RELATIONSHIP BETWEEN TRIPLE ION FORMATION-CONSTANTS AND THE SALT CONCENTRATION OF THE MINIMUM IN THE CONDUCTOMETRIC CURVES IN LOW-PERMITTIVITY SOLVENTS

Conductivities of a number of uni-univalent salts, including tetrabuty lammonium and lithium nitrophenolates, were measured at 25.0 degrees C in low-permittivity solvents such as tetrahydrofuran (THF, epsilon(r) = 7.58), 1,2-dimethoxyethane (DME, 7.2), chloroform (4.8), and ethyl acetate (6.0). Minima in the conductometric curves (Lambda-C-1/2) were observed for concentrations which were dependent upon both the salt a nd the solvent, C-min = 1.73 x 10(-4) mol dm(-3) for 2,4-(NO2)(2)C10H5 OLi (lithium 2,4-dinitro-1-naphtholate) in THF and 2.56 x 10(-2) mol d m(-3) for LiPic (lithium picrate) in DME. The observed molar conductiv ities including C-min could be completely explained by the formation o f ion pairs (M+ + X- reversible arrow MX, K-1), ''symmetrical'' triple ions (2M(+) + X- reversible arrow M2X-, K-2; M+ + 2X(-) MX2-, K-3; K- 2 = K-3), and (in some cases) additional formation of quadrupoles (2MX reversible arrow M2X2,K-41). A linear relationship (the slope of -1) between the triple ion formation constants (log(K-2/K-1)) and the salt concentrations at the minimum (log C-min) was given for all the salts in the various solvents, except for some systems in which a distinct formation of quadrupole takes place, e.g., LiNO3 in DME (K-1 = 3.16 x 10(10), K-2 = 4.5 x 10(13), and K-41 = 35). The formation of triple io ns might be attributed to the ion sizes in solutions in which Coulombi c interactions were the only main forces between ions (R4N+... X-). Ho wever, coordination (or covalent) bonding forces as well as Coulombic forces had to be considered for the lithium salts except for LiClO4 an d LiBF4). Gutmann's donor and acceptor numbers of solvents (and not th e permittivity) accounted for the larger difference of C-min, of lithi um salts in THF and DME. In mixed solvents of THF and 2-ethyl-1-hexano l (epsilon(r) = 7.58), the C-min values of LiNO3 and 2,4-(NO2)(2)C10H5 OLi increased with increasing contents of the hexanol, whereas the C-m in values of LiClO4 and Bu4NX (X- = NO3-, 2,4-(NO2)(2)C10H5O-, and ClO 4-) remained constant for 0-30 vol % hexanol added to THF. The positiv e shifts in C-min were explained quantitatively by the decrease in tri ple ion formation constants and/or by an increase in the quadrupole fo rmation constants.