Ammonium dihydrogenphosphate with dihydrogenphosphates of (Potassium, Rubidium, Cesium, and Thallium) and Water (IUPAC SDS Volume 66)
Evaluator
Jitka Eysseltová Department of Inorganic Chemistry, Charles University
Evaluation
7.7. NH4H2PO4-MH2PO4-H2O (where M = K, Rb, Cs, Tl)

All the articles dealing with the NH4H2PO4-KH2PO4-H2O system1-9 report a series of solid solutions as the equilibrium solid phases. Some comparisons are possible and they are shown on Fig. 19 and Fig. 20. For the 273 K data, Fig. 19, the values for the KH2PO4 content reported by Askenasy and Nessler1 are larger than those reported by Polosin and Ozolin4. This is true also for the binary solubility of KH2PO4 in water at 273 K, where the 15.5 100wi value is larger than the 11.74 100wi value recommended by IUPAC.10 It, therefore, seems likely that there is a systematic error in the analytical values for potassium and, consequently, the data in Ref.1 are rejected. The values at 298 K, Fig. 2, agree fairly well and may be tentatively accepted.

The NH4H2PO4-MH2PO4-H2O [M = Rb,11 Cs,12 Tl13] sytems are characterized by the formation of solid solutions. Earlier10 it was noted that the results in Refs. 11 and 12 were affected by unreliable analytical procedures and are, therefore, rejected. No independent solubililty data have, as yet, been reported for these systems.

Figures
Solubility graph
FIG. 19. Solubility in the NH4H2PO4–KH2PO4–H2O system at 273 K.
Solubility graph
FIG. 20. Solubility in the NH4H2PO4–KH2PO4–H2O system at 298 K.
Sources:

1 Askenasy, P.; Nessler, F.; Z. Anorg. Allg. Chem. 189, 305 (1930).
2 Dombrovskaya, N. S.; Zvorykin, A. Ya.; Kaliy 1937, 2, 24.
3 Zvorykin, A. Ya.; Kuznetsov, V. G.; Izv. AN USSR, Ser. Khim. 1938, 195.
4 Polosin, V. A.; Ozolin, R. K.; Kaliy 10, 31-34 (1937)
5 Polosin, V. A.; Ozolin, R. K.; Trudy TSKhA, Yubileynyi Sbornik 29-50 (1940).
6 Kuznetsov, D. I.; Kozhukhovskiy, A. A.; Borovaya, F. E.; Zh. Prikl. Khim. 21, 1278 (1948).
7 Shenkin, Ya. S.; Ruchnova, S. A.; Rodionova, N. A.; Zh. Neorg. Khim. 1972, 17, 3368.
8 Solov'ev, A. P.; Balashova, E. F.; Verendykina, N. A.; Zyuzina, L. F.; Vzaimodeistvie Khloridov Kaliya, Magniya, Ammoniya s ikh Nitratami i Fosfatami 1977, 3.
9 Bergman, A. G.; Gladkovskaya, A. A.; Galushkina, R. A.; Zh. Neorg. Khim. 1972, 17, 2055.
10 Eysseltová, J.; Dirkse, T. P.; Alkali Metal Orthophosphates (Vol. 31 of the Solubility Data Series), Pergamon Press, 1988.
11 Zvorykin, A. Ya.; Vetkina, L. S.; Zh. Neorg. Khim. 1961, 6, 2572.
12 Zvorykin, A. Ya.; Ratnikova, V. D.; Zh. Neorg. Khim. 1963, 8, 1018.
13 Bruzau, M.; Bull. Soc. Chim. Fr. 1948, 1177.