Determinação da primeira constante aparente de dissociação em águas de alta força iônica a 25,0ºC.

Abstract

The determination of the dissociation constant of acid or base systems or salts performed by manometric and thermodynamic methods can only be applied for low ionic strength and infinite dilution. In this thesis, it is proposed to determine at 25 ° C the first apparent dissociation constant of high ionic strength tertiary systems such that: (A) NaCl + NaHCO 3 + H 2 O; KCl + NaHC03 + H20; (c) CaCl2 + NaHC03 + H2O and (D) MgCl2 + NaHCO3 + H2O; that is, natural waters of high salinity. As an experimental method, Gran titration was used in the pH range in which the two mathematical functions of interest (F1 and F2) apply; that is, 3.00 ph 6.35. These mathematical functions allowed to obtain the following parameters: (i) coefficient of the operational activity of H +, fh +; (ii) the volume of strong acid (HCL) required to the equivalence point of H2CO3, V1 (ml) and (iii) the first apparent decomposition constant of the carbonic system with free and complex species, K, respectively. The pH was obtained with the use of a cell composed of a glass electrode coupled to the reference electrode type saturated calomel; that is, cell with liquid junction. The calibration of the electrode system was done, before each experiment with the test solutions, in a standard solution of low ionic strength. Due to the difference between the ionic matrices of standard solutions and test, the pH was interpreted in terms of the Liquid Junction Potential Residual (RPJL). The influence of this factor was also incorporated into the mathematical model of the F1 and F2 functions. In this work it was also proposed a modeling for high salinity tertiary systems where the Loewenthal & Marais theory (10) was applied to the carbonic system at low ionic strength and with certain modifications. These report the incorporation of ionic force, RPJL and ion pair effects in the equations that define these systems. The results obtained for fH + showed increasing values ​​with the increase of ionic strength in the range 0.1 - t * 3. 0. A similar relationship was obtained by Cavalcanto, 8 v H (4) for binary systems of chlorides 5 25 ° C and in the same ionic strength range of this research. The final experimental results obtained showed that the constant K 1a increases with the increase of ionic strength.