engleski

Net surface proton excess of smectites obtained from a combination of potentiometric acid-base, mass and electrolyte titrations

Difficulties in the determination of absolute net proton and hydroxide surface consumption of clays from potentiometric acid-base titration are due to the fact that unlike for oxide minerals curves of relative proton related surface charge densities for several electrolyte concentrations are shifted and do not exhibit a common intersection point. Moreover, the isoelectric point (pHiep) of clays is usually in the acidic region and differs from the point of zero net proton and hydroxide consumption (pHncph). As a simple experimental procedure for clays we propose to perform potentiometric mass titrations in combination with potentiometric electrolyte titrations for the determination of the pHncph at different electrolyte concentration. Potentiometric acid-base titrations (pH of the suspension monitored as a function of the volume of added acid or base) and mass titrations (pH of the suspension monitored as a function of the mass concentration of added solid) of two pretreated clay samples (SAz-1 and SWy-2) were carried out in sodium chloride media. Due to the pre-treatment samples SAz-1 and SWy-2 contain ≥ 92 wt.% montmorillonitic smectite. The mass titrations were supplemented by electrolyte titrations (pH of the suspension of high mass concentration measured as a function of electrolyte concentration). The observed decrease in the mass-titration endpoint pH due to additional salt clearly corroborates previous reports that the point of zero net proton and hydroxide consumption of smectites depends on the electrolyte concentration.

mass titration; montomorillonite; smectite; adsorption; acid-base properties; surface charge

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