Encyclopedia of Lipidomics

Living Edition
| Editors: Markus R. Wenk

Liquid Extraction: Acidic Extraction

  • Thusitha RupasingheEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-94-007-7864-1_93-1


Partition Coefficient Extraction Procedure Extraction Efficiency Distribution Coefficient Polar Lipid 
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The extraction efficiency of the analyte relies on the solubility of the analyte, during the liquid-liquid extraction. In a simple liquid-liquid extraction system, the partition coefficient is equal to the distribution ratio. Therefore, the distribution of the analyte between two phases does not depend on the composition of the two phases. In this situation, changes in pH of one phase do not affect the extraction the analyte. However, when the analyte undergoes an additional reaction within a phase, the distribution ratio may differ to the partition coefficient. If some ionic species are not soluble in the extract, higher extraction efficiency can be achieved by changing the pH of the raffinate. By changing the pH of the extraction system, the distribution coefficient of the solute can be changed to favor partitioning into the extract. The advantage of changing the pH of a liquid-liquid extraction system is that other functional groups are not affected by the changes in pH, so the separation of two layers remains unchanged (Wells 2003).

When the pH of the system affects the extraction of the analyte, in a weak acidic form (HAr), as shown in Fig. 1, the partition coefficient (KD) and the distribution ratio (D) are given as follows:
Fig. 1

Schematic diagram of solute’s partitioning of the weak acid, HA in the liquid- liquid extraction system.

$$ {\mathrm{K}}_{\mathrm{D}} = \left[{\mathrm{HA}}_{\mathrm{e}}\right]/\left[{\mathrm{HA}}_{\mathrm{r}}\right]\ \mathrm{and}\ \mathrm{D} = \left[{\mathrm{HA}}_{\mathrm{e}}\right]/\left\{\left[{\mathrm{HA}}_{\mathrm{r}}\right] + \left[{\mathrm{A}}^{-}\right]\right\} $$
The equilibrium constant, K a for the weak acid is given as \( {K}_a = \left[{\mathrm{H}}^3{\mathrm{O}}^{+}\right]\left[{\mathrm{A}}^{-}\left]/\right[{\mathrm{H}\mathrm{A}}_{\mathrm{r}}\right] \)

By simplifying the above two equations, D is given by \( \mathrm{D} = {\mathrm{K}}_{\mathrm{D}}\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]/\left(\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right] + {K}_a\right) \)

The above equation clearly demonstrates that the distribution ratio depends on the pH of the raffinate. By selecting an appropriate pH, the extraction efficiency of the analyte A could be improved significantly.

Bligh and Dyer (1957, 1959) have demonstrated that the use of hydrochloric acid in lipid extractions increases the extraction efficiency. Lipids associated with polar head groups, such as glycosylated sphingolipids, which are abundant in plant tissues, can be extracted with higher efficiency by using acid extraction. When using biphasic extraction procedures, glycosylated sphingolipids remain insoluble in the organic phase and are recovered in the aqueous phase. To overcome this loss of polar lipids during a biphasic extraction, a new extraction protocol adding acids to change the pH has been developed by Buré et al. (2014), demonstrating an overall improved extraction efficiency. However, acid hydrolysis of some lipid species such as plasmalogens has been reported by Frosolono and Rapports (1969). Therefore, chemical reactions that occur during the extraction procedure in the presence of acid should be carefully considered when establishing liquid-liquid extraction protocols for lipids.


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© Springer Science+Business Media Dordrecht 2016

Authors and Affiliations

  1. 1.The University of MelbourneParkvilleAustralia