Ammonium dihydrogenphosphate with Diammonium sulfate and Water (IUPAC SDS Volume 66)
Evaluator
Jitka Eysseltová Department of Inorganic Chemistry, Charles University
Evaluation
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 values in the table below.

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%.

Figure
Solubility graph
FIG. 17. Solubility of NH4H2PO4 in the NH4H2PO4–(NH4)2SO4–H2O system.
Sources:

1 Vol'fkovich, S. I.; Berlin, L. E.; Mantsev, B. M.; Tr. NIUIFa 1940, 228.
2 Uno, S.; Kogyo Kagaku Zashi 1940, 43, 399-402.
3 Bel'chev, F. V.; Bergman, A. G.; Zh. Prikl. Khim. 1944, 17, 520.
4 Bergman, A. G.; Tashtemirov, R.; Ukr. Khim. Zh. 1967, 33, 565.
5 Bayramova, R. M.; Agaev, A. I.; Uch. Zap. Azerb. Gos. Univ.; Ser. Khim. Nauk 1968, 8.
6 Shenkin, Ya. S.; Freyman, O. I.; Zh. Neorg. Khim. 1970, 15, 3151.
7 Akiyama, T.; Kanzaki, H.; Minagawa, S.; Nippon Dojo Hiryogaku Zasshi 49, 243 (1978).