TY - JOUR
T1 - Stability Constants for Mono- and Dioxalato-complexes of Y and the REE, Potentially Important Species in Groundwaters and Surface Freshwaters
AU - Schijf, J.
AU - Byrne, R. H.
PY - 2001/1/1
Y1 - 2001/1/1
N2 - We present the first measured set of stability constants for mono- and dioxalato-complexes of yttrium and all rare earths except Pm (Y+REE), Ox β n = [MOx n 3−2n ] [M 3+ ] −1 [Ox 2− ] −n (where [ ] ≡ concentrations, M ≡ Y+REE, and Ox 2− ≡ C 2 O 4 2− ). Aqueous solutions of Y+REE were titrated with oxalic acid in the presence of a cation-exchange resin, and Y+REE concentrations in the solution phase were measured by ICP-MS. This method allows investigation of all Y+REE simultaneously under identical conditions and is thus very sensitive to subtle inter-element variations in log Ox β n . Experiments were performed at a single ionic strength ( I = 0.05 M), but at two values of pH. Patterns of log Ox β 1 and log Ox β 2 , determined from our experiments, are similar in shape and reminiscent of those for carbonato-complexes. The average ratio of stepwise stability constants K 2 /K 1 = Ox β 2 /( Ox β 1 ) 2 is 0.05 ± 0.02 for Y+REE excluding La and Ce. Literature values of Ox β 1 (Eu) for 0.03 mol/L ≤ I ≤ 1 mol/L were used to derive the relation log Ox β 1 (Eu) = log Ox β 1 0 (Eu) − 6.132√ I /(1 + 1.47√ I ) + 0.902 I , where log Ox β 1 0 (Eu) is the stability constant at infinite dilution. Applying this relation to all Y+REE, the following values of log Ox β 1 0 (at zero ionic strength) were found: 6.66 (Y), 5.87 (La), 5.97 (Ce), 6.25 (Pr), 6.31 (Nd), 6.43 (Sm), 6.52 (Eu), 6.53 (Gd), 6.63 (Tb), 6.74 (Dy), 6.77 (Ho), 6.83 (Er), 6.89 (Tm), 6.95 (Yb), 6.96 (Lu). These values, which are based on direct measurements for each individual Y+REE, agree quite well with published extrapolations that are mostly based on linear free-energy relationships. Total oxalate concentrations of 10 −5 −10 −3 M have been reported for soil solutions. Free oxalate ions persist at much lower pH than free carbonate ions and a simple speciation model demonstrates that oxalato-complexes can dominate Y+REE speciation in mildly acidic groundwaters of low-to-moderate alkalinity.
AB - We present the first measured set of stability constants for mono- and dioxalato-complexes of yttrium and all rare earths except Pm (Y+REE), Ox β n = [MOx n 3−2n ] [M 3+ ] −1 [Ox 2− ] −n (where [ ] ≡ concentrations, M ≡ Y+REE, and Ox 2− ≡ C 2 O 4 2− ). Aqueous solutions of Y+REE were titrated with oxalic acid in the presence of a cation-exchange resin, and Y+REE concentrations in the solution phase were measured by ICP-MS. This method allows investigation of all Y+REE simultaneously under identical conditions and is thus very sensitive to subtle inter-element variations in log Ox β n . Experiments were performed at a single ionic strength ( I = 0.05 M), but at two values of pH. Patterns of log Ox β 1 and log Ox β 2 , determined from our experiments, are similar in shape and reminiscent of those for carbonato-complexes. The average ratio of stepwise stability constants K 2 /K 1 = Ox β 2 /( Ox β 1 ) 2 is 0.05 ± 0.02 for Y+REE excluding La and Ce. Literature values of Ox β 1 (Eu) for 0.03 mol/L ≤ I ≤ 1 mol/L were used to derive the relation log Ox β 1 (Eu) = log Ox β 1 0 (Eu) − 6.132√ I /(1 + 1.47√ I ) + 0.902 I , where log Ox β 1 0 (Eu) is the stability constant at infinite dilution. Applying this relation to all Y+REE, the following values of log Ox β 1 0 (at zero ionic strength) were found: 6.66 (Y), 5.87 (La), 5.97 (Ce), 6.25 (Pr), 6.31 (Nd), 6.43 (Sm), 6.52 (Eu), 6.53 (Gd), 6.63 (Tb), 6.74 (Dy), 6.77 (Ho), 6.83 (Er), 6.89 (Tm), 6.95 (Yb), 6.96 (Lu). These values, which are based on direct measurements for each individual Y+REE, agree quite well with published extrapolations that are mostly based on linear free-energy relationships. Total oxalate concentrations of 10 −5 −10 −3 M have been reported for soil solutions. Free oxalate ions persist at much lower pH than free carbonate ions and a simple speciation model demonstrates that oxalato-complexes can dominate Y+REE speciation in mildly acidic groundwaters of low-to-moderate alkalinity.
UR - https://digitalcommons.usf.edu/msc_facpub/1729
UR - https://doi.org/10.1016/S0016-7037(00)00591-3
U2 - 10.1016/S0016-7037(00)00591-3
DO - 10.1016/S0016-7037(00)00591-3
M3 - Article
VL - 65
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -