Practical basis for absolute values of the entropy

When SP [T] = SP [0] (condition 2), AS°[T] can be expressed as v (SP [T] — Sp [0]) that is, in terms of the observed quantities. We use the difference (Sp [T] — SP [0]) as the absolute value of the entropy, which is equivalent to assigning the value of zero to SP [0]. The two effects for which this assignment is valid are (1) the nuclear effects including those of nuclear spin, provided that the isothermal change of state does not involve a nuclear reaction and (2) the isotopic effects, provided there is no change in the isotopic composition of the substances. 

We thus can obtain a consistent set of absolute values of the entropy function for pure substances from thermal measurements alone on the practical basis of assigning the value of zero to the entropy function at 0 K with the exclusion of nuclear and isotopic effects, within the understanding of the third law as discussed in Section 15.4. The calculation of the entropy function of pure substances in the ideal gas state by the methods of statistical mechanics must be consistent with the practical basis. In addition to obtaining absolute values by the methods that have been discussed, values can also be obtained from equilibrium measurements from which ASe can be determined for some change of state. If all but one of the absolute values in the equivalent sum VjSP are known, then the value of that one can be calculated. 

---