Determinação experimental de constantes de dissociação de ácidos fracos orgânicos.

Abstract

Natural waters are governed mainly by the carbonic system; that is, they contain carbonic species free or associated with other species in solution. Certain natural waters, for example domestic wastewater, also contain volatile weak acids such as acetic, propionic, butyric, and the like. For the definition of the chemical balance of natural waters, it is necessary to know the dissociation constants pertinent to the reactions involved in the analyzed system. These constants undergo the influence of two parameters: (i) the ionic strength and (i i) the temperature. Literature is poor in terms of the dissociation constants of weak mono and multiprotic acid systems. The vast majority of data reported only at 25 ° C. The main objective of this thesis is to determine the dependence of the first dissociation constant of the carbonic system CH 2 CO 2 + C 0 2 (aq + H 2 O) and the dissociation constant of the acetic system The ionic strength range was 0.025 ^ I s 0.200 and the temperature range was 25-40 ° C, that is, the temperature commonly used in acid studies (natural and superficial wastewater). The experimental method used was Gran in the pH range corresponding to the two mathematical functions of interest (F1 and F2) / that is, 3.00 <pH 6.35 for the carbonic system and 2.00 ^ pH <5.20 for the acetic system. The interpretation of the straight lines obtained in the design of the functions F1 and F2 versus volume of mineral strong acid added (v ") was done through the statistical method of" Minimal Squares ". The application of the functions F- ^ and F2 allowed us to obtain the following parameters of interest to the research: (1) The coefficient of operational activity of the hydrogen ion in the molar scale, fHQp * (21 The volume of mineral strong acid (HCl) the equivalence point of interest, v (ml) and (31) The experimental dissociation constant of the system under investigation, K '. The pH values ​​were obtained through an electrochemical cell consisting of a glass electrode coupled to a reference electrode silver - plate chloride and with a salt bridge of saturated KCl, that is, cell with liquid junction. Although this type of electrochemical cell was used, it was not considered necessary to determine the Residual Potential of Liquid Junction (RPJL) because most of the test solutions were of low ionic strength. Due to the type of measurement used in this thesis, electroimetric measurement of operational p (activity) was considered the influence of the ionic force on the specific ion of measurement, ie, ion hydrogen. This was done by incorporating the H + ion operational activity coefficient into the equilibrium equations of the analyzed systems and in the theoretical discussion of the Great Method. In this work the modeling for the carbonic system (according to the theory developed by Loewenthal and Marais7) and the acetic system are also shown. The latter was made from the theory of weak monoprotic acid systems *** 7 '. The experimental results obtained for the carbonic and acetic systems showed that the ionic strength dependence is minimal when it comes to natural waters with low ionic strength. There was only a small increase in the dissociation constants surveyed with the increase in ionic strength. However, the temperature dependence is more pronounced, that is, the experimental dissociation constants increased significantly with increasing temperature.