in this high-temperature range, and can be specified approximately by the above relationships and averaged values of A8" and k from those in Table IV. This similar behavior has been observed from a conductance study of many 1-1 electmlytes,Qand also in the observed linear relationships of A. us. density, where the intercept and slope of eq 1-4 are approximately the same for the several 1-1 electrolytes studied.a-G Franck and Rothls have observed some apparent simplicity in the spectroscopic study of hydrogen bonding in water a t temperatures to 400". At constant density, they observe an approach to constancy (above 400") in v(max) of an absorption band considered to support a continuum model for water. The observed electrolyte equilibrium behavior in water from the conductance studies and this approach to constancy of v(max) in supercritical water perhaps may be related. The thermodynamic relationships presented herein might well be expected to exist at pressures approaching zero. With this assumption the fraction of ionization can easily be calculated for cases of immediate, applied interest, for example, in geochemistry or pressurized water technology where pressures of 200-400 bars might be the range t o be considered. Thus a t 200 bars and at 400°, sodium iodide (and the other 1-1 electrolytes investigated) is an extremely weak electrolyte, showing about the same extent of dissociation as water at 25". (13) E. U. Franclc and K. Roth, Discussions Faradau Soc., 43, 108 (1967).
