A determinação da alcalinidade total de águas naturais a 25ºC.

Abstract

The parameters Alkalinity and Acidity associated to pH terminations constitute the fundamental tools for the interpretation and chemical correction of natural terrestrial waters, ie meteoric, surface, underground and domestic wastewater. Alkalinity measurements commonly used in practice involve: (a) colorimetric measurements where a chemical is used that has the property of changing color at the desired pH or equivalence point; and (b) electrometric measurements where a specific electrode is used ion H +, coupled to a reference electrode with or without liquid junction. In both measurements, the observed pH value (also referred to as operating pH) is interpreted according to standardized methods in common use. However, such methods when applied to the determination of Total Alkalinity or Alkalinity of CO2 from natural waters containing the carbonic system (ie, H0, CO2, H2O) tend to incur considerable errors. In other words, the interpretation (whether mathematical or graphical) of these common methods generally does not allow the accurate realization of the value of the equivalence point of C02, that is, the pH established by the addition of x moles of a solution of CO2 to one liter of pure water. This is due to the fact that this equivalence point is influenced by the following factors: (1) Total concentration of the carbonic species in solution, CT, (2) temperature, T and (3) ionic strength, I. The parameter is the one with the greatest influence on the CO2 equivalence point Any addition or expulsion of CO2 during an electrometric titration will affect the determination of this equivalence point and, the calculated experimental value of Total Alkalinity of natural waters The main objective of this research is to show the application of mathematical functions developed by Gran (1952) for weak mono and multiprotonic acid systems, in adapting Gran's First Function to the carbonic system, that is, to apply Gran Titration to natural waters. In this way, a more accurate determination of Total Alkalinity of natural waters is possible. For this purpose, three types of water of low ionic strength (I <0.1) were used, and at a constant temperature of 25 ° C, namely: (1) Surface water treated and coming from the source Boqueirão, Campina Grande, PB; (2) Mineral water, india brand, Santa Rita Mineral Source, PB (3) Synthetic water composition NaCl + NaHCO3 + H2O. In addition to this objective, the work also shows the most appropriate ways to obtain more valuable results from the analysis of the operational pH values ​​by applying graphic methods (Straight function of Gran x added volume of strong mineral acid and apH / aV x aV ) and statistic (Minimum Square Method) The analysis performed on the experimental results with the theoretical ones (notably in the case of synthetic water) showed that Gran Titration actually presents the most accurate Alkalinity data since the method provides exactly the value of the volume of mineral strong acid required to the equivalence point of interest. The other advantages of applying Gran Titration to Experimental alkalinity, namely: (a) Its application was made considering the definition of pH in terms of H + ion operational activity; (b) the coefficient of operational activity of the hydrogen ion was obtained (which is not possible in the other titrations) and (c) the Residual Potential of Liquid Junction Potential (RPJL) was determined when using the electrochemical cell type Electrode of Glass coupled to the reference electrode with liquid junction. It is interesting to note that Residuals of Net Junction Potential were considered as e r r o in pH measurements and not were computed in the calculations of Alkalinity.