Partition equilibrium solvents

The relations (pA)a = (fijdp 5 (Fb) = (Pb)p would only by the merest chance form the solution of (2), hence there will not in general be a partition equilibrium between the ions when one is established between the neutral.molecules, but one solvent, say a, will contain more A ions than corresponds with ionic partition equilibrium. These will pass through the surface of contact into /3, and similarly B ions from /3 to a. The separation of the two kinds of ions will however set up an electrostatic field across the boundary, and the two kinds of ions collect there in two sheets very close together—in fact, we have an electrical... 

An electrolyte, e.g. HCl is shaken up with the two immiscible solvents, water and oil, until the partition equilibrium is arrived at. 

We have determined the ion-pair formation-partition equilibrium constant with picrate anion for a number of primary, secondary, tertiary and quaternary ammonium ions 23>. In aqueous media of pH 5-6, the ammonium ions and picrate are considered to exist almost completely as unpaired counter ions. When the aqueous solution is mixed with an immiscible organic solvent, the ions are partitioned into the organic phase as the ion pair. We expected that the steric effect of N-substituents in the ion-pair formation-partition equilibrium could be analyzed by a procedure similar to Eq. 24, and derived Eq. 27 for the set of quaternary ions 23). 

Automatic headspace samplers are available from manufacturers of gas chromatographs. These devices are based on the technique of sampling an amount of vapor above the sample itself. Samples are sealed, neat or in a suitable solvent, in containers, and hold at a preset temperature in a thermostatted liquid bath. The headspace vapor results as a partition equilibrium is established between the liquid or solid and the gaseous phase of the volatiles. As each sample is presented to the analyzer, the vessel is punctured and a portion of the headspace gas is withdrawn by a pneumatic injection technique and forced into the column. The main application for those samplers is in the routine analysis of low-boiling fractions in samples containing nonvolatile solids or high-boiling components. Some of the more popular applications today are ... 

For LLE the liquid sample is mixed with a larger volume of a non-polar organic solvent to induce a partition equilibrium of the analyte between the aqueous and organic layer. Typically, the latter one contains the major fraction of analytes and is further processed for LC-MS analysis. In contrast to protein precipitation, LLE thus requires a subsequent drying step by evaporation or with a gentle stream of nitrogen... 

The partition equilibrium depends on the solubility of the analyte in the organic solvent but more critical on the analyte s polarity that is determined by the functional groups and charge. As discussed above (Lipophilicity of TA) the log P value is a measure of extractability into an organic phase. The higher the log P value the more lipophilic is the compound (Table 1). 

In the above discussion it is assumed that during storage a partition equilibrium is established between the packaging and food. However, this is not always the case. Given the time ti/2, which is the time required for half of solvent contained in the packaging material to be transferred to the food, then one gets ... 

The partition coefficient (k/sw) is the ratio of concentrations of a specific compound in an immiscible solvent and water at equilibrium. Solvents with high partition coefficients can sequester the target compound, thus limiting its availability for the enzyme action in the aqueous phase [129], The partition coefficient for anthracene was determined in 15 solvents from different nature mineral oils, vegetable oils, alcohols, hydrocarbons, and others. The values of log K w obtained ranged from 3.7 (silicone oil) to 5.2 (undecanone). Among the solvents evaluated, two were selected for a further study the solvent with the lowest partition coefficient (silicone oil, log Ksw 3.7) and a solvent with an intermediate value (dodecane, log Ksw 4.5) [53],... 

In two-phase membrane systems, one of the surrounding phases is the same as the membrane phase, so there could be, for example, an organic solvent both in the membrane pores and in the acceptor, while the donor is aqueous or gaseous. There is only one phase boundary, and consequently, only one partition equilibrium. This technique is chemically analogous to LLE in separation funnels. 

Consider two pairs of solutions (of the same components), each in partition equilibrium, and let the concentrations of the solute in the first solvent A be c and c -f dc , and in the second solvent C(, and Cft+dcj,. Let all the solutions be present in such large volumes that the addition or withdrawal of 1 mol. of the solute does not appreciably alter the concentrations. 

Which extraction mode is the better remains a controversial issue. While the static mode provides longer contact between the sample and solvent, swells the matrix and facilitates penetration of the extractant in its interstices — thereby increasing its efficiency — the dynamic mode allows the analyte to be continuously exposed to the pure (clean) solvent, thus favouring displacement of the analyte s partitioning equilibrium to the mobile phase. Most SFE methods use both modes a static step is employed to ensure close contact between the sample and supercritical fluid without consuming much extractant that is followed by a dynamic step where the extracted analytes are driven to the restrictor and equilibrium is allowed to complete. 

All other variables being equal, a partitioned equilibrium for the analyte between the sample matrix and the extraction solvent is reached more quickly at higher temperature and pressure because the analyte solubilization kinetics are improved. Therefore, cycle time can be much shorter for ASE extractions relative to room-temperature/pressure-solvent extractions. If certain sample variables such as pore size or structure make rapid equilibrium questionable, it is simple to design a recovery versus extraction time experiment (the results of which are shown in Figure 9) so that variability and lower recovery due to a pre-equilibrium phase separation can be avoided. The desirable extraction duration is a trade-off between the recovery and the time required to achieve it and generally runs from 10 to 17 min. 

Numerical values for the structural parameters of QSAR are obtained from an evaluation of the effect of the substituent on the properties (e.g., the rate or equilibrium constants) of a model reaction. The classification of these parameters is therefore model dependent. The model reactions are chosen to represent the most pervasive types of physicochemical phenomena (e.g., dissociation reactions, hydrolysis, substitution reactions, partition between solvents). 

Liquid-liquid extraction (LLE) has been around for a long time and has been used extensively as an analytical sample pretreatment to remove unwanted matrix components [16,17]. It is based on the principles of differential solubility and partitioning equilibrium of analyte molecules between aqueous (the original sample) and organic phases. Depicted in Fig. 5, LLE initially involves pH adjustment of the sample with an appropriate buffer. This pH adjustment is intended to neutralize the molecule, making it more amenable to extraction. The next step is the addition of an immiscible organic extraction solvent, followed by agitation... 

Only the unionized form of a drug is extracted into the organic solvent. Therefore acidic drugs, which are unionized imder acidic conditions are extracted from acidified matrices into organic solvents basic drugs are likewise extracted from basified matrices. The optimal pH for acidic species is 1-2 pH units below their pK values, and for basic species it is 1-2 pH units above their pK values. Extraction can be difficult for compoimds which are soluble in water at all pH values, for example water-soluble amphoteric and neutral drugs. In some cases, the addition of buffer salts to the aqueous solution increases its ionic strength and hence its polarity. This tends to decrease the affinity of polar compoimds for the aqueous phase, and thus shifts the partition equilibrium in favor of extraction. 

In the dynamic extraction mode, the extractant is pumped through the sample at a preset flow rate. This mode allows the analyte to be exposed continuously to the pure (clean) solvent, thus favoring displacement of the analyte s partitioning equilibrium to the extractant. 

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