Sodium-potassium system, phase diagram

The sodium-potassium system has been the most fully explored because at one time it seemed possible that the eutectic mixture (NaK) might become the accepted coolant for fast nuclear reactors. At the eutectic composition, 67.8 at.% potassium, the mixture is liquid at temperatures down to -12.5°C. This low temperature relative to the melting points of the two pure metals (97.8 and 63.2°C, respectively) is remarkable and must be related to their different atomic sizes. An inflection in the phase diagram suggests the presence of a compound NaaK presumably the atoms can pack into a solid stmcture of this composition, and there is no evidence of NaaK... 

The composition data obtained for the series of mixed fatty acid-potassium soap systems, prepared by both the ethanol and petroleum ether routes, lend strong support to the formation of 1 to 1 acid-soap complexes. It is of interest to inquire into the phase relationships in these two-component systems. A phase diagram presented by McBain and Field (15) for the lauric acid-potassium laurate system shows that compound formation takes place between the two components at the 1 to 1 molar ratio, but the compound undergoes melting with decomposition at 91.3 °C. [A similar type of phase behavior has been reported by us for the sodium alkyl sulfate-alkyl alcohol (9) and sodium alkyl sulfonate-alkyl alcohol (12) systems, but in these cases the stoichiometry is 2 to 1]. 

Figure 16.2. Some phase diagrams, (a) The water end of the system potassium chloride and water, (b) The water end of the system sodium chloride and water, (c) The water end of the system magnesium sulfate and water the heptahydrate goes to the mono at 150°C, and to anhydrous at 200°C. (d) /3-methylnaphthalene and /S-chloronaphthalene form solid solutions, (e) Mixtures of formamide and pyridine form a simple eutectic, (f) These mixtures form binary eutectics at the indicated temperatures and a ternary eutectic at mol fractions 0.392 dibenzyl, 0.338 diphenyl, and 0.27 naphthalene.

The sodium polyphosphate systems produce glasses very easily while it is difficult to make potassium glasses. The milky glassy masses of potassium metaphosphate compositions obtained by quenching the potassium melts are rich in potassium trimetaphosphate, though the phase diagram for the system contains none. Potassium cyclic triphosphate can be made thermally by dehydrating monopotassium orthophosphate with urea at temperatures less than 300 C,... 

The melting point of pure zirconium trichloride could not be obtained because of disproportionation. Attempts to determine the phase diagram for the sodium chloride system failed because of excessive disproportionation. In the potassium chloride-zirconium trichloride system the eutectic temperature was 581° 2°C. A mixed system was used, however, since it was found that the stability of the trichloride was satisfactory in a 50 50 sodium chloride potassium chloride melt. Powder patterns of the frozen melt at 15 mole% zirconium trichloride which had been at 750°C for 24 hours showed no evidence of the dichloride and tetrachloride. There was evidence for slight solubility of the trichloride in the solid at this composition. The reaction of zirconium metal with the trichloride in the equimolar sodium-potassium chloride melt revealed that even after the trichloride had been reduced to the dichloride, zirconium metal dissolved to the extent of 5 x lO" or 10 mole fraction of excess metal in the solution. Zirconium metal did not, however, appear to dissolve in the melt in the absence of zirconium dichloride. 

Unlike the sodium systems, melt history has less influence on potassium Kurrol s salt systems mentioned above. Potassium trimetaphosphate is not a phase diagram entity. Potassium phosphate melts with R values of unity are very difficult to quench to glasses, because they crystallize very rapidly. If potassium phosphate melts are cooled slowly, long-chain polyphosphates, [KPOsln, are the only crystals to form. No other crystals are thermodynamically stable in this portion of their phase diagram and Kurrol s salts dominate a large area of this diagram. 

The electrical properties of the alkali metal salt complexes are of interest as solid electrodes for batteries (27, 27a). There is particular interest in alkali metal-thiocyanate salt complexes this interest is related to the anion having a high lyotropic number and a correspondingly high conductivity (see Table 4). The complete phase diagram has been published for the potassium thiocyanate/ and sodium/thiocyanate/polyethylene glycol system (28), and various thermodynamic properties have... 

If we refer to the MR -2R -3R diagram, the pole MR (feldspar) would become sodic as temperature increases while the mixed layered phase becomes potassic. If we consider calcium in this system, it will not form a feldspar as does sodium and must enter into solution or be precipitated as carbonate when montmorillonite layers decrease in the mixed layered phase. In either event the net effect is to reduce the potassium part... 

---