A new apparatus for measuring the steady-state temperatur difference b
etween two saturated solutions of the inert salt, one of them continin
g a foreign substance, is discribed. The three phases are, the vapour
of the solvent, a saturated solution of a supporting (inert) salts, an
d the solid phase of the inert salt. The mixtures are kept on the top
of temperature sensors and are first separated off from each other and
the common atmosphere of the measuring cell using a specially formed
lid. After balancing of the Wheatstone bridge, the lid is lifted and t
he establishing of the temperature difference is recorded using the in
creasing bridge potential difference Delta U, which is proportional to
the total concentration of all foreign species. All dissolved ions an
d molecules are to be considered as foreign species expect the ions of
the inert salts. Three Phase Vapour Tensiometry (TPVT) was successful
ly used to detect binuclear species formed in the titration of diaqua-
Cr(III) complexes with ethyltrimethylammonium hydroxide in saturated s
olution of ethyltrimethylammonium perchlorate. TPVT titrations of lant
hanum nitrate with water in acetone saturated with lithium nitrate and
tetraheptylammonium nitrate, respectively, gave a hydration number of
9 for La(III).