FUNDAMENTAL EQUATIONS OF IDEAL GASES 355 



Clearly p > p,at. for P > psat- In the case of water at zero 

 degrees under a pressure of 100 atm. there is obtained from (41) 



P/Psat. = 1.084. 



The effect (Poynting effect) is small, but in exact determina- 

 tions of vapor pressure, as by the "streaming" method, the 

 effect must be considered (the vapor pressure of water at zero 

 degrees is altered by roughly one tenth percent per atmosphere 

 pressure).* 



15. Defect in the Sum Rule for Vapor Pressures. The rule 

 that the total pressure over a liquid phase mixture of mutually 

 immiscible substances is given by summing the separate vapor 

 pressures suffers from the fact that the gases are actually not 

 ideal. Thus ammonia deviates at one atmosphere and zero 

 degrees by 1.6 per cent from the ideal pressure. A mixture of 

 nitrogen and ammonia in equal molal proportions, however, 

 exerts a pressure, at zero degrees and about one atmosphere. 



* The method of passing a neutral gas over liquids and subsequently 

 absorbing the vapor out of a known volume of the gas mixture has been 

 much employed in determinations of vapor pressures where the latter 

 are small. In utilizing such data to compute vapor pressures the 

 relation of the mass of the vapor to the mass of the neutral gas must be 

 accurately known. Frequently the perfect gas laws have been invoked 

 to compute the pressure of the vapor in the neutral-gas-vapor mixture. 

 If, however, precise results are desired this procedure is inexact owing 

 to the fact that Dalton's rule of mixtures may not be as close an approxi- 

 mation as desirable. See Eli Lurie and L. J. Gillespie, J. Am. Chem. 

 Soc, 49, 1146, (1927), also Phys. Rev., 34, 1605, (1929) and Phijs. Rev., 36, 

 121, (1930). The disability of the method, due to the failure of Dalton's 

 law, might be avoided by passing the neutral gas through a saturation 

 apparatus containing pure water and then through a similar apparatus 

 in series with the first but containing the solution of interest. The 

 temperature of the latter could then be raised until suitable tests showed 

 that the content of water in the neutral gas was the same after each 

 saturation apparatus. Determinations at several temperatures would 

 then establish the vapor pressures of the solution from the known values 

 for pure water. It can be shown that strictly the "Dalton defect" is not 

 precisely the same in both saturations because of the temperature 

 difference, but the error thus made can be shown to be exceedingly 

 small. 



