Kinetics and mechanisms of radiolytic degradation of nitrobenzene in aqueous solutions

Steady-state radiolysis experiments were performed to gain insight into the kinetics and mechanisms of nitrobenzene (NB) degradation in aqueous solutions. The degradation of NB under gamma-ray irradiation followed pseudo-first-order kinetics, and the pseudo first-order rate constant and the initial G value of NB decomposition were functionally related to both the initial NB concentration and the irradiation dose rate. Under oxidative conditions, complete mineralization of NB was achieved, whereas no total organic carbon reduction was observed under reductive conditions. The radiolytic products of NB under various conditions were identified using FTIR and GC-MS analyses. The mechanisms behind the radiolytic degradation of NB under both oxidative and reductive conditions were proposed schematically in light of the degradation products observed. In addition, calculations based on ab initio molecular orbital and density functional theory provided support for the proposed mechanisms and the preferred pathways among all the possible reactions.     

Kinetics and mechanism of degradation of dichlorvos in aqueous solutions containing reduced sulfur species

Reactions of dichlorvos with five reduced sulfur species (hydrogen sulfide, bisulfide, thiosulfate, thiophenol, and thiophenolate) were examined in well-defined anoxic aqueous solutions to investigate their role in its degradation. Reactions were monitored at varying concentrations of reduced sulfur species over pH range to obtain the second-order reaction rate constants. Experiments at 25 degrees C demonstrated that degradation of dichlorvos promoted by bisufide, thiosulfate, and thiophenolate were of much greater importance than hydrolysis under the experimental conditions in our study. In contrast, hydrogen sulfide and thiophenol were not effective in the degradation of dichlorvos. The activation parameters of the reaction of dichlorvos with bisulfide, thiosulfate, and thiophenolate were also determined from the measured second-order rate constants over a temperature range of 12-50 degrees C. The relative reactivity of the reduced sulfur species decreases in the following order: PhS- > HS- approximately equal to S2O3(2-). When the second-order rate constants at 25 degrees C are multiplied by the environmentally relevant concentration of the reduced sulfur species, predicted half-lives of dichlorvos ranged from hours to days. The results indicated that reduced sulfur species could play a very important role in the chemical fate of dichlorvos in coastal marine environments.     

Initial steps of the metal-catalysed degradation of L-dehydroascorbic acid in acidic aqueous solutions

The initial steps of the degradation of L-de- hydroascorbic acid (L-DHA) in acidic aqueous solutions and the catalytic effect of different transition metal ions on this reaction were studied. The main product was identified as 3,6-furanosido-2,3-hexodiulosonic acid-2-hydrate (compound I) by GLC-MS and ¹³C-NMR, formed by lactone hydrolysis and hydration of the carbonyl group in the C-2 position of L-dehydroascorbic acid. In addition, number of other compounds were detected; they are formed from compound I by simple enolisation, lactonisation, hydration and dehydration reactions as well as by cleavage and formation of cyclic half acetal bonds. The chemical structures of these compounds were tentatively deduced by the mass spectra of their TMS derivatives and a reaction scheme for their formation is proposed. The velocity and the direction of the reactions were found to be strongly influenced by the presence of catalytic amounts of different transition metal ions. It was concluded that in acidic medium, in contrast to the situation in neutral and alkaline solutions, the opening of the lactone ring of L-DHA is, to a certain degree, a reversible reaction.     

Photocatalytic degradation of acid blue 80 in aqueous solutions containing TiO2 suspensions

The photocatalytic degradation of the anthraquinonic dye Acid Blue 80 in aqueous solutions containing TiO2 dispersions has been investigated. The process has been monitored by following either the disappearance of the dye (via HPLC) and the formation of its end-products (via IC, GC, and TOC analysis). Although a relatively fast decolorization of the solutions has been observed, the mineralization is slower, and the presence of residual organic compounds was evidenced even after long term irradiation, confirming the relevant stability of anthraquinone derivatives. The identification of various unstable intermediates formed after low irradiation times was performed by HPLC-MS, allowing us to give insight into the early steps of the degradation process which mainly involve C-N bonds breaking and substrate hydroxylation. Complete and relatively fast mineralization of the substrate was achieved by irradiating the semiconductor dispersions in the presence of added K2S2O8.     

Kinetics of the reaction of sorbic acid with sulphite species.

Under aerobic conditions, the reaction of sorbic acid with sulphite species, S(IV), involves a pH-dependent oxidative mechanism with the loss of S(IV). Sorbic acid is essential to the process. In the absence of air, a much slower nucleophilic addition of sulphite ion to undissociated sorbic acid takes place.     

Kinetics of ascorbic acid degradation in potato blanching

Degradation kinetics of ascorbic acid was investigated on potato strips wrapped in aluminium foil and blanched for 5–60 min at different temperatures between 65°C and 100°C. The results confirmed that first order kinetics is adequate in describing the degradation reaction. The value of the rate constant was found to pass through a maximum at 85–90°C, indicating the inactivation of oxidative enzymes around these temperatures. Comparison of the degradation losses based on the present investigation with results on total loss of ascorbic acid during blanching reported in the literature showed that thermal degradation accounts for a considerable portion of the total loss.     

Kinematic viscosity and water activity of aqueous solutions of glycerol and sodium chloride

During osmotic dehydration, a removal of water from foodstuffs is achieved with a decrease in the water activity of the food sample. This dehydration process involves the utilization of highly concentrated solutions with one or several solutes that increase considerably the viscosity of the liquid phase. This property is fundamental in the studies of mass and momentum transfer processes. Glycerol and sodium chloride are studied as systems employed in this type of process due to the advantages for the final dehydrated product. Kinematic viscosities of binary and ternary aqueous solutions of these solutes were measured at various concentrations (from 0 to up 5.0 mol kg^–1) and temperatures (from 20 up to 50 °C). Water activities for each indicated solution at 25 °C are also reported. Experimental data for both physical properties were simultaneously correlated with concentration and temperature for binary and ternary solutions with a suitable accuracy. Additionally, relationships between kinematic viscosities and water activities were established.     

Fungal metabolites of sorbic acid

A number of fungal detoxification reactions of sorbic acid have been reviewed. These include decarboxylation to give trans-1,3-pentadiene, esterification to give ethyl sorbate, reduction to give 4-hexenol and 4-hexenoic acid. It was shown that seven Penicillium species could convert sorbic acid into 1,3-pentadiene whilst P. bilaii, P. fellutanum and P. glabrum did not. However, most Eurotium species were unable to bring about this conversion. Considerable differences in the resistance of two isolates of P. crustosum to sorbic acid were found. An isolate from coconut was more resistant than one isolated from hazelnuts. Both sorbic acid and caproic acid (hexanoic) brought about disorganization of the mitochondrial membranes in P. crustosum. It is suggested that these lipophilic acids inhibit growth by interfering with the electrochemical membrane potential across the mitochondrial membranes.     

Enzymatic determination of sorbic acid

 An enzymatic method for the determination of sorbic acid based on the spectrophotometric measurement of sorbyl coenzyme A (sorbyl CoA) at 300 nm was developed. Sorbic acid is converted to sorbyl CoA with acyl CoA synthetase (EC 6.2.1.3) in the presence of coenzyme A and adenosine-5′-triphosphate. The reaction is made quantitative by irreversibly hydrolyzing pyrophosphate formed during the acylation reaction to phosphate in the presence of inorganic pyrophosphatase (EC 3.6.1.1). The absorbance measured at 300 nm is specific for sorbyl CoA and its value is proportional to the amount of sorbic acid in the sample solution. Accuracy, repeatability and recovery rates after standard addition to sample solutions were evaluated. All steps of the enzymatic analysis were carried out on an automatic centrifugal analyzer. The method is fast, precise and reliable, and therefore well suited for routine determinations, especially for high sample throughputs. Received: 7 June 1999 / Revised version: 12 August 1999     

Nucleophilic reactions of sorbic acid

The conjugated dienoic acid structure of sorbic acid renders it susceptible to nucleophilic attack. Nucleophiles known to react with sorbic acid include sulphite ion and amines. These attack the molecule in position 5 and, in the cse of amines, cyclization to form substituted dihydropyridones may follow. Recent investigations show that thiols in general can also add to sorbic acid. Cysteine, for example, reacts slowly with sorbic acid at 80 degrees C and pH 5.5, leading to 5-substituted 3-hexenoic acid. In general, reaction products are difficult to isolate from aqueous reaction mixtures as they are susceptible to acid- and base-catalysed hydrolysis. A synthesis of model compounds may be carried out by reaction of sorbate esters with the appropriate thiol (or its ester if it is an acid) in the presence of the corresponding sodium alkoxide. It is interesting that alkyl thiols give di-adducts with sorbate ester whilst low molecular weight thiols containing an oxygen atom give a monoadduct. The mechanism of this reaction and its implications to the preparation of samples for toxicological evaluation are discussed. The reaction of sorbic acid with nitrite ion is unusual and its mechanism is considered.     

甘油酯化反应动力学研究

甘油酯化是高酸值油脂降酸值的有效方法,可用于生物柴油的制备。采用大豆油与油酸的混合物为模型化合物,考察了反应温度对甘油酯化产物分布的影响,建立了甘油酯化二级反应动力学模型,并关联出相关反应的动力学参数速率常数k和反应活化能Ea;通过该模型预测了反应温度、甘油与游离脂肪酸摩尔比和原料油酸值对降酸效果的影响。结果表明:模型值与实验值有较好的一致性;降酸反应对反应温度有较高的依赖性,反应温度越高、甘油与游离脂肪酸摩尔比越大及原料油初始酸值越高,降酸速率越快,降酸反应越彻底。研究结果将有助于揭示甘油酯化反应机理,为甘油酯化反应器的设计提供依据。     

GC-MS联用测定酸乳饮料中山梨酸含量

介绍了气相色谱、质谱联用测定酸乳饮料中山梨酸含量的方法。此方法定性、定量分析准确、简便,可为食品中防腐剂的分析检测提供参考。     

Technical note: Kinetics of sorbic acid loss during storage of peaches preserved by combined factors

It was found that the loss of sorbic acid in stored peaches preserved by combined factors (water activity, pH, additives) followed first order reaction kinetics. Rate constants and activation energy (14 kcal/mol) were almost identical to those previously reported for sorbic acid loss in concentrated glucose solutions of identical water activity and pH.     

山梨酸的开发与应用

介绍了山梨酸的理化性能,对生产它的几种方法进行了论述,并阐述了其实际应用。     

肉制品中苯甲酸、山梨酸的测定

采用沉淀剂沉淀、微波超声、离心分离相结合的方法,提取肉制品中苯甲酸、山梨酸,用高效液相色谱法测定,再用保留时间定性,外标法定量。该方法能有效地排除肉制品中蛋白质、脂肪等大分子物质对检测的影响,最低检测限为0.5mg/kg,回收率为97.6%~103.9%之间,苯甲酸、山梨酸RSD分别为0.17%、0.29%,因此能广泛的应用到肉制品的检验中。