Reciprocal salt-pairs transition point

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 sal -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. 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 ... 

Transition Interval.—double salt, we learned (p. 242), 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 of the double salt be obtained. A similar behaviour is also found in the case of reciprocal salt-pairs. In the case of each salt-pair there will be a certain range of temperature, called the transition interval, within which, if excess of the salt-pair is brought into contact with water, interaction will occur and one of the salts of the reciprocal salt-pair will be deposited. For the salt-pair which is stable below the transition point, the transition interval will extend down to a certain temperature below the transition point and for the salt-pair which is stable above the transition point, the transition interval will extend up to a certain temperature above the transition point. Only when the temperature is below the lower limit or above the upper limit of the transition interval, will it 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 will give rise to a third salt by double decomposition, in this case to sodium chloride. ... 

Formation of Double Salts.—In many cases of four-component systems, the transition point is not a point at which one salt-pair passes into its reciprocal, but one at which a double salt is formed. Thus, at 4 4° Glauber s salt and potassium chloride form glaserite and sodium chloride, according to the equation... 

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