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Thursday 13 May 2021

Water Soluble Amine Bases for Extracting Hydrophobic Acids into Water



A few organic acids are so hydrophobic, high-melting, or both, that they do not form water-soluble sodium or potassium salts. Two examples are pamoic and orotic acids. If you require that such an acid give a salt that either dissolves in water or is extracted into an alkaline solution, resort must be made to very hydrophilic amines as the neutralizing base.

 

Once such a salt is taken up in the water the hydrophobic acid can be precipitated in free acid form by protonation with hydrophilic acids such as phosphoric, oxalic, citric or tartaric acids that are 'comfortable' existing as salts in water with the very hydrophilic amines.


When it is not to be precipitated the free acid can be extracted into a fresh organic water-immiscible solvent in which it is soluble by neutralizing with one of these hydrophilic acids.


 Of course, the same shifting with appropriate cheaper sodium or potassium alkali is preferable when that works.


Ethanolamine

Miscible with methanol, acetone, glycerin; solubility at 25 °C: in benzene: 1.4%, in ether: 2.1%, in carbon tetrachloride: 0.2%, in n-heptane: less than 0.1%. Immiscible with ether.         


Diethanolamine

Miscible with methanol. acetone, alcohol, chloroform, glycerin. Solubility at 25 °C in benzene: 4.2%; in ether: 0.8%; in carbon tetrachloride: <0.1%; in heptane: <0.1%. Slightly soluble to insoluble in petroleum ether.


Triethanolamine

Miscible with water, methanol, acetone; soluble in benzene (4.2%), ether (1.6%), carbon tetrachloride (0.4%), n-heptane (<0.1%)


Tromethamine

79.1 mg/mL in ethylene glycol; 26 mg/mL in methanol; 14.6 mg/mL in anhyd. ethanol; 22 mg/mL in 95% ethanol; 14 mg/mL in dimethyl formamide; 20 mg/mL in acetone; 0.5 mg/mL in ethyl acetate; 0.4 mg/mL in olive oil; 0.1 mg/mL in cyclohexane; 0.05 mg/mL in chloroform; less than 0.05 mg/mL in carbon tetrachloride


N, N-Dimethylethanolamine

Miscible with alcohol, ether; Miscible with acetone, benzene