| 2. |
- Elmroth, Sofi K.C., et al.
(författare)
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Kinetics and Mechanism for Reaction between Ammine- and Haloamminegold(III) Complexes and Thiocyanate. Competitive Electron Transfer and Substitution
- 1989
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 28:14, s. 2703-2710
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Tidskriftsartikel (refereegranskat)abstract
- The reactions in acidic aqueous solution between thiocyanate and each of the gold(III) complexes Au(NH3)43+, trans-Au(NH3)2C12+, and trans-Au(NH3)2Br2+ have been studied by use of potentiometric pH measurements and sequential-mixing stopped-flow spectrophotometry. The reactions give a common gold(I) product whereas the rate-controlling steps are different. The reaction between Au(NH3)43+ and thiocyanate takes place via rate-controlling substitution of an ammine ligand by thiocyanate with k = 7.6 +- 0.1 M-I s-l, DeltaHo = 61+- 1 kJ mol-I, and DeltaSo= 26+-3 J mo1-1 K-1 at 25.0 "C, followed by rapid reduction to gold(1) with the overall stoichiometry 3Au(NH3)43+ + 6SCN- + 4H+ + 4H20 = 2Au(SCN)2- + Au(SCN)(CN)- + S042- + 12NH4+ (i)For trans-Au(NH3)2X2+ (X = CI, Br), thiocyanate replaces halide in two rapid consecutive and reversible substitution steps without an observable solvent path prior to the slower reduction: trans-Au(NH3)2X2+ = Au(NH3)2XSCN+ = trans-Au(NH3)2(SCN)2+ (ii) Second-order rate constants (M-I s-I) at 2.0 oC are as follows: for X = CI; k1 = (9.0+-1.4) x I03, k-1 = (0.6+-0.2), k2 = (1.56+-0.21) X I05, k-2 = (3.4+-0.6) x 102; for X = Br, k1= (8.9+-0.3) x 104, k-1 = (1.32+-0.20) x 103, k2 = (1.4+-0.4) x 105, k-2 = (1.0+-0.7) x 104. Temperature variation of k, gave the following values: for X = CI; DeltaHo = 33+-7 kJ mol-', DeltaSo = -48+-21 J K-1 mol-1; for X = Br, DeltaHo = 30+-11 kJ mol-1, deltaSo = -50+-30 J K-1 mo1-1 at 25.0 oC. Parametrization of the substitution rate constants shows that the nature of the entering ligand is even more important than the trans effect for these complexes, in marked contrast to isoelectronic Pt(II) complexes. The relative stability constants for these short-lived complexes, K, = k,/k,, were obtained from the rate constants and are as follows: for X = CI, K1 = (1.5+-0.5) X 104, K2 = (4.6+-0.5) X 102; for X = Br, K1 = 67+-12, K2 = 12+-3. The ratio KI/K2 shows a nonstatistical distribution for the chloro-thiocyanato system, indicating a increased thermodynamic stability for the complex trans-Au(NH3)2CISCN+, whereas the bromo-thiocyanato system is approximately statistically distributed. An UV-vis spectrum for the intermediate short-lived complex trans-Au(NH3)2BrSCN+ was calculated from continuous-flow spectra. Reduction to gold(1) takes place via three parallel paths subsequent to establishment of the rapid substitution equilibria (ii). Each gold(III) complex trans-Au(NH3)2X(2-n)(SCN)n+ is reduced by outer-sphere thiocyanate in second-order reactions. The second-order rate constants, krn (n = 0, 1, 2), at 25.0 oC are as follows: for X = CI, kr1 = (2.7+-0.5) x 103, kr2 = (2.2+-0.4) x 102; for X = Br, kr0 = 10+-5, kr1 = (3.0+-0.5) x 102, kr2 = (2.5+-0.4) x 102 M-1 s-1, Temperature variation of kr2 gave DeltaHo = 66+-4 kJ mol-1 and DeltaSo = 21+-12 J mol-1 K-1 at 25.0 oC. The mixed chloro- and bromo-thiocyanato complexes are reduced most rapidly, indicating that an asymmetric distribution of electrons along the trans-axis facilitates reduction. It is concluded that reduction takes place by attack of outer-sphere thiocyanate on the sulfur of a coordinated thiocyanate. In keeping herewith, the two complexes trans-Au(NH3)2XSCN' (X = CI, Br), which contain a loosely bound halide ligand in the ground state, also substitute this halide ligand for thiocyanate most rapidly (k2). A unified mechanism for competitive electron transfer and ligand substitution for the reaction between gold(III) complexes and reducing ligands is suggested
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| 3. |
- Jiang, Yuan, et al.
(författare)
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The influence of different ultrafiltration set-ups on the mineral partitioning between skim milk streams
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Ingår i: International Journal of Dairy Technology. - 1364-727X.
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the partitioning of minerals in ultrafiltration (UF) streams of skim milk using cross-flow (CF-UF) and dead-end (DE-UF) was investigated using 50 kDa membrane at different temperatures (5, 10, 25, 35, 55°C). CF-UF showed higher protein retention (6.02–10.9%) compared with DE-UF (3.44–4.70%), along with more retention of calcium, with values ranging from 29.8 to 41.5 and 57.8 to 112 mM for DE-UF and CF-UF, respectively. Ionic calcium in permeates varied less for CF-UF (2.48–1.18 mM) than for DE-UF (3.11–0.98 mM). The insights provided in this study can be exploited to tailor adjustments of minerals in milk streams using UF.
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| 4. |
- Moltke Sörensen, Ann-Dorit, et al.
(författare)
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Interactions between Iron, Phenolic Compounds, Emulsifiers, and pH in Omega-3-Enriched Oil-in-Water Emulsions
- 2008
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Ingår i: Journal of Agricultural and Food Chemistry. - : American Chemical Society (ACS). - 0021-8561 .- 1520-5118. ; 56:5, s. 1740-1750
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Tidskriftsartikel (refereegranskat)abstract
- The behavior of antioxidants in emulsions is influenced by several factors such as pH and emulsifier type. This study aimed to evaluate the interaction between selected food emulsifiers, phenolic compounds, iron, and pH and their effect on the oxidative stability of n-3 polyunsaturated lipids in a 10% oil-in-water emulsion. The emulsifiers tested were Tween 80 and Citrem, and the phenolic compounds were naringenin, rutin, caffeic acid, and coumaric acid. Lipid oxidation was evaluated at all levels, that is, formation of radicals (ESR), hydroperoxides (PV), and secondary volatile oxidation products. When iron was present, the pH was crucial for the formation of lipid oxidation products. At pH 3 some phenolic compounds, especially caffeic acid, reduced Fe3+ to Fe2+, and Fe2+ increased lipid oxidation at this pH compared to pH 6. Among the evaluated phenols, caffeic acid had the most significant effects, as caffeic acid was found to be prooxidative irrespective of pH, emulsifier type, and presence of iron, although the degrees of lipid oxidation were different at the different experimental conditions. The other evaluated phenols were prooxidative at pH 3 in Citrem-stabilized emulsions and had no significant effect at pH 6 in Citrem- or Tween-stabilized emulsions on the basis of the formation of volatiles. The results indicated that phenol−iron complexes/nanoparticles were formed at pH 6.
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