Kinetics and mechanism of the reaction between dimethylformamide and chromium(VI)

The kinetics of oxidation of N,N‐dimethylformamide by chromium(VI) has been studied spectrophotometrically in aqueous perchloric acid media at 20°C. The rate showed a first‐order dependence on both [Cr(VI)] and [DMF], and increased markedly with increasing [H⁺]. The order with respect to [HClO₄] was found to lie between 1 and 2. The rate was found to be independent of ionic strength as well as of any inhibition effect of Mn(II). The formation of superoxochromium(III) ion was detected in an aerated solution of chromium(VI), DMF and HClO₄. The proposed mechanism, involving two reaction pathways, leads to the rate law, rate = K_a1 [HCrO₄⁻] [DMF] (k_I K_a2 [H⁺]²+k_II[H⁺]). The first pathway, with rate constant k_I, involves the formation of chromium(V) and a free radical. The second pathway, with rate constant k_II, involves the formation of Cr(IV), CO₂ and dimethylamine. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 409–415, 1999     

Method for the determination of chromium (VI) as chromium (VI)-dibenzyidithiocarbamate

Dibenzyldithiocarbamic acid (DBDC) exhibits the ability to speciate between chromium(VI) and chromium(III), since only the chromium(VI) will form complexes with DBDC. The complex is then extracted into an organic solvent and assayed using an ultraviolet-visible (UV-VIS) spectrophotometer at 498.8 nm. Using 250 ml of aqueous sample detection limits less than 1 ng/ml are possible, while the linear range extends to 500 gmg/ml when working at 498.8 nm. Oxidation of the chromium(III) to chromium (VI) using cerium (IV) enables the determination of total chromium and subsequently the chromium (III) in solution. Evaluation of the method with a standard reference material produced only 4.81 part per thousand error in the determination of chromium(VI).     

Chromium(VI), chromium(V) and chromium(IV) oxidation of 12-tungstocobaltate(II)

The reaction between Cr^VI and 12-tungstocobaltate(II) was carried out in 2.0 mol dm^–3 HCl and followed a simple second order rate law. The reaction was catalysed by hydrogen ion due to the formation of active H₂CrO₄ and was inhibited by chloride ion as, in its presence, conversion of the active species into inactive chlorochromate occurs. Chromium(V) and chromium(IV) were generated in situ by the use of Cr^VI—V^IV or Cr^VI—2-ethyl-2-hydroxybutyric acid and Cr^VI—i-PrOH reactions respectively, and the oxidation of 12-tungstocobaltate(II) by these atypical oxidation states, was also studied. The rate constants for the oxidation of 12-tungstocobaltate(II) by Cr^VI, Cr^V and Cr^IV were found to be in the ratio 1:1.2:5.2 respectively. The ionic strength did not affect the reaction, while decrease in the solvent polarity increased the rate of the reaction. The activation parameters were also determined and the values H , G and S were found to be 52.4 ± 6 kJ mol^–1, 100.8 ± 7 kJ mol^–1, –151.7 ± 10 J K^–1 mol^–1 respectively, supporting the mechanism proposed.     

Electron transfer reaction in the chromium(VI)-manganese(II) system in the presence of ethylenediaminetetraacetic acid (EDTA)

Upon addition of Cr ^VI to a solution of ethylenediaminetetraacetic acid (EDTA) and Mn ^II, a transient species appears which has an absorption maximum at 500 nm. Kinetic studies of the outer-sphere oxidation of the Mn ^II-EDTA complex with the Cr ^VI-EDTA complex have been investigated by visible spectrophotometry at 25 °C. The formation of a transient species has been characterized spectrophotometrically and the encounter complex formation constants have been determined (K_OS = 1.75 × 10² and 1.66 × 10³ mol^-2 dm⁶ for [EDTA] and [Mn^II] variations, respectively). The effect of total [EDTA], [Mn^II], [Cr ^VI] and [HClO₄] on the rate of the reaction was determined. On addition of HClO₄, there is a decrease in the rate constants. The reaction product is the Cr^III-EDTA complex with λ_max = 400 and 550 nm. On the basis of the various observations and product characterization a most plausible outer-sphere mechanism has been envisaged.     

One- and two-electron reduction of chromium(VI) by polyaminocarboxylatocobaltate(II) complexes and the formation of chromium(V) and chromium(IV)

The kinetics of the oxidation of Co^IILⁿ complexes {where L = ethylenediaminetetraacetate (EDTA), diethylenetriaminepentaacetate (DTPA), or N-(2-hydroxyethyl)ethylenediaminetriacetate (HEDTA)} by Cr^VI were studied under pseudo-first-order conditions with [Co^IILⁿ] ? [Cr^VI]. The kinetics showed first-order dependence on [Cr^VI]. The rate constant, k _obs, decreases with increasing concentration of [Cr^VI]. At constant [H⁺], ionic strength, and temperature, the rate law is described by Eq. (i)

$$ - {\text{d}}\left[ {{\text{Cr}}^{\text{VI}} } \right] / {\text{dt}} = \left\{ {{\text{k}}_{ 2} \left[ {{\text{Co}}^{\text{II}} {\text{L}}^{\text{n}} } \right]{\text{ + k}}_{ 3} \left[ {{\text{Co}}^{\text{II}} {\text{L}}^{\text{n}} } \right]^{ 2} } \right\}\left[ {{\text{HCrO}}_{4}^{ - } } \right] $$

(i)

Both k ₂ and k ₃ showed acid-dependent and acid-independent pathways. The direct conversion Co^IILⁿ to Co^IIIL^m is ruled out by spectrophotometric and ESR spectroscopic measurements that showed the formation of initial reaction intermediate(s). The rate law is consistent with one-electron and concurrent two-electron transfers leading to the formation of Cr^V and Cr^IV, respectively. An inner-sphere process, at least for the first term, leading to the formation of a relatively stable Cr^V species is almost certain. The kinetic term showing second-order dependence on [Co^IILⁿ], most likely, involves concurrent two-electron transfer leading to the formation of Cr^IV. The type of rate law and the proposed mechanism, reported here, depart from the well-established rate laws observed and mechanisms proposed for the oxidation of one-electron reductants by Cr^VI.

     

Kinetics of the reaction of chromium(VI) with tris(1,10-phenanthroline)iron(II) ions in acidic solutions. Anion and medium effects: perchlorate versus triflate

Reinvestigation of the reaction between title reagents in aqueous acidic triflate and perchlorate media revealed an unusual difference: the reaction is strictly first-order with respect to the concentration of Fe(phen)3(2+) (phen = 1,10-phenanthroline) in the triflate medium but shows an additional, but we believe artifactual, higher-order term in the perchlorate medium. We postulate that the apparent orders with respect to [Fe(phen)3(2+)] in (H/Li)ClO4 do not indicate the actual chemical mechanism but, in whole or in part, the orders, particularly the higher-order component, reflect an interaction specific to Fe(phen)3(2+) or Fe(phen)3(3+) and ClO4(-) in solution. Data in (H/Li)O3SCF3 solutions indicate that, in the absence of added Fe(phen)3(3+), the first of the three sequential electron-transfer steps is rate controlling. Reactions started in the presence of the product Fe(phen)3(3+) occur somewhat more slowly, suggesting the first electron transfer is reversible. This finding allows the relative rate constants for Cr(V) oxidation and reduction to be evaluated, with limited precision, by two methods of analysis. The dependences on [Cr(VI)] can be resolved into contributions from the species HCrO4(-) and Cr2O7(2-), each of which in turn depends on [H+]. The reaction mechanism is discussed in light of the data obtained in the triflate medium. Further, the rate constants for certain steps can be considered in light of E0 for the Cr(VI)/(V) couple.     

Indirect spectrophotometric determination of traces of antimony(III) based on its oxidation by chromium(VI) and reaction of chromium(VI) with diphenylcarbazide

An indirect method for the determination of antimony(III) is described. Antimony(III) is oxidized to antimony(V) by chromium(VI) and the excess of chromium(VI) is then determined spectrophotometrically with diphenylcarbazide. Optimal conditions were established for both the determination of antimony(III) and the elimination or reduction of interferences. Antimony(III) can be determined quickly and easily in the range 0.05–5 mg l^?1; the relative standard deviation is 2% for 1.0 mg l^?1 antimony(III). The method is applicable to marine sediments and geothermal waters.     

Spectrophotometric determination of chromium(III, VI) by use of chromium(III, VI)-Chrome AzurolS-cetylpyridinium bromide-hydroxylamine hydrochloride-zinc(II) system

This paper shows that the sensitivity of the Cr(III, VI)—Chrome Azurol S (CAS)-cetylpyridinium bromide (CPB)—hydroxylamine hydrochloride system can be increased and the wavelength of maximum absorption slightly shifted by addition of zinc(II) and that the analytical data are practically identical for both Cr(III) and Cr(VI), indicating that under the conditions used both initial oxidation states of chromium yield the same final oxidation state, Cr(III). On the basis of the Cr(III, VI)—CAS—CPB—NH₂OH·HCl—Zn systems a new, highly sensitive and selective method for spectrophotometric determination of microamounts of Cr(III, VI) has been developed, with molar absorptivity of 1.27 × 10⁵ 1. mole⁻¹ . cm⁻¹ for the complex at 620 nm and linear calibration up to 0.4 μg/ml chromium. Various foreign ions do not interfere. The method can be applied to direct determination of chromium in steels.     

Kinetics of chromium(VI) adsorption from model solutions on iron oxide

Adsorption of Cr(VI) on Fe₂O₃ from model solutions with various Cr(VI) concentrations was studied. The adsorption capacity was determined, the constants of chromium(VI) adsorption on iron(III) oxide for the pseudo-second-order model were calculated, and the diffusion coefficients for the process were evaluated.     

Spectral studies on the reaction of chromium (VI) compounds with aminophosphonic esters

The studies on reaction of newly obtained aminophosphonic acid diethyl ester derivatives of fluorene with Cr₂O₇ ^2–, CrO₄ ^2–, CrO₃Cl^– and CrO₃ have been investigated using electronic, infrared,Raman and NMR spectral methods. It has been found that the resulting compounds are of the type (AH)₂Cr₂O₇, whereA stands for the organic part of the molecule. The organic cation and solvent effects on the electronic states of pseudotetrahedrally arranged Cr(VI) anions are discussed.

---

Spektroskopische Untersuchungen über Reaktionen von Chrom(VI)-Verbindungen mit Aminophosphonsäure-Estern

Zusammenfassung Es wurden Elektronen-, Ultrarot-,Raman- und NMR-spektroskopische Untersuchungen beschrieben, die an neuen Verbindungen durchgeführt wurden, welche als Reaktionsprodukte von Cr₂O₇ ^2–, CrO₄ ^2–, CrO₃Cl^– und CrO₃ mit Aminophosphonsäurediethylesterderivaten von Fluorene synthetisiert worden waren. Es wurde festgestellt, daß diese Verbindungen mit einer allgemeinen Formel (AH)₂Cr₂O₇ beschrieben können werden, in derA den organischen Teil der Verbindungen bedeutet. Der Einfluß des organischen Kations wie auch der von Lösungsmitteln auf die Elektronenzustände des pseudotetraedrischen Cr(VI)-Anions wurde gleichfalls untersucht.

     

Micellar catalysis on the redox reaction of glycolic acid with chromium(VI)

Chromium(VI) oxidation of glycolic acid in the absence and presence of cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) followed the same mechanism as shown by kinetic study. The reaction followed second‐order kinetics, first‐order in each reactant. The oxidation is strongly catalyzed by manganese(II) and cationic micelles of CTAB or CPB. The catalytic effect of micelles can be fitted to a model in which the reaction rate depends upon the concentration of both reactants in the micellar pseudophase. Some added inorganic salts (NaCl, NaBr, NaNO₃, and Na₂SO₄) reduce the micellar catalysis by excluding glycolic acid from the reaction site. The reactivity of glycolic acid towards chromium(VI) has been discussed and also compared with those obtained previously for the reaction between chromium(VI) and the reductants oxalic and lactic acids. On the basis of the observed results, probable mechanisms have been proposed. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 377–386, 2001     

Cr（VI）与牛血清白蛋白的相互作用

采用荧光光谱、紫外光谱、CD光谱法研究了K2Cr2O7与牛血清白蛋白(BSA)的相互作用。实验结果表明, 铬(Ⅵ)使BSA的紫外吸收降低,峰位红移,表明铬(Ⅵ)与BSA发生较强的相互作用;铬(Ⅵ)酸根离子与BSA形成基态复合物导致BSA内源荧光猝灭,猝灭机理主要为静态猝灭。测定了不同温度下该反应的热力学参数,ΔGθ＜0,ΔHθ和ΔSθ分别为–12.60 kJ/mol 和 56.60 J/(mol·k), 表明上述作用过程是一个熵增加、自由能降低的自发分子间作用过程,铬(Ⅵ)酸根离子与BSA之间以静电作用力为主;非辐射能量转移机理确定了铬(Ⅵ)与牛血清白蛋白中色氨酸残基之间的距离 r＝2.85 nm;同步荧光和CD光谱研究表明,铬(Ⅵ)使BSA的二级结构发生改变,α–螺旋含量降低,色氨酸残基所处微环境的极性减小。     

Ion exchange recovery of chromium (VI) and manganese (II) from aqueous solutions

Sorption recovery of toxic ions – chromium (VI) and manganese (II) – from aqueous solutions with different acidity (0.001–0.5 M HCl) was investigated on cation and anion exchangers synthesized with long-chained cross-linking agents (LCA). The initial concentrations of Cr(VI) and Mn(II) were 1 g/L and 5 g/L, respectively. It was shown that the resins with LCA possess high ionic permeability due to the elasticity of polymeric skeleton. High selectivity and good kinetic properties of these sorbents allowed to achieve quantitative (∼100%) recovery and separation of manganese (II) and chromium (VI) in counter-current columns, which results in the complete purification of solutions from toxicants (below the maximum permissible limits), whereas the valuable components (chromium and manganese) can be returned back to industrial process.     

Potentiometric flow titration of iron(II) and chromium(VI) based on flow rate ratio of a titrant to a sample

A new potentiometric flow titration has been proposed based on the relationship of the flow rates between titrant and sample solutions. A sample solution is pumped at a constant flow rate. The flow rate of the titrant solution is gradually increased at regular time intervals and a flow rate for the titrant solution in the vicinity of the equivalence point is obtained. The concentration of the sample is calculated by C(S) (mol l(-1))=(R(T) (ml min(-1))xC(T) (mol l(-1)))/R(S) (ml min(-1)), where C(S), C(T), R(S), and R(T) denote the unknown sample concentration, titrant concentration in the reservoir, the flow rate of the sample solution which is a constant rate, and the flow rate of the titrant solution at an inflection point, respectively. The potentiometric flow titration of iron(II) with cerium(IV) and of chromium(VI) with iron(II) has been presented. The titration time of the proposed method is about 10 min per sample. An R.S.D. of the method is 0.77% for seven determinations of 1x10(-3) mol l(-1) iron(II). Similarly, the flow titration of chromium(VI) with iron(II) is carried out over the range 1x10(-4)-1x10(-3) mol l(-1) chromium(VI) and is successfully applied to the determination of chromium in high carbon ferrochromium.     

Sorption of chromium(III) and chromium(VI) on lead sulfide

The sorption of chromium(III) and chromium(VI) on lead sulfide has been investigated in dependence on pH, time of sorption and the concentrations of sorbate and sorbent. The mechanisms of the sorption of Cr³⁺ and CrO ₄ ^2– traces on lead sulfide are discussed; a difference between CrO ₄ ^2– sorption on PbS and -Fe₂O₃ has been found. Sulfates and molybdates affect the removal of chromates from aqueous solutions. Lead sulfide carrier prepared in this work was also used for the preconcentration of chromium(III) and chromium(VI) from tap water.     

Kinetic spectrophotometric determination of antimony(III) based on induced oxidation of tris(1,10-phenanthroline)iron(II) by chromium(VI)

The method involves the measurement of the extent of the induced reaction, which ceases a few seconds after initiation. Antimony(III) can be determined in the range 0.4–10 μg ml^-1. The standard deviation is ±0.25 μg. The method is applied to marine sediments.     

Sorption of chromium(VI) and chromium(III) on aluminium hydroxide

Factors that influence the sorption of Cr(VI) and Cr(III) on aluminium hydroxide were investigated. The sorption of chromates decreases as the pH of the suspension increases. The mechanism of CrO ₄ ^2– sorption was interpreted in terms of reactions between chromates and –OH and/or H₂O groups at the hydroxide/liquid interface. It has been shown that chromates are more tightly sorbed on aluminium hydroxide compared to other anions, e.g. chlorides. On the other hand, specifically absorbed anions, such as molybdates, compete strongly with chromates for the sorption sites. The sorption of chromium(III) increases with the pH of the suspension. Also, the sorption of chromium(III) is suppressed in the presence of citrate ions. The best conditions for the fixation of Cr(VI) and Cr(III) by aluminium hydroxide are presented.