Retraction from aqueous solutions

Figure 13. Wettability of various monolayers on platinum prepared by retraction from aqueous solution [43]...

The pH of a non-aqueous solution can be below 0 pH for an acidic solvent, such as acetic acid, and be above 14 pH for a basic solvent, such as an amine. The pH measurements can only be compared to measurements in the same solvent because the dissociation constants, tlie solvent s dielectric constant, and ionic mobility will all be different. The neutral point is not necessarily 7 pH. For example, the neutral point of methanol is 8.42 pH and ethanol is 9.55 pH at 25 C on the water-based scale [Ref, 4.14]. The buffer solvent should be as similar as possible to the process solvent. If a batch process moves from an aqueous to a non-aquetms composition, the readings cannot be compared between phases but an automatic retractable assembly can enable cleaning, buffer, and soaking solutions that match up sensor calibration and conditioning with the batch phase. If the water content drops below o Vo, the solution can be considered to be non-aqueous [Ref. 4.2]... 

LPME or single-drop microextraction (SDME) involves extraction of organic contaminants from an aqueous donor solution into a microdrop (typically 1 jjlL) of an organic acceptor solvent suspended from the tip of a microsyringe. After extracting for a preset period of time, the microdrop is retracted back into the microsyringe and transferred to a gas chromatograph for further analysis [114]. Various types of pesticides have been extracted by LPME from water samples for determination by GC [115-121]. Recently, an automated d)mamic liquid-liquid-liquid microextraction (D-LLLME) system was developed for the extraction of phenoxy acid herbicides from environmental waters and their determination by LC [122]. 

Most of the examples discussed are related to reversible (equilibrium) conditions. The behavior of the adsorption layer formed on hydrophobic methylated surfaces in aqueous surfactant solutions is shown in Figure 4.26a. Compression of the particles resulted in the retraction of the adsorption layer, which returned once the particles were separated. The situation is principally different in the case of adsorption from a nonpolar phase on polar particles, as is schematically shown in Figure 4.27b. The fixation of the adsorption layer by chemisorption, as in the case of the adsorption of amines on silicate glass, resulted in an adsorption layer that had its own strength, and a certain... 

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