7.5.NH4H2PO4-(NH4)2SO4-H2O Six articles1-6 report solubility data for this system. In addition, Akiyama, et al.7 present graphical data along with a smoothing Eq. [1]
100wi = A/(m + B) [1]
where 100wi is the solubility of NH4H2PO4 expressed as mass %; m is the mole ratio SO4/PO4 in the saturated solution; and A and B are constants having the following table. A comparison of the data at 263 K, 273 K, 293 K and 298 K, as well as calculated values from Eq. [1] is given on Fig. 17. Only at 293 K are the data from different authors in agreement with each other. Therefore, data at 293 K are accepted tentatively while the data at the other temperatures can be evaluated only after the results of additional work are available. An isotherm at 293 K has been reported,1 but when this is compared with Fig. 17 its values appear to have a systematic error. Furthermore, the invariant points described in one article4 do not correspond with results reported by others and, therefore, are rejected. The smoothing equation of Akiyama, et al.7 seems to fit the experimentally determined data fairly well, especially when the concentration of (NH4)2SO4 is less than 25 mass %.