CONCENTRATION CELLS; MEMBRANE POTENTIALS 185 



Measurement of the equilibrium constant, in the usual manner, gives a 

 measure of AF, since 



-AF = RT\n K 



eq 



Further, 



d In K eq AH 



dT " ~RT~ 2 



and therefore measurement of the equilibrium constant at several tempera- 

 tures allows evaluation of A //by an alternative method. 



The Third Law, stated early in this chapter, provides another avenue for 

 the determination of the thermodynamic energies. The law says that the 

 entropy of all elements in their stable states (viz., S °) is zero at absolute zero 

 temperature (where all molecular motion ceases). Thus the entropy of all 

 pure substances at 0°K is also zero. Further, the entropy at the normal body 

 temperature of 37°C is the sum of all the little ways heat energy can be 

 stored by the material; and it can be evaluated from the heat capacity, C. , of 

 the substance measured at different temperatures from 37° C down to abso- 

 lute zero. Within the past 25 years, literally thousands of "third-law en- 

 tropies" have been so evaluated. Table 7-5 lists some of these values for 

 biologically important molecules. Then, as Szent-Gyorgyi, 13 the energetic 

 contemporary physiologist, so aptly stated in the quotation which opened 



TABLE 7-5. Some Free Energies of Formation and Third Law Entropies. 



-Af f ° (Cal/mole) S Q (cal/deg mole) 



H 2 0(1) 56.7 16.75 



H 2 0(g) 54.7 45.13 



NaCl(s) 91.7 



C 2 H 5 OH 40.2 38.4 



C 12 H 22 O n (sucrose) 371.6 



C0 2 (g) 51.08 



HAc 94.5 38.0 



this chapter, a large, formal system of very useful numbers has been calcu- 

 lated and tabulated from known experimental results. The National Bureau 

 of Standards, Washington, D. C, has published handbooks of useful data. 

 Tables 7-1 and 7-3, as well as 7-5, present very carefully selected samples, 

 of biological and medical interest. 



CONCENTRATION CELLS; MEMBRANE POTENTIALS 



If two vessels containing different concentrations (two glass vessels con- 

 taining 2 at different pressures joined by a closed stopcock; or two salt 



