Anal. Chem. 1989, 6 1 , 1971-1977
1971
Ionic Equilibria of Picric Acid in Mixed Amphiprotic Solvents. The 2-MethoxyethanoVWater Solvent System Andrea Marchetti, Emanuele Picchioni, Lorenzo Tassi, and Giuseppe Tosi*
University of Modena, Department of Chemistry, via G . Campi, 183, 41100 Modena, Italy
Picric acld Is the gulde solute used In our studies In order to verb the applicability of an emplrkal treatmen! that descrlbes the dependence of the dlssoclatlon constant on temperature and composltlon of the solvent mlxture ( x Is mole fractlon). The 2-methoxyethanoVwater solvent system has been employed In thls work operatlng at 19 temperatures ranging from -10 to +80 'C; by use of the conductometric method the dissociation constant values of plcrlc acld have been evaluated by the Fuoss-Hsla equation. The data have been fltted by three emplrlcal equations that represent the functlons K = K ( T ) , K = K ( x ) , and K = K ( T , x ) . The K = K ( T , x ) equatlon In Its complete form Is composed of 20 terms, some of whkh can be elhnlnated because of small statistkal weigM; the number and type of these terms vary on passlng from one solvent system to another, and the best-flttlng form, whkh In thls case represents 13 terms, Is suggested. The posslblllty of extrapolatlng K values of plcrk acM In a pure water solutlon Is lnvestlgated and compared wlth similar previous calculatlons.
INTRODUCTION The importance of the study of dissociating behavior of weak electrolytes is demonstrated by the increasing interest and noticeable effort of many research workers. However, the role of the solvents in ionization and dissociation processes of weak solutes is still not well understood, because of the complexity of physicochemical phenomena that take place in solution (1). The use of binary solvent systems (or higher mixtures of miscible components) apparently can make this problem difficult. On the contrary, it represents an useful and powerful tool of experimental investigation, making possible the continuous variation of some physicochemical properties such as density, viscosity, and dielectric constant; all these parameters play, in fact, a significant role in the ionization and dissociation processes. As regards our specific interest in obtaining a calculation model able to predict acid-base titration curves in nonaqueous solvents, in their mixtures and in mixtures with water (2-7), we need to know at first some characteristic properties of the systems under study, such as the dissociation constants of the utilized electrolytes, the association constants of the adducts formed during the titrations, and the ionic mobilities in solution. Recently we have performed a series of preliminary investigations on picric acid, chosen as the guide solute, in some solvents, i.e. pure 1,2-ethanediol (Gliet), pure 2-methoxyethanol (Gliem), their binary mixtures (8-lo), and the binary system Gliet/water (11). The dissociation constants of the above solute were evaluated by the conductometric method a t 19 temperatures in the range between -10 and +80 "C. In connection with our previous studies we report here the dissociation constant determination of the same solute in the binary system Gliem/water using the same conductometric technique in the temperature range above mentioned in the pure solvents and in their six mixtures. In the case of pure 0003-2700/89/0361-1971$01.50/0
water and of another particularly water rich mixture we have encountered some difficulties in evaluation and interpretation of experimental data (see the discussion below). This study is, consequently, a part of systematic research regarding both the influence of the solvents on the dissociation constants of weak electrolytes and the correlation between the feasibility of conductometric titrations of such solutes under suitable experimental conditions and the properties of the solvent systems employed (2-11).
EXPERIMENTAL SECTION Materials. The 2,4,6-trinitrophenol(picric acid), supplied by BDH (purum ca. 95%), was twice purified by recrystallization from hot ethanol and diethyl ether (mp 122 "C; lit. 122-3 "C) (12). The solvent 2-methoxyethanol (containing
