MEASUREMENT OF THE TEMPERATURE OF MAXIMUM DENSITY OF AQUEOUS-SOLUTIONS OF SOME SALTS AND ACIDS

The temperatures of maximum density (theta) of aqueous solutions of al kali-metal and ammonium halides and mineral acids have been measured a t low concentrations (0.002-0.08 mol kg(-1)). Plots of the shift in th eta (Delta theta(obs)) against concentration yielded smaller slopes (t ermed the Despretz constant, K-m or K-D) than those evaluated from lit erature data. In addition, K-m is found to have different values at lo w and high concentrations. The earlier hypothesis of a linear relation ship between Delta theta(obs) and concentration (Despretz law) over a wide range of concentration is disproved, a finding that is supported by theoretical considerations. The results have been used to define li miting ionic Despretz constants for various ions. The individual limit ing ionic Despretz constants computed using the additivity principle a re found to be in the order F- < Cl- < Br- less than or equal to HSO3- < l(-) < ClO4- < SO42- for anions and H+ < Li+ < NH4+ < K+ < Na+ < Rb + < Cs+ for cations. Limiting ionic K-m values correlate better with t he effective ionic radii of ions computed by Leyendekkers using the Ta mmann-Tait-Gibson (TTG) model of aqueous solution properties, than Pau ling ionic radii. From a plot of K-m against effective ionic radius it has been possible for the first time to rationalize the theta behavio ur of cations of tetraalkylammonium salts, which are known to be struc ture stabilizers and whose theta behaviour could not be explained prop erly by earlier workers.