Determinação da alcalinidade de águas de alta salinidade e alta força iônica a 25ºc.

Abstract

The main objective of this work is to present a very precise method for the determination of Total Alkalinity or simply Alkalinity of high salinity and high ionic water. This method is called Gran Titration (1952) and has numerous advantages over the other titrations employed in practice ("verbi gratia": the Conventional Colorimetric and Electrometric Methods). These advantages are related to the inclusion of several effects beyond those commonly known (influence of Temperature, Ionic Force and Total Concentration of Species in solution) in the equations that define the Gran functions as well as in the experimental methodology of this titration. These effects that are difficult to circumvent by other titrations are: (i) the formation of ionic or complex pairs due to the presence of sufficiently concentrated species that also affect the activity of the high salinity water species under study; (ii) the influence of hydration effects and ionic volume on the activity of high ionic strength and high salinity water species; and (iii) the pH interpretation regarding the effect of the Liquid Junction Potential Residual, RPJL. This residual is due to the use of electrochemical cell with liquid junction; ie with reference electrode with saturated potassium chloride solution, KCl. The high salinity water and high ionic strength used in the experiments performed was defined by the system H2C03 + CO2 (aq) + H2O + Dissolved Salts or H2CO * 3 + H2O + SD. This means that this water, in addition to incorporating the carbon system, takes into account the salts defined by the univalent chlorides KCl and NaCl and alkaline earth CaCl2 and Mg Cl2. The temperature for the investigated solutions was kept constant at 25 ° C. However, the concentration of dissolved salts was purposely increased in order to verify their influence on the experimental results obtained from alkalinity and the operational activity coefficient of ion hydrogen (resulting from the use of Gran Titration). Initially, the modeling of the system under study was shown, based on that developed by Loewenthal & Marais (1976) for the carbonic system. Then, besides showing the theoretical determination of Alkalinity, the following items were included in this thesis: (1) the determination of the First Function of Gran, F1, which serves to calculate the experimental Alkalinity; (2) the theoretical development of the titles employed in practice; (3) the experimental methodology employed for each method used and finally (4) the results obtained from the alkalinity according to each method applied and the discussion of these results. In the particular case of the experimental results of the hydrogen ion operational activity coefficient, f H + op, it can be said that increasing values ​​were obtained with increasing ionic strength in the ranges used for each type of high salinity water analyzed. Similar relationships were obtained by Cavalcanti, B. & Loewenthal, R. (1981) and Guimarães, P. & Cavalcanti, B. (1983) at 25 ° C in the same ionic range of this research. However, the experimental Alkalinity values ​​remained practically constant with the increase of ionic strength in all three methods used (Colorimetric, Electrometric and Gran Titration). Such observation was also verified in the work of Guimarães & Cavalcanti (1983). The final discussion of this research showed that Gran Titration is actually the method that allows the most accurate alkalinity of high salinity and high ionic strength waters. This is due to the fact that it circumvents all adverse effects by obtaining this parameter. This work is expected to be of use for further research involving both high salinity and high ionic water as well as those requiring a more accurate determination of H2CO * 3 Alkalinity or simply Alkalinity.