The Phase Rule and Its Applications

by Alexander Findlay

Transition Point.—In the case of the formation of double salts from two single salts, we saw that there was a point—the quintuple point—at which five phases could coexist. This point we also saw to be a transition point, on one side of which the double salt, on the other side the two single salts in contact with solution, were found to be the stable system. A similar behaviour is found in the case of reciprocal salt-pairs. The four-component system, two reciprocal salt-pairs and water, can give rise to an invariant system in which the six phases, four salts, solution, vapour, can coexist; the temperature at which this is possible constitutes a sextuple point. Now, this sextuple point is also a transition point, on the one side of which the one salt-pair, on the other side the reciprocal salt-pair, is stable in contact with solution.

The sextuple point is the point of intersection of the curves of six univariant systems, viz. four solubility curves with three solid phases each, a vapour-pressure curve for the system: two reciprocal salt-pairs—vapour; and a transition curve for the condensed system: two reciprocal salt-pairs—solution. If we omit the vapour phase and work under atmospheric pressure (in open vessels), we find that the transition point is the point of intersection of four solubility curves.

Just as in the case of three-component systems we saw that the presence of one of the single salts along with the double salt was necessary in order to give a univariant system, so in the four-component systems the presence of a third salt is necessary as solid phase along with one of the salt-pairs. In the case of the reciprocal salt-pairs mentioned above, the transition point would be the point of intersection of the solubility curves of the systems with the following groups of salts as solid phases: Below the transition point: NH₄Cl + NaNO₃ + NaCl; NH₄Cl + NaNO₃ + NH₄NO₃; above the transition point: NaCl + NH₄NO₃ + NaNO₃; NaCl + NH₄NO₃ + NH₄Cl. From this we see that the two salts NH₄Cl and NaNO₃ would be able to exist together with solution below the transition point, but not above it. This transition point has not been determined.

Formation of Double Salts.—In all cases of four-component systems so far studied, the transition points have not been points at which one salt-pair passed into its reciprocal, but at which a double salt was formed. Thus, at 4.4° Glauber's salt and potassium chloride form glaserite and sodium chloride, according to the equation

2Na₂SO₄,10H₂O + 3KCl = K₃Na(SO₄)₂ + 3NaCl + 20H₂O

Above the transition point, therefore, there would be K₃Na(SO₄)₂, NaCl and KCl; and it may be considered that at a higher temperature the double salt would interact with the potassium chloride according to the equation

K₃Na(SO₄)₂ + KCl = 2K₂SO₄ + NaCl

thus giving the reciprocal of the original salt-pair. This point has, however, not been experimentally realized.[^[384]]

Transition Interval.—A double salt, we learned (p. [277]), when brought in contact with water at the transition point undergoes partial decomposition with separation of one of the constituent salts; and only after a certain range of temperature (transition interval) has been passed, can a pure saturated solution be obtained. A similar behaviour is also found in the case of reciprocal salt-pairs. If one of the salt-pairs is brought in contact with water at the transition point, interaction will occur and one of the salts of the reciprocal salt-pair will be deposited; and this will be the case throughout a certain range of temperature, after which it will be possible to prepare a solution saturated only for the one salt-pair. In the case of ammonium chloride and sodium nitrate the lower limit of the transition interval is 5.5°, so that above this temperature and up to that of the transition point (unknown), ammonium chloride and sodium nitrate in contact with water would give rise to a third salt by double decomposition, in this case to sodium chloride.[^[385]]