تسجيل الدخول

تسجيل

LIQUID PHASE EXTRACTION: POLARITY

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  p123-125

2026-07-04

23

LIQUID PHASE EXTRACTION: POLARITY

Polarity is the tendency of the compound to behave like a miniature magnet, with a positive side and a negative side. Not all compounds have this property. It depends upon their structure and symmetry. Figure 2 gives examples of polar and nonpolar compounds. Figure 2 (a) shows examples of nonpolar compounds and Figure 2 (b) shows polar compounds. Most organic compounds tend to be nonpolar or slightly polar, whereas inorganic compounds can range from nonpolar to very polar. Compounds that are symmetric tend to be nonpolar. Polarity is caused by an excess of electron density on one side of a molecule and therefore a deficiency on the other side. The side with the excess electron density has a net negative charge and the other side has a net positive charge because it is deficient in electrons. The degree of polarity exhibited by a substance determines its solubility in a given solvent. A simple rule about solubility and polarity is like dissolves like. This means that polar compounds have a greater affinity for other polar compounds and, in the case of solubility, it means that polar solutes will dissolve more readily and to a greater degree in polar solvents. This property is exploited in liquid extractions. If an analyte contains a mixture of substances where some are polar and others are nonpolar, they can be separated into two groups using a polar solvent such as water to dissolve the polar substances and a nonpolar sol vent such as methylene chloride to dissolve the nonpolar substances. Sometimes the polarity of a substance can be manipulated chemically in such a way so as to not change its basic nature. For example, cocaine is, like most organic compounds, fairly nonpolar. It can be made more polar by reacting it with a dilute acid, forming a salt. Salts are generally quite polar substances. The free cocaine can be recovered at any time with the use of an alkaline substance that reverses the action of the acid.

FIGURE 1 Immiscible liquids. The two liquids do not mix with one another. The less dense liquid sits on top of the more dense one. One is more polar than the other. The more polar analytes tend to dissolve in the more polar liquid.

FIGURE 2 Polar and nonpolar compounds. Compounds that exhibit certain types of symmetry are nonpolar. The electrons pull equally in all directions in nonpolar compounds. Examples are shown in (a). Polar compounds have an unequal pull of electrons in one direction. This creates positive and negative sides of the molecule. This can be most easily seen in the water molecule in (b).