There are two reports of the temperature and composition of the invariant points in the title system.^2,4 However, while these two reports agree with respect to the temperatures of the invariant points, they do not agree about the compositions. For the invariant point at about 301.8 K they even disagree with respect to the nature of the equilibrium solid phase. In view of the region of existence of (NH₄)₂HPO₂·2H₂O (see p. 14) it is probable that the correct composition of the solid is that reported by Bergman and Velikanova,² but additional experimental work is needed before any recommendation can be made about the composition of the invariant solutions. The isotherms measured at 323 K¹ and 283 K³ cannot help to solve this problem.

Experimental Data:   (Notes on the Nomenclature)