9.5. (NH4)2HPO4 - (NH4)2SO4 - H2O Only a very limited amount of data is available for this system. There is a report of the 298 K isotherm.1 Another report2 presents the values for the isotherms at 273 K, 283 K and 293 K. And Akiyama, et al.3 have developed the following smoothing equation for this system:
S = A /(M + 1.2) [1]
where S is the solubility of (NH4)2HPO4 expressed as mass %; m is the mole ratio SO4/PO4 in the saturated solution; and A is a constant having the following values see in the table below.
A graphical comparison of some of the data at 293 K and 298 K as well as results calculated from Eq. [1] are shown on Fig. 27. From this comparison it appears, again, as if the work of Vol'fkovich, et al.1 is affected with a systematic error.