Abstract: A series of hitherto unknown isomeric monobutyl esters of octylphosphonates was investigated for the structure-reactivity relationship in cobalt and nickel extraction. Linear regression analyses of the apparent extraction equilibrium constants with σ^p constants and Charton's steric parameters v of substituents directly linked to the coordinating moiety of the acidic phosphonates P(O)OH gave, as a rule, excellent results. Since the σ~p values of sudstituents of the compounds under investigation are similar, the correlation analysis between extraction constant and Charton's parameter v also gave satisfactory results. Based on these observation, it was suggested that in the serics of acidic mono-alkyl alkylphosphonates, steric effect plays a predominating rule in the extractive separation of cobalt and nickel.

Key words: Extraction, Codalt ion, Nickel ion, Phosphonic acid ester, Structure-property correlation