Potentiometric Acid-base Titrations Research Articles

Water-Enhanced Direct Air Capture of Carbon Dioxide in Metal-Organic Frameworks.

We have developed two series of amine-functionalized zirconium (Zr) metal-organic framework-808 (MOF-808), which were produced by postsynthetic modifications to have either amino acids coordinated to Zr ions (MOF-808-AAs) or polyamines covalently bound to the chloro-functionalized structure (MOF-808-PAs). These MOF variants were comprehensively characterized by liquid-state 1H nuclear magnetic resonance (NMR) measurements and potentiometric acid-base titration to determine the amounts of amines, energy-dispersive X-ray spectroscopy to assess the extent of covalent substitution by polyamines, powder X-ray diffraction analysis to verify the maintenance of the MOF crystallinity and structure after postsynthetic modifications, nitrogen sorption isotherm measurements to confirm retention of the porosity, and water sorption isotherm measurements to find the water uptake in the pores of each member of the series. Evaluation and testing of these compounds in direct air capture (DAC) of CO2 showed improved CO2 capture performance for the functionalized forms, especially under humid conditions: In dry conditions, the l-lysine- and tris(3-aminopropyl)amine-functionalized variants, termed as MOF-808-Lys and MOF-808-TAPA, exhibited the highest CO2 uptakes at 400 ppm, measuring 0.612 and 0.498 mmol g-1, and further capacity enhancement was achieved by introducing 50% relative humidity, resulting in remarkable uptakes of 1.205 and 0.872 mmol g-1 corresponding to 97 and 75% increase compared to the dry uptakes, respectively. The mechanism underlying the enhanced uptake efficiency was revealed by 13C solid-state NMR and temperature-programmed desorption measurements, indicating the formation of bicarbonate species, and therefore a stoichiometry of 1:1 CO2 to each amine site.

Mechanochemical Synthesis of Ethoxyaminohumic Acids and Surface-Active Properties of Their Solutions at Solution–Air Interface

Ethoxyamine derivatives of humic acids have been obtained by mechanochemical synthesis via the simultaneous interaction of humic acids with poly(ethylene glycol) (PEG-6000 and PEG-1500) and an aminating reagent (urea, hydroperitum, or cyanoguanidine) in a vibrating apparatus. Reaction products have been characterized by IR spectroscopy, acid–base potentiometric titration, and viscometry. Tensiometric and rheological characteristics of the surface layers of solutions of salts of the synthesized derivatives of humic acids have been studied by the pendant drop and oscillating pendant drop methods. The solutions of the salts of ethoxyaminohumic acids have been found to exhibit a pronounced surface activity at the air–water interface. The experimental dependences of the viscoelastic modulus on the surface pressure and the concentration of the solutions of ethoxyaminohumic acid salts are in satisfactory agreement with the functions calculated in terms of the theoretical model of bimolecular adsorption. The presence of amino groups in the structure of ethoxyaminohumic acids predetermines their high solubility in the acidic pH region. The simultaneous incorporation of ethoxy and amino groups into humic acid macromolecules makes it possible to obtain a novel type of surfactants, which combine three functions, i.e., the functions of anionic, cationic, and nonionic surfactants.

Using a novel approach to characterize the surface reactivities of silica-rich ferrihydrite and biogenic cyanobacteria-ferrihydrite aggregates and the implications for Archean ocean geochemistry

Precambrian banded iron formations (BIF) are iron- and silica-rich chemical sedimentary rocks that are commonly used as paleo-redox proxies for Archean and Paleoproterozoic seawater geochemistry. At the onset of the Great Oxidation Event (herein GOE) around 2.4 Ga, cyanobacteria flourished with increasing nutrient fluxes due to oxidative weathering on land. In turn, this led to increased primary productivity that facilitated the permanent shift from a reducing Earth atmosphere to an oxidizing one. Interestingly, the duration of GOE also overlapped with one of the most prolific periods of BIF deposition.It is widely accepted that cyanobacteria were likely responsible for BIF formation during the GOE. Oxidation of dissolved Fe(II) by oxygen produced from cyanobacteria forms a metastable and amorphous mineral phase ferrihydrite, Fe(OH)3. As an essential component in both ancient BIF deposits and various modern ecosystems, the surface reactivity of ferrihydrite has been extensively studied under different conditions (i.e., pH and ionic strengths). Not only are the highly reactive surfaces of ferrihydrite particles important shuttles for trace element transport from the water column to the sediment pile, but previous studies have also demonstrated that cyanobacterial cells and ferrihydrite tend to aggregate at seawater pH. This means that ferrihydrite was also a vector for the transport of organic carbon to the seafloor. However, a complicating issue is how co-ions affect the surface reactivity of ferrihydrite, specifically dissolved silica which was abundant in ancient seawater. Although previous studies have demonstrated that silica can passivate the surface reactivity of ferrihydrite, what remains unclear is how silica impacts ferrihydrite-biomass aggregation. To fill this knowledge gap, we formed both silica-spiked ferrihydrite and cyanobacteria-ferrihydrite aggregates in situ and subsequently conducted empirical potentiometric acid-base titrations and Cd adsorption experiments on the fresh aggregate samples at three different ionic strengths (0.56 M, 0.1 M and 0.01 M). We minimized sample processing (i.e., drying and powdering) to a simple washing step, in which the aggregate pellets remained hydrated to avoid any mineral transformation thus altering their true surface reactivity in seawater. Experimental results were then fitted with non-electrostatic model to predict both surface charges and metal-adsorption behavior of ferrihydrite aggregates. Different from previous surface-complexation modelling studies, here we used a novel and more powerful modelling program called Phreefit. It utilizes the global optimization algorithms instead of more commonly used Newton-Raphson method in FITEQL program, which is often too limited for precisely modelling complex systems such as the two samples in this study. Furthermore, we also measured the surface charges of both samples over the pH range from 3 to 9 on a Malvern Zetasizer and characterized the surface functional groups through Fourier-Transform Infrared Spectroscopy to help with our interpretation of the experimental data.Preliminary results show that cyanobacteria-ferrihydrite aggregates formed primarily due to ionic bridging. Cyanobacterial cells likely facilitated the precipitation of dissolved silica. Findings from titration and Cd adsorption experiments indicate that the surface reactivity and capacity of both silica-rich ferrihydrite cyanobacteria-ferrihydrite aggregates to adsorb trace elements differ from their individual components, likely due to site blockage. This distinction is particularly prominent when considering the expected Archean seawater pH from 6 to 8. This disparity implies that the biogenic ferrihydrite aggregates do not exhibit an additive surface reactivity, which is in agreement with similar previous studies. Our combined results are crucial to accurately predict the adsorption of trace elements onto the aggregate surface and, ultimately, comprehend the archive of trace elements in sedimentary rocks used to reconstruct Precambrian ocean chemistry.

The impact of aggregation between clay and phytoplanktonic cyanobacteria on trace elemental cycling in coastal environments

Clay minerals and cyanobacteria occur ubiquitously in nature, and both independently influence trace metal cycling due to their high specific surface areas and reactivity. Although the surface reactivity of each is well-studied, the reactivity of clay-cyanobacterial aggregates has received less attention. Here, we performed potentiometric acid-base titrations and Cd adsorption experiments on various concentrations of clay-cyanobacterial aggregates using three clay minerals (kaolinite, illite, and montmorillonite) and two model planktonic cyanobacteria, the freshwater species Synechocystis sp. PCC 6803 and marine species Synechococcus sp. PCC 7002. A component additivity surface complexation modeling (CA-SCM) approach was applied to study the impacts of the aggregation processes on the total surface reactivity. Results demonstrate that the CA-SCM approach effectively predicted the acid-base titration and Cd adsorption behaviors of aggregates at low aggregate concentrations (up to 2.5 g of clay/L for acid-base titration and 0.1 g of clay/L for Cd adsorption), indicating the additivity of the reactivity for the two components. However, at higher aggregate concentrations (25 g of clay/L for acid-base titration and 10 g of clay/L for Cd adsorption), the CA-SCM approach overestimates the acid-base titration and Cd adsorption behaviors for the aggregates, indicating significant surface site blockage during aggregation. Our SEM images show that both cyanobacterial species were covered by layered clay minerals at the surfaces and formed relatively larger aggregates at high aggregate concentrations than at low aggregate concentrations. Based on the combined results, we propose that the interaction between clays and cyanobacteria is relatively weak at low aggregate concentrations (e.g., <10 g of aggregate/L), but that significant obstruction of surface adsorption sites occurs at high aggregate concentrations (e.g., >10 g of aggregate/L), possibly due to the encrustation of bacteria by clay minerals. The implication of this work is that where turbid rivers discharge into estuaries or deltas with high primary phytoplankton productivity, aggregation (or flocculation) processes will cause the depression of surface reactivity of the aggregates and lead to a decrease in metal-binding capacity. Therefore, depending on whether the trace elements are nutrients (e.g., phosphorous) or potentially toxic (e.g., cadmium), this process has the potential to impact the marine biosphere. To ground truth this hypothesis, we further analyzed the correlation between total organic carbon (TOC) content and trace metal compositions in a compiled shale database and the results show clear negative correlation between Cd concentration and the concentration of TOC, Al and Si. Although this offers only an indirect assessment of the role of clay-plankton aggregation, as a first-order test it does indeed show that the aggregation between clay minerals and phytoplanktonic bacteria in estuaries is an essential factor that influences trace-elemental cycling.

Design of hierarchically porous Zr-MOFs with reo topology and confined PMA for ultra-efficient oxidation desulfurization

It is challenging but desirable to synthesize hierarchically porous MOFs (HP-MOFs) with well-defined topology. Herein, a series of HP-Zr-MOFs (H-UiO-66-X) with different topologies were synthesized through an in situ self-assembly template strategy. Further, H3PMo12O40 (PMA) was dispersed within H-UiO-66-X to illustrate the confinement effect of HP-MOFs on PMA as ultra-efficient ODS catalysts. Potentiometric acid–base titration is used to evaluate three typical types of Brønsted acid protons (μ3–OH, Zr-OH2, and Zr-OH) presented in the nodes of defect HP-MOFs according to their different pKa values (∼3, ∼6 and ∼8). By quantifying the amount of defect sites (Zr-OH2 and Zr-OH) resulting from either missing Zr6O8 clusters or missing linkers in the HP-MOFs, the information about the true topology of HP-MOFs is provided. The Zr-OH2 defects are correlated to missing Zr6O8 cluster and the Zr-OH defects are related to missing linkers. The loaded PMA molecules mainly occupy the Zr-OH2 defect sites by the substitution of H2O molecules and still maintain the Keggin structure. The results give deep insight on the structure-activity relationship for PMA confined in HP-Zr-MOFs with clear topologies.

Absolute and Relative Positioning of Natural Organic Matter Acid-Base Potentiometric Titration Curves: Implications for the Evaluation of the Density of Charged Reactive Sites.

Potentiometric acid-base titration curves collected on humic (nano)particles as a function of pH and salt concentration reflect the electrostatics of the particles and the amount of chemical charges (Q) they carry. In turn, the interpretation of titration data helps quantify their reactivity toward metals provided that both intrinsic chemical and nonspecific electrostatic contributions to proton binding are correctly unraveled. Establishing a titration curve requires several steps, i.e., blank subtraction, relative curve positioning with respect to the electrolyte concentration, and absolute curve positioning achieved by the estimation of particle charge Q0 at low pH. Failure to properly establish each step may lead to the misevaluation of nanoparticle charging behavior. Here, we report (i) a simple procedure to measure and position titration curves for humic substances (HS) versus salt concentration and (ii) an original approach for absolute curve positioning upon the exploitation of proton affinity spectra. The latter do not depend on Q0 and they thus constrain the titration data analysis using the soft Poisson-Boltzmann-based titration (SPBT) formalism for nanoparticles in the thick electric double-layer regime. We illustrate the benefits of our approach by analyzing titration measurements for a large range of humic nanoparticles and by comparing the outcome with results from the literature.

Monitoring Cu(II)-insulin and Mn(II)-insulin complexes using potentiometric, chromatographic, UV–vis absorption and fluorescence emission spectroscopic techniques

In the present work, Cu(II)-insulin and Mn(II)-insulin complexes were investigated using potentiometric acid-base titration, UV–vis absorption and fluorescence (FL) emission spectroscopy, and liquid chromatography (UFLC). Cu(II) and Mn(II) ions are essential nutritional trace elements. They have important activities with enzymes and proteins in the human body. Insulin is a peptide hormone that regulates glucose level in the body. Cu(II) and Mn(II) metal ions are bound to insulin molecules, and their protein complexes form. The binding behaviors of the essential nutritional metal ions to insulin molecules have physiological importance in a human body or drug formulations. The binding properties of Cu(II) and Mn(II) ions to insulin molecule were examined using different methods and compared with each other. It was shown that the Cu(II)-insulin complex formation could be monitored by potentiometric acid-base titration, UV–vis absorption, UFLC and FL methods. However, UV–vis absorption, UFLC and FL methods are not suitable for the monitoring Mn(II)-insulin complexes. The binding mechanism of Cu(II) ions is different than the binding mechanism of Mn(II) ions. The Cu(II) binding is governed by the chelating mechanism, while the Mn(II) binding is by ionic interaction. The acid-base potentiometric titration method is a cheap practical method to check whether the binding of metal ions to protein is or not as well as Cu(II) and Mn(II) metal ions. UV–vis absorption spectroscopy can be used for protein-metal ion interactions via a chelating mechanism.

Acid-base properties of Synechococcus-derived organic matter

Marine cyanobacteria play an important role in trace metal cycling in the oceans. However, the influence of bacterial detritus following cell lysis – that includes both particulate organic matter (POM) and dissolved organic matter (DOM) – is less understood. Previous investigations have shown that the ratio of bacterial detritus to living cells increases significantly with ocean depth, indicating that the former plays a central role in trace metal cycling and eventual sequestration in deep marine sediments. Thus, in this study we produced detritus from the mechanical lysis of the common marine cyanobacterium Synechococcus sp. PCC 7002 (referred to as cyPOM and cyDOM), and then measured their buffering capacity through proton adsorption and release via potentiometric acid-base titrations. Both cyPOM and cyDOM were best modelled using a 2-site non-electrostatic protonation model. The cyPOM fraction possessed ligands with pKa values of 5.78 (±0.07) and 9.01 (±0.29) with corresponding site concentrations of 41.8 (±8.15) µmol g−1 and 41.2 (±5.28) µmol g−1, while cyDOM had pKa values of 4.89 (±0.22) and 6.80 (±0.22) and corresponding site concentrations of 42.8 (±14.6) µmol g−1 and 37.3 (±16.5) µmol g−1. As a comparison, intact Synechococcus cells were previously reported to be best modelled by invoking 3-sites with overall ligand densities more than double that of cyPOM and cyDOM. Trace metal (Cd, Co, Cu, Ni, and Zn) adsorption experiments indicate that cyPOM has a lower affinity towards trace metals than intact Synechococcus cells, evidenced by lower trace metals adsorption per gram of dry mass and therefore less trace metal sequestration potential. This implies that intact Synechococcus cells are chiefly responsible for trace metal sequestration, even at high cyPOM proportions (10:1 – cyPOM:Synechococcus) that are more representative of POM that reaches the seafloor. The cyDOM-trace metal association could not be determined experimentally; thus we applied existing thermodynamic data from modern marine DOM studies acquired via potentiometric titrations and metal-binding constants. Unlike cyPOM, metal speciation is almost entirely accounted for in the dissolved fraction (>99.9%). These results further the view that DOM is the most important microbial fraction in controlling metals speciation in the water column.

Разработка комплекса моющих средств для мембранных установок в молочной промышленности

Aim. Development of a complex of detergents (acidic, alkaline, enzymatic, chlorine-containing, surfactant-containing additives) for membrane units used in the dairy industry. Materials and Methods. Methods for controlling the concentration of active substances in concentrates of agents are given on the example of KATELON 202 - determination of total acidity by the method of acid-base potentiometric titration (in terms of HNO3) and KATELON 114 – determination of the mass fraction of active chlorine by redox titration. Results. A complex of detergents for membrane plants (ultrafiltration (UV, UF), microfiltration (MF, MF), nanofiltration (NF, NF), reverse osmosis (RO, RO)), used in the dairy industry and including alkaline non-foaming, medium-, low-foam products KATELON 109, KATELON 110, KATELON 115; alkaline chlorine-containing non-foamy agent KATELON 114; acid non-foamy KATELON 202, KATELON 207, KATELON 208; high-foam neutral additives KATELON 308 and KATELON 309 – enhancers of the washing effect for KATELON 109, KATELON 110, KATELON 115; preservative non-foaming additive KATELON 310 for protection from microbiological contamination; enzyme medium supplement KATELON 601. The composition of all products, their brief characteristics and physicochemical indicators, as well as an approximate program of washing with a complex of developed preparations are given. The dependences of pH and electrical conductivity were obtained for KATELON 109, KATELON 110, KATELON 115, KATELON 202, KATELON 207, KATELON 208. Conclusion. The complex of detergents is successfully used at Kopylsky Butter Cheese Plant, Lyubansky Cheese Making Plant, Minsk Dairy Plant No. 1, Verkhnedvinsky Butter and Cheese Plant, Molodechno Dairy Plant, Volozhin production site of Minsk Dairy Plant No. 1, Tolochin Branch of Lepel Dairy Canning Plant.

Insights into the improving mechanism of defect-mediated As(V) adsorption on hematite nanoplates

The fate of As(V) in subsurface environments is strongly affected by ubiquitous iron oxides. Defects are commonly present in natural hematite, while the impacts of defects on the active sites and complexation mechanism of hematite for As(V) remain poorly understood. In this study, the defect-rich hematite was employed to investigate the surface charge characteristics and As(V) adsorption behavior using potentiometric acid-base titration and CD-MUSIC model in comparison with corresponding defect-poor hematite. The total arsenate-active site density (5.7 sites/nm2) on defective hematite includes 1.2 sites/nm2 of original sites and 4.5 sites/nm2 of Fe vacancy-induced sites. The result revealed that the vacant Fe3+ sites in defective hematite was compensated by the protons in solution, thus resulting in a considerable increase in site density as well as positive charge. The CD-MUSIC modeling results demonstrated that the presence of Fe vacancies in hematite is beneficial to the improvement in affinity constants for both monodentate and bidentate arsenate complexes. The high adsorption capacity of defective hematite (2.60 μmol/m2) compared to defect-free hematite (1.33 μmol/m2) is attributed to its large affinity constants as well as its more active surface sites, thereby playing a vital role in reducing the threats of heavy metals in the environment.

Postsynthetically Modified Polymers of Intrinsic Microporosity (PIMs) for Capturing Toxic Gases.

Polymers of intrinsic microporosity (PIMs) are promising materials for gas adsorption because of their high surface area, processability, and tailorable backbone. Specifically, nitrile groups on the backbone of PIM-1, an archetypal PIM, can be converted to other functional groups to selectively capture targeted gas molecules. Despite these appealing features of PIMs, their potential has mainly only been realized for the separation of nontoxic gases. Here, we prepared PIM-1 materials modified with carboxylic acid and amidoxime functional groups and investigated their performance as adsorbents for the capture of ammonia (NH3) and sulfur dioxide (SO2) gases. After determining the Brønsted acidity or basicity of the PIMs from potentiometric acid-base titrations, which can be correlated with affinity for acidic or basic toxic gases, we explored the uptake capacity toward NH3 and SO2, respectively. Gas sorption studies revealed that the carboxylated PIM showed higher affinity toward NH3 through the incorporation of Brønsted acid sites, while the amidoxime functionalized PIM exhibited affinity toward SO2 through the installed of slightly basic functional groups. Overall, this study highlights new insight into PIMs as solid sorbent materials for capturing toxic gases, which can be transferred to their potential use in practical applications, such as personal protective equipment or air filtration.

Development of the methods for determination of the active components in acidic and alkaline detergents

The goal of the study is to develop methods for determination of the active components in acidic and alkaline detergents of complex composition. The objects of analysis are acidic (KATELON 202 – 208) and alkaline (KATELON 111 – 115) detergents of complex composition, which, in addition to easily detectable components (KOH and/or NaOH, HNO3, and H3PO4), contain weak organic acids (lactic, glycolic, acetic), including polybasic acids — aminotrimethylenephosphonic and diethylenetriaminepenta (methylenephosphonic) acids or salts — tetrasodium salt of glutamic acid N,N-diacetate, trisodium salt of methylglycine diacetic acid. An acid-base potentiometric titration was carried out for each of the listed objects and calculation formulas were derived to determine the concentration of the active substances (except for KATELON 115) in terms of nitric or phosphoric acid for acidic detergents or sodium or potassium hydroxide for alkaline detergents, respectively. A specific titration procedure (with indication of the sample weight, the pH value at the end of titration and corresponding calculation formula) is given in the application data sheet for each detergent along with the metrological characteristics of the methods.

Acid Neutralizing Capacity of Selected Antacid Suspensions Available in the Ghanaian Market

Antacids are substances commonly used by patients to obtain fast symptomatic relief from dyspepsia. They are weak bases which neutralize excess gastric acid and subsequently raise the pH of the gastric contents. The potency of the antacids depends mainly on their acid neutralization capacity (ANC) and this can vary from one brand to another. Several dosage forms of antacids are available for use by patients. However, In Ghana, suspensions are the commonest dosage form of antacids which is preferred by patients.&#x0D; The objective of this study was to determine the acid neutralizing capacity of six (6) randomly selected brands of antacid suspensions on the Ghanaian market using potentiometric acid-base titration. The samples were coded A-F to avoid any bias in the study. All the sampled brands had more than one year to expiry as indicated on their label.&#x0D; Brand D had the highest ANC of 29.70 mEq/dose whiles brand A had the lowest ANC of 11.25 mEq/dose. From the results obtained, it can be inferred that acid neutralization can be more effective and rapidly achieved with liquid antacids containing a high amount of magnesium hydroxide and aluminium Hydroxide. Hence, for acute symptomatic relief from dyspepsia, antacids containing a higher concentration of magnesium hydroxide and aluminium hydroxide would be most beneficial to patients.

Experimental evidence supports early silica cementation of the Ediacara Biota

Abstract Casts and molds of soft-bodied organisms in Ediacaran sandstones (“Ediacara-style” fossilization) have played an important role in reconstruction of the emergence and radiation of early complex macroscopic life. However, the preservational processes responsible for the Ediacara fossil record are still vigorously debated. Whereas classic studies proposed fossilization via rapid sulfide mineralization of carcass and matground surfaces, a more recent view posits silica as the key mineral involved in their preservation. We performed experiments in which a variety of soft-bodied organisms were exposed to silica-rich solutions at concentrations considered characteristic of Ediacaran seawater (2 mM). Our results document continuous precipitation of amorphous silica onto the surfaces of these organic tissues under constant and normal marine pH values (7.8). Mineral formation was accompanied by a progressive decrease in the dissolved silica (DSi) concentration of the experimental solution to levels well below amorphous silica saturation. Additionally, we find that the magnitude of silica precipitation is correlated to each organism’s functional-group chemistry, as measured by potentiometric acid-base titrations. We suggest that a wide range of soft-bodied organisms were prone to silicification in Ediacaran marine environments characterized by anactualistically high DSi concentrations. This provides further support for the model that the extraordinary moldic preservation of the Ediacara Biota was promoted by early silica cementation and that this mode of preservation can offer an accurate glimpse into the composition of those early animal ecosystems.

U(VI) sorption onto natural sorbents

This investigation made it possible to compare the sorption capacity of mineral and organic sorbents in respect to U(VI). In order to establish the patterns of sorption, natural sorbents with a high content of rock-forming mineral were used: glauconite, diatomite, zeolite, peat, brown and hard coals, shungite. Each sample was characterized using X-ray diffraction, X-ray fluorescence method, scanning electron microscope, XPS spectra, potentiometric acid–base titration and surface area measurement. The partitioning of U(VI) was determined by sequential extraction technique. It was shown that the highest sorption ability and strongest interaction towards U(VI) is common to peat and brown coal.

Electrosurface Properties of Detonation Nanodiamond Hydrosols as Depending on the Size of Dispersed Particles

The electrosurface properties have been experimentally studied for different monodisperse fractions of hydrosol particles of detonation nanodiamond (DND) obtained by air annealing of particle agglomerates, their deagglomeration by sonication in demineralized water, and subsequent centrifugation of resulting hydrosols. The acid-base potentiometric titration and laser Doppler electrophoresis have been employed to obtain the pH dependences of the surface charge density and electrophoretic mobility of the nanoparticles in hydrosols containing 0.001 M KCl within a pH range of 3.5–10.5. A decrease in the average size of DND nanoparticles leads to an increase in the absolute values of the density of their negative surface charge. The high surface potential values calculated for these DND samples are independent of the nanoparticle size. The absolute values of the electrokinetic charge density, which are an order of magnitude lower than those of the surface charge, increase significantly with a decrease in the particle size.

Estimating the thermodynamic equilibrium constants of metal oxide particles through a general electrophoresis model

We propose an efficient and convenient procedure for estimating the thermodynamic equilibrium constants of the surface dissociation/association reactions of metal oxide particles through electrophoresis measurements and a general electrophoresis model, which takes account of essentially all the relevant factors. These constants are usually estimated experimentally through a tedious potentiometric acid-base titration procedure. In addition, since several assumptions need be made in applying this procedure, the results obtained can be unreliable, especially when the difference between the equilibrium constant of the dissociation reaction and that of the association reaction is small. Another merit of the procedure proposed is that the site density of the surface dissociation/associations functional groups need not be known in advance so that it becomes much more convenient and efficient than previous procedures. The applicability of the present procedure is examined by applying it to the cases of SiO2 and TiO2 nanoparticles dispersed in an aqueous NaCl solution.

Striking analogies and dissimilarities between graphene oxides and humic acids: pH-dependent charging and colloidal stability

This study provides a comparative framework on the elucidation of analogies and differences in the interfacial protolytic processes and the associated colloidal behaviour of a typical humic acid (HA) and a set of single-layer graphene oxide (SLGO) and (multi-layered) graphite oxide samples in aqueous electrolyte media. The pH dependence of the surface charge densities of HA and SLGO was explored at three different salt concentrations by potentiometric acid-base titration, along with simultaneous determination of zeta potential and hydrodynamic sizes. Charging curves obtained in the pH range of 3 to 10 by cyclic titrations reveal the presence of a small hysteresis, proving the chemical stability of SLGO and graphite oxides in weakly acidic and alkaline solutions. HA and SLGO display a parallel shift of the pH-dependencies of their negative charge densities with increasing ionic strength, demonstrating a unique combination of particle and polyelectrolyte-like behaviour, which is absent for multi-layered graphite oxide exhibiting charging curves that resemble to “classical” colloidal particles. An accurate purification of SLGO results in inherent change in its surface properties; however, the salt tolerance of aqueous HA solutions is still superior to that of SLGO dispersions.

Surface charge properties of variable charge soils influenced by environmental factors

The variable charge soils that have been developed in tropical and subtropical zones are unique soils compared with the constant charge soils that have been developed in Europe and North America, especially the sensitivity of the particle surface charges to environmental factors in the variable charge soils. In this study, to avoid the interference of side reactions, experiments were performed using a fast potentiometric acid-base titration under non-equilibrium condition, and simulation by the surface complexation model to investigate the effects of environmental factors, including the impact of CO2 (in nitrogen/in air), the impact of different ionic strengths, the impact of dry-wet rotation (different soil-to-liquid (S/L) ratios), and the impact of seasonal temperature variation (different reaction temperatures), on the surface acidity constants and on the protonation/deprotonation processes in a typical variable charge soil collected in southern China. The titration results demonstrate that the observed measured apparent pH when surface charge equal to zero (pHQ=0) were significantly negative shifted in air, and negative correlations were proven between the S/L ratios, the reaction temperatures and the observed pHQ=0 values. The simulation results from the fast titration prove that changing the S/L ratios modifies the surface species concentration (≡SOH, ≡SOH2+, and ≡SO−), further influencing the modification of the surface charge properties of the soil. Meanwhile, an increase in temperature not only increases the amount of the surface species concentration but also causes an enlargement of the protonation/deprotonation processing range, which may increase the acid-base buffer capacity of soils. Taken together, these results can help us to further understand the natural soil acidification processes occurring in variable charge soils, especially in the southern areas of China.

Synthesis of a Novel Sensor Electrode Based on Bromothymol Blue as an Indicator Electrode in Potentiometric Acid-Base Titration in Aqueous Solution

Synthesis of a Novel Sensor Electrode Based on Bromothymol Blue as an Indicator Electrode in Potentiometric Acid-Base Titration in Aqueous Solution