Buffering reactions involving minerals

Be able to set up the equations necessary to compute the buffer capacity of a water as a function of pH from its chemical analysis, given that reactions involving minerals contribute to the buffer capacity. 

Shortly after the discovery of hydrogen chloride and hydrogen fluoride in the Venusian atmosphere in 1967, chemists begin to investigate possible atmospheric/surface interactions that would buffer (control) the amount of both gases in the atmosphere. One of the hypothesized reactions involves the interaction of HCl with the mineral nepheline (NaAlSiOJ, as follows ... 

Note that reactions that involve the precipitation and dissolution of the minerals, with the exception of gypsum, in Davis and Ashenberg s (1989) simulations consume or produce protons. Therefore, they more or less buffer the pH changes upon the mixing of the pH 11.3 tailings fluid with the pH 2.7 pit lake water. It should also be noted that equilibrium with atmospheric CO2 also constitutes a buffer reaction. 

Stoichiometry (28) is followed under neutral or in alkaline aqueous conditions and (29) in concentrated mineral acids. In acid solution reaction (28) is powerfully inhibited and in the absence of general acids or bases the rate of hydrolysis is a function of pH. At pH >5.0 the reaction is first-order in OH but below this value there is a region where the rate of hydrolysis is largely independent of pH followed by a region where the rate falls as [H30+] increases. The kinetic data at various temperatures both with pure water and buffer solutions, the solvent isotope effect and the rate increase of the 4-chloro derivative ( 2-fold) are compatible with the interpretation of the hydrolysis in terms of two mechanisms. These are a dominant bimolecular reaction between hydroxide ion and acyl cyanide at pH >5.0 and a dominant water reaction at lower pH, the latter susceptible to general base catalysis and inhibition by acids. The data at pH <5.0 can be rationalised by a carbonyl addition intermediate and are compatible with a two-step, but not one-step, cyclic mechanism for hydration. Benzoyl cyanide is more reactive towards water than benzoyl fluoride, but less reactive than benzoyl chloride and anhydride, an unexpected result since HCN has a smaller dissociation constant than HF or RC02H. There are no grounds, however, to suspect that an ionisation mechanism is involved. 

The reason for the poor correlation between percent base saturation and pH is apparent in the assumptions needed to obtain equation 5.11 that is, the H -Ca exchange model oversimplifies the buffering response of soil solids to acid inputs. Specifically, the complicating involvement of in soil acidity must be considered. Reaction 5.9 cannot proceed very far before adsorbed H" begins to attack and dissolve the mineral on which it is adsorbed. This serves to consume H+, releasing structural A1 into soluble form ... 

Souchay and Graizon [151] considered that two electrons were involved in the reduction of benzophenone semicarbazone. Lund [28] demonstrated a four-electron mechanism for the electrode reaction with initial cleavage of the N —N bond in benzaldehyde semicarbazone, as with benzophenone semicarbazone in mineral acid solution. In the reduction of benzophenone semicarbazone in a solution buffered at pH 4.0 1-benzhydrylsemicarbazide, i. e., the product from the addition of two electrons, was isolated with 30% yield. The semicarbazones of cinnamaldehyde and benzalacetone can be reduced both in the protonated and in the unprotonated form four electrons are required in alkaline solution, but it has not been possible to determine... 

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