Potentiometric Acid-base Titrations Research Articles

Synthesis and Characterization of PAMAM Dendrimer-Based Multifunctional Nanodevices for Targeting αvβ3 Integrins

We have synthesized a stable and clinically relevant nanodevice (cRGD-BT-ND; ND for short) that exhibits superior binding to the biologic target alphavbeta3 integrins, when either compared to the same free cRGD peptide or to the biotinylated nanodevice without covalently attached peptides (BT-ND). Selective targeting of alphavbeta3 integrins was achieved by coupling cyclic cRGD peptides to the nanodevice (ND) surface, while biotin groups (BT) were used for amplified detection of bound cRGD-BT-ND by anti-biotin antibody or avidin linked to horseradish peroxidase after binding. The synthesis involved the following steps: the amino-terminated ethylenediamine core generation 5 poly(amidoamine) (PAMAM_E5.NH2) dendrimer was first partially acetylated and then biotinylated, and residual primary amine termini were converted to succinamic acid groups (SAH), some of which finally were conjugated with cRGD peptide residues through the amino group of the lysine side chain. The starting material and all derivatives were extensively characterized by polyacrylamide gel electrophoresis (PAGE), size exclusion chromatography (SEC), potentiometric acid-base titration, MALDI-TOF, and NMR. Cytotoxicity of all dendrimer derivatives was examined in B16F10 melanoma cell cultures using the XTT colorimetric assay for cellular viability. Binding of nanodevices to the biological target was determined using plates coated with human alphavbeta3 integrin and alphavbeta3 receptor expressing human dermal microvascular endothelial cells (HDMECs). The PAMAM_E5.(NHAc)72(NHBT)8(NHSAH)35(NHSA-cR GD)4 nanodevice is nontoxic within physiologic concentration ranges and specifically binds to the alphavbeta3 integrins, apparently much stronger than the cyclic cRGD peptide itself.

Enhancing chemical analysis with signal derivatization using simple available software packages

Derivative techniques for analytical signal processing are useful for solving some noise and signal resolution problems in various fields of study such as titrimetry, spectrophotometry, chromatography and electrochemistry. The broad use of these techniques, however, is often limited by costly inflexible built-in software packages in commercial analytical instruments. We propose here the application of commercial simple software packages such as Microsoft® Excel and Microcal Origin for signal smoothing and fitting, and for obtaining derivative analytical signals in batch and flow-based analyses, including potentiometric titration, spectrophotometry, chromatography, voltammetry and sequential injection analysis (SIA). The worldwide (especially Excel) software packages are easy-to-use for less experienced users and have also capabilities for advanced users, and therefore employing such packages can result in expansion of useful derivative techniques. We demonstrate application of the available package-aided derivative capabilities for enhancing some chemical analyses, including potentiometric acid–base titration, Bradford assay of protein, chromatographic separation of ajmaline and reserpine and anodic stripping voltammetry of copper. The derivative signals from smoothed and fitted curves offer better accuracy and precision, even for non-resolving peaks and tailing peaks. In some cases, the optimization of experimental conditions is not further required, which can lead to fast method development.

Electrochemical determination of acidity level and dissociation of formic acid/water mixtures as solvent

The autoprotolysis constant K HS of formic acid/water mixtures as solvent has been calculated from acid–base potentiometric titration curves. A correlation of the acidity scale p K HS of each medium versus pure water has been implemented owing to the Strehlow R 0(H +) electrochemical redox function. The results show that formic acid/water mixtures are much more dissociated than pure water; such media are sufficiently dissociated to allow electrochemical measures without addition of an electrolyte. It has also been shown that for a same H + concentration the activity of protons increases with formic acid concentration. For more than 80 wt.% of formic acid the acidity is sufficiently increased to locate the whole acidity scale p K HS in the super acid medium of the generalized acidity scale p H H 2 O .

Acid-base chemistry of montmorillonitic and beidellitic-montmorillonitic smectite

Suspension of a Tunisian purified smectite and American montmorillonite are studied by acid-base potentiometric and mass titrations. These experimental methods are used to determine the point of zero net proton charge (PZNPC). A very good agreement is observed between the two kinds of experiments. The two Namontmorillonites, studied at different ionic strengths, present proton adsorption vs. pH curves with a common crossing point. The PZNPC of the edge sites are 8.02 for Tunisian purified smectite and 8.11 for pure American montmorillonite. By analyzing the proton adsorption or desorption (H+ vs. pH) curves, one may assume the presence of four active sites at the surface. The montmorillonite surface undergoes two successive protonations and two successive deprotonations. Below pH A1OH groups) occur simultaneously. For pH > PZNPC and in alkaline pH range, deprotonation of surface hydroxyl groups exposed at the edge sites (>SiOH, and >A1OH at high pH) of montmorillonite platelets causes an overall negative charge.

Spectroscopic and Potentiometrie Studies on Derivatized Natural Humic Acid

Isolated soil humic acid (HA) and commercial Aldrich HA were derivatized by esterification with methanol-thionyl and acetylation with acetic anhidride, in order to obtain derivatives with selectively blocked carboxyl and phenol groups, respectively. Results obtained by FT-IR spectroscopy and potentiometry show that the methanol-thionyl procedure is a selective, specific and efficient route for blocking carboxyl groups. The good correlation between results obtained by direct potentiometry after HA esterification and by classical calcium-acetate and baryta exchange methods suggests that esterification followed by direct acid-base potentiometric titration can be used as a method for the estimation of carboxyl and phenol group contents. Phenol groups can not be specifically identified by the acetylation method, due to the low selectivity of the acetylation method. The average values of apparent and intrinsic pK of underivatized and derivatized HAs confirm decrease in ionizable groups content due to derivatization and their values are related to the different chemical structures of the acids.

Characterization of copper adsorption on oxidized activated carbon

This paper discusses Cu(II) ion adsorption onto oxidized activated carbon with HNO3. The modifications introduced on the activated carbon (AC) surface were characterized by surface area measurement, FT-IR spectroscopy, titration by the traditional Boehm procedure, and potentiometric titration. A nonlinear method for fitting acid-base potentiometric titration data was applied to determine the concentration and pKa distributions of the ionizable acidic sites on AC, before and after HNO3 oxidation. Adsorption studies were conducted in batch mode for the concentration range 0.08 to 1.75 mmol L-1 and pH = 4.0. Results showed that the acid treatment of AC leads to a decrease in apparent surface areas and produces slight modifications in pore structure. However, oxidation strongly influenced the AC surface chemical structures by increasing the concentration of acidic groups with pKa < 6.0, which significantly improved Cu(II) ion adsorption. The uptake enhanced for oxidized carbon was related to the presence of acidic groups with pKa lower than 4, revealing the important role of acidic groups in adsorption by ion-exchange mechanisms.

A comparison of the properties of polyurethane immobilised Sphagnum moss, seaweed, sunflower waste and maize for the biosorption of Cu, Pb, Zn and Ni in continuous flow packed columns

The biosorption of Cu, Pb, Zn and Ni from a mixed solution of the metals was investigated in continuous flow packed columns containing polyurethane immobilised biomass. The characteristics and biosorption properties of Sphagnum moss, the brown seaweed Ascophyllum nodosum, waste biomass from the preparation of sunflower oil, and whole plant maize were compared. All the biomass types showed a preference for the sequestration of Pb followed by Cu, with Ni and Zn having roughly equal affinity. With continuous metal loading to the column there was an initial binding of all metals and then a displacement of the lower affinity metals by those with a high affinity. This led to a chromatographic effect in the column with breakthrough concentrations for low-affinity metals higher than the concentration in the feed. A similar phenomenon was found on desorption using acidic solutions where low-affinity metals were desorbed preferentially. The results also indicated that despite competitive displacement of one metal species by another the biomass appeared to succeed in retaining some low-affinity metal species indicating that there may be selective sites present with different affinity characteristics. When using a multi-metal solution with Cu, Pb, Zn and Ni at equal 10 mg l −1 concentrations as column influent, the total quantities of metal sequestered were: seaweed, 117.3 mg g −1; sunflower waste, 33.2 mg g −1; Sphagnum moss, 32.5 mg g −1; and maize, 2.3 mg g −1. The use of an acid base potentiometric titration showed a relationship between the number of acid functional groups and biosorption capacity, although this was not proportional for the biomass types studied. It can, however, be used in conjunction with a simple classification of metals into high and low-affinity bands to make a preliminary assessment of a biosorption system.

Analysis of poly(amidoamine)-succinamic acid dendrimers by slab-gel electrophoresis and capillary zone electrophoresis

Ethylenediamine (EDA)-core poly(amidoamine) (PAMAM) succinamic acid dendrimers (Ex.SAH, where x refers to the generation) were synthesized and analyzed by polyacrylamide gel electrophoresis (PAGE), size-exclusion chromatography (SEC), potentiometric acid-base titration, and capillary zone electrophoresis (CZE). Various generations (E1.SAH-E7.SAH) PAMAMs and a succinamic acid terminated core-shell tecto(dendrimer) (E5(E3.SAH)(n)) were first analyzed by PAGE. PAGE results show that the relative mobilities of generation 2 to generation 7 dendrimers decreased with the increasing number of generations. The molecular mass of a generation 5 core generation 3 shell tecto(dendrimer) (denoted as E5(E3.SAH)(n)) was determined to be between the Mw of E6.SAH and E7.SAH. CZE analysis allowed the evaluation of electrophoretic properties of given-generation dendrimers. The electrophoretic mobilities of individual generations PAMAM polyanions are similar, indicating that the separation mainly depends on their approximately identical charge/mass ratio. The E5(E3.SAH)(n) tectodendrimer had a lower electrophoretic mobility, which was consistent with its lower charge/mass ratio. The combination of PAGE and CZE analysis provides an alternative and effective way to characterize this group of PAMAM-succinamic acid dendrimers.

Effects of pH and ionic strength on electrokinetic properties of imogolite

Imogolite, an aluminosilicate with thread-like morphology and a nano-size tubular tunnel, is a unique clay mineral not only in morphology but also in colloidal stability. This clay is normally contained in volcanic ash soil and important in characterizing the physical and chemical properties of volcanic ash soil. In the present study, we performed measurements of acid–base potentiometric titration, electrophoresis, sediment volume and electrical conductivity for suspensions of natural imogolite. From these results, we clarified: (i) point of zero charge (PZC) existed near pH 6, where the imogolite has the same amount of positive and negative charges. An exchangeable cation was able to penetrate into the tube, but the counter-ion inside the tube played a small roll in the electrokinetic phenomena. (ii) The sediment volume increased with an increase in pH. The formed flocs were bulky at a high pH in accordance with reduction in the electrical repulsive force. This behavior was more pronounced with an increase in ionic strength. (iii) Surface conductmetry was very useful for detecting the mobile ion in the electrical double layer near the outer surface of the tube. The electrical surface conductance, the sediment volume and the electrophoretic mobility indicated that only the outer mobile ions contributed to the electrokinetic phenomena.

Biosorption of cadmium by the protonated macroalga Sargassum muticum: Binding analysis with a nonideal, competitive, and thermodynamically consistent adsorption (NICCA) model

Protonated biomass of the seaweed Sargassum muticum was investigated for its ability to remove cadmium(II) from aqueous solutions. In this work, a nonideal, semiempirical, thermodynamically consistent (NICCA) isotherm was proposed to fit the experimental ion binding data obtained in NaNO 3 0.05 mol L −1. This model describes the competition between protons and metal ions satisfactorily. Moreover, it reflects the complexity of the macromolecular systems that take part in biosorption considering the heterogeneity of the sorbent. It was demonstrated in this work that the NICCA isotherm constitutes a great improvement with respect to a simpler Langmuir competitive equation, which was not able to describe all the experimental data satisfactorily. Potentiometric acid–base titrations in the absence of cadmium were made to estimate the maximum amount of acid functional groups (2.61 mmol g −1) and the conditional proton binding parameters, log K ˜ H (3.8) and m H (0.54). The values of the binding parameters for the cadmium ion were chosen to provide the best simultaneous description of the isotherm at pH 4.5, as well as the dependence of cadmium adsorption on pH. Values of log K ˜ Cd (3.1), n Cd (1.8), and p (0.19) in the case of the NICCA isotherm or log K Cd (2.94–3.4) for Langmuir competitive models were obtained. Kinetic experiments were performed at two different pH values (3.0 and 4.5), establishing the time dependence that represents the sorption of cadmium with a pseudo-second-order kinetic model. It was observed that 4 h is enough to ensure that the equilibrium uptake was reached.

Effects of pH and ionic strength on the adsorption of phosphate and arsenate at the goethite–water interface

The surface properties of a well-crystallized synthetic goethite have been studied by acid–base potentiometric titrations, electrophoresis, and phosphate and arsenate adsorption isotherms at different pH and electrolyte concentrations. The PZC and IEP of the studied goethite were 9.3 ± 0.1 and 9.3 ± 0.2 , respectively. Phosphate and arsenate adsorption decrease as the pH increases in either 0.1 or 0.01 M KNO 3 solutions. Phosphate adsorption is more sensitive to changes in pH and ionic strength than that of arsenate. The combined effects of pH and ionic strength result in higher phosphate adsorption in acidic media at most ionic strengths, but result in lower phosphate adsorption in basic media and low ionic strengths. The CD-MUSIC model yields rather good fit of the experimental data. For phosphate it was necessary to postulate the presence of three inner-sphere surface complexes (monodentate nonprotonated, bidentate nonprotonated, and bidentate protonated). In contrast, arsenate could be well described by postulating only the presence of the two bidenate species. A small improvement of the arsenate adsorption data could be achieved by assuming the presence of a monodentate protonated species. Model predictions are in agreement with spectroscopic evidence, which suggest, especially for the case of arsenate, that mainly bidentate inner-sphere complexes are formed at the goethite–water interface.

Competitive sorption of protons and metal cations onto kaolinite: experiments and modeling

Competitive sorption of protons, Cu, and Pb onto kaolinite (KGa-2) was investigated over wide concentration ranges and quantitatively described using three different models based on surface complexation and cation exchange reactions. In all models, two types of binding sites were assumed for kaolinite: edge sites (SOH 0.5−) with pH-dependent charge and face sites (X −) with permanent negative charge. In a first step, proton sorption was measured by potentiometric acid–base titrations of kaolinite dispersed in 0.01, 0.03, and 0.1 M NaNO 3 electrolyte solutions. The acid–base titration data were fitted to obtain site densities and protonation constants for the edge and face sites, respectively. In a second step, the sorption of Cu and Pb onto kaolinite was investigated at fixed pH values by metal titration using ion-selective electrodes for Cu 2+ and Pb 2+, respectively, and by independent batch sorption experiments. Our metal sorption data cover a range of pH 4–8 for Cu and pH 4–6 for Pb, three different ionic strengths (0.01, 0.03, and 0.1 M NaNO 3), and up to eight orders of magnitude in free metal ion activity. An additional experiment was conducted to explore the sorption competition between Cu and Pb. In all three models, sorption of protons and metal cations to the edge sites of kaolinite was described with a 1-p K basic Stern (BS) approach. The three models differed only in the description of cation sorption to the face sites. In the first model (BS/GT), we used a Gaines–Thomas (GT) cation exchange equation for the face sites. This model yielded a satisfactory description of Cu sorption, but failed to describe Pb sorption isotherms at pH 4, 5, and 6. In the second model (BS/BS), we replaced the Gaines–Thomas equation by a basic Stern surface complexation formulation, thereby introducing electrostatic terms for sorption to face sites and allowing for free binding sites X −. This did not improve the fits of Cu or Pb sorption to kaolinite, however. In the third model (BS/BS ext), we extended the BS/BS-model by introducing additional monodentate sorption complexes at face sites (X Cu + and X PbNO 3). This model described both Cu and Pb sorption very well over the entire range in metal concentrations and pH. It also correctly predicted the competitive effect of Pb on sorption of Cu. Model calculations with all three models suggested that Cu and Pb were sorbed mainly to face sites at low pH, while sorption to edge sites dominated at high pH values.

Surface-charging behavior of Zn–Cr layered double hydroxide

A Zn-Cr layered double hydroxide (LDH) having the formula Zn(2)Cr(OH)(6)Cl(0.7)(CO(3))(0.15)2.1H(2)O was synthesized and characterized by powder X-ray diffraction, infrared spectroscopy, acid-base potentiometric titration, mass titration, electrophoretic mobility, and modeling of the electrical double layer. Adsorption of alizarin was also performed in order to show some particular features of the HDL. Net hydroxyl adsorption, which increases with increasing pH and decreasing supporting electrolyte concentration, takes place above pH 5. The electrophoretic mobility of the particles was always positive and it decreased when the pH was higher than 9. An isoelectric point of 12 could be estimated by extrapolating the data. The modified MUSIC model was used to estimate deprotonation constants of surface groups and different adsorption models were compared. Good fit of hydroxyl adsorption and electrophoresis could be achieved by considering both OH(-)/Cl(-) exchange at structural sites and proton desorption from surface hydroxyl groups. The modeling, in agreement with alizarin adsorption, indicates that most of the structural positive charge of the LDH is screened at the surface by exchanged anions and negatively charged surface groups. It also suggests that only structural charge sites initially neutralized by chloride ions are active for anion exchange. The remaining sites are blocked by carbonate and do not participate in the exchange.

Acid-base characteristics of carbon adsorbents, determined by potentiometric titration

For initial carbons of varied origin and carbons oxidized with various oxidizing agents, the continuous distribution function of surface groups with respect to the protonation constants was determined by acid-base potentiometric titration.

Lead Adsorption on a Soil: A Polarographic Study

AbstractThe adsorption of lead by a non‐contaminated loamy sand soil (Guarda, Portugal) was studied by voltammetric titrations using differential pulse polarography for pH values of 6.0, 6.8 and 7.2 and I of 0.5, 0.1 and 0.01 mol L−1. After lead or soil additions, residual lead in solution was measured in the presence of the soil particles after an equilibration period, thus with minimum sample manipulation. The characteristics of the surface groups were studied by acid base potentiometric titrations. Lead retention by the soil is influenced both by pH and ionic strength of the medium. From the voltammetric data surface constants and total available binding groups have been estimated according to a complex surface model for the different experimental conditions and the results interpreted in terms of the surface characteristics of the soil and the support medium. Surface binding capacities in the range 1 to 70 mmol Pb kg−1 were found depending on the pH and the ionic strength. The behavior found is in agreement with what is known in soil chemistry thus supporting the conclusion that voltammetric methods are quite appropriate for determining the extent of interaction between metal ions and soils.

Kinetics of Mercuration of 2- and 4-Methyl-containing Salts of Heterocyclic Cations

Kinetics of the monomercuration by the active methyl groups of 2- and 4-methyl-containing salts of quinolinium, pyridinium, benzothiazolium, and pyrrilium with mercury(II) acetate and trifluoroacetate in a mixture of anhydrous ethanol and acetonitrile (3:1) at 50-70°C were studied by means of acid-base potentiometric titration. The apparent second-order rate constants of these reactions were estimated, and factors affecting their rate were considered. A mechanism of the reactions was offered.

The Determination of the pKa of Multiprotic, Weak Acids by Analyzing Potentiometric Acid-Base Titration Data with Difference Plots

This paper discusses the pKa determination of mono-, di-, and triprotic weak acids with the help of difference (Bjerrum) plots, and the effect of strong acid–base concentration errors, ligand weight errors, and nonlinear electrode response. Experimental examples are given for the titration of an acidic heterocycle, as well as glycine, ethylenediamine, and tris(2-aminoethyl)amine (the last two after addition of excess HCl to ensure complete protonation) with standarized NaOH. The analysis procedure makes use of Microsoft Excel spreadsheets and nonlinear least squares curve fitting of the experimental data to the theoretical Bjerrum function. In addition to providing pKa values for mono- and multiprotic acids, this approach has been found suitable for detecting small errors in parameters, such as strong acid and ligand concentration, and corrections can often become necessary to achieve the best fit. Difference plots allow the pKa values of monoprotic and multiprotic weak acids to be determined rapidly and with good precision.

Surface Site Densities of Uranium Oxides: UO2, U3O8

ABSTRACTThe estimation of the surface site density for UO2 (main component of the SF matrix) is an important aspect to take into account in the development of radiolytical models for the dissolution of the spent nuclear fuel (SF). Also, other oxides can be formed on the SF surface due to the effect of radiolytically-formed oxidizing species. Due to the lack or reliable data in literature we have studied the surface site densities of two uranium oxides: UO2 and U3O8.For this determination, the knowledge of both the reactive surface area and the surface charge of the solid are necessary. In this work the reactive surface area of the two solids was determined by the BET method while the surface charge was determined by potentiometric acid-base titrations of suspensions of the solids at different ionic strengths (0.001, 0.01, and 0.1 mol·dm−3) under N2 atmosphere.The amount of adsorbed protons was calculated by subtracting blank titration from thesolid suspension titration. The single-site nonelectrostatic model (NEM) was used to describe titration data. Uranium speciation in solution was included in the model as well.The surface area values obtained were 0.15 ± 0.01 m2·g−1 for UO2 and 0.77 ± 0.02 m2·g−1 for U3O8, while the surface acidic site densities were determined to be 165 ± 10 sites·nm−2 and 48 ± 3 sites·nm−2 for UO2 and U3O8, respectively.

Curious aligned growth of carbon nanotubes under applied electric field

Magnetic multiwalled carbon nanotube/iron oxide composite (Fe3O4/MWCNT) was synthesized as an adsorbent for the preconcentration of U(VI) from aqueous solutions. The surface properties of Fe3O4/MWCNT were characterized via adopting FTIR, SEM and potentiometric acid–base titration. The sorption behaviors of U(VI) on the surface of Fe3O4/MWCNT were investigated under environmental conditions by using batch technique, such as pH, ionic strength and contact time. The pH-dependent U(VI) sorption behavior on Fe3O4/MWCNT illustrated that sorption mechanism of U(VI) was achieved by outer-sphere surface complexation at low pH values, while the sorption of U(VI) was achieved by inner-sphere surface complexation and simultaneous precipitation at high pH values. Besides, the pH-dependent sorption also demonstrated an optimal and feasible pH value of 7.0 by adopting Fe3O4/MWCNT in the preconcentration of U(VI) from aqueous solutions. The sorption kinetic experimental data could be well simulated by using the pseudo-second-order pattern. The Langmuir and Freundlich patterns were employed to simulate sorption isotherms of U(VI) at three different temperatures, the experimental results demonstrated that sorption process was favorable at higher environmental temperature. The maximum sorption capacity of U(VI) on Fe3O4/MWCNT was higher than that of major materials reported. Related experimental data further indicated that Fe3O4/MWCNT had satisfactory treatment performance for the simulated wastewater. The Fe3O4/MWCNT particles having strong magnetism could be favorably and easily separated from aqueous solution under an external magnetic field. The correlative experimental results further demonstrated that the Fe3O4/MWCNT composite could be a promising adsorbent for the preconcentration of radionuclides from large volumes of aqueous solution.

The use of the Nb–Nb 2O 5 as an oxide ion indicator electrode in fused salts

The use of the Nb–Nb 2O 5 electrode as an oxide ion indicator electrode in potentiometric titrations between acids and bases in fused salts was investigated. Two oxide ion acceptors (acids), NaPO 3 and Na 4P 2O 7 were titrated with oxide ion donors (bases) in fused KNO 3. The reaction was carried out at 625 K and the bases used in these titrations are K 2CO 3 and Na 2O 2. The electrode was found to serve as an excellent indicator for acid base potentiometric titrations in fused salts. It serves also as an oxide ion indicator for the titration of mixtures of acids in which chemical transformations are taking place during the course of titration. The experimental results were discussed in terms of the potential arrests and potential shifts upon the addition of the oxide ion donor to the acid.