Association and colloid-chemical properties of humic acids in aqueous solutions

Colloid-chemical properties of humic acids in aqueous solutions have been studied as functions of their concentration, medium pH, and counterion nature. It has been established that, at pH < 4, Na-form humic acids exist in aqueous solutions as 30–120-nm aggregates. At pH > 4, the state of humic acids depends on the solution concentration. At low concentrations, they exist as individual molecules; as the concentration is increased, they associate to form supramolecular structures at 5 mg/dm³ and micelles at 8 g/dm³. An increase in the counterion charge has been shown to decrease the critical concentration of supramolecular structuring. Supramolecular structures have increased adsorbability on clay minerals and promote the manifestation of solubilizing properties of humic acids in solutions even at a concentration of 5 mg/dm³, which is three orders of magnitude lower than the critical micelle concentration (8 g/dm³).     

Lignite humic acids aggregates studied by high resolution ultrasonic spectroscopy

The thermodynamic stability of lignite humic acids (sodium salt) aggregates was studied by high resolution ultrasonic spectroscopy within the temperature interval from 5 to 90°C. The changes in differential ultrasonic velocity (U12) showed strong differences among humic solutions within the concentration range from 0.005 to 10 g L⁻¹. Measurement revealed several transitions which were attributed to the weakening of humic secondary structure. Concentration around 1 g L⁻¹ seemed to be a limit under which the change of the prevalence and importance of hydration occurred. Above this concentration the difference in U12 decreased following the temperature increase which was explained as a dominance of hydrophilic hydration. In contrast, below this concentration, the temperature dependence of U12 resulted in increasing tendency which was attributed to the prevalence of hydrophobic hydration, i.e. uncovering of apolar groups towards surrounding water. Additional experiments in which the humic sample was modified by hydrochloric acid resulted in a slight structural stabilization which lead to the conclusion that humic micelle-like subaggregates form an open-layer assemblies easily accessible for interaction with an extraneous molecule. That was partly verified by addition of propionic acid which brought about even larger reconformation of humic aggregates and exhibition of polar groups towards hydration water. The reversible changes in humate solutions induced by elevated temperatures provided the evidence about the existence of significant physical interactions among humic molecules resulting in formation of various kinds of aggregates. The nature of aggregates, mainly the stability and conformation, strongly depends on the concentration. Evidently, the changes observed in this work cannot be simply explained as expansions or conformational changes of macromolecular coils.     

Temperature dependence of density, viscosity, ultrasonic velocity, and other properties of substituted pyrazoles in acetone-water mixtures

The density, viscosity and ultrasonic velocity of some substituted pyrazoles viz. 5-(2-hydroxyphenyl)-3-(pyridin-3-yl)-4-benzoylpyrazol, 5-(2-hydroxyphenyl)-3-(3-nitrophenyl)-4-(3-pyridinoyl)-pyrazol, 5-(2-hydroxyphenyl)-3-(3-nitrophenyl)-4-benzoylpyrazol and 5-(2-hydroxyphenyl)-3-phenyl-4-(3-pyridinoyl)-pyrazole have been measured in 70: 30 (vol/vol) acetone-water mixture at 298, 303, 308, and 313 K for 0.01 mol dm^?3 concentration of pyrazoles. The acoustical parameters such as adiabatic compressibility (??_s), relative association (R _A), specific acoustic impedance (Z), apparent molar volume (?_v), apparent molar adiabatic compressibility (?_K), and intermolecular free length (L _f) were calculated from the experimental densities and velocities. The changes in acoustical properties have been used to interpret the molecular interactions in solutions. The activation energies of viscous flow of pyrazole solutions were determined from the data of viscosity at different temperature.     

The Analysis of Galvanizing Preflux Solutions for Zinc by Flow Injection-Flame Atomic Absorption Spectrometry

Abstract

A direct flow injection flame atomic absorption spectrometry method of determining zinc salt solutions with concentrations of 100's g dm^?3 is reported. It was shown that high concentrations of KCl, NaCl and NH₄Cl do not significantly interfere with the determination of zinc in both the mg dm^?3 and g dm^?3 concentration ranges. Where g dm^?3 concentrations were determined a secondary spectral line at 307.6 nm was found satisfactory. Galvanizing preflux solutions were analyzed both after dilution to the mg dm^?3 range at 213.9 nm and directly injected at 307.6 nm. Precisions of better than 6.6% rsd were observed by the direct method compared with better than 4.8 by dilution techniques. Concentrations up to 110 g dm^?3 in zinc were determined the total salt content being greater than 300 g dm^?3.     

Intermolecular/interionic interactions in l-leucine-, l-asparagine-, and glycylglycine-aqueous electrolyte systems

Ultrasonic velocity and density values have been measured for ternary systems (amino acid/di-peptide + salt + water): l-leucine/l-asparagine/glycylglycine each in 1.5 M aqueous solutions of NaCl or NaNO₃ or KNO₃ used as solvents for several concentrations of amino acids/di-peptide at different temperatures in the range of 298.15-323.15 K. The ultrasonic velocity values have been found to increase with increase in amino acids/di-peptide concentration and temperature in all the systems. The increase in ultrasonic velocity with increase in concentration has been discussed in terms of electrostatic interactions occurring between terminal groups of zwitterions (NH₄⁺ and COO⁻) and Na⁺, K⁺, Cl⁻, NO₃⁻ ions. The interactions of water dipoles with cations/anions and with zwitterions have also been taken into consideration. It has been observed that the ion-zwitterion and ion-dipole attractive forces are stronger than those of ion-hydrophobic repulsive forces. These interactions comprehensively introduce the cohesion into solutions under investigation. The cohesive forces are further enhanced on successive increases in solute concentration. Using ultrasonic velocity and density data, the parameters such as isentropic compressibility (κ_s), change (Δκ_s) and relative change (Δκ_s/κ₀) in isentropic compressibility, specific acoustic impedance (Z) and relative association (RA) have been computed. The isentropic compressibility values decrease with increase in the concentration of solutes as well as with temperature. The decrease in κ_s values with increase in concentration of l-leucine, l-asparagine and glycylglycine in 1.5 M aqueous solutions of NaCl, NaNO₃ and KNO₃ have been explained in terms of an increase in the number of incompressible molecules/zwitterions in solutions and the formation of compact zwitterions-water dipole and zwitterions-ions structures in solutions. The decrease in κ_s values with increase in temperature has been attributed to the corresponding decrease of κ_relax. (relaxational part of compressibility)_, which is dominant over the corresponding increase of κ_∞ (instantaneous part of compressibility). The trends of variations of Δκ_s, Δκ_s/κ₀, Z and RA with change of concentration and temperature have also been interpreted in terms of various intermolecular/interionic interactions existing in the systems.     

Oxyhumolite influence on adsorption and desorption of phosphate on blast furnace slag in the process of two-stage selective adsorption of Cu(II) and phosphate

Two-stage adsorption was used for selective removal of Cu(II) and phosphate from aqueous solutions. In the first stage, adsorption of Cu(II) and phosphate on oxyhumolite (OX) was examined. The pseudo second-order equation was found to be the best fit for the kinetic adsorption data. The adsorption capacity of OX for Cu(II) and phosphate depends on the adsorption time, the equilibrium pH influences only the adsorption of Cu(II). The high adsorption efficiency (E = 95 %, pH 3.5, 0.5 g of the solid sorbent and 50 cm³ of the solution, c = 4 mmol dm^?3) of OX for Cu(II) is caused by the presence of humic acids (HA). In the second stage, blast furnace slag (BFS) and activated blast furnace slag (BFS-A) were used to remove phosphates. The presence of OX in the first stage positively influences the adsorption efficiency of sorbents in the second stage due to the soluble humic compounds and residues of humic acids (HA) which support the precipitation of Ca-phosphates on BFS and the ions exchange reactions on BFS-A. Adsorption equilibrium of phosphate on both slags at 298 K can be well described by the Langmuir isotherm equation. Desorption of Cu(II) from OX was around 70 %. The presence of OX in the first stage also influences the desorption of phosphate bound in the second stage. Desorption efficiency of both slags for phosphate was about 60 %.     

Electrochemical Determination of Vitamin D3 in Pharmaceutical Products by Using Boron Doped Diamond Electrode

A sensitive square‐wave voltammetry method was developed to determine cholecalciferol (vitamin D₃) in pharmaceutical products at boron‐doped diamond electrode as a working electrode. Vitamin D₃ provided a well‐defined voltammetric peak at around +1.00 V (vs. Ag/AgCl, 3.5 mol dm^?3) in 0.02 mol dm^?3 Britton‐Robinson buffer pH 5.0 prepared in 50 % ethanol. The influence of various factors such as type and pH of the supporting electrolyte, scan rate and square‐wave parameters were studied and optimized. Under optimum conditions, the oxidation peak current increased linearly with the concentration of vitamin D₃ over the range of 2 to 200 μmol dm^?3. The calculated limit of detection and limit of quantitation were 0.17 μmol dm^?3 and 0.51 μmol dm^?3, respectively. The boron‐doped diamond electrode exhibited specific recognition capability for cholecalciferol amongst possible interferences, and the determination of vitamin D₃ was possible in samples such as commercial pharmaceutical products without complicated sample pretreatments.     

Fast visualization of the mass transfer processes at the electrode/electrolyte interface with a Mach-Zehnder interferometer

A Mach-Zehnder interfeometer is employed to visualize the mass transfer processes at the electrode/electrolyte interface during the potentiodynamic sweep of the Pt electrode in 0.1 mol dm^?3 K₄Fe(CN)₆ with 0.5 mol dm^?3 KCl solution at 20 mV s^?1. The changes of solution??s refractive index, brought about by the mass transfer during the reaction, can be recorded in situ in interferograms. The distributions of the optical path difference are obtained by numerical reconstruction of interferograms to reflect changes of solution??s refractive index and the mass transfer processes. The mass transfer of [Fe(CN)₆]^4? and [Fe(CN)₆]^3? is presented visually. This method provides a new approach to detect the mass transfer processes at the electrode/electrolyte interface in real-time.     

A simplified procedure for the determination of butyltin species in water

A method is described for the analysis of solutions containing inorganic tin and butyltin compounds. It can be used to determine total tin at a concentration of 20 ng dm^?3 using a 5 dm³ sample. The method is based on solvent extraction with dichloromethane containing tropolone and determination of the tin as inorganic tin by atomic absorption spectroscopy using electrothermal atomization. The extracted butyltin compounds can be separated by paper chromatography and the tin content of the individual spot determined as above. Observations on the stability of butyltin compounds in water at the ～2 mg dm^?3 (Sn) are included.     

Construction and Application of Flow Enzymatic Biosensor Based of Silver Solid Amalgam Electrode for Determination of Sarcosine

In this paper, the flow amperometric enzymatic biosensor based on polished silver solid amalgam electrode for determination of sarcosine in model sample under flow injection analysis conditions is presented. The biosensor works on principle of electrochemical detection of oxygen decrease during enzymatic reaction which is directly proportional to the concentration of sarcosine in sample. The whole preparation process takes about 3 h. The RSD of repeatability of 10 consecutive measurements is 1.6 % (c_sarcosine=1.0×10^?4 mol dm^?3). Under optimal conditions the calibration dependence was linear in the range 7.5×10^?6–5.0×10^?4 mol dm^?3 and limit of detection was 2.0×10^?6 mol dm^?3.     

Aqueous size-exclusion chromatography of humic acids on a sephadex gel column with diluted phosphate buffers as eluents

The elution behavior of humic acids on a Sephadex gel column is senstive to the composition, concentration, and pH of the eluent. the concentrations of the eluent in the literature are too high to obtain the correct molecular size distribution of humic acids. By reducing the concentration of phosphate buffer eluents to about a hundredth of conventional concentrations, the correct distribution is obtained. In the proposed method, 1 ml of 0.1% sample solution is introduced into a Sephadex G-50 column (2.2-cm diameter, 55 cm long) and humic acids are eluted with a 10^?3 M phosphate buffer solution (pH 7–9) at a flow rate of 1 ml min^?1.     

Optimization of Liquid Chromatography and Micellar Electrokinetic Chromatography for the Determination of Atrazine and its First Degradation Products in Humic Waters Without Sample Preparation

Liquid chromatographic method and micellar electrokinetic chromatographic method were optimized for determination of atrazine, desethylatrazine, desisopropylatrazine, hydroxyatrazine and their polar degradation products in solutions with humic acid without previous sample preparation step. Reversed-phase HPLC method was satisfactory in terms of repeatability and detection limits, which were ± 1.7–12.5% (RSD) and 0.1–0.5 mg L^?1, respectively. However, the most polar products could not be separated from the front peak pertaining to humic acid. With MEKC, excellent separation of both chloro and hydroxy degradation products and parent compounds was achieved in a single analysis, and possible interferences of humic acid were successfully avoided by its retention at the anode. Drawbacks were detection limits, estimated to be 2–4 mg L^?1, and RSD of the migration times was 20% compared to 0.5% with HPLC method. HPLC method was used to monitor degradation of atrazine and its first degradation products in the presence of humic acids, and MEKC was used for confirmation purposes.     

Use of ultrasound spectroscopy to examine the effect of cysteine on β-lactoglobulin interactions

We investigated the effects of cysteine on β-lactoglobulin interactions using ultrasound spectroscopy, rheological measurements, and differential scanning calorimetry. Changes in ultrasonic velocity and attenuation were monitored using ultrasound spectroscopy, and we discuss the effects of cysteine on gel formation together with the results obtained using other methods. A decrease in ultrasonic velocity occurred around 54 °C, suggesting that the compressibility of the system increases at approximately this temperature. An increase in ultrasonic attenuation was observed at approximately 54 °C, which is much lower than the commonly observed denaturation temperature of 75–80 °C. The temperature coincided with the onset of phase transition by differential scanning calorimetry and the initial rise in temperature of dynamic modulus for rheological measurements under heat treatment. We conclude that cysteine promotes the polymerization processes of denatured proteins during the initial stage of gelation. The ultrasonic spectroscopic analysis is a useful tool to monitor protein molecule interactions prior to gelation.     

Effect of mechanochemical modification on the surfactant and structural properties of humic and himatomelanic acids

The structural properties of humic and himatomelanic acids are studied by means of ¹Н NMR spectroscopy and differential thermal analysis after mechanochemical modification of peat. The relationship between the structural modification of humic and himatomelanic acids and their surfactant properties in aqueous solutions is established. It is shown that the critical micelle concentration of transformed himatomelanic acids is halved in comparison to the initial sample, while the adsorption equilibrium constant grows by 9 times.     

Quasichemical interpretation of the ultrasonic velocity in ternary aqueous systems

The quasichemical model of hydration have been used to calculate the speed of ultrasound in binary solutions of water and nonelectrolyte. The model has been confined to systems that exhibit a maximum in the ultrasonic velocity vs. nonelectrolyte concentration. The parameters of the model are the hydration equilibrium constant, the nonelectrolyte hydration number, and the molar volume and compressibility of the hydrated nonelectrolyte. These have been fitted to experimental results by the method of least squares. The model calculations reproduce qualitatively the ultrasonic velocity as a function of nonelectrolyte concentration. The calculated maximum of the ultrasonic velocity is generally too low, but the nonelectrolyte concentration at which this maximum occurs agrees well with experiment.Addition of a third component shift the velocity maximum. The quasichemical model has also been used to calculate this shift. These calculations have been based on the parameters developed for the binary system. The shift on the nonelectrolyte concentration scale is reproduced satisfactorily, but the shift of the maximal value of the ultrasonic velocity is less accurately predicted by the model.     

Acoustic absorption and dynamic compressibility studies of dilute and concentrated solutions of polystyrene in toluene and cyclohexane

Acoustic absorption and adiabatic compressibility measurements are reported on solutions of polystyrene (M_n = 89,000) in toluene and cyclohexane. The data in toluene cover a temperature range from 293 to 343°K and a concentration range of 10–400 Kg m^?3 (1–40 wt%). The dependence of acoustic absorption on concentration was found to be linear up to 100 kg m^?3, which corresponds to the concentration at which polymer–polymer interactions cause significant changes in the specific viscosity-concentration relationship. Up to 200 kg m^?3 the data could be fitted to computations based on an artificial separation of the dispersion into contributions from viscoelastic and segmental processes, using parameters obtained from a study of narrow molecular weight distribution samples at 25 kg m^?3. However, neither approach was capable of describing dispersions in the 300, 400 kg m^?3 solutions. The modification of the relaxation spectrum observed at the highest concentrations is ascribed to volume and entropy changes associated with alterations of the local environment around a segment of the polymer chain. These changes have their origin in interchain penetration and polymer–polymer contacts, and indicate that ‘entanglement’ is primarily entropic in effect. The adiabatic compressibility exhibited similar deviations from a simple concentration dependence, and allowed estimation of an incompressible volume increment associated with polymer–polymer interactions in the high-concentration entangled matrix. However, the adiabatic compressibilities of solutions of polystyrene, 10–15 kg m^?3, in cyclohexane showed no deviations from simple behavior in the region of the theta temperature. Measurements of the adiabatic compressibility of polystyrene in mixtures of cyclohexane-toluene have been used to obtain the relative magnitude of solvent and polymer contributions to the excess compressibility.     

Studies on thermo-acoustic parameters of poly(ethylene glycol)- 400 at different temperatures

The ultrasonic velocity and density have been measured at different temperatures between 299 and 363 K for the pure liquid sample, poly(ethylene glycol) with average molecular mass 400 g mol^?1 (PEG 400). From these, isentropic compressibility (β), intermolecular free length (L _f), acoustic impedance (Z), molar volume (V _m), Schaff’s available volume V _a(s), molar sound velocity (R _a), and molar compressibility (W) have been evaluated. The variations of these parameters with the temperature of the sample have been studied. Data so obtained are employed to compute other thermodynamic parameters. Variations in various parameters with respect to temperature are discussed in the light of the results obtained.     

Flow-injection analysis with a coated tubular solid-state copper(II)-selective electrode

The incorporation, behaviour and suitability of a home-made coated tubular solid-state copper(II)-selective electrode into the conduits of a flow-injection system is described. The compact tubular sensor (volume 7.8 μl) is constructed from a copper tube and Tygon tubing, treated with ammonium sulphide to give the copper/copper sulphide electrode and conditioned in an ascorbic acid medium. Interferences of foreign ions are similar to those found in batch analysis, but are less severe in the flow-injection system. Changes in carrier stream pH between 1 and 3 affected the electrode response, but sample pH has no or little influence in the range 1–7 depending on the copper(II) ion concentration in the samples. With 30-μl samples the flow-through electrode system covers a working range up to 5000 mg dm^?3 with a detection limit of 0.5 mg dm^?3. The system is suitable for the determination of copper(II) in effluent and tap water (relative standard deviation < 1.75% for 0.5–912 mg dm^?3 copper) and acidic copper sulphate plating solutions (relative standard deviation < 1.21% for 87–4135 mg dm^?3 copper) at a sample frequency of about 80 h^?1. The results obtained agree well with results by a standard atomic absorption spectrometric method.     

Extraction of small amounts of dialkylphosphorodithioates with tetraphenylarsonium

The extraction behaviour of ion-pairs formed by dialkylphosphorodithioates with tetraphenylarsonium cation was examined for the dichloromethane—water system. The mole ratio method, with spectrophotometric and conductimetric measurements of the organic phase, showed the mole ratio of the ion-pair to be 1:1. Dissociation of the ion-pairs occurred at concentrations in the aqueous phase lower than 10^?1 mol dm^?3 for the dimethyl compound and 6 × 10^?4 mol dm^?3 for the diethyl analogue. Under favourable experimental conditions, dialkylphosphorodithioates can be collected from aqueous solutions at concentrations not exceeding 1 μg cm^?3 (ca. 5 × 10^?6 mol dm^?3), with extraction efficiencies of 86–95%. Dialkylphosphorothioates are extracted much less efficiently; diethylphosphate and inorganic phosphate are not extracted.     

Pulse radiolysis studies of intermolecular charge transfers involving tryptophan and three-electron-bonded intermediates derived from methionine

The oxidation processes of the radiation-generated, three-electron-bonded intermediates AcMet₂ [S??S]⁺ and AcMet [S??Br] were investigated by pulse radiolysis via their reactions with tryptophan (TrpH). These intermediates were derived from N-acetyl-methionine amide (N-AcMetNH₂) and N-acetyl-methionine methyl ester (N-AcMetOMe). The bimolecular rate constant k of the reaction between each intermediate and l-tryptophan (TrpH) was measured. For N-AcMetNH₂, k for the reaction of AcMet₂ [S??S]⁺ with TrpH were 3.4?×?10⁸ and 2.2?×?10⁸?dm³?mol^?1?s^?1 at pH?=?1 and 4.5, respectively. For N-AcMetOMe, k for the reaction of AcMet₂ [S??S]⁺ with TrpH were 4.0?×?10⁸ and 2.8?×?10⁸?dm³?mol^?1?s^?1 at pH 1 and 4.5, respectively. The rate constants for the intermolecular transformation of Met [S??Br] into TrpH⁺ or Trp were also estimated. For N-AcMetNH₂, k for the reaction of AcMet₂ [S??Br] with TrpH were 2.6?×?10⁸ and 3.3?×?10⁸?dm³?mol^?1?s^?1 at pH 1 and 4.5, respectively. Related mechanisms were discussed.