https://doi.org/10.1351/goldbook.15344
Isothermal, isobaric reversible reaction between two phases, a saturated liquid and a solid, that results, on removal of the solvent component of a ternary system in one new solid phase. For example, \[\ce{l + \upalpha <=> \upbeta}\] where the forward arrow indicates the direction of removal of the solvent component. The system is in equilibrium along the peritonic line, on which is found the peritonic composition or point, an isothermal, isobaric invariant of the system which lies between the compositions of phases \(\ce{l}\) and \(\upalpha\).
Examples:
- In an ionic system, \[\ce{AX.p H2O(s) + B^{z+}(aq, sat) +} (q - p)\ce{H2O(l) BX.q H2O(s) + A^{z+}(aq, sat)}\] The two salts may also contain the same ions.
Note 1: Named from a combination of eutonic and peritectic.
Note 2: The very general terms transition point or composition are deprecated, as they can refer to many other types of equilibria. Historically, the use of transition point in this case dates back to at least. - System \(\ce{KCl + MgCl2 + H2O}\).
See also: dystonic reaction, eutonic reaction