**3.3 Dialysis method**

*Phytochemicals in Human Health*

*3.2.1 Solvent precipitation method*

*Flow charts of common polarity gradient extraction method.*

**Figure 1.**

separated with this method. For example, see [12].

*3.2.2 Exclusive reagent precipitation method*

The solubility of some components in the mixed component solution can be changed by adding a specific solvent that can be mutually soluble with the solution, so it can be precipitated from the solution. The gradual precipitation by changing the polarity or amount of solvent added is called fractional precipitation. For example, using water as an extracting solvent to extract phytochemicals, ethanol is added to the water extracting concentrate to make its alcohol content more than 80%, and then polysaccharides, proteins, starch, gum, and so on will be precipitated and removed after filtration. The preceding procedure is called water extraction and ethanol precipitation. Crude polysaccharides from plants are often

Some reagents could react selectively with certain chemical constituents to produce reversible precipitation, and the separation aims are achieved, which is called the exclusive reagent precipitation method. For example, alkaloid precipitation reagents such as Reynolds ammonium salt can precipitate after reacting with alkaloids, which can be used to separate alkaloids and nonalkaloids, or watersoluble alkaloids and other alkaloids. As another example, reactions of cholesterol and sterol saponins could form precipitation, which can separate them from

triterpene saponins. Additionally, gelatin can precipitate tannins, which can be used to separate or remove tannins. In practical application, appropriate precipitation reagents should be selected according to the properties of target constituents and

Adding inorganic salts to a certain concentration or saturated state in the water extract of plants can reduce the solubility of some components in water, thus they could be separated from water-soluble compounds. The inorganic salts commonly used for salting out are sodium chloride, sodium sulfate, magnesium sulfate, ferric

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impurities in plants.

*3.2.3 Salting out method*

It is a method to let substances selectively penetrate through natural or synthetic semi-permeable membranes (or dialysis bags) under the action of concentration difference, pressure difference, or potential difference, so as to achieve the purpose of separation, classification, purification, or concentration. For example, when saponins, proteins, polypeptides, polysaccharides, and other substances in plants are separated and purified, dialysis can be used to remove inorganic salts, monosaccharides, and other impurities. On the contrary, large molecular impurities can also be left in the semipermeable membrane, while small molecular substances can be separated and purified through the semi-permeable membrane into the solution outside the membrane [14].
