Complexes of chromium(III) with nitrilotriacetic acid and their extraction by solutions of aliquat-336 in dichloro-ethane

It has been shown that chromium(III) and nitrilotriacetic acid (NTA; H₃Z) form only a series of 1:1 complexes in solution. The violet uncharged species [CrZ(H₂O)₂] cannot be extracted by a solution of the long-chain quaternary ammonium chloride, Aliquat-336, in dichloroethane. Its acid dissociation constant (pK₁ = 5.87) was determined spectrophotometrically. On increasing the pH of the solution the blue anion CrZ(H₂O)(OH)^- of pK₂=8.74 and its conjugate base the binegative anion CrZ(OH)^2-₂ of pK₃= 11.81 are successively formed. Both of these anionic species are extracted by Aliquat-336 with maximal extraction (ca. 60%) at about pH 9. At still higher pH values the olive-green anion, CrZ(OH)₃^3-of much lower or negligible extractability, is formed.