Evaluation of Acidic-Alkaline Properties of Various Minerals through Thermodynamic Constants of their Hydrolytic Dissociation

Abstract

During the last decades a serious effort was mounted to calculate acidic-alkaline properties of minerals as an important characteristic of their stability both in melt and solution systems (D.S. Korzhinsky, A.A. Marakushev, V.A. Zarikov, D.Burt et al . ) . In particular, main ideas were selected in the special book : ' A c i d a l k a l i n e p r o p e r t i e s . . . ' (1982) . Nevertheless, the problem of the general thermodynamic description and comparison of various mineral groups and species is open now from this point of view. In this relation rather simple and effective approach is suggested below to create the wide system of conventional numerical values. There are several data bases including sets of thermodynamic constants or other characteristics of hydrolytic mineral reactions, such as Karpov (1981), Bowers et al. (1984), Ball, Nordstrom (1991). Some universal characteristics of different types of dissolution reactions can be calculated with the help of these or other data bases, Depending on the chemical environment, these characteristics reflect various types of interactions. The simplest type of reactions, chosen from Bowers et al.(1984) and illustrated in the Tablel (first column), has been used in this study. Taking such ideal acid solution without complexand secondary phase formation, but with some real level, e.g. 10-4mol/kgH20, for every produced aqua-ion activities, we calculate the necessary pH value, providing equil ibrium. Ion act ivi ty products, commonly known in chemistry, have been based on the same conception of standard state of solutions. The proposed values can be named as conventional indices of acidic-alkaline stability (CIAAS) of minerals in solutions. To check the accuracy of pH calculation, some reactions of hydrolytic dissociation have been calculated with the help of different data bases (Table 1, pHI 3). In addition to demonstrate the efficiency of the suggested approach, just same indices for a set of calcium minerals are presented in the right column of Table 2. Really, the good agreement between their decrease from top to bottom along with the drop of mineral solubility is obvious. In the first stage the hydrolytic dissociation of 40 oxides , s i l icates , a lumosi l i ca tes , carbonates , sulphates and sulphides has been estimated. We can repeat, that these reactions are true only in rather acid solutions and inconsistent with the near-neutral, and especially alkaline pH values. Moreover, it is well