Negative Salt Effect Research Articles

Determination of triazines in soil by microwave-assisted extraction followed by solid-phase microextraction and gas chromatography–mass spectrometry

A method for determining triazine herbicides in soil samples that combines microwave-assisted extraction with solid-phase microextraction is described. Water containing 1% methanol was employed as extractant. The parameters of solid-phase microextraction and microwave-assisted extraction were investigated. In solid-phase microextraction, particular attention was paid to the negative effect of salt on fiber stability. Our experiments showed that this effect could be effectively reduced by simply washing the fiber with deionized water. The selected triazines could be efficiently extracted by the aqueous extractant at 105 °C for 3 min, with 80% output of maximum power (1200 W). The extraction procedure provided good precision (<7%) and recoveries (76.1–87.2%). The limits of detection were in the range 2–4 μg/kg. Compared with conventional liquid extraction, microwave-assisted extraction–solid-phase microextraction was more efficient, accurate and faster, and used a very small amount of organic solvent (only 250 μL methanol). The extraction of aged spiked soil samples indicated that, although the recoveries were lower than those of freshly spiked samples, they were nevertheless satisfactory for the quantitative analysis of real-world samples.

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXVIII. Effect of the Nature of the Verdazyl Indicator on Salt Effects in Dehydrobromination of 3-Bromocyclohexene in Nitrobenzene

The salt effect on the rate of dehydrobromination of 3-bromocyclohexene in PhNO2 depends on the nature of the verdazyl indicator. With triphenylverdazyl and its chloro and nitro derivatives in the presence of Et4NClO4, a normal salt effect is observed, in the presence of bromides, a superposition of normal and special salt effects, while in the presence of chlorides, a superposition of normal and special negative salt effects. With the dimethoxy verdazyl derivative, a normal salt effect is always observed.

Ionic and osmotic components of salt stress specifically modulate net ion fluxes from bean leaf mesophyll

ABSTRACTIonic mechanisms of salt stress perception were investigated by non‐invasive measurements of net H+, K+, Ca2+, Na+, and Cl− fluxes from leaf mesophyll of broad bean (Vicia faba L.) plants using vibrating ion‐selective microelectrodes (the MIFE technique). Treatment with 90 m M NaCl led to a significant increase in the net K+ efflux and enhanced activity of the plasma membrane H+‐pump. Both these events were effectively prevented by high (10 m M) Ca2+ concentrations in the bath. At the same time, no significant difference in the net Na+ flux has been found between low‐ and high‐calcium treatments. It is likely that plasma membrane K+ and H+ transporters, but not the VIC channels, play the key role in the amelioration of negative salt effects by Ca2+ in the bean mesophyll. Experiments with isotonic mannitol application showed that cell ionic responses to hyperosmotic treatment are highly stress‐specific. The most striking difference in response was shown by K+ fluxes, which varied from an increased net K+ efflux (NaCl treatment) to a net K+ influx (mannitol treatment). It is concluded that different ionic mechanisms are involved in the perception of the ‘ionic’ and ‘osmotic’ components of salt stress.

Kinetics of Reduction of Toluidine Blue with Sulfite—Kinetic Salt Effect in Elucidation of Mechanism

The reaction between toluidine blue (TB+) and the sulfite ion is an ideal kinetic experiment in an undergraduate physical chemistry laboratory, to show the dependence of the reaction rate on the concentrations of reactants and ionic strength. Using excess concentrations of sulfite, the depletion of [TB+] can be monitored spectrophotometrically at 560 nm. The reaction follows pseudo-first-order kinetics. The overall reaction is second order, with first-order dependence on both sulfite and TB+ concentrations. The experiments with varying ionic strength demonstrate how kinetic salt effect can be used to predict the nature of charged species involved in the rate-limiting step. The observed negative salt effect can be interpreted as the involvement of each of TB+ and sulfite ions in the rate-limiting step. On the basis of the reaction orders and kinetic salt effect, the reaction mechanism is suggested. The theory of kinetic salt effect and some typical students' results are also described.

Studies on toluidine blue reaction with sulfite in aqueous solution and role of Cu(II) as promoter

The kinetic and mechanistic details of the reaction between toluidine blue and sulfite were studied spectrophotometrically by monitoring the depletion of toluidine blue at 633 nm. The reaction had first-order dependence on both the reactants, a stoichiometric ratio of 1:1 and a negative salt effect indicating the participation of TB+ and SO32− ions in the rate-limiting step. The reaction products were leuco base of toluidine blue and sulfate. The reaction was pH dependent and hence studied at its optimum pH 7.2. Cu(II) acted as promoter by facilitating the formation of a ternary complex with the reactants. The protonation and stability constants of the reactants were determined and used in validating the mechanism. The activation parameters for the reactions in absence and presence of Cu(II) were determined and compared with other systems. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 539–549, 1999

Studies on toluidine blue reaction with sulfite in aqueous solution and role of Cu(II) as promoter

The kinetic and mechanistic details of the reaction between toluidine blue and sulfite were studied spectrophotometrically by monitoring the depletion of toluidine blue at 633 nm. The reaction had first-order dependence on both the reactants, a stoichiometric ratio of 1:1 and a negative salt effect indicating the participation of TB+ and SO32− ions in the rate-limiting step. The reaction products were leuco base of toluidine blue and sulfate. The reaction was pH dependent and hence studied at its optimum pH 7.2. Cu(II) acted as promoter by facilitating the formation of a ternary complex with the reactants. The protonation and stability constants of the reactants were determined and used in validating the mechanism. The activation parameters for the reactions in absence and presence of Cu(II) were determined and compared with other systems. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 539–549, 1999

Salt Effects on the Kinetics of the Proton-Transfer Reactions in the 3,5-Dinitrohydroxybenzoic Acid/Ammonia System

A T-jump study of the 3,5-dinitrohydroxybenzoic acid (DNSA)/ammonia system has been undertaken, as a function of the nature and concentration of the added salt. Rate constants for the different possible proton-transfer reactions have been obtained from the experimental relaxation curves. The mechanism has been analyzed with the aid of a theoretical model based on Marcus- like free energy surfaces for each elementary step. Despite possible uncertainties in the estimations and approximations used, two conclusions seem to be well founded: first, the stepwise character of the proton-transfer reactions involving DNSA, and second, the possible change in the mechanism connected with the change in the final proton acceptor in the deprotonation of DNSA. This result, not previously assessed in the literature, is relevant for analyzing the salt effects, as is discussed in this work, and also for the hypothetical application of such a system in a molecular protonic device, an area of current research interest. Finally, the use of the well-known plot of log(k) versus ΔpKa for analyzing the negative or positive salt effects on the proton-transfer reactions is suggested.

Concentrated salt effects on the rates of solvolyses involving carbocations as reaction intermediates in acetone–water mixed solvents

Extensive studies have been carried out on the concentrated salt effects on the solvolysis reaction rates of aliphatic halides and related compounds (RX) in acetone–water mixed solvents. In 90 vol% acetone– 10 vol% water solution, the pseudo-first-order rate constant (k/s–1) of a typical SN1 substrate, tert-butyl chloride, at 50 °C was increased exponentially by the addition of M+ClO4– (M+ = Li+, Na+: up to 4.0 mol dm–3) and M2+(ClO4–)2 (M2+ = Mg2+, Ba2+: up to 2.0 mol dm–3); the extent of the cation effects increased as Na+ ⩽⩽⩽ Li+ 1.0 mol dm–3 Et4NBr decreased the solvolysis rate markedly, whereas it was increased slightly by lower Et4NBr concentrations. The positive effects of metal ions for typical SN1 substrates were explained by the change of solvent structure and by a “chemical” interaction between the anions from the substrates (R+–X–) and M+ or M2+ in the presence of very concentrated salts; the negative effects of nonmetallic salts should have been brought about by the decrease in activity of H2O. The solvolysis rate of 2-adamantyl tosylate (C10H15OTs) in 50 vol% acetone–water solution at 50 °C was also increased exponentially by the addition of LiClO4, whereas those of typical SN2 substrates, methyl tosylate (CH3OTs) and ethyl bromide, were decreased by the addition of LiClO4. On the other hand, for isopropyl bromide and benzyl chloride, the solvolysis rates were not changed by the addition of LiClO4. A good linearity was observed between the increase in log (k/s–1) in the presence of 1.0 mol dm–3 LiClO4 and the m-values of the substrates (by Grunwald–Winstein). It is proposed that one could simply distinguish SN1 from SN2 reactions merely by observing a substantial increase in the solvolysis rate constant at 1.0 mol dm–3 LiClO4 in aqueous mixed solvents. The salt effects on the solvolysis rates of sulfonyl chlorides in 50% acetone–water at 35 °C were very different from those for substrates with carbocations as reaction intermediates.

Kinetics and mechanism of zinc(II), cadmium(II), and mercury(II) incorporation into meso tetrameta‐tolueneporphyrin

The kinetics of Zn(II), Cd(II), and Hg(II) incorporation into meso tetrameta-tolueneporphyri n (H2T(m-CH3)PP) in acetone have been studied by means of stopped-flow method. A unified reaction mechanism was proposed and the kinetic parameters were obtained by nonlinear least-square methods. The effect of ionic strength (I) on Cd(II)/H2T(m-CH3)PP was investigated. It has been found that there is a negative kinetic salt effect and the relationship of rate constants with ionic strength was obtained. Some solvent effects have also been investigated in this article. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 277–283, 1998.

Acetylcholinesterase as polyelectrolyte: interaction with multivalent cationic inhibitors

Influence of inorganic salts on the interaction of cobra venom acetylcholinesterase (EC 3.1.1.7) with hexamethonium and gallamine has been studied. The observed negative electrostatic salt effect in the dissociation constant of the enzyme-ligand complex, K D, has been described by equation p K D = p K D° − Z L Ψ +Z log[Me +Z] following from Manning's polyelectrolyte theory, where Ψ +Z is the fraction of condensed counterions Me +Z per one negative charge of the polyanionic enzyme. The Z L Ψ +Z values for the complex formation between native acetylcholinesterase and hexamethonium ( Z L = + 2) or gallamine ( Z L = + 3) were in quantitative agreement with those predicted by the theory making use of Ψ +1 = 0.50 found earlier from the influence of salts upon the hydrolysis of acetylcholine by the enzyme. Increase in the number of negative charges in acetylcholinesterase by its modification with pyromellitic dianhydride resulted in an increase of Ψ +1 to 0.6. The data show that the influence of salts on the electrostatic contribution to the energy of binding of cationic substrates and inhibitors by acetylcholinesterase can be quantitatively described proceeding from the counterion condensation model of Manning by using only one empirical parameter Ψ +1 for a given subtype or modified form of the enzyme.

Pericarp Thickness and Other Kernel Physical Characteristics Relate to Microwave Popping Quality of Popcorn

ABSTRACTThe popping performance of 18 popcorn hybrids was studied using 2 microwave treatments, with and without salt, and 1 conventional method. Differences in popping quality indices between the 2 microwave treatments, indicated a negative effect of salt on popping for the majority of hybrids. Pericarp thickness, kernel volume, sphericity, diameter ratio, thousand kernel weight, hardness, and density were investigated. Pericarp thickness gave highest correlation with expansion volume in microwave and conventional popping while sphericity correlated slightly better in unsalted microwave popping. Regression models, which predict expansion volume in terms of pericarp thickness and kernel shape and size, were developed for both microwave and conventional popping.

Degradation Kinetics and Mechanisms of Moricizine Hydrochloride in Acidic Medium

AbstractThe degradation kinetics of moricizine hydrochloride (1) were examined over a pH range of 0.6 to 6.0 at an ionic strength of 0.3 and 60°C. The disappearance of intact 1 was followed by a stability‐indicating HPLC assay. The degradation products, which had approximate solubilities of <100 μg/mL, precipitated in aqueous solution. The precipitate was collected for HPLC analysis and identification of degradation products. Degradation of 1 was catalyzed by acetate and phosphate buffers and was pH dependent, with the pH of the minimum rate constant located between 2.8 and 3.2. At pH 0.6‐2.0, 1 degraded via amide hydrolysis to yield first ethyl (10H‐phenothiazin‐2‐yl) carbamate (2), an amide hydrolysis product, which further oxidized in parallel to give ethyl (3‐oxo‐3H‐phenothiazin‐2‐yl) carbamate (3), ethyl (10H‐phenothiazin‐2‐yl) carbamate S‐oxide (4), and diethyl (3, 10′‐bi‐10H‐phenothiazine‐2,2′‐diyl)bis(carbamate) (5), the dimer of the amide hydrolysis product. At pH 2.2‐6.0, 1 degraded via a reverse Mannich reaction, to form the reverse Mannich product ethyl [10‐(1‐oxo‐2‐propenyl)‐10H‐phenothiazin‐2‐yl] carbamate (6), and by parallel reaction via the described amide hydrolysis pathway. The dimer of the amide hydrolysis product was not detectable at pH >2.8. At pH >4.0, the reverse Mannich product was the predominant degradation product. Degradation of 1 was subject to positive and negative kinetic salt effects at pH 1.0 and 4.0, respectively. Arrhenius plots determined at pH 1.0 and 6.0 were linear between 37 and 70°C. Predicted shelf lives at room temperature (22 °C) at pH 1.0 and 6.0 were identical to the shelf lives determined experimentally.

A kinetic approach for the mechanism of malachite green‐peroxydisulphate reaction in aqueous solution

AbstractThe oxidation kinetics of malachite green (MG) with peroxydisulphate ion (PDS) were investigated by monitoring the absorbance change at 618 nm, in aqueous solutions under buffered and unbuffered conditions. Under both conditions, the reaction had first order dependence on reductant and a fractional order (one half) dependence on the oxidant. The stoichiometric ratio between MG and PDS was 1:2. The reaction had a negative salt effect. Reaction rates were enhanced under both high or low pH relative to neutral conditions. A plausible mechanism is proposed.

Molecular basis of ionic strength effects: interaction of enzyme and sulfate ion in CO2 hydration and HCO3- dehydration reactions catalyzed by carbonic anhydrase II.

CO2 hydration and HCO3- dehydration reactions catalyzed by carbonic anhydrase II have been examined at various concentrations of sodium sulfate with a stopped-flow technique. We find that at low ionic strength CO2 hydration and HCO3- dehydration reaction rates remain unaffected by varying the salt concentration at pH higher than 7.0, while the reaction rates decrease with increasing ionic strength at low pH. For CO2 hydration reactions, salt effects reside only in the kcat term, not in the Km term, whereas for HCO3- dehydration reactions, salt effects reside only in the Km term, not in the kcat term. In this regime, the salt concentration dependence of the turnover rate for CO2 hydration at low pH is attributed to an electrostatic effect on the ionization constants of the enzyme and/or enzyme-substrate complex, which in turn affect the pH profile of kcat. The rates of the bimolecular interaction between the uncharged CO2 molecule and carbonic anhydrase II at high pH are unaffected by low salt concentration while the rates of the bimolecular interaction of HCO3- with enzyme at low pH decrease with increasing salt concentration, consistent with a negative salt effect on an electrostatically enhanced diffusion of the negatively charged substrate to the positively charged active site. These bimolecular reactions between enzyme and substrate at low ionic strength obey rate equations derived from the Debye-Hückel limiting law and the transition-state theory. Simple linear relationships between the logarithm of the catalytic constant, log kdenz, and the square root of the ionic strength were established.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics and mechanism of transesterification of phenyl salicylate

AbstractThe first‐order rate constants, k1, obtained for methanolysis and ethanolysis of phenyl salicylate (PSH) in aqueous mixed solvents, fit to a relationship: k1 = A1 · [ROH]T/(1 + A2[ROH]T) where A1 and A2 are the unknown parameters and [ROH]T is the total concentration of alkanol. It is proposed that the alkanolysis of PSH involves the preequilibrium formation of monomeric ROH from polymeric (ROH)n, followed by an intramolecular general base‐catalysed nucleophilic attack by monomeric ROH on the carbonyl carbon of the ester. In the mixed solvents containing alkanol and MeCN, the k1 – [ROH]T profiles obtained in the presence of K+ ions are different from those obtained in the presence of Na+ ions which could be attributed to the cation‐induced changes in the alkanol structure. Negative KCl salt effect has been observed on methanolysis of PSH, while it is essentially unaffected by the presence of tetraalkylammonium iodide salts (R4NI). The rates of ethanolysis of PSH have been found to increase with increase in [R4NI] and this increase becomes more pronounced with increasing hydrophobic surface area of R4NI. The rate constants for methanolysis of PSH in aqueous mixed solvents containing 80% MeOH (v/v) are independent of [ŌH] within the [ŌH] range of 0.01 to 0.15 M. The rate of methanolysis could not be detected within ca. 47 h in mixed solvents containing 96% HeOH (3.8% MeCN and 0.2% H2O), 80% MeOH (19.8% MeCN and 0.2% H2O), and 0.022 M HCl. It is concluded that for efficient transesterification, PSH should exist in ionized form. The reaction of PSH with MeOH is ca. 400 times faster than that with t‐BuOH which could be ascribed to the most likely steric effect. The values of ΔH* and ΔS* obtained for methanolysis and ethanolysis are essentially independent of [ROH] within the ROH content of 20% to 96% for MeOH and 50% to 96% for EtOH. The effect of organic co‐solvent on rate of hydrolysis of PSH could be explained in terms of organic co‐solvent‐induced water polarization.

Kinetic studies on the hydrolysis of sodium tripolyphosphate in sterile solution

The hydrolysis of sodium tripolyphosphate was investigated in sterile aqueous solution between pH 3 and 7 and at temperatures between 40 and 70°C. The decomposition proceeds in two steps; the first is the formation of pyrophosphate and the second is its subsequent dissociation into orthophosphate. All kinetic investigations were thus made by measuring the increase in orthophosphate concentration by colorimetric measurement. The main objective of these measurements was the establishment of a rate law; it is for that reason that conditions were selected which are somewhat different from those incurred in practice. The first reaction step is always faster than the second. The ratio of the two rate constants lies typically between 4 and 7. The resulting rate laws accord with pseudo first order kinetics. Calculations based on experimentally found rate constants predict half-life of the tripolyphosphate between several hours and 270 days depending on pH and temperature. The lowest reaction rates were found at high pH and low temperature. The reaction rate depends on kind and concentration of buffer chosen. This is due to negative salt effect and other catalytic properties of the solution. For similar reasons the half-life in natural water and in activated sludge is reduced by at least two orders of magnitude.

Peroxo salts as initiators of vinyl polymerization, 5. Kinetics of polymerization of N‐vinyl‐2‐pyrrolidone by peroxodisulfate in aqueous solution. Effect of Ag+ and Cu2+ ions

AbstractThe kinetics of the polymerization of N‐vinyl‐2‐pyrrolidone by peroxodisulfate ion in aqueous solution in the presence and absence of metal ions Ag+ and Cu2+ is studied. The rate laws for the polymerization are established and the kp/kt1/2 values computed. A negative salt effect is observed for the Ag+ catalysed process, indicating the formation of a complex of the type AgS2O8− (aq). Cu2+ ions, however, are found to reduce the rate of polymerization.

ChemInform Abstract: KINETIC SALT EFFECTS IN THE ACID HYDROLYSIS OF POTASSIUM ETHYL MALONATE IN WATER AND IN 50% DIOXANE‐WATER MIXTURE

The rates of the acid hydrolysis of potassium ethyl malonate in presence of KCl, NaCl, Li2SO4 and MgSO4 in water and in 50% by weight of dioxane—water mixture were measured. The concentration of the added salts covered the range 0–1M. A negative salt effect on the rate of the half-ester hydrolysis was observed. The applicability ofBronsted's equation was tested and deviations found were explained as due among other reasons, to the operation of an iondipole type of interaction. The correlation between the hydration numbers as well as the radii of the cations of the added electrolytes and the reaction rate were discussed. The activation energy was found to increase with an increase of the concentration of electrolyte due to the relative changes in solvation of the transition state as a result of hydration of the electrolytes added. The thermodynamic properties of the activated complex were calculated and discussed as functions of the salt concentration. A remarkable linear compensation between ΔH≠ and ΔS≠ was found.

Kinetic salt effects in the acid hydrolysis of potassium ethyl malonate in water and in 50% dioxane?Water mixture

The rates of the acid hydrolysis of potassium ethyl malonate in presence of KCl, NaCl, Li2SO4 and MgSO4 in water and in 50% by weight of dioxane—water mixture were measured. The concentration of the added salts covered the range 0–1M. A negative salt effect on the rate of the half-ester hydrolysis was observed. The applicability ofBronsted's equation was tested and deviations found were explained as due among other reasons, to the operation of an iondipole type of interaction. The correlation between the hydration numbers as well as the radii of the cations of the added electrolytes and the reaction rate were discussed. The activation energy was found to increase with an increase of the concentration of electrolyte due to the relative changes in solvation of the transition state as a result of hydration of the electrolytes added. The thermodynamic properties of the activated complex were calculated and discussed as functions of the salt concentration. A remarkable linear compensation between ΔH ≠ and ΔS ≠ was found.

The hydrolysis and photolysis rates of nitrapyrin in dilute aqueous solution

The hydrolysis rate of nitrapyrin (2-chloro-6-(trichloromethyl)pyridine), the active ingredient of N-SERVE® nitrogen stabilizer (a registered trademark of The Dow Chemical Company, Midland, Michigan), in buffered, distilled water followed simple first-order kinetics over the concentration range, 6.2×10−7 to 8.7×10−5M. The only product of the reaction was 6-chloropicolinic acid. The rate of the reaction decreased with increasing buffer concentration at 35°C (M buffer concentration-half-life): 0.005M − 1.7 days, 0.02M−2.0 days, 0.067M−4.0 days. The ramifications of this negative salt effect are discussed. The hydrolysis rate was independent of pH over the range, 3.2 to 8.4. Additional data were obtained for rates at 25° and 45°C. The activation energy for the reaction under these conditions was 25.0 kcal/mole.