PEO-[M(CN)5NO](x-) (M = Fe, Mn, or Cr) interaction as a driving force in the partitioning of the pentacyanonitrosylmetallate anion in ATPS: strong effect of the central atom

The partitioning behavior of pentacyanonitrosilmetallate complexesFe(CN) 5NO] (2-), Mn(CN) 5NO] (3-), and Cr(CN) 5NO] (3-)has been studied in aqueous two-phase systems (ATPS) formed by adding poly(ethylene oxide) (PEO; 4000 g mol (-1)) to an aqueous salt solution (Li 2SO 4, Na 2SO 4, CuSO 4, or ZnSO 4). The complexes partition coefficients ( K complex) in each of these ATPS have been determined as a function of increasing tie-line length (TLL) and temperature. Unlike the partition behavior of most ions, Fe(CN) 5NO] (2-) and Mn(CN) 5NO] (3-) anions are concentrated in the polymer-rich phase with K values depending on the nature of the central atom as follows: K F e ( C N ) 5 N O ] 2 - > K M n ( C N ) 5 N O ] 3 - > K C r ( C N ) 5 N O ] 3 - . The effect of ATPS salts in the complex partitioning behavior has also been verified following the order Li 2SO 4 > Na 2SO 4 > ZnSO 4. Thermodynamic analysis revealed that the presence of anions in the polymer-rich phase is caused by an EO-M(CN) 5NO] ( x- ) (M = Fe, Mn, or Cr) enthalpic interaction. However, when this enthalpic interaction is weak, as in the case of the Cr(CN) 5NO] (3-) anion ( K C r ( C N ) 5 N O ] 3 - < 1), entropic driving forces dominate the transfer process, then causing the anions to concentrate in the salt-rich phase.