J. W . .LARSON,P. CERUTTI,H. K. GARBER,AKD L. G. HEPLER
2902
For wide variation in particle size, a number of threecoordinated particles may exist in addition to the fourcoordination matrix. The uncertainty in coordination number would appear to be h0.5, influencing the pendular-ring condensate by about 1275, which is sufficient to modify the residual adsorption by one order of magnitude. Contact-angle measurements obtained a t saturation pressure were applied to the three-phase line of contact existing at reduced pressure. Where finite adsorption does exist, the spreading pressure n will decrease for any reduced-pressure situation. The dependence of equilibrium contact angle on P / P Ovia the spreading pressure is given by the Gibbs equation PP
when inserted in Young's equation 0 = cos-'
{
YSV
- YSL
YLv
}
= cos-' {YE
}
- YsL - = YLV
(6)
The extent of this variation in 0 has been examined'* for a number of model systems spanning acceptable ranges in n. The decreased 0 at reduced PIPQwhich enhances condensation is not sufficiently large to influence the pendular-ring-condensate quantities within the uncertainty imposed by particle-size distribution and coordination number.
Although the true adsorption at high relative pressures cannot be resolved from these calculations, the following conclusions can be drawn from the study. (1) The major contribution to the measured isotherms at high relative pressures is the pendular-ring condensate. ( 2 ) The best calculations are not sufficiently accurate to permit a realistic estimate of the extent) of true adsorption at high relative pressures. (3) Proposed adsorption models for these systems must be constructed from the isotherm region at low PIP0 for studies on these powders or from studies on suitable substrates where capillary condensation cannot contribute (planar interfaces). (4) Finite contact-angle systems show greatly enhanced pendular-ring condensation to adsorption and might be the most appropriate systems for studying capillary condensation, with as few complications as possible arising from adsorption. ( 5 ) Thermodynamic functions for adsorbed molecules derived from the measured (condensed fluid plus adsorption) isotherm are in serious error.
Acknowledgment. One of the authors (J. W. W.) wishes to acknowledge the permission granted by Mobil Research and Development Corp. to publish this work. The other author (W. H. W.) wishes to express appreciation to the Robert A. Welch Foundation for their continued interest and support. (12) J. W. Whalen, unpublished data.
Electrode Potentials and Thermodynamic Data for Aqueous Ions. Copper, Zinc, Cadmium, Iron, Cobalt, and Nickel by J. W. Larson, P. Cerutti, H. K. Garber, andiL. G. Hepler Department of Chemistry, Carnegie-Mellon University, Pittsburgh, Pennsylvania, and Department of Chemistry, University of Louisville, Louisville, Kentucky (Received February 1.4, 1068)
Heats of solution of CuS04(c), ZnS04(c),CdSOdLc), hydrates of these compounds, and hydrates of FeSOe(c) have been measured. These data lead to AH,', Szo,and AGi" values for the Mz+(aq) ions. Standard potentials are calculated from the AGtO values and are compared with standard potentials previously derived from electrochemical measurements. Of particular interest is our Eo = 0.473 V for Fe-Fe2"(aq) compared with Eo 0.409, 0.440, and 0.467 V from earlier electrochemical measurements. We also obtain AGrO, AH
