AbstractThe extraction by TBP of nitrato complexes of metals occurs mainly by the formation of nonconducting complexes in which the oxygen of the PO group is covalently bound to the metal, e.g., P==0→M. In other TBP complexes, this O atom is bonded to hydrogen, e.g., to a hydrogen atom of water, of an undissociated acid, or of the hydronium ion.Three features in the extraction of metal nitrates at trace concentration from nitric acid concentrations>7Mwhich await interpretation are the second increase in the distribution coefficient,DM; the decrease in the magnitude of this second increase as the fraction of inert diluent increases; and the change in the temperature coefficient ofDMfrom negative to positive. Extraction (i) by bonding of the phosphoryl oxygen to an aquo group (of the aquonitrato metal complex), or (ii) by nitrato acids, do not explain these features.Measurements of the conductivity and viscosity of 100% TBP-HNO3-H2O phases are consistent with the existence of three steps as the ratio HNO3/TBP increases. In the first step, ions, postulated as (TBP·H2O·H)3O+and (TBP·H)2(H2O·H)O+, are formed. In the second step, the molar conductivity decreases as the predominant species becomes TBP·HNO3. In the third step the molar conductivity and the water content increase by the formation of ions such as (TBP·H)(H2O·H)(HNO3·H)O+, in which a nitric acid molecule is bonded to the hydronium ion: the second increase inDMfor certain metals is explained by there being similar bonding, through the oxygen of a nitrato group of the metal complex, in place of the HNO3in this complex ion when HNO3/TBP is>1. The positive temperature coefficient shown by this form of extraction of metal nitrates is also shown in this region by the extraction of nitric acid, the conductivity, and the water content.