In order to obtain more pure substances, it is generally necessary to extract them for two or three times, which can be proved by the formula: ∫ distribution constant K=Ca/Cb(Ca is the concentration of solute in the stock solution and Cb is the concentration of solute in the extractant). Let the volume (mL) of aqueous solution be V0, V be the volume (mL) of extractant used each time, and m0 be the mass (g) of organic compounds dissolved in water. M 1 and M 1...M are the mass (g) of the organic matter left in the water after extraction 1 to n times respectively, then: one extraction: k = c0/c1= (m1/v0)/[ M 1=m0KV0/(KV0+V) secondary extraction: K==(m2/V0)/[(m 1-m2)/V], m2 = m65438+kv0/(kv0+v) = m0 [ 0? 5∴n Extraction times: m = m0 [kv0/(kv0+v)] n: kv0/(kv0+v)] n