NOTES
July, 1956 0.05', and the resistance measured at 20.0 and 35.0'. The specific conductance of the solvent was negligible in comparison with that of the solution measured. The concentration of the solution was calculated by assuming that .the trimesitylboron was converted quantitatively to sodium trimesitylboron, and that the volume of the solution does not differ appreciably from that of the pure solvent.
1021
1800
80..
1600
Acknowledgment.-The authors wish to thank the Research Corporation for the Frederick Gardner Cottrell grants which supported this work.
1400
-:
PHASE EQUILIBRIUM I N T H E SYSTEM NiO-H20i BYL. A. R O M O ~
5
1200 -2
& 1000 0
r;l
Department of
Geochemistry The Pennsylvania State univ'ersity Park, P a . Received March 6, 1966
University,
&
5
800
2
The isobaric decomposition of Ni(0H)Z has been investigated by Huttig and Peters at various tem600 peratures.3 They found that the hydroxide decomposes gradually at a constant pressure of 10 400 mm. giving a series of hydrates which are converted into NiO a t a temperature of 230". More recently, it has been reported that the complete con200 version of the hydroxide into the oxide takes place a t a slightly higher t e m p e r a t ~ r e . ~ Since Ni(OH)2 is used as a catalyst5-' in reactions at fairly high pressures as well as high tem200 peratures, it is very important that some information be obtained on the phase equilibrium and stability relations of Ni(OH)2 at various pressures Fig. 1.-Phase and temperatures. Experimental The hydrothermal runs were made in "test-tube" bombs which are attachable to a high pressure source of water by means of suitable fittings.8 They were heated to the desired temperature by means of furnaces provided with a temperature control. The starting material used in all runs was Ni(0H)z prepared from NiNOs in solution by precipitation with 0.1 N NaOH. Crystallization of this material was obtained by treating it hydrothermally a t 200' and 10,000 p.s.i. for a period of 3 days. Small amounts of the hydroxide were packed in small envelopes made of silver foil. These envelopes were dropped into the "test-tube" bombs which were then joined to the high water pressure pump and heated a t the desired temperature a t a given pressure. After several trials, it was found that 24-hour runs were adequate. The products obtained on quenching the bombs were examined by means of X-ray diffraction. The readings of temperature and pressure are believed to be accurate within the limits of =k5' and fl kg./cm.*, respectively.
b
250 275 300 325 Temperature, 'C. equilibrium dia ram of the system NiOHzO: 0, Na(OHfz; 0 , NiO. 225
the water vapor pressure, thus, the phase equilibria relations can be represented by the p-t diagram. It is interesting to indicate that in some cases it was possible to determine the p-t values at which both NiO and Ni(OH)2 coexisted in equilibrium with water vapor. This finding indicated that the univariant line represents the equilibrium boundary with a high degree of accuracy. It must be emphasized that in no case a conversion of NiO into Ni(OH)2 was recorded, even in cases where long time (10 days) runs were made. The temperature of conversion of Ni(OH)* into NiO in open air as determined by differential thermal analysis is 255 f 5". (b) Heat of Reaction.-The AH value was calculated by means of the Clapeyron-Clausius Results equation. The accuracy of the value calculated (a) Phase Equilibrium.-In the system NiO- from this equation depends on two factors: (a) HzO there are two components and three phases. the accuracy of t,he dp/dT values, ie., the slope This according to the phase rule gives a univariant of the univariant curve and (b) the value of 3-phase equilibrium system. The composition of AST which is the change in molar volumes of the the two phases is constant depending only on components of the system as a function of pressure and temperature. In this case, it can be stated (1) The experimental work waa carried out in the Department of that the A p / A T at various values is highly accuGeochemistry, The Pennsylvania State University, University Park, rate, their error being within the magnitude of the Pennsylvania. (2) Pigments Department, du Pont de Nemours, Wilrnington, error of measurement of temperature and presDelaware. sure. This is confirmed by the fact that it was (3) G. F. Hiittig and A. Z. Peters, Anorg. allgem. Chem., 189, 183 possible to record p-t values where both phases (1930). coexisted in equilibrium. Changes in volume per (4) A. Merlin and 6. Teichner, C07npt. rend., 286, 1892 (1953). (5) 8. J. Vles, Ree. trau. chin.Pays-Baa. 46, 743 (1925). grain of NiO and Ni(OH)2 are not known. How(8) K.Chakravarty and I. Ch. Ghosh, J . Ind. Chem. Soc., 4, 431 ever, if one considers that the specific volume of (1927). H2O changes considerably as the pressure and (7) A. Quartaroli, Gazz. c h i n . ilal., 67, 234 (1927). temperature are raised, it need not be considered (8) R.Roy and E. F. Osborn, Econ. Geol.,47, 717 (1952).
NOTES
1022
Vol. 60
critical the comparatively small changes in specific carbon with negative “net” heat but was subsevolume of the solid phases. Values for the molar quently withdrawn as inadequate for treatment of a volumes of water vapor were obtained from the localized, hydrogen bonded a d ~ o r b a t e . ~ Handbook of Physical Constants (for P < 100 Data are now available which permit a roughly atm.)g, and the tables given by KennedylO (for quantitative definition of the isolation requirement P > 100 atm.). The value of A V was defined as by determining, from a break-down of the adsorbate entropy, the minimum distance between sites A P e V&o(g) V N i O ( s ) - rNNi(BO)n(a) The A€€ calculated from values of Ap/AT taken required to prevent interaction of adsorbed molealong a hydrothermal pressure range of 10 to 1800 cules. The heat of immersion of Graphon in water kg./cm.2 with a corresponding temperature change combined with an adsorption isotherm a t the same temperature yielded a negative “net” heat of of 250 to 310” is 12.0 1.5 kcal. mole-]. adsorption5 (supporting the results of references Acknowledgment.-The author wishes to thank 3a and 3b). Application of the BET equation to Dr. R. Roy for reading the manuscript. data for the adsorption of water and of nitrogen on (9) R. W. Goranson, “Handbook of Physical Constants,” Geol. Graphon indicated that only about 1/1500 of the SOC.Am., Section 14, 211-212, 1942. total surface was receptive t o water molecules. (IO) G. C. Kennedy, A m . J . Sei., 44, 100 (1950). The authors recognized the implication that the active sites were isolated, preventing the interaction between adsorbed.molecules which, when present, NEGATIVE “NET” HEATS OF ADSORPTION’ contributes strongly to the heat of adsorption. The indicate6 surface area per active site is about BY DONALD GRAHAM 16,000 A.2 corresponding t o gn average distance Contribution N o . 178 from Jackson Laboratory, E . I . du P o d de Nemours and Company, WaEminoton, Delaware between sites of about 130 A. It is, therefore, Received March 8 , 1068 reasonable to conclude that a molecule displaced Discussion.-The “net” heat of adsorption of a from any site will re-enter the gas phase with vapor on a solid is the difference between the heat negligible probability of moving directly to anof adsorption and the latent heat of condensation. other site. The term was first used in the belief that liquefacThe entropy analysis is made at the coverage of tion was the first step in adsorption.2 We now minimum integral molar entropy (e l), as this know that this belief was incorrect but “net” minimizes the error in estimating the configuraheats are still widely reported and are useful when tional contribution. The observed integral encorrectly interpreted. Negative values for the tropy a t this point was 29.3 e x . The authors of “net” heat were first observed in the adsorption reference 5 state that heterogeneity precludes any of the first monolayer of water on charcoal, both appreciable configurational term. However, the by isosteric calculation3& and by calorimetry. 3b nature of the bond and the shape of the entropy Some confusion has arisen in the treatment of such plot indicate that the active sites were much more negative “net” heats and a clearer understanding of uniform than they believed and that the effect of configuration must be considered. Ideally, the the relations involved is needed. Negative “net” heats of adsorption result from integral entropy of configuration a t e = 1 would a combination of two conditions: (1) An adsorptive be 0, but in real systems there is a residual convapor with a latent heat of condensation higher tribution at the entropy minimum due to some than that heat of its adsorption which does not second layer deposition before the first monolayer include interaction between adsorbed molecules. is complete. This quantity has been estimated t o (2) Prevention (or a t least severe limitation) of be about one entropy unit6 leaving 28.3 e.u. for the interaction between adsorbed molecules. degrees of freedom associated with the motion of The widely discussed concept of a BET “C” the adsorbate molecules. The adsorbed molecules constant less than unity as a criterion of negative can rotate freely in the plane of the surface but “net” heat has failed to meet the test of experiment. there may be some hindrance in the other two Also, the type 3 isotherm associated with a frac- coordinates. The barrier is probably small as tional ‘(C” constant represents strong lateral acetone adsorbed on mercury with a comparable interaction. This concept is therefore in direct heat apparently retains almost unhindered rotaconflict with the required limitation of lateral tion? Loss of only about 2 units of rotational interaction. entropy is therefore assumed, leaving 8.4 e.u. for A second serious obstacle to understanding and adsorbate rotation. general acceptance of negative “net” heats of Part of the translational entropy of the gas is adsorption has been the accompanying requirement converted to what may be treated as a weak vibrathat the entropy of the two-dimensional adsorbed tion perpendicular to the adsorbent surface. film must be greater than that of the corresponding has developed a method for estimating liquid. A two-dimensional gas model was proposed .Elemball’ to explain the high entropy of water adsorbed on this contribution. For water on Graphon at 25”, his method gives a value of 2.6 e.u.
+
*
-
(1) Presented before the Division of Colloid Chemistry, at the 128th Nstional Meeting of the American Chemical Society in Minneapolis, Rlinneaota, September, 1955. (2) A. B. Lamb and A. S. Coolidge, J . A m . Chem. SOC.,42, 1146
(1920). (3) (a) A. S. Coolidge, ibid., 49, 708 (1927); (h) F. G. Keyes and bI. J. Marshal, ibid., 49, 15fi (1927).
(4) J. H. deBoer, “The Dynainioal Character of Adsorption,” Oxford University Press, London, 1953, p. 53, 234. (5) G. J . Young, J. J. Chessiok, F. H. Healey and A. C. Zettlcnioyer, THISJOURNAL, 58,313 (1954). (6) L. E. Drain, Science Prog., 42, 608 (1954). (7) C. Rentball, Proc. Rou. Soe. (London),A190, 117 (1047).
c
